食品和生物分离过程（英文）：32874_09

Chapter 9 Solids separation processes M. J. LEWIS, Department of Food Science and Technology, The University of Reading, RG6 6AP 9.1 INTRODUCTION This chapter will cover the separations involving solid foods, together with the properties of those solids which will influence that separation. Some mention will also be made of handling and transporting solids and preparatory processes, such as size reduction. The separation of solids from liquids and solids from gases is not covered in detail in this chapter, although a summary of the methods based on sedimentation and filtration is given in Table 9.1. In these applications, the term solids refers to discrete particles suspended within the fluid and not those dissolved or in the colloidal form, for which a range of other operations for their removal or separation is available. The objective may be to recover the solid for further processing or to remove the solid which may be contaminating the liquid or gas. The method selected also depends upon whether the solid is to be retained or discarded. To illustrate some of the difficulties in selecting solids separation methods, the re- moval of solids from gases will be further illustrated. This can be achieved by classifiers, cyclones, bag filters or electrostatic precipitators. In cyclones on milk powder plant, particles less than 5-10 pm may be lost. Cyclone losses of 0.35-1.0% of total production have been cited for dairy products. Such losses are now unacceptable for environmental reasons. High-efficiency cyclones have been used, whereby secondary air is introduced into the cyclone to increase the efficiency. However, these cyclones are not very success- ful with powders containing fat, as considerable free fat is generated and the powder sticks to the interior surface of the drier. Therefore it is not possible to install a milk drier where the powder recovery system consists of cyclones alone. Wet systems such as scrubbers have been installed, using the pasteurised milk, prior to evaporation, as the scrubbing liquid, thereby recovering the fines and heat. From a recovery standpoint, this would seem an excellent solution. However, from a hygiene and quality standpoint, these proved almost impossible to operate without bacteriological contamination. Most of these have now been removed (Knipschildt, 1986). The solution to the problem has been provided by bag filters, which are capable of reducing the particle concentration from 244 M. J. Lewis Table 9.1. Summary of mechanical solid separation techniques Solids from liquids Sedimentation: Principles: gravity, centrifugal, electrostatic, magnetic centrifugation Examples: gravity settlers, centrifugal clarifiers, hydrocyclones; use of chemical floc- Filtration: (see also Chapter 8; fat fractionation) Principles: gravity, vacuum, pressure and centrifugal Examples: sand and cake filters, rotary vacuum filters, cartridge and plate and frame filters, microfilters (Chapter 5); use of filter aids culants or air flotation Solids from gases Principles: sedimentation and filtration Examples: cyclones, bag filters, electrostatic precipitators 200 mg m-3 to below 10 mg m-3 air. The powder can be recovered from the bags and the ‘clean air’ can be used for heat exchange. Further details are provided by Knipschildt (1986). However, rather than removing all the particles, there may be a requirement to fractionate the powder, based on particle size (see Sections 9.3 and 9.4). This example illustrates the theme for this chapter, where the main emphasis is placed on the separation of components from within a solid matrix. Solids come in many forms, shapes and sizes, so the first part of the chapter will be devoted to discussion of the main properties of solid foods which will influence the different types of separation processes. 9.2 PHYSICAL PROPERTIES OF SOLIDS Solids come in a wide variety of shapes and sizes. All solid foods are particulate in nature and there are a wide range of sizes and shapes to contend with. Some examples are illustrated from the different food sectors in Table 9.2. It should be noted that although all these foods are regarded as solids, their moisture content may range from less than 10% to greater than 90%. Their moisture content and chemical composition can be found from foods composition tables, for example Paul and Southgate (1978) (see also Chapter 2). Indeed, one of the main objectives is often to remove selected components from the food. Some operations where separations from solids is involved and constitutes an impor- tant part of the process are: cleaning of agricultural produce (see Section 9.6.3); sorting and size grading, particularly for quality grading of fruit and vegetables; peeling of vegetables, dehulling of cereals and legumes and deboning or shelling of meat and fish; fractionation or recovery of the main components within the foods, e.g. proteins, fat, carbohydrates and minerals. Solids separation processes 245 Table 9.2. Some examples of solid foods Fruit: apples, oranges, grapes, blackcurrants, pears, bananas Vegetables: potatoes, carrots, sprouts, peas Cereals and legumes: rice, wheat, soyabeans, cowpeas, sorghum Animal produce: large carcasses, small joints, minced meats, fish fillets, prawns, shrimps Beverages: coffee beans, tea leaves, instant powders and granules Other powders: milled products, powders produced by drying and grinding methods and other shellfish A special range of operations and an area of increasing interest is concerned with the separation or fractionation of solids, in their particulate or powder form, and their recov- ery from other materials. In this chapter, emphasis will be placed on the separation of powders, based on factors such as size and shape, density differences, flow properties, colour and electrostatic charge, An important pretreatment for many such operations is size reduction. Methods of size reduction are discussed in Section 9.3.1. Size reduction increases the surface area and the surface area to volume ratio, thereby enhancing rates of heat and mass transfer. However, in some cases very fine powders provide processing problems, and size enlargement or agglomeration may be used to improve flow characteristics and wettability. Many foods which are solid in appearance, will also flow if the shear force provided is great enough, e.g. butter, spreads and starch doughs. This behaviour is known as plasticity. The flow behaviour of powders is also important and is discussed in more detail in Section 9.2.7. Some of the important physical properties of solid foods are listed in Table 9.3. These are discussed in more detail by Lewis (1990), Jowitt et al. (1983, 1987), Mohsenin (1984, 1986) and Peleg and Bagley (1983). Many of these properties are influenced by the chemical composition of the food, and in particular its moisture content. Of special interest in this context is the behaviour of particulate systems and the separation of mixtures. Many such separations are based on density differences. In some cases the powders may be subjected to various forces, gravitational, which are slow Table 9.3. Physical properties of solids Appearance, size, shape, size distribution, colour Specific gravity, particle density, bulk density, porosity, overrun (for aerated products) Thermal properties; specific heat, latent heat, thermal conductivity, thermal diffusivity, Rheologial properties; plasticity, elasticity, viscoelasticity, hardness Electrical conductance or resistance, electrical charge, dielectric constant, dielectric loss Diffusion and mass transfer characteristics specific enthalpy factor 246 M. J. Lewis compared to centrifugal forces, drag forces or electrical, electrostatic or magnetic forces. Also, the flow characteristics and behaviour of food powders are markedly different to those of fluids. Some of the physical properties of food powders will now be considered in more detail, especially those which will influence the effectiveness, quality and nature of the separation process. 9.2.1 Classification of powders Powders can be characterised in a large number of ways; Peleg (1983) gives some examples: by usage: e.g. flours, beverages, spices, sweeteners; by major component: e.g. starchy, proteinaceous, fatty; by process: e.g. ground powders, freeze-dried, agglomerated; by size: e.g. fine, coarse; by moisture sorption characteristics: e.g. hygroscopic; by flowability : free flowing, sticky, very cohesive. Further classification could be by hardness, by explosion potential or by microbial hazards. Hayes (1987) summarises a detailed system used for characterising a wide range of food powders based on density, size, flowability, abrasiveness, a range of miscellane- ous properties and hazards such as flammability, explosiveness and corrosive nature. Some important physical, chemical and functional properties of powders are given in Table 9.4. For products such as beverages, the palatability and sensory characteristics of the reconstituted products are important and may be variables considered when grading these products. Care should also be taken to ensure that the microbial count is within acceptable limits for the products. Determination of some of these properties for milk powders is described in publica- tions by the Society of Dairy Technology (SDT, 1980), and Schubert (1987a). Table 9.4. Factors contributing to the quality of powders Appearance Size and shape Wettability Sinkability Solubility Dispersibility Bulk density and particle density Palatability Nutrient content Microbiological quality Solids separation processes 247 9.2.2 Particle size and particle size distribution As mentioned earlier, food powders come in a wide range of sizes and shapes. Uniform shapes, such as spheres, can be characterised by one dimension, i.e. the diameter, whereas two or more measurements may be required for more complex shapes. Whatever the shape, there are several methods available to characterise the size and particle size distri- bution. Virtually all operations that result in the production of a powder, e.g. milling or spray drying, will give rise to a product with a distribution of particle sizes and this distribution is of extreme importance and will affect the bulk properties. Particle size may range over several orders of magnitude, ranging from less than 1 pm to as large as hundreds or even thousands of microns for some large granules. Particle size can be measured in principle by measuring any physical property which correlates with the geometric dimensions of the sample. According to Schubert (1987a) the attributes used to characterise particles may be classified as follows: geometric characteristics, such as linear dimensions, areas or volumes; mass; settling rates; interference techniques such as electrical field interference and light or laser scattering or diffraction. Based on these attributes, the following methods have been used for food materials: microscopy or other image scanning techniques; wet and dry sieving methods; electrical impedance methods such as the Coulter counter; laser diffraction patterns, such as the Malvern, Northrup and Cilas instruments. Since particles can vary in both shape and size, different methods of particle size analysis do not always give consistent results, both because of the different physical principles being exploited, but also because size and shape are interrelated. Sampling is also important to ensure that a representative sample is taken, usually by the method of quartering. Whatever method of measurement is used, a large number of particles must be meas- ured in order to ascertain the particle size distribution. It has been suggested for light microscopy that 200 measurements are made on each of three separate slides (Cloutt, 1983); this makes the method very tedious. The simplest way to present such results is in the form of a distribution curve, the two most common being in the form of either a frequency distribution (histogram) or a cumulative distribution (see Fig. 9.1). The cumu- lative distribution can be based on percentage oversize or percentage undersize. Percent- age undersize is used more often. The method used for data collection may give a distribution in terms of number of particles (for example by counting) or the mass (weight) of particles (for example by sieving). If the number of particles is known, the distribution can be represented by a frequency distribution. Table 9.5 gives some typical figures for the number of particles collected by microscopical examination, arranged into numbers falling within different size ranges (0- 10 pm) etc., together with the frequency distribution and cumulative number distribution undersize. 248 M. J. Lewis 100 80 60 10 40 20 0 LL *OB 0 0 20 40 Size (prn) 60 80 100 0 Fig. 9.1. (a) Frequency distribution (F), (b) cumulative distribution (0: see also data in Table 9.5. Table 9.5. Frequency distribution Size range Mean diameter Number Frequency Cumulative Cumulative OLm) of range (pm) in range distribution distribution volume distribution 0 to 10 5 5 1.8 1.8 0 10 to 20 15 15 5.6 7.4 0 20to30 25 35 12.9 20.3 2.0 30to40 35 50 18.4 38.7 7.0 40to50 45 55 20.3 59.0 20.0 50to60 55 50 18.5 77.5 41 .O 60to70 65 32 11.8 89.3 64.0 70to 80 75 20 7.4 96.7 85.0 80to90 85 8 2.9 99.6 98.0 90to 100 95 1 0.4 100.0 100.0 >loo 0 27 1 - mean diameter = 45.96 pm; d2/1 = 53.29 pm; d3/2 = 58.86 pm Cumulative number frequency indicates the percentage of the total number less than the mean diameter of the range. Cumulative volume represents the percentage of the total volume less than the mean diameter of the range. Other values which may be calculated from the distribution include the mean diameter and the median diameter and the standard deviation, which gives an indication of the spread. The simplest is the mean diameter, defined as C nidi /C ni 7 Solids separation processes 249 where ni is the number of particles in class i and di is the mean diameter of class i. The median diameter is the diameter which cuts the cumulative distribution in half. The d2/1 ratios and d3/2 ratios are also calculated. However, one widely used characteristic is the Sauter mean particle diameter (d& This is calculated from d3p = xnid:/xnid,? (9.1) This gives the diameter of the particle having the same surface area to volume ratio as the entire dispersion. The surface area/volume ratio = 6/d3/2. (9.2) Rates of heat transfer and mass transfer are proportional to the surface area to volume ratio. Therefore the surface area exposed has a big influence on physical properties, e.g. wettability, dispersion, dissolution and chemical reactions, such as oxidation, as well as the forces acting at the surface of powders. Equation (9.2) demonstrates that decreasing d312 will increase the surface area to volume ratio. Such data can be converted to frequency or cumulative distribution based on surface area or volume, by calculating the surface area and volume of each range. These cumula- tive distributions based on numbers and volume are compared in Fig. 9.2. This distinction is made because the shape of a numbers distribution and a mass or volume distribution is quite different because the area and volume distributions are most influenced by the larger diameter particles, since the volume = 4 m3. For example, it can be seen that only 10.7% of the particles are greater than 65 pm, whereas on a volume basis, 36% by volume are greater than 65 pm (Fig. 9.2). The weight fraction distribution would be similar to the volume fraction distribution, provided that the solid density is independent of particle size. The volume distribution is a common form of presentation in emulsion science, since it is often the larger particles which are likely to cause separation problems. Therefore it can be very informative to know what fractions by volume are bigger than a particular size. For example, in cream separation in milk, problems may arise from a relatively small number of large fat globules. 100 40 0 liL 20 0 0 20 40 60 80 100 Size (pm) Fig. 9.2. Comparison of volume distribution (V) and cumulative number distribution (N). See also data in Table 9.5. 250 M. J. Lewis Most of the discussion has focused upon spherical particles or those closely approxi- mating to these. However, the particle shape is also very likely to be important and a wide variety of shapes are also found. Irregular-shaped objects are more complicated to define and a number of characteristic dimensions have been used to represent them. Some are given in Table 9.6. Table 9.6. Characteristic diameters for irregular shaped particles 4 Surface diameter dv Volume diameter dd Drag diameter The diameter of a sphere having the same surface area as the particle The diameter of a sphere having the same volume as the particle The diameter of the particle having the same resistance to motion as the particle in a fluid of the same density and viscosity The width of the minimum square aperture through which the particle will pass. 4 Sieve diameter Other dimensions include the free-falling diameter and Stokes diameter, the projected area diameter and the specific surface diameter. In many cases the shape is more complex and a large number of dimensions would be required to describe the size and shape. Image analysis methods, whereby an image of the object is transferred to a computer screen and software is available to do any number of manipulations and calculations on the shape, are useful for this. The particle size and distribution has a pronounced effect on interparticle adhesion, which will affect some of the bulk properties, such as bulk density, porosity, flowability and wettability (see Section 9.2.5). 9.2.3 Particle density The density of an individual particle is important as it will determine whether the compo- nent will float or sink in water or any other solvent; the particle may or may not contain air. It can be measured using a specific gravity bottle, using a fluid in which it will not dissolve. Alternatively, it may be measured by flotation principles. However, surface forces may start to predominate for fine powders. In the absence of air, the particle density can be estimated from the following equation, based on the mass fractions and densities of the food components. P = 1/ [ (M1 lP1+ M2 / P2 + * * * + Mtl lPn ,I (9.3) where Ml is the mass fraction of component 1, p1 is the density of component 1 and n is the number of components. Data on mass fractions can be found from the Composition of Solids separation processes 25 1 Foods Tables (Paul and Southgate, 1978). A simple two-component model can be used (n = 2; water and solids) or a multicomponent system. The density of the major components are given as (kg m-3) (Peleg, 1983): water 1000 salt 2160 fat 900-950 citric acid 1540 protein 1400 cellulose 1270-1 6 10 sucrose 1590 starch 1500 glucose 1560 It is noteworthy that all solid components except fat are substantially more dense than water. However the differences between protein and the various types of carbohydrates are less marked, although minerals are much higher. In comparison air has a density of 1.27 kg m-3. This equation is not applicable where there is a substantial volume fraction of air in the particle. Any deviation between the experimentally determined value and the value calculated from the above equation may mean that there is substantial air within the solid. An estimate of the volume fraction of air (V,) can be made from (9.4) P= &pa + V,P~ =VaPa +(l-va)Ps where pa = density of air; ps = density of solid (estimated using eq. (9.2)) and p = true solid density, measured experimentally. This volume fraction (V,) of air is sometimes known as the internal porosity. Many other foods contain substantial amounts of air, for example mechanically worked doughs. One solution to determine the unaerated density is to measure the dough density at different pressures and extrapolate back to zero pressure (absolute) to obtain the unaerated density. This methodology could then be used to determine the extent of aeration during the mixing process. Note that from the compositional data, the calculated particle density of an apple is about 1064 kg m-3. Most apples float in water, indicating a density less than 1000 kg m-3, Mohsenin (1986) quotes a value of 846 kg m-3, suggesting an air content of about 20%. One important objective of blanching is to remove as much air as possible from fruit and vegetables prior to heat-treatment in sealed containers, to prevent exces- sive pressure development during their thermal processing. Data on the amount of air in fruits and vegetables are scarce in the food literature. There is evidence that this air is quickly displaced by water during soaking. Data on particle densities are provided by Lewis (1990), Mohsenin (1986), and Hayes (1987). Note that if the food is frozen, the density of ice should be substituted (916 kg m-3 at 0°C). However, not all the water is likely to be frozen, even at -30°C. The particle density of dehydrated powders is considerably affected by the conditions of spray drying. Increasing the solids content of the feed to the drier will result in higher particle densities and bulk densities. High particle densities will enhance sinkability and reconstitution properties. Differences in particle densities are exploited for several clean- ing and separation techniques, e.g. flotation, sedimentation and air classification. 252 M. J. Lewis 9.2.4 Forces of adhesion There will be interactions between particles, known as forces of adhesion and also be- tween particles and the walls of containing vessels. These forces of attraction will influ- ence how the material packs and how it will flow. Some of the mechanisms for adhesive forces have been described as liquid bridging by surface moisture or melted fat; electrostatic charges; molecular forces, such as Van der Waals and electrostatic forces; crystalline surface energy. Schubert (1987a) describes some of the models that have been used to quantify these forces, and the limitations of such models. There is some indication that interparticle adhesion increases with time, as the material consolidates. Flowability may be time-dependent and decrease with time. 9.2.5 Bulk properties Although the discussion so far has focused on individual particles, the behaviour of the collective mass of particles or bulk is very important in most operations. The bulk properties of fine powders are dependent upon geometry, size, surface characteristics, chemical composition, moisture content and processing history. Therefore it is difficult to put precise values on them and any cited values should be regarded as applying only to that specific circumstance, Peleg (1983). The term cohesive is used to describe the behaviour of powders, as they are influenced by forces of attraction (or repulsion) between particles. For powders that are cohesive, the ratio of the interparticle forces to the particles’ own weight is large. This ratio is also inversely proportional to the square of the particle size, which explains why small particles adhere to each other more strongly than large particles. Schubert (1987a) states that the majority of food particles are non-cohesive (and thus free flowing) only when the particle size exceeds 100 pm. Increase in moisture content makes powders more cohesive and increases the size at which the transition from cohesive to non-cohesive takes place. Some of the bulk properties will be considered in more detail. 9.2.6 Bulk density and porosity The bulk density is an important property, especially for storage and transportation, rather than separation processes. It is defined as the mass divided by the total volume occupied by the material. This total volume includes air trapped between the particles. The volume fraction trapped between the particles is known as the porosity (E), where E = PS - pb/ps (9.5) where ps and pb are measured solid and bulk densities. Methods for determining bulk density are described by the Society of Dairy Technology (1980) and Niro (1978). Terms used depend upon the method of determination and include loose bulk density and com- pacted and compressed bulk densities. Some bulk densities of powders are given in Table 9.7. Further values are given by Peleg (19831, Hayes (1987) and Schubert (1987a). Peleg (1983) argues that the relatively Solids separation processes 253 Table 9.7. Bulk density of some powders Bulk density Bulk density Powder (kg m-3) Powder (kg m-3> Oats 513 Milk 610 Wheat 785 Salt (granulated) 960 Flour 449 Sugar (granulated) 800 Cocoa 480 Sugar (powdered) 480 Coffee (instant) 330 Wheat flour 480 Coffee (ground and roasted) 330 Yeast (baker’s) 520 Corn starch 560 Egg (whole) 340 From the data of Milson and Kirk (1980). From the data of Peleg (1983). Taken from Lewis (1990) (with courtesy of Prentice Hall). low bulk density of many food powders cannot be explained solely by geometrical considerations. As mentioned, most food powders are known to be cohesive. Therefore open bed structures supported by interparticle forces are very likely to occur. Such materials are likely to have a low bulk density and high porosity. Factors that increase cohesiveness and interparticle forces are likely to decrease the bulk density. Moisture sorption tends to increased cohesiveness, mainly due to interparticle liquid bridges. Anticaking agents are believed to work by reducing cohesive forces and thereby increasing bulk density. Peleg (1983) provides data for the cohesiveness of some powders together with the effects of some anticaking agents on the bulk density of some food powders. Powders can be compressed either by tapping or mechanical compression, as in tableting. The forces involved in compression are much higher than those in tapping or mechanical vibration. The ratio of tapped bulk density to the loose bulk density is referred to as the Hausner ratio. Hayes (1987) quotes the following ranges, together with some values for some food powders: 1 .o- 1.1 free flowing 1.1-1.25 medium flowing 1.25-1.4 difficult >1.4 very difficult. Hayes also refers to another index, termed the ‘Novadel Tap Test’, which is related to the percentage volume decrease on tapping. The larger the volume decrease, the poorer is the flowability. Peleg (1983) states that the Hausner ratio may be used for flowability index in powders, where friction is the major obstacle to flow, but that there is no evidence that it is useful for cohesive powders. When powders are compressed the powder bed deforms and a number of mechanisms are involved, including spatial rearrangement of the particles without deformation 254 M. J. Lewis together with those brought about by fragmentation and plastic deformation of the particles. For cohesive powders, the open structure supported by interparticle forces is relatively easily overcome by the compressive force and there is a relatively large change of bulk density with pressure. Non-cohesive powders show relatively little change of buik density with pressure. An empirical relationship of the following form is found to fit experimental data well: p~ =a + b log s (9.6) where p~ is the bulk density, a and b are constants and s is the shear stress. The constant b is defined as the compressibility. High values of b indicate a cohesive powder, whereas low values indicate a non-cohesive powder. Some values for different powders are given by Peleg (1983). The use of anticaking agents was found to reduce the compressibility. 9.2.7 Flowability The flowability of powders is very important in their handling. Some indices of flow- ability have already been discussed. Generally flowability increases with increasing particle size and decreasing moisture content, As well as compressibility and cohesiveness, other factors used to assess flow- ability are as follows: Slide angle. This is measured by placing the powder sample on a flat smooth horizon- tal surface, which is then slowly inclined until the powder begins to move. The angle at which movement occurs is known as the slide angle. Angle of repose. This is useful in the design of powder handling systems. Its value depends upon the method of determination, which is usually by forming a heap. Other methods involve a bed rupture or a rotating drum method. Its magnitude is affected by frictional forces and interparticle attractive forces, which become dominant in cohe- sive powders. According to Carr (1976), angles of up to 35' indicate free flowability; 35-45" indicates some cohesiveness; 45-55' indicates cohesiveness or loss of free flowability ; >55' indicates very high cohesiveness, very limited or zero flow These parameters are empirical in nature and often the results are not applicable, when conditions are changed (Peleg, 1977). Peleg (1977) and Schubert (1987a) have described a more fundamental method for looking at the flow behaviour of powders, based on the work of Jenike, described by Leniger and Beverloo (1975). A flow cell is used, where the powder is first consolidated to a particular bulk density and porosity (see Fig. 9.3(a)). It is then subjected to a compressive force (N) and the shear force (S) required to cause the powder to yield and shear is determined. These readings are converted to a normal stress (0) (N/A) and a shear stress (7) (SIA). This procedure of determining the shear stress is repeated for a number of different normal stress values. The information is presented on a plot of shear stress against normal stress and gives the yield locus, for that particular porosity. Figure 9.3(b) shows the data obtained for a non-cohesive powder, which can be characterised by the angle of friction (a). Also in all cases a large angle of friction, Solids separation processes 255 tN --t Tp Tp 0 0 (4 (b) (c) (4 Tb Fc (J1 (J Fig. 9.3. Solid characterisation: (a) Jenike flow cell; (b) normal stress against shear stress, for a non-cohesive powder, a = angle of friction; (c) yield locus for a cohesive powder for powders compacted to different initial porosities; porosity 1 > 3; (d) Mohrs circles, showing the unconfined yield stress (f,) and major consolidation stress (01). indicating high interparticle friction, does not always mean poor flowability, for example dry sand has a high value but flows quite well. Figure 9.3(c) shows the yield locus for a cohesive powder, at a particular porosity. However, if the porosity of the sample is increased, the yield locus will change. There- fore there are a family of curves at different porosities. Also the curves do not pass through the origin. This yield locus data therefore describes the flow behaviour of pow ders. This data is used to determine the unconfined yield stress (f,) and the major consolida- tion stress ((T~), by application of Mohrs circles (see Peleg, 1977; Schubert, 1987a; Leniger and Beverloo, 1975). The ratio of ol/fc is termed the Jenike flow function, which has also been used as an indicator of the flowability of powders. Its values correspond to the following character- istics: <2 very cohesive, non-flowing 24 cohesive 4-10 easy flowing >10 free flowing. This more fundamental information is extremely useful for designing hoppers, bins, pneumatic conveying systems and dispensers. Similar measurements can be made using a more sophisticated annular flow cell, which is capable of reliable shear force determina- tions at low normal stresses. The hydrodynamics of powder flow are different to that for liquids. The pressure does not increase linearly with height, rather it is almost independent. Also they can resist appreciable shear stress and can, when compacted, form mechanically stable structures 256 M. J. Lewis that may halt flow. Also any pressure or compaction can increase the mechanical strength and hence the flowability. 9.3 SEPARATION OF PARTICULATES AND POWDERS This section will be most concerned with the separation or recovery of solids from within a solid matrix or from a particulate system. The main emphasis will be those in fine particulate form, so the production of material in a form suitable for separations is often crucial for the process. In this respect size reduction and milling equipment is important. 9.3.1 Size reduction Size reduction is a very important preliminary operation for separation processes for many cereals, legumes and other commodity crops, as well as for extraction operations, e.g. tea and coffee, or expression processes, e.g. fruit juice expulsion or oil extraction. Sugar is one example of a commodity that comes in a range of particle sizes, e.g. granular, caster, and icing sugars. Some data on sieve size measurements of different sugars is cited by Hayes (1987). The term ‘crushing’ is applied to the reduction of coarse material down to a size of about 3 mm, whereas ‘grinding’ is commonly used for the production of finer powdered material. The degree of size reduction can be characterised by the size reduction ratio (SRR), where (9.7) average size of feed average size of product SRR = Several stages may be required if the overall size reduction is large. The main forces involved are compressive forces, impact forces and shear or attrition forces. Usually there is a predominant force involved for each type of equipment, al- though the other forces may be involved to a lesser extent. The fracture resistance in- creases with decreasing particle size. Aspects which need to be considered in the selection of the most appropriate equip- ment for size reduction are the particle size range required and the hardness of the material. Hardness can be measured in Mohs, whose scale ranges between 0 and 8.5. On this hardness scale, most foods are either very soft (~1.5 Moh); soft (1.5 to 2.5 Moh) or medium hard (2.5 to 4.5 Moh). More details are provided by Hayes (1987) and Christison (1991). Very soft materials such as dried fruit, dried plant material, meat and fish may be processed with a Colworth stomacher down to 100 pm, or high-speed cutters, such as a bowl choppers. Other mills for processing grain cereals, legumes, salt, and sugar include the follow- ing: (1) Hummer mills. These are very much general-purpose mills. Size reduction is mainly due to impact forces. They are widely used for peppers and other spices, sugar and dried milk powder. (2) Roller mills. These can be one or several sets of rollers; size reduction is by Solids separation processes 257 compressive forces; size reduction ratio is usually below 5. These are widely used for the milling of wheat and refining of chocolate. (Size range 10-1000 pm.) (3) Disc attrition mills. These come in a number of designs. Simple disc mills have two discs, one of which is stationary and the other moving. The speed is relatively slow, with a peripheral velocity of 4-8 m s-'. The feed material enters at the centre of the discs and the discs are profiled in such a way as to cause grinding to occur as the material falls radially across the grinding discs. (Size range down to 100 pm.) On the other hand, impact pulveriser mills, such as pin disc or stud mills, operate at high rotational speeds, creating peripheral velocities up to 200 m s-*. In this case the discs contain pins or studs, which intermesh. In the simple design there is one stationary and one moving disc, whereas in other designs both discs move. These types of mill can produce very fine powders, suitable for air classification (see Section 9.4). Another high-speed mill is the high-speed rotor mill, which is a variant of the hammer mill. A rotor with a series of hardened blades rotates at speeds in excess of 15 000 r.p.m. and the fines pass through a sieve ring, fitted round the circumference. (4) Bull mills. This is a tumbling mill and is used for very fine grinding processes. It comprises a horizontal slow-speed rotating cylinder which contains steel balls of flint stones; the balls are normally 25-150 mm in diameter. The mechanism is by impact and shear. The optimum speed of rotation is about 75% of the critical speed, which is defined as the speed which causes the steel balls to centrifuge. Two or more mill types may be required to achieve the desired level of size reduction. The size rediction achieved often depends on whether the discharge product is released immediately or whether it is restricted by use of a screen. In the latter case the residence time within the action zone is increased until the particle is smaller than that of the screen. A third alternative is to allow all the particles to leave unrestricted and to separate them externally, recycling oversize particles for further milling. The particle size required also affects the cost of milling and the energy requirement: the latter is based on the following equation: dE - K, dD D" -- - where dE is the energy required to produce a small change in diameter dD and K, is a characteristic of the material. The three main equations result from different values of n. (Note: n is a power-law exponent.) n = 1: E = K,, ln[Dl/D2]; Kick's law Bond's law n = 7: 3 E = 2K,[L--&] D20.5 n=2: E = Kn,[&-+] Rittinger's law 258 M. J. Lewis where D2 is the final diameter and D1 is the initial diameter. Energy requirements are well in excess of those required to produce a new surface, as much energy is lost in friction and other inefficiencies. Care must be taken to ensure that the sample does not get too hot during size reduction. Wet milling can be achieved by wetting the material and the feedstock is ground in a suspension in the liquid, which is often water. Energy requirements are usually slightly higher than for dry milling but a finer powder is obtained and dust problems are elimi- nated. The wear and tear on the mill is also higher. Often wet milling is useful as part of an extraction process, whereby soluble components are transferred from the solid to the liquid phase. Wet milling is popular for corn milling. More information on size reduction is provided by Brennan et al. (1990), Loncin and Merson (1979), and Christison (1991). One very pertinent comment about milling is that the weakness of a material may be at the juncture of different components, thereby initiating a crude form of fractionation process, which can be further exploited. 9.3.2 Sieving Sieving is probably the easiest and most popular method for size analysis and separation of the components within powders. A sieve is an open container which has uniform square openings in the base. The screen aperture is defined as the space between the individual wires of a wire mesh screen, and the mesh number is the number of wires per linear inch. However, this nomenclature has now been deleted from the latest British Standards although it may still be regularly encountered. A whole range of standard sieves are available up to 25 mm in size. Sieves for powders and agglomerates may be from a few millimetres, down to about 20 pm. Sieves in common use are produced to a number of standards, e.g. BS 410,1969 (see Table 9.8), IS0 R 565 and ASTM Ell 81. In most cases the screen interval, which is the ratio of successive sizes in a test series, is either 2, 2°.5 or 2°.25. The high mesh sieves may be too fragile for some applications. A more robust set consists of the Institute of Mining (IMM) screens, where the thickness of the wire is approximately the same as the aperture size. The Tyler series is another used in America. Coulson and Richardson (1978) summarise these. Hayes (1987) also gives a summary of the most widely used mesh sizes, the smallest being about 44 pm. These sizes are slightly higher than both the British and American standards. Some information is also provided by Christison (1991). A number of special sieves are available for particle size measure- ment below 50 pn. Complications arise below this size for reasons discussed later. A single sieve separates a particulate material into two fractions. When sieving materials of a non-spherical nature, the situation is complicated by the fact that particles with a size close to that of the nominal aperture of the test sieve may pass through only when presented in a favourable position. Such particles, sometimes termed ‘near-aperture particles’, may partially block or blind the sieve aperture and reduce its effective area. Therefore some particles less than the nominal sieve diameter will be retained by the sieve. Particles much smaller than the sieve nominal diameter pass through fairly rapidly, whereas those close to the sieve diameter take a much longer time and a small fraction may never pass through. The effectiveness of a sieving process depends upon the amount Solids separation processes 259 Table 9.8. Common mesh sizes Mesh size Aperture (mm) Mesh size Aperture (pm) 4 4.00 44 355 6 2.80 52 315 7 2.24 60 250 8 2.00 72 224 10 1.80 85 180 12 1.40 100 160 14 1.25 120 125 16 1 .oo 150 112 18 0.90 170 90 22 0.7 1 200 71 25 0.63 240 63 30 0.50 300 56 36 0.45 350 45 400 36 of material placed on the sieve, the type of movement imparted to the sieve and the time allowed for the process. A small charge will result in more effective sieving, but care should be taken to ensure that a uniform sample is taken, particularly for analysis. The criteria may well be different for analysis and separation. The sieving time can be af- fected by the following factors (British Standards Institute 1796, 1989): the material characteristics, e.g. fineness, particle shape, size distribution, density; intensity of sieving; nominal aperture size of the test sieve; characteristics of sieving medium; humidity of the air. Materials can also be sieved in a liquid, the procedure being referred to as wet sieving. Wet sieving is used for extremely fine particles, e.g. below 50 pm, or particles that become electrically charged. One advantage is that it reduces inter-particle adhesion. Also powders that cannot be dispersed or materials in liquid suspension should be sieved wet in order to facilitate dispersion of the primary particles, which may cause the coarser particles to agglomerate or may be difficult to disperse. A wide variety of dispersant liquids is available, for example ethanol, isobutanol or benzene, for wheat flours, and octanol for milk powder. More details are provided by Schubert (1987a). Usually a number of sieves are clamped together, with the largest on the top. The material is placed in the top sieve and the sieves are subjected to a vibratory mechanism. In this way a distribution of particle sizes can be determined. This method is recommended for particle size estimation between 100 and 1000 pm. More sophisticated equipment available for research and development relies on vibrations caused by electro- magnetic or sonic mechanisms (Christison, 1991). 260 M. J. Lewis A special type of sieve is the air jet sieve, in which a reduced pressure is applied to the underside of the sieve. A jet of air is discharged upwards from a radial slotted arm rotating continuously under the screen and this helps prevent blinding by fluidising the particles that are likely to cause blinding. A diagram of the air jet sieve is shown in Fig. 9.4. One separation is performed at a time. This is very useful for analyses involving smaller particles. Fig. 9.4. Air-jet sieve (courtesy of Hosokawa Micron Ltd). Test sieving is camed out on a wide variety of food materials and for many different purposes, for particle size analysis, for producing materials with a known particle size distribution or for supplying materials of a specified size range. Problems encountered with sieving result from sample stickiness, sieve blockage and agglomeration. Strumpf (1986) contests that these problems increase exponentially as the sieve size decreases. One of the main applications of sieving is in the flour industry, to separate the differ- ent fractions of flour. Names of particular sieving processes include scalping, to separate the break stock from the remainder of the break grind; dusting, bolting and dressing, which involves sieving flour from the coarser particles; and grading, which is classifying mixtures of semolina, middlings and dunst into fractions of restricted particle size range. Semolina, middlings and dunst are terms used to describe fractions of rolled endosperm of decreasing particle size (as determined by sieving). Definitions are provided by Kent (1983). Other terms sometimes used are scalping, for removing large particles, and dedusting, for removing small particles, Brennan ef al. (1990). 9.4 AIR CLASSIFICATION 9.4.1 Introduction Air classification is a means of using a gaseous entraining medium, which is usually air, to separate a particulate feed material into a coarse and fine stream, on a dry basis. According to Klumpar et al. (1986), classifier designers take advantage of the following phenomena to achieve this objective: Solids separation processes 261 small particles fall more slowly in air than large particles; larger particles have a greater centrifugal force in cyclonic flow than smaller particles; smaller particles have less inertia and can change their direction of flow easier than large particles; larger particles require a higher conveying velocity; larger particles have a larger probability of colliding with a rotating blade. Separation is based mainly upon particle size, although other particle properties, such as shape, density, electric, magnetic and surface properties may play a part. The procedure of winnowing or aspiration is a traditional process to separate chaff from grain after threshing and is one of the simplest forms of air classification. The chaff is dispersed in the wind or by using an air stream. This principle is used on vining and combining machines for harvesting peas and grain. Another simple form of classification involves subjecting a powder, containing a range of particle sizes, to an upward airstream of constant and uniform velocity (Fig. 93a)). Some of the particles will become fluidised, some will be conveyed and carried away by the air stream and others will remain stationary. Consequently, some degree of separation is achieved. In principle, the remaining material could then be subjected to a higher velocity, removing another fraction, and this process could be continued. The forces involved in this simple process are the drag forces acting on the particles due to the air stream, which counteract that due to gravity. An alternative system is to use a long shallow, slightly sloping tray (see Fig. 9.5(b)). This type of separation also forms the basis of the simple zig-zag separator, illustrated in Fig. 9.5(c) and Fig. 9.6, which can be single or multiple tube. This is used for separating particles in the range 0.1 to 10 mm. In this case the separation is further enhanced by tortuous passages and collision surfaces, which are particularly effective at removing the larger and more dense particles. It can be used for dedusting operations and is also capable of removing shells or hulls from disintegrated peanuts, cottonseed, rapeseed or cocoa beans. However, the major interest in air classification is that it provides a means of separat- ing small particles which cannot be readily achieved by sieving, i.e. below 50 pm. Thus a 4 /-heed <7 Fines q-TJ 1 -1 F 4,,$ Feed 11 Fines F 7Farse Coarse (4 (b) (C) Fig. 9.5. Simple clnssifiers: (a) for aspiration F = fan; (b) for fractionation L = large; S = small particles; (c) zig-zag classifier. 262 M. J. Lewis Fig. 9.6. Commercial zig-zag classifier (courtesy of Hosokawa Micron Ltd). powder subject to air classification can be separated into two streams, one primarily below the required particle size and the other predominantly above it. The required particle size is referred to as the cut size and is discussed in more detail in Section 9.4.3. Cut sizes of interest in food processing operations may range between 2 and 50 pm. 9.4.2 Commercial air classifiers In many commercial air classifiers, the gravitational force used in the examples cited above is supplemented by a centrifugal force, induced by subjecting the particles to circular motion. This is essential for separating small particles and speeds up the separa- tion process. This centrifugal force is produced by a spiral or vortex air flow pattern, promoted by a tangential air inlet and the use of directional vanes or baffles. In addition there may also be a rotating disc or turbine, which further accelerates both the air and particles and increases the centrifugal force produced. This force acts toward the outside of the chamber, and counteracts the drag force which is produced by the air moving in a spiral direction toward the centre of the classifying chamber. The mode of operation of a typical classifier is as follows. The inlet air is mixed with the material to be separated. The feed particles are subjected to a centrifugal force originating from a revolving rotor and a drag force produced by the air current, which moves in a spiral direction toward the central shaft. The separation is based on differen- tial mass, density and shape. The larger and more dense particles are influenced by the mass-dependent centrifugal forces and move toward the outside of the chamber, where they are removed by a discharge worm-screw conveyor or some other means. The smaller, lighter particles are more subject to the frictional forces of the air current and move with the air stream, leaving from the centre of the classifying chamber, into a cyclone, where they are separated from the air. The relative magnitude of the two forces can be changed by altering the rotational speed of the disc and the air velocity; changing either will change the cut-size. Classifiers with the facility to change these factors inde- pendently, will be capable of improved operational flexibility. Although the bulk of the separation takes place within the classifying chamber, some preliminary removal of the coarser particles may be achieved outside the main classifying chamber. The disc or Solids separation processes 263 turbine can be mounted on a horizontal or vertical axis. The latter produces a centrifugal force in the horizontal plane and favours greater throughput, but less precision in cut-size (Fedoc, 1993). Some large capacity classifiers have several turbines mounted in one unit. Air classifiers are categorised by reference to a number of factors, such as: the forces acting upon the particles; e.g. the presence or absence of a rotor, the drag force of the air and the presence of collision forces, which hinder larger particles; the relative velocity and direction of the air and particles, controlled by their respec- tive feed systems; directional devices such as vanes, cones or zig-zag plates, which allow a change in direction and provide collision surfaces; location of the fan and fines collection device, whether they are situated internally or externally. Other important features are the capacity of the classifier and the energy utilisation. A comprehensive treatment of classifier types and their operating principles has been reviewed by Klumpar et al. (1986). For some classifiers processing coal dust and cement, flow rates of over 100 tonnes h-' can be handled. Laboratory classifiers are available which will handle batches of as little as 50 g of samples and which will separate 2-6 kg h-' on a continuous basis. Larger classifiers handling foods can process more than 5 tonnes h-I . One machine which has been widely used for food processing is the Alpine Mikroplex classifier (Fig. 9.7), which is rated up to 1.6 t h-' and a maximum energy consumption of 19 kW. This design uses a rotor on a horizontal axis. Material is fed by gravity between the fan and the vertical distributor plate. The air flow rate and direction of flow can be changed by vanes within the machine, and this is sufficient to change the cut size, which is also affected by feed rate, The cut size range is 3-30pm. The coarse material is removed by a discharge worm conveyor. It is categorised as a free vortex machine. Such machines are now largely being replaced by forced vortex machines (Fedoc, 1993), Coarse fraction knife edge Inlet for material to be classified Coarse fraction discharge worm Air guide vane Classifying chamber Spiral airflow Fig. 9.7. Alpine Microplex classifier (frontal cross-section view) (courtesy of Hosokawa Micron Ltd). 264 M. J. Lewis whereby the force is provided by means of a rotor or turbine, which disperses the particles into an air stream applied by a suction fan. These types of equipment operate under a slight vacuum. Some examples are shown in Fig. 9.8. 9.4.3 Process characterisation In most cases, air classification work is empirical in nature because of the difficulties in quantifying the forces acting upon a particle, with any degree of accuracy. One method of characterising the separation is by means of the cut size. Ideally, all particles below the cut size end up in the fines and all particles above the cut size end up in the coarse stream (see Fig 9.9(a)). However, there will always be a small fraction of particles smaller than the cut size in the coarse stream and an equally small proportion of particles larger than the cut size in the fines stream. The extent of this overlap and the cut size can be determined by measuring the particle size distribution of the coarse and fine streams (see Section 9.2.2), and presenting the data for both streams as a weight fre- quency distribution. The cut size is defned as that size where the weight of particles below the cut size in the coarse fraction is the same as the weight of coarse particles above that size in the fines stream (see Fig. 9.9(b)). The yields of fines (Yf) and coarse (Y,) streams need to be known. If they are equal, the point of overlap (Fig. 9.9(b)) gives the cut size. If they are not equal, which is most likely to be the case, the frequency distribution for the fine stream must be multiplied by the yield for the fine stream, and that for the coarse stream by the yield for the coarse stream (Fig. 9.9(c)). The cut size is given by the point of intersection of these curves. Factors which influence the cut size are the dimensions of the classifying chamber, peripheral forces and the spiral gradient. The cut point can be adjusted by varying the rotor speed, air velocity, vane setting and feeding rate. By equating these forces when they are in equilibrium, an equation for the cut size (d) can be derived. This is based on Stokes’ equation: d2 = [18pu,r/pu~] (9.8) where p = viscosity of air u, = radial speed of air r = clearance of classifier wheel p = particle density up = peripheral speed of particle (equivalent to rotational speed). Although it is not possible to predict the movement of a particle by Stokes’ law in an air classifier, because the forces acting upon the particle depend upon its position in the classifier, the equation is useful in that it predicts how the two main parameters, air flow rate and rotational speed, may influence the cut size. It predicts that increasing the air flow rate (u,) increases the cut size, whereas increasing the rotational speed (up) de- creases the cut size. The cut size for most operations is in the sub-sieve size range. Solids separation processes 265 Hosokawa Turboplex Ultra-Fine Classifier Fig. 9.8. Selection of air classifiers (courtesy of Hosokawa Micron Ltd). 266 M. J. Lewis IuuAdhk (4 (b) Size (c) Fig. 9.9. Cut size determination: (a) ideal separation; (b) real separation, weight frequency distribution; (c) weight frequency distribution corrected for yield. Therefore air classification provides an excellent means of separating powders, based on cut sizes below 40 pm. However, the cut size alone does not provide information on how sharp the separation is. An alternative method of evaluation is to determine the grade efficiency, which also has the advantage of indicating the sharpness of the separation. The particle frequency distribution is determined by weight for the coarse stream (qc(x)) and feed material (4L.x)) (see Fig. 9.9(b)). The yield is determined for the coarse stream Yc. The grade efficiency T(x) indicates for any particle size x, the mass fraction of feed material appearing in the coarse fraction. Thus ycqc (x) T(x) = ~ 4f (XI Thus grade efficiency can be plotted against particle size (see Fig. 9.10). The cut size is where the T(x) = 0.5, indicating the size of the particles, half of which appear in the coarse stream and therefore by difference, half of which appear in the fine stream. The sharpness of the separation is measured by the ratio k = [~&x75~], i.e. the ratio of the sizes giving grade efficiencies of 0.25 and 0.75 respectively. Ideally k = 1 .O. The best industrial air classifiers achieve k = 0.7, but typically commercial air classifi- ers show k values from 0.3 to 0.6 (Schubert, 1987b). qJ7 c Size 0 Fig. 9.10. Grade efficiency vs particle size: (a) ideal separation; (b) and (c) decreasing sharpness. Solids separation processes 267 Another factor used to define the separation achieved is the protein shift, which is widely used in those operations where fractionation of protein occurs. The protein shift (S,) for a simple separation is defined as (9.9) (Cp - Cpo) y s, = CPO where C, = proteins in fines, C,, = protein in flour and Y is the dry weight yield of fines. Table 9.9. Schubert (1987b) has shown that protein shift is largely independent of yield. Table 9.9. Protein shifts produced by air classification of flours from different cereals and legumes This can be determined by analysis of the resulting streams. Some values are shown in Flour Processing Protein shift (%) Barley Roller-milled 19 Barley Pin-milled 28 Malted barley Commercial grind 8 Malted barley Pin-milled 18 Oats Pin-milled 27-32 Triticale Pin-milled 28-36 Rice Turbo-milled 8-10 Potato Pin-milled 22-25 White bean Turbo-milled 22 Field bean Pin-milled 42 Fababean Pin-milled 45 Taken from Sosulski (1983a) (with courtesy of Chapman Hall). In an ideal separation all the protein will finish up in one stream, i.e. the fines. Under these conditions, the ideal protein content in the fines (CPldeA, ) will be C,,/Y, provided the yield is greater than the initial protein content. It is possible to compare the measured separation with the ideal case by defining an efficiency of protein enrichment (epe), where (CP - CPO) (CP,deai - CPO) epe = Values range from 0 to 1, the higher the value the more efficient is the separation process. Eliminating (CPideal ) gives (CP -Cp0)J cpou - Y) epe = (9.10) 268 M. J. Lewis However, this is little used, compared to protein shift. ing relationship: Schubert (1987b) gives an alternative method of analysing the data, using the follow- c, = [CPO/Y"] (9.11) where m is a measure of the separation. If the protein content in the fines is plotted against the yield (Y), a characteristic curve is obtained, which can be used to determine m. m = 1 m = 0 applies to an ideal curve, where C, = [Cpo/Y] no separation, C, = C,, This approach can also be used to measure the effectiveness of the separation. Higher values of m imply better separation. Examples are given for protein extraction from spent grain by air classification (m = 0.12) and wet processing (m = 0.48). The reason that wet processing is more effective is that the protein strongly adheres to the larger husks and is not effectively moved by air classification. The presence of a liquid breaks down these adhesive forces and improves the quality of the separation. 9.4.4 Applications Air classifiers are designed to grade endosperm and cotyledon particles in the subsieve size range of 2-60 pm, into subgroups based on differential mass, density and shape. This is below the particle size conveniently handled by sieves. With most cereals the separation of starch and protein is based primarily on size and shape rather than density, even though the density difference is significant. However, these subgroups may still represent subcellular structures which differ substantially in their physical and chemical properties. Most of the applications have been concerned with fractionation of the components of cereals and legumes, in particular the starch and protein fractions. The simplest process involves a single pass through the air classifier after size reduction. An alternative procedure is to have a double pass, where the coarse stream from the first stage of separation is further milled and reclassified to produce a second fines stream (Fig. 9.1 1). This results in two fines streams, which may be handled separately or recom- bined, and one coarse stream. Comparison of results from different workers is not straightforward, because not all the information is always presented. The quality of the separation will be influenced by the particle size range, cut point, yield, moisture content, feed rate, classifier type and operational conditions (see Section 9.4.3). 9.4.5 Cereal separations Mature endosperms of most cereals are composed of thin-walled cells which are approxi- mately 100 to 150 pm in diameter, with the cell walls being only 3-7 ,um in diameter (Kent, 1983). Most of the cell volume is occupied by starch in the form of granules, which is embedded in a protein matrix. The main starch granules are spherical or lenticu- lar in nature with a diameter 15-40 pm, although wheat, barley and oat endosperms also Solids separation processes 269 Feed M - - Fines M - Coarse Fig. 9.1 1, Double classification process: M - mill; A - air classifier; C - cyclone. contain a low proportion by weight of small spherical granules with a diameter of 1-10 pm. The protein matrix is often referred to as interstitial or wedge protein. In cereal flour milling it was discovered at an early stage that the finer flour contained more protein, which led to the development in some countries of commercial procedures for protein displacement milling of soft wheat flours, whereby finely ground flours were air classified into a light fine fraction which has double the protein content of the original wheat flour and a coarse starch fraction with some specific advantages in certain applica- tions. Obviously the quality of the separation depends upon the cereal type and variety and the method of milling. During conventional roller milling of wheat, the endosperm particles are separated from the bran while progressively being reduced in size to pass through sieves having apertures of 100-150pm. The resulting flour consists of particles with a range in diameter of 2-200 pm. Kent (1983) made some observations on the differences between hard wheat and soft wheat. Hard wheats contain higher protein contents 12-14% and are physically more difficult to break. Soft wheats have a lower protein content and are more easily reduced in size. In hard wheats, the majority of the particles are shattered cell fragments over 50 pm in diameter, whereas in soft wheats the protein matrix is partially disintegrated to yield more of the detached starch granules and wedges of protein matrix containing variable amounts of small starch granules and cell wall fragments. There was also a difference in the amount of protein bound to starch between hard and soft wheat. It is now common practice to use impact mills such as the pin disc mill on powders as a pretreatment to air classification, as these are capable of producing significant proportions below 20 pm. Impact mills consist of two sets of pins. In the simple design only one set of pins rotates. Finer flours result when both sets rotate in opposite directions at different speeds (6000-1 8 000 r.p.m.). An optimum impact velocity for disintegrating most endosperm is about 200 m s-’, which is not high enough to damage the starch granules extensively. Damaged starch granules would adversely affect the baking performance and functionality of the starch fraction. Impact mills are best for soft and brittle products because of the high wear and tear. Particle size was also found to be dependent on moisture content, feed rate, nature of the grinding surface and size of the screen selected. 270 M. J. Lewis Jones et al. (1959) showed that there were three principal groups of particles in pin- milled wheat flour: large particles of either cell wall material including starch and pro- tein, or larger detached starch granules or seed coat (>40pm); medium-sized starch granules, some with adherent protein (15-40 pm), and small chips of free wedge protein and detached small starch granules (<15 pm). The main mechanism of separation was size rather than density differences. Kent (1965) fractionated hard (13.6% protein) and soft (7.6% protein) wheats, using 17 and 35 pm as the cut-off to distinguish between the fractions. The yield of the fine fraction (0-17 pm) was 7% for the soft wheat, but only 1% for the hard wheat and the protein content in the fine fraction had increased much more for the soft wheat, to 14.5%, compared to 17.1% in the hard wheat. The yield of intermediate fraction for soft wheat (17-35 pm) was 45% and the protein fraction was reduced from 7.6 to 5.3%. The yield of the coarse fraction (over 35 pm) was 48%, but the protein content was higher than the original, at 8.9%. Further size reduction, using an impact mill, increased the fine fraction yield for the soft wheats from 7 to 20% and the protein content from 14.5 to 15.7%. Thus the amount of protein associated with the fine fraction had increased by 300%. For wheat it can be seen that a limited fractionation of its major components can be achieved and that the protein content can be doubled in the fine fraction for soft wheats. As cut size increased above 19 pm, the yield of fines increased but the protein content of the fines started to decrease (Jones, 1960; Sullivan et al., 1960). Repeated classifica- tion, performed by remilling the coarse fraction and reclassifying it at the same cut point (four times) produced protein shifts ranging between 20 and 34%, and (eight times) ranging between 30 and 60%. These improvements were more marked for soft wheat than for hard wheat (Stringfellow et al., 1963-64). One problem could be starch damage. Some of the protein was found to be bound to the starch. Sosulski (1983) summarises the use of air classification for cereals. Since then, more attention has been devoted to hard wheats. For these, attrition milling has been found to be more effective than pin disc milling, and protein enriched fractions have been used as gluten replacers (Sosulski et al., 1988a). Further work on hard wheat classifica- tion and baking properties of the resulting fractions has been reported by Nowakowski et al. (1987). Vose (1978) provides data on some protein shifts for barley, malted barley and oats (see Table 9.9). Protein shifts were higher for flours produced by pin-milling. 9.4.6 Legumes Sosulski (1983a) has reviewed the use of air classification for fractionating proteins in some legumes. He produced data which gave a measure of the efficiency of the process, in comparison with those for some cereals. Comparison was made using the degree of protein shift, which is based on the yields of coarse/fine fractions and their protein contents relative to the parent flour (see eq. (9.9)). Some values are given in Table 9.9. High protein shifts are indicative of good separations. The results confirm that pin- milling as a pretreatment improves the quality of the separation and roller-milling and turbo-milling are less effective because they fail to release much protein. Results with air classification of potato protein were not very promising, but the protein shifts of greater than 40% for legumes were very encouraging. Unlike most cereals, the starch granules of Solids separation processes 27 1 legumes do not exhibit a bimodal distribution or variable distribution and the average dimensions are 16-21 pm in width and 23-28 pm in length (Biliaderis et al., 1981). Pin- milling has no effect on the starch granules but reduces other cell structures to below 5 pm. Trials with field peas, which contain 1627% protein, have proved successful and Sosulski (1983a) reported results for separation of field peas containing 21% protein, into 25% of fines, containing 60% protein. The coarse fraction was pin-milled a second time to release more protein and reclassified, giving an additional 10% of fines, containing 46% protein. The resulting final coarse fraction contained only 3% protein, giving an overall recovery of over 90%. Further pin-milling and reclassification, in order to further reduce the protein content of the starch fraction and improve its functionality, resulted in marginal reduction in the protein, but considerable increase in the amount of starch damage. The residual protein could also be removed by water washing, which gave a product containing less than 0.06% nitrogen. Experience has shown that it is not necessary to remove the hulls prior to pin-milling. However, if not removed, they will finish up in the coarse fraction and modify its composition, as they comprise a significant part of the seed weight, about 8% for field peas and 13% for fababeans. They may cause problems if they contain anti-nutritional factors and it is advisable to remove them for fababeans to avoid problems from con- densed tannins. Fababeans contain 28-32% protein, which is substantially more than field peas and makes them an attractive proposition for air classification. They have also been success- fully fractionated. Work with field beans showed that protein fractions at yields of 20-30% and containing 50-60% protein could be obtained in the fines. However, the coarse fractions still contained substantial quantities of protein, giving a considerable reduction in the overall recovery. Results for a number of other legumes are recorded in Table 9.10. Tyler et al. (1984) examined the effect of cut size on the separation of several legumes. In general an increase in the cut size resulted in an increased yield of the fines fraction and protein recovery in the fines, but a decrease in protein concentration. How- ever, the amount of starch in the fine stream increased, together with a decrease in starch separation efficiency. In fact, for legumes, air classification can be regarded as a means of separating starch, as most of the non-starchy materials finish in the fines (Han and Khan, 1990a). For pea flour it was found that most of the lipid fraction was found in the fine stream, but fibre was distributed between the fractions (Wright et al., 1984). Cloutt et al. (1986) investigated starch size distribution for cowpeas, fababeans and pigeon pea and found considerable differences between them. Cloutt et al. (1987) looked at the effects of cut size on the separation characteristics of the same three legumes. For each one there was a good relationship between the fines yield and protein content of the fines. This relationship could be established to permit protein content to be estimated from the dry weight yield. Han and Khan (1990b) have evaluated the effects of dry roasting on the fractionation process and functional properties of the separated fractions and found for the protein fraction that roasting reduced nitrogen solubility index and foaming properties. h B r: y=?z ?g?a$? Ye 85 x !i I Irnrnrn rn~rnrnrnrn Lt: cd gg 8 c.l Z?? "V?Z$Z 2 .3 0 0 cd .s m a .s .e Y a, Y !a a E I I*ww ~wwwww VI -0 c cd TOW "\ql?W"l? h 3 0-0 0rn-lO-N G I lior-00 cor-wwww r( E CI sz 9.2 $2 i "1PO "?".099" CA% ;;$ e c Il0Nrn rnornm*= 8s Q I IZVZ "OF""? ;5 rno-ooc1 .s 5 ag ?9TOo9 Zpa,,, T-Cr?"F m-g 6 s3zrn-e sg .9z a% 22 oc C( TP09 09?09TTl? &$ q I lggz rnrn-*rnN 5.2 bbrnrlio0 e 2% % .s "r?O\W" W\q"\qr?? "rnQ\SGG A2 Q ZTSZ8 WbW 2LF: 0% eT G gzggg ZZggZG a .% ea %5 .s," zs m m .Y g c Ilr-li3f3 222222 k-g >la c c1Cu~OQ' 0bCuN-r- -00 r-w NN NNNNNc1 g& c s c $ t'Vi"O\" 909"\q.?V! 5 .z m 0** 0*W000 $ 0 * Z?bV)F W09OTb09 5 2 +OQN rnCurnQO0 0 c 3 scd e3 s 33Ecaa,a, as a 8s z 2 Aa.->p3iz-- 0 3s a 8%' g a maw22 QJEALE '3 2 0-0 2- Y oa 2G e a, ';j 5- M a,3 *-c 0 .e Y .e .o Y brnrnbr- NWbrn~CI a 00 g 0 "g .- c 0a .e a, a rl*Nc'IN NNc1NrnN * 2 Q\ 0) .= g.5$xs8&PCzPM s - 6! B E, 2 ; 2 g s f .- .E 3 Q' (pI m v .- 2 2 E g e* -0 g 8 2 Solids separation processes 273 The functional properties of air-classified legumes have been reported to be very good. These have been evaluated for field peas and fababeans by Sosulski and McCurdy (1987) for the fine fraction. They reported good solubility properties and that water holding capacity and oil absorption increased with protein content. They also showed good emul- sification, whippability and foam stability. Han and Khan (1990b) evaluated the func- tional properties for the fine and coarse fractions. In general, they were affected by the heat treatment, the ratio of protein to starch in the fractions and the presence of other components such as lipids. Aguilera et al. (1984) considered that there was potential for using both the protein-rich and starch-rich fractions in extruded products. Sosulski et al. (1988b) contrasted the relative efficiencies of wet and dry processing methods; dry processing resulted in 75-85% recovery of protein for field beans and 93- 98% recovery of starch. The protein concentrates from dry processing had higher whippability and foam stability, but lower water hydration and oil absorption capacity. For cowpeas, protein fractions produced by dry processing generally produced superior products to those obtained from wet processing (Ningsanond and Ooraikul, 1989). The binding of phytic acid to protein was found not to be affected by dehulling or air classification (Carnovale et al., 1988). Tyler and Punchack (1984) found that milling and air classification was not influenced by the state of maturity, when peas were harvested at different dates. Sosulski et al. (1987) reported that dehulling and air classification improved the storage properties of cowpea products. 9.4.7 Other applications Air classification and sieving was found useful in the preparation of oat-bran, which is rich in beta-glucan (Wood et al., 1989). It has also been used to remove gossypol from cottonseed protein (Quang et al., 1988). Potato granules produced by spray drying from a wet milling process have been successfully classified. Granules containing 10% protein were converted to a fine stream containing 38% protein (Fedoc, 1993). Air classification of spent brewers grain is described by Schubert (1987b). The recovery of protein was poor because of the strong forces of attraction between the protein and the husks. For rapeseed extracts, a shift of 11.5-17.2% protein in the fines was found. However, phytic acid and glucosinolates were also concentrated. (King and Deitz (1987)). It was also found that steaming, crushing and air classification was an effective method for removing fibre. Ground amaranth has also been air classified, for addition to breads (Sanchez- Marroquin et al., 1985). 9.5 WET SEPARATION PROCESSES As well as dry separation, wet separation techniques are available, most of which are dependent upon differential solubilities and precipitation methods. Of special interest is the recovery of protein from a solid matrix. 274 M. J. Lewis 9.5.1 Protein recovery Food composition tables simply give the protein content of foods, so we may be forgiven for assuming that protein is a single entity. In fact most foods contain many protein fractions; therefore there are many objectives in recovering them, the main ones being as follows: (1) (2) (3) (4) to recover all the protein from foods or their by-products to improve functional properties and reduce waste; to separate proteins from toxic components within the food; to recover specific biologically active proteins, such as enzymes, insulin and hor- mones; to fractionate proteins; for example albumins are soluble in water and globulins in salt solutions. Deutscher (1990) provides a thorough review of protein purification methods. In terms of selling price, there may be up to seven orders of magnitude difference between some of the proteins currently available. The solubility of a protein in solution depends primarily upon the properties of its exposed surface groups, the type of solvent, its temperature, pH and polarity level, Le. dielectric constant, and the type and concentration of dissolved ions. Water is the sim- plest extractant, and its pH is adjusted to be well away from the isoelectric point of the protein, which is in the region of pH 4 to 5 for many proteins. Dilute solutions of neutral salts are also used for salting-in, as these are thought to interact with surface charged groups, thereby improving solubility. Once in solution, it may then be required to aggre- gate and precipitate the protein. Methods available are: lowering the temperature to reduce protein solubility; adjustment of pH to the isoelectric point; addition of non-polar solvents to reduce the attraction of surface polar groups with water, to encourage hydrogen bonding between surface polar groups; unfolding (denaturation) and hydrophobic interactions; addition of large quantities of very polar solvents, which also causes unfolding, hydro- gen bonding between surface polar groups and hydrophobic interactions; increasing the levels of salts (salting out), whose ions bind more readily to water and allow hydrogen bonding between exposed polar groups; raising the temperature to cause thermal denaturation to take place. The principles are reviewed by Brocklebank (1987). Some specific examples will be taken from legume and cereal processing. 9.5.2 Soya processing The terminology used for the different grades of proteins includes flours, grits, concentrates and isolates. Concentrates and isolates are protein enriched, where concentrate applies to products with greater than 70% protein on a dry weight basis (dwb), whereas isolates applies to greater than 90%. Solids separation processes 275 Soyabeans contain about 40% protein and 20% fat (dry weight basis). The beans are cracked, dehulled and flaked prior to oil removal by solvent extraction; the defatted flakes are toasted to remove solvent and inactivate the antinutritional compounds. Undesirable changes may also take place, such as colour development (darkening) and protein denaturation. These are reduced by minimising retention times and operating at low temperatures. Desolventisation is by direct steam heating or by passing superheated solvent vapour through the flakes, which evaporates the bulk of the remaining solvent from the flakes. Protein flakes contain about 50% protein. Flakes are further milled to produce grits or flour, the only difference being the final particle size. Concentrates (70% protein) are produced by one of three methods, which involve the washing out of non- protein material: extraction with alcohol (60-80%); extraction with water acidified to pH 4.5 to minimise protein loss; water extraction with highly toasted flakes to minimise protein extraction. In all cases the aim is to maximise the extraction of sugars and other soluble components and minimise the loss of protein. Isolates are produced by aqueous extraction of proteins at elevated pH, followed by isoelectric precipitation using acid, producing a protein curd, which after washing con- tains over 90% protein. Greater than 90% protein is extracted at pH 8. The pH of minimum solubility is 4.2-4.6, where about 10% of the protein is soluble and not recovered. Functional properties such as solubility, whipping ability, emulsification capacity and gelation can be modified by various chemical, enzymatic and thermal treat- ments, before drying. The proteins in soyabeans, and many other legumes and cereals are characterised by a variety of means, for example their solubilities in different solvents (see Section 9.4.5), or according to their molecular weight, determined by ultracentrifugation or SDS electrophoresis (Pearson, 1983; Deutscher, 1990). Note that isolates can be produced by other methods, such as ultrafiltration and diafiltration (Chapter 4), or ion exchange (Chapter 6). The principle of minimum solubility has also been applied in the procedure for the production of spun soya fibres, which involves solubilisation at low pH, followed by extrusion into an acid bath. The fibres produced are further stretched in a heated bath. Texturisation of soyabeans can also be achieved by thermoplastic extrusion, by use of high temperatures and pressures. At the end of the extruder barrel the pressure is released and expansion occurs. Similar principles of washing out or solubilisation and isoelectric precipitation can be applied to the production of concentrates and isolates from rapeseed, cottonseed, lupins and other protein sources (Hudson, 1983). Sosulski (1983b) noted that rapeseed meal gave lower extraction rates, a poorer recovery at low pH and a darker product, compared to soyameal, with only about a 50% overall yield of protein. Glucosinolates and some of the other toxic components can be removed by 80% ethanol. Some lupin seed varieties have the highest protein contents amongst the legumes (up to 45%), but are extremely variable in their composition. They can also be high in alkaloids, which are bitter and need removing. In some cases the fat content may be greater than 15% (Cerletti, 1983). 276 M. J. Lewis There has also been considerable work done on extracting protein from leafy materials (Humphries, 1982). 9.5.3 Wheat protein An interesting example for cereals is the separation of the protein fractions in wheat. Traditionally, wheat proteins have been classified according to their extractability in various solvents. Schofield and Booth (1983) describe five fractions which can be produced by sequential fractionation: Albumins (1) and globulins (2) are extracted in dilute saline; on dialysis with water the albumins remain soluble and the globulins precipitate. Gliadins (3) are soluble in concentrated aqueous alcohol. Glutenins (4) are extracted in dilute aqueous acid or alkali. A residual fraction (5) is not extracted under any of these conditions. All fractions are heterogeneous, with overlap between the samples. The main fractions are the glutenins and gliadins, which each constitute about 35% of the total protein. Both these fractions are insoluble in water and a crude preparation can be produced by washing in water or saline. The protein ‘fraction’ of greatest technological significance is gluten, produced by washing a flour dough with excess water. From the above discussion it would be expected to be heterogeneous in nature, comprising mainly glutenins, gliadins and residual protein, with small amounts of albumins and globulins. Although characterised as a protein fraction, it may also contain up to 20% of other components, such as starch, lipid and hemicelluloses, depending upon the production conditions. The gliadins confer extensibility while the glutenins and residue protein confer elasticity. More detailed analysis of these fractions is provided by Schofield and Booth (1983). There is considerable demand for gluten products and production has expanded regularly as new uses for gluten and starch develop. Techniques involved in producing gluten are mostly based on extraction processes, from either a dough or a batter made from flour, rather than whole grain. In these processes, the gluten network is allowed to develop during the extraction process. Requirements for raw materials are that the protein content should be high and the flour should be of consistent quality to ensure a uniform end-product. The other important raw material is water; with soft water the gluten is soft and slimy and the starch is removed less easily. Therefore hard water is preferred, perhaps suggesting a role for calcium in protein stability. Extraction from dough or batter uses considerable water and several alternative procedures have been investigated to reduce water utilisation. Some processes rely on separating some of the starch before the gluten network fully develops. Centrifugation of flour slurry, not fully hydrated, produces (based on density differences) a starch-rich fraction and a second fraction containing most of the protein, with a protein content of 2040%. This can be spray dried to produce a protein enriched fraction, but with a much lower protein content than normal gluten. Alternatively, the protein rich fraction can be further sheared and milled, which allows the gluten to develop, and this is removed by screens and further washed. Wet milling of whole wheat Solids separation processes 277 has also been practised. Wheat grains are macerated with water, the bran screened-off and starch and gluten separated. Other proposed methods involve chemical dispersion, for example in weak acids such as acetic acid, or in dilute ammonium hydroxide solutions. Non-aqueous separations have been proposed, using fluids with different densities, such as fluorinated hydrocarbons. Drying of gluten is important as gluten deteriorates rapidly if kept wet. Heat damage should be avoided to ensure good quality; quality is based on its protein content, physical characteristics and end-use performance. 9.5.4 Other applications These examples illustrate some of the techniques that have been investigated to concen- trate or recover protein from cereals and legumes. Similar approaches can be used for animal protein. Mackie (1983) gives a comprehensive review on the recovery of fish protein from a wide range of raw materials. Early fish protein concentrates were produced by solvent extraction to remove fat and water, followed by air drying to remove residual solvent. Bones may or may not be removed, leading to higher ash contents if they were not. One of their major problems was almost a complete lack of any functional properties. A similar procedure to that for soya, for the production of fish protein fibre from alkali extracts of fish processing wastes is also described. The handling of by- products from meat, fish and poultry are considered in more detail by Ockerman and Hansen (1 988). Topics covered include mechanical deboning, rendering for fat extraction and waste meal production, and the extraction of gelatine and other food and non-food materials. Another basis for separation is the ability of the water or solvent to reduce interparticle adhesion. Schubert (1987b) reports that interparticle adhesion is about one order of magnitude less in liquids than in gases. Therefore for strongly cohesive materials, there is potential for improving separations by dispersing the material in a liquid, followed by sieving or separating by sedimentation under those conditions. However, water costs and dewatering costs may be high. One example investigated was the extraction of protein from spent brewer’s grain. This material contains about 20% dry matter, of which up to 28% is protein which has been heat denatured. This tends to adhere strongly to the much larger husks which contain little protein. Wet processing using water to reduce interparticle adhesion was evaluated. Additional water was added and the mixture subjected to moderate shear and then separated. The protein enrichment was about 65%. Water utilisation was high. In an alternative process a specially designed screw press was used to separate the husks from the protein fraction. The relationship between initial protein content, final protein content and yield could be described by the characteristic equation (see eq. (9.1 1)) c, = [cpO/ytn] where m = 0.48. In both cases the degree of protein enrichment was much higher than for dry processing, whereby the whole mass was dried, milled and subject to air classifica- tion; the protein enrichment was between 30 and 40% and m = 0.1. 278 M. J. Lewis 9.6 SOME MISCELLANEOUS SOLIDS SEPARATIONS 9.6.1 Dehulling In many countries of the world, legumes are initially processed by removing the seed coat or hull and splitting the seed into its dicotyledenous components. Removal of the hull brings about some of the following advantages: a reduction in fibre and tannin content, and improvements in appearance, cooking quality, texture, palatability and digestibility. Removing the hulls from many legumes is a tedious task. Legumes are often soaked and dehulled manually and redried. This method is probably the only one which removes all the hulls and consequently is used for estimating the hull content of seeds and also the theoretical yield of dehulled product, which usually range between 85 and 95%. Some values for hull content, expressed on a dry weight basis, are given in Table 9.1 1 (Reichert et al., 1984). Table 9.11. Dehulling performance of some different legumes Hull Dehulling Hull Intact content % Yield efficiency (DE) adhesion seeds % (a) (b) (c) (d) (e) soya bean 8.27 88.7 0.72 1 91.3 fababean 11.92 83.3 0.71 1 59.9 field pea 7.74 87.3 0.61 1 47.6 lentil 8.47 85.4 0.54 1 98.2 mung bean 8.95 74.2 0.33 2 18.1 cowpea (black-eyed) 5.24 79.6 0.25 2 18.7 cowpea (brown) 3.24 78.3 0.11 2 19.3 (a) Dry weight basis. (b) (c) (d) (e) Adapted from tables in Reichert et nl. (1984). kidney bean 8.47 84.2 0.51 2 0 The yield of dehulled grain when 90% of the hull has been removed from the seed. DE = hull removed (g/ 100 g seed)/( 100 - yield) (g/100 g seed). 1 designates loose adhesion between hull and cotyledon, whereas 2 denotes a tight binding. The weight percentage of seeds which have cotyledons bound together after dehulling. For most commercial dehulling applications, abrasion or attrition mills are used, with attrition mills, e.g. plate mills, being favoured where the hull is less firmly attached to the seed coat. One problem with abrasion mills is that the yields are much lower than the theoretical yields and losses are higher because cotyledon material is lost with the hulls, sometimes as high as 30%. Soaking methods and residual mechanical hull removal are methods still widely used to evaluate the efficiency of hull removal. One problem with dehulling comes from the different size and shapes of the legumes, with dehullers de- signed specifically for each crop. There is some interest in a universal dehuller. Reichert et al. (1984) also examined the dehulling performance of a multipurpose disc attrition mill with a variety of legumes. Their characteristics are compared in terms of yields and dehulling performance (see Table 9.1 1 footnotes for definitions). Solids separation processes 279 Although there was little difference in the yields, which was the weight of dehulled product recovered when 90% of the hulls were removed, the dehulling performance, which gives the proportion of hulls in the abraded fines, gives a better indication of performance (see definition). Values ranged from 0.72 for soya bean down to 0.11 for brown cowpeas. Thus for soyabeans the abraded fines contains 72% hull materials, whereas for brown cowpeas it was only 11 %, indicating big losses of cotyledon materials in the hulls. Statistical analysis showed that the factors most responsible for differences in dehulling performance were seed hardness and resistance to splitting. 9.6.2 Peeling Peeling is an important process for many processed and convenience fruit and vegetables. Mechanisms involved in peeling are abrasion, chemical cleaning, including caustic (lye) or brine and thermal peeling. Often more than one mechanism is involved and often spray washing is required to remove any loosely attached peel. All peeling operations generate solid waste and may cause damage to the material. Abrasive peeling. The food is fed into a rotating bowl, which is lined with an abrasive material, such as carborundum. Rollers can also be used. The abrasion rubs off the skin, which is removed by water. Claimed advantages are (Fellows, 1990) low energy costs, minimal thermal damage, low capital costs. Drawbacks include higher product losses (up to 25%), production of large volumes of dilute wastes and relatively low throughputs. Some irregular shaped materials, for example potatoes with eyes, may need some manual inspection and finishing. Onion skins are easily removed by abrasion peeling. Knives may be used for citrus fruits. Chemical peeling. A dilute solution of sodium hydroxide (1 to 2%) is heated to 100- 120°C and contacted with the food for a short time period. Water sprays are then used to dislodge the skin. This was once popular for root vegetables, but it can cause some discoloration. It has now been largely replaced by steam peeling. The use of a more concentrated lye solution (10%) is known as dry caustic cleaning (Fellows, 1990) and reduces water consumption and produces a more concentrated waste for disposal. Brine solutions are also sometimes used. Thermal peeling. The food is fed in batches into a pressure vessel, which rotates slowly. High-pressure steam is fed into the vessel and rapid heating occurs at the surface, within 15-30 s, but not in the bulk, due to the low thermal conductivity of the food, thereby minimising chemical reactions, including cooking, in the bulk of the food. The pressure is suddenly released, causing boiling of the liquid under the skin and flashing-off of the skin, which is removed with the condensed steam. Additional water sprays may be required. This method is increasing in popularity; it produces good quality products, with little damage, at high throughputs. There is minimum water utilisation and minimum losses. Flame peeling, using temperatures of 1000°C, has been used for onions. 9.6.3 Cleaning of raw materials Contaminants on food raw materials can be of various origins: mineral - soil, stones, sand, metal, oil; plant - twigs, leaves, husks, skins; 280 M. J. Lewis animal - faeces, hair, insects, eggs; chemical - pesticides, fertilisers, other contaminants; microbial - yeasts, moulds, bacteria and metabolic by-products, e.g. mycotoxins, e.g. patulin in apples. One of the first preliminary operations must be to remove these. Important considera- tions are high efficiency of removal, combined with minimising loss and damage and further recontamination of components. A combination of methods is used, including dry cleaning and wet cleaning. Aspiration to remove dust and light contaminants has already been described. Screening is widely used for removing contaminants considerably different in size to the food being treated. Sieves are available up to an aperture size of 25 mm and screens for larger sizes (see Section 9.3.2). Also disc separators, where the shape of the disc matches the shape of the food, can be used for separating seeds from grain. It is important to be able to detect the presence of metal fragments and remove them from the raw material, to prevent damage to the food processing equipment and contami- nation of the final product. Metal detectors may also be incorporated at the end of the packaging line. Magnetic materials can be removed by powerful magnets, which can be permanent magnets or electromagnets. Non-ferrous metals, such as aluminium, are detected by passing the material through a strong electromagnetic field. This field is distorted and initiates a warning signal. X-rays have also been used for products in sealed containers. It has been reported that modem cocoa processing leads to iron contamination levels of 200 mg/kg in cocoa mass and greater than 300 mg/kg in cocoa powder; this arises from hammer mills, impact mills and the agitator blades of rotating ball mills. Between 5 and 15% of the metal was greater than 75 pm. Improvement in design of this equipment was considered to be the best way of reducing this (List and Thiede, 1987). Krishnan and Berlage (1984) looked at the principle of separating walnuts by a magnetic field. Iron dust with gelatin or a magnetic solution was added to the whole nut. This was cracked and the shell separated from the meat using a permanent magnet. Electrostatic methods for cleaning materials are available, which take advantage of the differences in electrostatic charge of materials under controlled humidity conditions. The solid is fed from a hopper onto a drum, rotating at 70-350 r.p.m., which is either charged to a potential of 5-20 kV or earthed and the oppositely charged particles are separated as they are more strongly attracted to the drum. They are removed from the drum by a scraper. This method can be used to remove dust and stalk from tea fannings and also some unwanted seeds from cereals and oilseeds (Brennan et al., 1990). The Dodder mill uses a roller coated with a velvet-type material, which will attract particles, such as seeds, and remove them from cereals, due to differences in their surface properties. Wet methods are also widely used for cleaning purposes. Heavily soiled vegetables can be simply presoaked in water; the process helped by agitation. A more efficient process is spray washing, which uses high-pressure sprays and requires smaller volumes of water. The principles of flotation are used for cleaning vegetables; heavy particles such as soil, metal or glass sink, the vegetables are neutrally buoyant and straw and grass float. Screens may also be incorporated to remove oversize material. In wet-processing Solids separation processes 28 1 applications, the microbial quality of water supply and the additional costs of the water and effluent treatment need to be considered. Peeling or dehulling, as well as removing the outer layers, will also remove any disorders associated with them. 9.6.4 Sorting and grading Sorting and grading are important preliminary operations. Sorting is normally reserved for processes which separate foods into categories based on a single physical property, such as size, shape, weight or colour. Grading, on the other hand, is a quality separation and a number of factors may have to be assessed. Some examples are colour, absence of blemishes, flavour and texture. Food grading is usually done manually, by trained experts, because it is not usually possible to link quality with one physical property. Some examples are meat grading and inspection, fish grading, horticultural products, tea and cheese. However, the food analyst is always seeking for instrumental techniques for assessment of these sensory attributes which contribute towards the character of the food. For example the dielectric properties of fish have been found to change as the fish becomes less fresh. Consequently a wide range of instrumental methods has been evaluated for measurement both on-line and in the laboratory, of properties of foods that correlate with the sensory characteristics of appearance, colour, flavour and texture. Appropriate instrumentation and sensors have been reviewed by Kress-Rogers (1993). Equipment for size sorting based on rollers and screens, which provide either a fixed or a variable size aperture, are discussed in more detail by Fellows (1990) and Brennan et al. (1990). Sorting by weight is important for high value products such as eggs, and some tropical fruits. Image analysis is being increasingly investigated in this respect. Colour sorting and grading Foods can be sorted on the basis of their colour, for example removing discoloured baked beans, prior to them being blanched. One of the most common applications is to pick out miscoloured pieces and the simplest method is by manual inspection, as the food passes by the inspectors, on conveyor belts. Colour sorters have been available for over 40 years and one widely used application range is for particles in the range 2-10 mm. Some examples are: rice, baked beans and other legumes, peanuts and roasted coffee beans. Throughputs range from about 10 kg h-' up to 10 000 kg h-', with many applications between 100 and 1000 kg h-'. These are based on a sensor located above the conveyor. The feed is divided into lanes or channels. Mohsenin (1984) summarises the sequence of operations as singularisation into discrete units, acceleration to present a substantial number of units to the system per unit time, presentation of each unit before the sensors, evaluation and comparison to some predetermined standard, and segregation to separate each unit according to its colour or other specified standard. Figure 9.12 illustrates the layout of such a sorter. Colour sorting relies on the optical properties and reflectance of the samples. The principle is that the light source is directed on the material and the reflected light is measured by a photodetector and compared with preset standards. Materials outside the range are rejected. The incident light and reflected light may or may not be filtered, to 282 M. J. Lewis Chute dust n t product Reject product Fig. 9.12. Diagram of colour sorter (with courtesy of Sortex Ltd). allow only selected wavelengths to reach the detector. Monochromatic sorting uses only one selected waveband range and is used where there is sufficient difference between the reflectivity of acceptable and unacceptable products within the selected waveband. Unfortunately it is not always possible to find a single section of the spectrum where this is the case, so it is necessary to resort to a more complicated procedure which involves measuring at two selected wavebands. This is known as bichromatic sorting. Usually a ratio of the signals from the two wavebands will facilitate sorting of the materials. In some cases dual monochromatic sorting is used, where it might be important to reject more than one type of defect. A discrepancy in either signal will cause the item to be rejected. More detail is provided by Low and Maughan (1993). Other factors to be considered are the natural variations in colour that are found for each product. The feed rate is also important and there may be problems operating in a dusty or humid environ- ment. As in most applications, such machines are never 100% efficient in terms of either removing all defective items or rejecting acceptable items. Improving the sensitivity of the detection unit increases the efficiency for removing defective items, but also means that a greater proportion of acceptable items will be rejected. There is also a recommended flow rate range for each machine. Increasing the flow rate within this range usually leads to a greater loss of acceptable material. In most cases, the overall performance improves if the material has been cleaned and size graded prior to colour sorting. Some typical examples of removal efficiencies for defective items are Solids separation processes 283 green coffee: removing defective beans and foreign material, 90% at 900 kg h-l; white beans: removing discoloured beans and foreign material, 97% at 1500 kg h-l; frozen peas: removing foreign material, 99% at 10 000 kg h-'; (Low and Maughan, 1993). Other applications are for sorting of fruit, picking out bruised, damaged or mouldy fruit. Colour measurement is used for control purposes; for example controlling the energy input into baking ovens to ensure a product of uniform colour from the oven. Transmittance methods form the basis of egg inspection and have been used to distin- guish between cherries with and without pips. Future developments will combine colour sorting with vision analysis, whereby sort- ing will be based on colour differences, size and shape. REFERENCES Aguilera, J. M., Crisafulli, E. B., Lusas, E. W., Vebersax, M. A. and Zabik, M. E. (1984) Biliaderis, C. 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