Charrlng Heo t Water Caking I Dry matter loss1 104 TECHNOLOGY OF CEREALS are shown in Fig. 5.1, should be borne in mind 35' 0 H r [\ throughout. Moisture content and storage E 30' 5 temperature e c7' Moisture content is expressed as a percentage of the grains' wet weight. The safe moisture contents for storage vary according to the type of cereal but it is widely assumed that they are equivalent to the equilibrium moisture content of the respective grains at 75% RH and 25°C (Table 5.1). 50 io0 I50 ?d TABLE 5.1 Equtlibnum Moisture Contents of Grains at 75% RH and 25°C Cereal Type Moisture ?LO Barley Maize 14.3 Oats 13.4 Rye 14.9 (25"-28"C) F~~ 5.2 potential storage the (number of days) of wheat grain as a function of temperature ("C) and moisture content ("h), the germination rate maintained being 70%. From 14.3 (250-280c) Guilbot, (1963) Producteur Agncole Francats, Suppl. mai ITCF, Paris. Rice 14.0 40' Sorghum 15.3 (u) h ?? red 14.7 (rt) + white 15.0 (rt) s Wheat e 30" durum 14.1 (u) 3 g 200 G 100 ?! u i Based on values in Bushuk and Lee, 1978. I In temperate regions the moisture contents at which grain is stored are closer to those described moisture contents cannot be considered alone as the deleterious effects of excessive dampness are affected critically by ambient temperature and the composition of the surrounding atmosphere. The increase in relative humidity of the interseed atmosphere with temperature, is slight. It amounts 10°C drop in temperature. temperature as they affect storage of wheat is The relationship depicted takes account only of the maintenance of grain quality as assessed by grain viability. The relationship is also import- ant, however, through its effects on infesting organisms, as Fig. 5.3 shows. The values used in Figs 5.2 and 5.3 refer as wet rather than dry. The significance of 0" 5 io 15 20 25 Moisture content % 0 Good conservation H Insects m Germination n Moulds to about 0.6-0.7% moisture increase for each FIG 5.3 Risks to which stored cereal grain is exposed as a %. From Burges, H. D. and Burrel, M. J. (1964) J. Sci The relationship between moisture content and function of grain temperature in "C and moisture content in shown in Fig. 5.2. Food AFC. 1: 32-50. to sound clean samples. Broken grains are almost always present to some extent as a result of damage during harvesting or transferring to stores. In broken grains endogenous enzymes and their substrates, kept separate in the whole grain, STORAGE AND PRE-PROCESSING 105 can achieve contact and lead to necrotic deteriora- radicle, and leaves and stem develop from the tion. Further, the most nutritious elements of the plumule (see Ch. 2). Hydrolytic enzymes are grain, endosperm and embryo, are exposed released into the starchy endosperm, and these to moisture, micro-organisms and animal pests catalyze the breakdown of stored nutrients into whereas in the whole grain they are protected a soluble form available to the developing plantlet. by fruit coat, seed coats and possibly husks. The conditions required for germination are Impurities can also reduce storage time in that also conducive to other, more serious, hazards weeds present in the crop ripen and dry at a such as excessive mould growth. They would different rate from the crop itself. Hence, still rarely occur throughout a well managed store green plant material - with a relatively high but could develop in pockets due to moisture moisture content - can carry excessive moisture migration. Deterioration results from loss of into store even when mixed with dry grain. weight due to enzyme activity and a loss of quality resulting from excessive enzyme activity in the products of processing. These problems would apply even if the germination process were termin- Changes during storage in the grains ated through turning the grain. Having germinated themselves in store the grain would also be useless for seed Respiration purposes as the process cannot be restarted. In a natural atmosphere gaseous exchange will Microbial infestation occur in a stored cereal crop. This is due to respiration and it involves a depletion in atmos- pheric oxygen and an increase in carbon dioxide Fungal spores and mycelia, bacteria and yeasts with the liberation of water, and energy (as heat). are present on the surfaces of all cereal crops. Respiration rates measured in a store normally During storage they respire and, given adequate include a major contribution from micro-organisms moisture, temperature and oxygen, they grow that are invariably present at harvest; neverthe- and reproduce, causing serious deterioration in less even ripe dry grain, suitable for storing, grains. contains living tissues in which respiration takes A distinction may be drawn between those that place, albeit at a very slow rate. The aleurone attack developing and mature grain in the field and embryo are the tissues involved and, like and those that arise during storage. Field fungi other organisms present, their rate of respiration thrive in a relative humidity (RH) of 90-100% increases with moisture content and temperature. while storage fungi require 70-90% RH. Several Respiration is a means of releasing energy from investigations have shown that a RH of 75% stored nutrients (mainly carbohydrates) and a is required for germination of fungal spores consequence of long storage is a loss of weight. (Pomeranz, 1974). Under conditions unfavourable to respiration this Storage fungi are predominately of the genera may, however, be insignificant and under any Aspergillus, of which there are five or six groups, circumstances it is likely to be of little consequence and Penicillium, the species of which are more in relation to other storage hazards. Respiration clearly defined. Some of the more common storage can be reduced by artificially depleting the oxygen fungi and the minimum relative humidity in in the atmosphere. which they can thrive are listed in Table 5.2. As with other spoilage agents dependent upon a minimum moisture content, fungi may be a problem even when the overall moisture content Germination Germination of grain is an essential and natural in the store is below the safe level. This can phase in the development of a new generation of result from air movements leading to moisture plant. It involves the initiation of growth of the migration. Warm air moving to a cooler area will embryo into a plantlet. Roots develop from the give up moisture to grains, thus remaining 106 TECHNOLOGY OF CEREALS TABLE 5.2 Approximate Minimum Equilibruim Relative Humidity for Growth of Common Storage Fungz Thermophilic fungi die at 60°C and the process thermophilic yeasts up to 70°C. Mould RH(%) limit In recent years attention has been given to the Aspergillus halophiticus 68* toxic products of fungi such as Aspergillus flavus A. restictus group 70* and Fusarium miliforme which produce aflatoxin A. glaucus group 73* and zearalenone (see Ch. 14). A. chevalieri 71t A. repens 71t Insects and arachnids A. candidus group 80* A. candidus 75t Insects that infest stored grains belong to the A. ochraseus group 80* A. jlavus group 85* A. jlavus 78t beetle or moth orders: they include those capable A. nidulans 78t of attacking whole grain (primary pests) and those 80-90* 82t that feed on grain already attacked by other pests A. fumigatus Penicillium spp. P. cyclopium 82t (secondary pests). All arachnid pests belong to P. martensii 79t the order Acarina (mites) and include primary 83t and secondary pests. Most of the common insects P. islandicum and mites are cosmopolitan species found through- OUt the world where grain is harvested and stored (Storey, 1987). Insects and mites can be easily distinguished as arachnids have eight legs and insects, in their most conspicuous form, have six. Reference to the most conspicuous form is neces- sary as some insects (including those that infest grain) develop through a series of metamorphic forms. There are four stages: the egg, the larva, the pupa and the adult or imago. Although some female insects lay eggs without mating having occurred, this is less usual than true sexual reproduction, and this and dispersion are the two principal functions of the adult. Large numbers of eggs are produced and these are very small. Those of primary pests may be deposited by the female imago inside grains, in holes bored for the purpose prior to the egg laying. Under suitable conditions eggs hatch and from each a larva is kept going by spore forming bacteria and * Christensen, C.M. and Kaufmann, H.H. (1974) Micro- flora. In: Storage of Cereal Grain and thezr Products. Christensen, C. M., (Ed.) Amer. Assoc. of Cereal Chemists Inc. St. Paul, MN. U.S.A. t Ayerst, G. (1969). The effects of moisture and temperature on growth and spore germination in some fungi. J. Stored Prod. Res. 5: 127-141. Table adapted from Bothast, R. J. (1978). in equilibrium with them. Unless temperature gradients are extreme the exchanges occur in the vapour phase; nevertheless, variations in moisture content up to 10% within a store are possible. If mould growth continues in the presence of oxygen, fungal respiration increases, producing more heat and water. If the moisture content is allowed to rise to 30% a succession of progressively heat-tolerant micro-organisms arises. Above 40°C mesophilic organisms give way to thermophiles. TABLE 5.3 Alphabetical List of Primary Insect and Arachnid Pests Systematic name Common name Family Acarus siro. L Grain (or flour) mite Acarid a e Cryptolestes ferrugineus Stephens Rust red grain beetle Cucujidae Rhyzopertha dominica F. Lesser grain borer Bostrichidae Sitophilus granarius L. Grain weevil Curculionidae Stiophilus oryzea L. Rice weevil Sitophilus zeamais Motschulsky Maize weevil Sitotroga cerealella Olivier Angoumois grain moth Gelechiidae 107STORAGE AND PRE-PROCESSING FIG 5.4 Acarus siro, the flour mite (top), Sitophilus granarius, the grain weevil, and Gryptolestes ferrugineus, the rust-red grain beetle (bottom). Crown Copyright Central Science Laboratory 1993. 108 TECHNOLOGY OF CEREALS TABLE 5.4 Alphabetical List of Most Important Secondary Insect Pests Systematic Name Common Name Family Anagasta kuehniella Zella* Mediterranean flour moth Phycitidae Gadra cautella Walker Almond moth " Cyptolestes pusillus Schonherr Flat grain beetle Cucu jidae Cyptolestes turacus Grouv Flour mill beetle " Ephestia elutella Hubner Tobacco moth Phycitidae Oyzaephilus surinamensis L. Saw-toothed grain beetle Cucujidea Oryzaephilus mercator Fauv. Plodia interpunctella Hubner Indian meal moth Phycitidae Tenebriodes mauritanicus L. Cadelle Ostomatidae Tribolium castaneum Herbst. Red flour beetle Tenebrionidae Tribolium confusum Duval Trogodenna granarium Everto Khapra beetle Dermestidae Merchant grain beetle " Confused flour beetle " * Formerly Ephestia kuehniella. FIG 5.5 Oryzaephilus Laboratory 1993. 108 TECHNOLOGY OF CEREALS TABLE 5.4 Alphabetical List of Most Important Secondary Insect Pests emerges. The larva is the form most damaging to the stored crop as it feeds voraciously. In consequence it grows rapidly, passing through a series of moults during which its soft cuticle is shed, thus facilitating further growth. Finally pupation occurs; the pupa, chrysalis or cocoon does not eat and appears inactive. However , changes continue and the final metamorphosis leads to the emergence of the adult form. The life cycle of mites is simpler as eggs hatch into nymphs which resemble the adult form, although there are only six legs present at this stage. By a series of four moults the adult form is achieved. The time taken for development of both insects and mites is influenced by temperature, the greater the temperature the more rapid the deve- lopment up to the maximum tolerated by the species. The primary pests -those attacking whole grains- are given in Table 5.3. The three most damaging of these pests in the U.K. are shown in Fig. 5.4. The most important secondary insect pests ~ those feeding only on damaged or previously attacked grains -are given in Table 5.4. The saw-toothed grain beetle is shown in Fig. 5.5. FiG 5.5 Oryzaephilus surinamensis. Saw-toothed grain beetle. Crown Copyright. Central Science Laboratory 1993. STORAGE AND PRE-PROCESSING 109 FiG 5.6 X-ray photograph of wheat grains, two uninfested (bottom left), the others showing cavities caused by insect infestation. An insect is visible within one of the cavities. (Part of a picture in J . Photograph. Sci., 1954, 2: 113; reproduced by counesy of Prof. G. A. G. Mitchell and the Editor of Journal of Photographic Science.) even though only a hollow bran coat may remain. Detection by means of soft X-rays is possible (Fig. 5.6). Those insects listed in the tables are considered major pests. They are particularly well adapted to life in the grain bin and are responsible for most of the insect damage to stored grain and cereal products. Minor pests occur mainly in stores in which grain has started to deteriorate due to other causes, while incidental pests include those that arrive by chance and need not even be able to feed on grains. For further information on minor and incidental insect pests specialist works such as Christensen (1974) should be consulted. Among the major primary pests five species develop inside grains. Weevils (grain, rice and maize) lay eggs inside while lesser grain borers and Angoumois grain moths deposit eggs outside but their newly hatched larvae promptly tunnel into grains. The presence of the insect and the damage it causes may not be evident from outside Damage caused by insects and mites Serious grain losses due to consumption of grain by insects and mites occurs only after prolonged storage under suitably warm conditions. They are most serious in hot climates. Other problems caused by insects include creation ofhot- spots around insect populations where metabolic activity leads to local heating. Moisture move- ments and condensation in cooler areas results in caking, and encourages fungal infestation (see Fig. 5.1). Introduction of insects and mites from wheat stores to flour mills can cause serious deteriora- tion in the products. Mite excreta taints flour with 110 TECHNOLOGY OF CEREALS a minty smell and hairs from the animals’ bodies however, have several advantageous features. can cause skin and lung disorders in workers Thus underground stores provide protection from handling infected flour. Silk from the larvae of temperature fluctuations, the most successful the Mediterranean flour moth webs together simple ones being found in hot dry regions. They causing agglomeration of grains and blockages in are filled, to leave little air space, and sealed, to handling and processing equipment. In tropical approach the concept of hermetic storage under countries termites can weaken the structure of a which insects and moulds rapidly use up oxygen, store, leading to its collapse. giving rise to high C02 content of the intergrain atmosphere. In more humid regions ventilation is desirable as the crop may have to be stored before reaching a safe moisture level. Such a Vertebrate pests The principal vertebrate pests in cereal stores system is suited to cob maize rather than threshed are rodents and birds. In many countries the three grains, as adequate space for air movement within main rodent species involved are: the store is essential. Clearly the requirements of ventilation and exclusion of insects are not immediately compatible and hence careful design Rattus nomegzcus - the Norway, common or brown rat; is essential. Rattus rattus -the roof, ships or black rat; Storage of maize as cobs is practised now largely by small scale growers producing for the Mus musculus - the house mouse. Apart from consuming grains, particularly the requirements of the local community. It was at embryo of maize, rodents cause spoilage through one time adopted more widely even in highly their excretions which contain micro-organisms commercial practice, much as small grain cereals pathogenic to man. These include salmonellosis, were stored unthrashed in ricks. murine typhus, rat-bite fever and Weil’s disease. In the commercial context stores are needed Rodents also damage stores’ structural elements, for three purposes: 1. Holding stocks on the farm prior to sale. containers, water pipes and electric cables. 2. Centralization before distribution or processing In well-managed stores access by rodents is denied and good housekeeping practice, such during the year following harvest. as removal of grain spillages, maintenance of 3. Storage of annual surpluses over a longer uncluttered surroundings and regular inspec- period. tions, prevent problems. The same is true of birds. These are serious pests only when access Farm stores may consist of any available space is easy, as for example in hot countries where that will keep out the elements. The facilities for grain may be left to dry in the sun. Damage to protection against mould and pests are very drains and blockage of pipes by nests can variable. Stores range from small wooden enclos- give rise to secondary storage problems through ures in the barn, to round steel bins holding 25- promoting local dampness in some stores. 80 tonnes, to silos of larger capacities. Good on- farm storage facilities allow farmers to choose the time to sell, to receive the best prices. The degree of centralization depends upon the Design of storage facilities The requirements of long term safe storage are marketing regime within the country of produc- protection against dampness caused by weather tion. In North America, Country elevators and or other sources, micro-organisms, destructively Terminal elevators with storage capacity up to high temperature, insects, rodents and birds, 500,000 tonnes exist. The country elevators provide objectionable odours and contaminants and un- a local staging en route to terminal elevators which authorized disturbance. Clearly the simplest stores include high-capacity equipment for cleaning, such as piles on the ground, unprotected, are suit- drying and conditioning of grain. The term able for short periods only. Other simple stores, ‘elevator’ is applied to the entire facility although STORAGE AND PRE-PROCESSING 111 Settling is a continuous process arising in part from the collapse of hulls, brush hairs, embryo tips etc. Control of pests and spoilage of grains in store Deterioration in store is less likely if care is taken to ensure that the grain is in a suitable condition for storing. Criteria for the latter include a suitably low moisture content, a low mould count and freedom from insects. Wheat contain- ing live insects can be sterilized by passage through an entoleter (Fig. 5.7), run at about 1450 rev/min. (BP 965267 recommends speeds of 3500 rev/min for conditioned wheat, 1700 rev/min for dry wheat.) Hollow grains and insects may be broken up and can be removed by subsequent it refers literally to the mechanism (normally belt and bucket) by which grains are raised to a level from which they can be deposited into the large capacity silos invariably found on the sites. Elevators are associated with good transport facilities by road, rail, water or all three. Many are capable of loading grain into vessels at a rate of 2,750 t/h. It is sometimes necessary to provide storage for grain beyond the normal capacity of an elevator facility or elsewhere. In such conditions a relatively inexpensive expedient is the flat store. This is little more than a cover for a pile of dry grain adopting its natural form as poured. Such a form is described by the angle of repose. In the case of wheat the angle is 27" to the horizontal, hence flat stores have roofs close to this angle. Very temporary stores may make use of inflatable covers. aspiration. Flat stores are easy to fill but, as they have flat floors, removal of stocks is more difficult, usually requiring the use of mechanical shovels. In con- trast, silos usually have a floor formed like a conical hopper whose walls make an angle greater than 27" to the horizontal. Piles created by grains falling freely from a central spout are not uniform as whole grains tend to roll from the apex down the sloping surfaces. Small impurities and broken grains roll less readily and thus become trapped in the central core of the pile. Such a core is described as the spoutline. As the interstices can amount to 30% of the occupied space, fines in the spoutline can reach that level. Because air circulation and hence heat loss is prevented, the spoutline can be associated with early deterioration through overheating. The diameter of the spoutline is proportional to the width of the bin. Also in contrast to tall tower-like stores, flat stores require little strength in the side walls. In a silo much of the pressure of the column of grain is borne not by the floor but by the side walls. This is because each grain rests on several grains below it so that some of the weight is distributed laterally until it reaches the walls and, by friction, rests on them. In all stores some settling occurs ~~~~~~~~~~~~~~~ff~~~~~~~~~~. R+M is re1ative1y dense and sett1ing may be Only 6oh of volume but oats may pack as much as 28%. FIG 5.7 Diagrammatic section through an Entoleter Aspirator. 1, fixh ide~; 2, 3, e; 4< swkg mm; -5< .ph discharge over 6, cone; 7, valve controlling air flow. Arrows indicate path taken by air. (Reproduced from Milling, 1969, Oct 10, by courtesy of the Editor.) 112 TECHNOLOGY OF CEREALS The store itself should provide protection from Pesticides used to control insects, during storage weather (particularly wet) and intrusion by insects of cereal grains, are of two types. Those that are and rodents. High temperatures are undesirable designed as a respiratory poison, and are hence and variation should be reduced to a minimum applied as gas or volatile liquid, are described as as this can lead to local accumulation of moisture. fumigants. Those designed to kill by contact or All spoilage agents depend upon respiration ingestion are described as insecticides. They may and hence a depletion of oxygen inhibits their be applied in liquid or solid form. proliferation and activity. To achieve this it is Of the gaseous fumigants, methyl bromide and necessary to provide a seal around the grain and phosphine (PH3) are the main examples. Examples a minimal headspace. In a sealed store oxygen of ‘liquid’ fumigants are mixtures of 1,2 dichloro- depletion can be achieved by natural or artificial ethane and tetrachloromethane: although the means. Natural depletion results from respiration most effective fumigant is methyl bromide, this which in most organisms consumes oxygen and gas does not penetrate bulk grain well and -the produces carbon dioxide. Artificial atmosphere use of a carrier gas such as tetrachloromethane control comes about by flushing of interstitial and is an alternative to the fan-assisted circulatory head spaces with a gas other than oxygen, usually systems required if methyl bromide is used alone. nitrogen or COz as these are relatively inexpensive. Few stores have the necessary fans. Complete removal of oxygen is not possible. The period of treatment required depends Experiments carried out in artificial conditions in upon the susceptibility of the species of insects the U.K. showed that baking properties of wheats present to the fumigant. For example a three day were maintained for eighteen years in low oxygen exposure to phosphine may eliminate the saw- conditions. At ambient temperatures germinative toothed grain beetle but six days at low temperature energy was seriously reduced and although this may be needed to kill the grain weevil. reduction was prevented by storage at 5°C this ‘Liquid’ fumigants penetrate bulks well. The was the only advantage of low temperature in proportions need to be adjusted to suit the depth addition to oxygen depletion recorded (Pixton, of the grain stored. Up to three metres deep a 1980). 3: 1 mixture of 1,2 dich1oroethane:tetrachloro- Sealed conditions are unusual and prevention methane is suitable but for penetration to a depth of spoilage in many cases depends upon careful of 50 m equal proportions are needed. Fumigation maintaining of the stored grains’ condition, and requires the stores to be sealed to prevent escape prophylactic treatments with chemicals. Fortun- of the toxic fumes. ately, nearly all threats to grain quality cause Pesticide residues temperature rises and monitoring of temperature, through incorporation of thermocouples, can reveal a great deal about condition. Some of the various types of pesticide (herbi- Forced ventilation can reduce temperatures cides, fungicides, insecticides and rodenticides) but it may be necessary to remove the cause by used in the field or in storage, may persist in use of chemical treatments. Such treatments are grains being processed or indeed into foods as relevant primarily when the problem is caused consumed. In the U. K. the maximum residue limits by insects. Because of the possible persistence of (MRLs) permitted are mostly defined in EC pesticides on cereals, their use in stores is increas- Directive 86/232 and amendment 88/298, which ingly becoming regarded as a last resort. In most came into force on 29 July 1988. Additional countries strict codes of practice apply to their MRLs came into force in December 1988 refer- use. In the U.K. the legislation is contained in ring to pesticides that have been refused approval the Food and Environment Protection Act 1985, in the U.K. but are used elsewhere; or those that the Control of Pesticides Regulations 1986 and have been consistently found in U.K. monitoring, HSE Guidance Notes (E440/85) on Occupational where the limits provide a check that good agri- Exposure Limits. cultural practice is being observed (see Table 5.5). STORAGE AND PRE-PROCESSING 113 TABLE 5.5 in residue level was also noted. Of the 1340 samples tested between ~~~il 1990 and March 1992, none approached the MRL for organophos- The Maximum Residue Limits for Pesticides (mglkg) in Cereals Excluding Rice (UK) Aldrin and Dieldrin 0.01 Captafol 0.05 phorus or other classes of pesticide. Carbaryl 0.5 Carbendazim 0.5 Carbon disulphide 0.1 Carbon tetrachloride 0.1 Chlordane 0.02 Chlorpyrifos methyl 10 DDT (total) 0.05 Diazinon 0.05 Before processing of cereals it is necessary to Dichlorvos 2 Endosulfan 0. I* Endrin 0.01 Ethylene dibromide 0.05 Entrimfos 10 Fenitrothion 10 carry out certain treatments on them. In cases Hexachlorobenzene 0.01 a and where grains undergo a period of storage before Hydrogen cyanide 15 Heptachlor 0.01 Mercury compounds 0.02 Malathion 8 The treatments include drying, cleaning, grading, 0.1 Methacrafos lo conditioning and blending. Of these, drying is Trichlorphon 0.1 Pyrethrins 3 likely to be carried out on the farm but the remainder are more likely to be performed at the the marketing system in use. Drying Monitoring of residues of fumigants applied at Purchasing-contracts typically stipulate an acceptable range of moisture contents correspond- ing to that required for safe storage. These may be considerably lower than the moisture content at harvest and some means of drying must be used. At its simplest, and in suitable climates, drying can consist of spreading the grain to dry in the sun assisted by frequent turning to expose all grains. Such treatment is unlikely to cause damage to grain beyond that which any handling imposes. More sophisticated drying methods employing artificially heated air or surfaces have inherent dangers that must be avoided if grain quality is to be maintained. In this context quality may be defined in several ways but the most fundamental is retention of the ability to germinate. This test is particularly relevant to malting barley as the malting process requires grain to be viable. In other cereals viability provides an indication that other quality factors have not diminished during drying. In milling-wheat, the most damaging change is denaturation of gluten and the consequent deterioration in baking quality. Aside from a germination test other, more rapid, tests include those using 2,3,5-tri-phenyltetrazolium chloride. A 0.2% aqueous solution applied to longitudinally Preprocessing treatments Y-He~cHorocYclohe~e 0.1 B-HexacHor~Yclohex~e 0.02 use, Some or all the treatments may take place Inorganic bromide 50 Hydrogen phosphide 0.1 before entering the store- Methyl bromide Pirimiphos-methyl 10 Phosphamidon 0.05 * 0.2 for maize. Italic type indicates limits are set by Good Agricultural mi11 Or the e1evator Or receiva1 as appropriate to Practice. Others are set by detection limit. Source: Osborne et al. (1988). levels specified by manufacturers in an experiment, revealed that only traces remained in cooked products. They were associated mostly with the bran fractions. Insecticide residues of laboratory-milled wheat grains, treated at manufacturers' recommended rates, were four times as concentrated in bran and fine wheatfeed as in white flour. In commer- cia1 samples the germ contained five times as much as in the white flour. The milling process removed only about 10% of the residue on the whole grain, however only 50-70% of that in white flour survived bread baking. Overall, the results suggested that insecticides applied at recommended rates are unlikely to lead to residues above the MRLs of Codex Alimentarius (Osborne et al., 1988). In the U.K. less than one quarter of stored grain is treated with pesticides (MAFF, 1991, H-GCA, 1991). As part of a surveillance pro- gramme, surveys have been carried out, since 1987, by the Flour Milling and Baking Research Association, in association with the National Association of British and Irish Millers, on samples representing those purchased by all flour millers. Residues have been low throughout but a decline 116 108 v 100- e! 92- a e, 84- +, f 60- 52 44 a $ 76- 0 68 remote grains and the introduction of a mixing mechanism minimizes the effects of this gradient. The diagrams in Fig. 5.9, show the differences. Many types of hot air drier exist, they are frequently classified as batch or continuous but they may also be distinguished on the basis of the direction of airflow in relation of that of the grain (Nellist, 1978). Thus in the Crossflow type, air flows across the path of the grains. The layer of grain adjacent to the air inlet is soon dried and - A its temperature rapidly approaches that of the - inlet air. The grain on the exhaust side remains - - - D C 11111111 STORAGE AND PRE-PROCESSING 115 Receiving bin Receiving bin Grain (Movement Grain (Movement Adjustable speed FIG 5.9 Non-mixing (left) and mixing (right) type columnar driers. (Wasserman and Calderwood, 1972). grain, but the heat and moisture transfer that occurs ensures that grain temperature does not rise to the inlet air temperature and that the air temperature falls rapidly. For the final phase of drying, the air and grain are almost at the same temperature. Advantages of this design are that high drying-air temperatures can be used to give high initial drying rates without overheating the grain and the period during which the grain and air are in temperature equilibrium is thought to help temper the grains and relieve stress cracks. In Counterflow driers air travels in the opposite direction to the grain and the dried grain temper- ature approaches that of the inlet air. The air tends to exhaust near to saturation, and drying is therefore efficient. The temperature of the hot air at inlet and the final grain temperature are almost the same (Nellist, 1978). Separations mites. Cleaning may occur before storage, or shortly before processing, or both. Cleaning before storage has the advantage that it helps to minimize deterioration in store which is aggravated by the parts. Receival at harvest time however is often accompanied by pressure to deal with grains rapidly and hence time for cleaning may not be available. Even grains cleaned before storage may require cleaning again before processing to remove undesirable elements resulting from the storage itself. Impurities, together with damaged and shrunken and broken grains, are collectively known as ‘screenings’ in the U.K., ‘Bezatz’ on the continent of Europe or ‘dockage’ in the U.S.A. Use of one term in the following includes all. ,mpurities Frequently encountered components of screen- ings may be classified according to their composi- tion thus: Vegetable matter - weed seeds, grains of other cereals, plant residues such as straw, chaff and sticks. - fungal impurities - bunt balls, ergot. - rodent excreta and hairs, insects and insect frass, Mineral matter - mud, dust, stones, sand, metal objects, nails, nuts etc. - string and twine. Miscel- laneous rubbish. Purity of cereal samples can be improved by cleaning and by separating. Cleaning involves the removal of material adhering to the surface of grains, while separation involves the removal of freely assorting material. Considerable ingenuity has been demonstrated in the design of devices Animal matter Other ,-ss&ense cf: .l>s9ken gr2ins, &sr and.-gXxa p!azs.. 116 TECHNOLOGY OF CEREALS for eliminating impurities by both methods and as a result a wide range of machines is produced by a number of manufacturers. Wide though the range is, the principles involved are few. Thus cleaning depends upon abrasion, amhim or impact, and separation upon discrimination by size, shape, specific gravity, composition and texture. No single machine can perform all the necessary operations and it is customary for parcels of cereal to pass through a series of operations based on the above principles. Metals Because of the potential danger to be caused to machinery by hard objects, these are usually among the first to be removed. Thus metals are removed by devices capable of detecting their composition, installed in spouting through which the grains are directed early in the process. Ferrous metals are attracted by strong magnets. Their removal from the magnetic surface by grain flow cannot be avoided so magnets are designed to revolve allowing removed metal objects to fall into a reservoir out of the grain stream. Non- ferrous metals are detected by the interruption which they cause to an electric field when passing through a metal detector. The interruption rapidly activates a mechanism which temporarily diverts the stream containing the offending item. Destoners Stones, sand and string also constitute potentially damaging impurities and hence these are also removed early. A single separator is capable of removing these and other less hazardous materials such as straw and some seeds of other cereals. Size and shape are the distinguishing criteria here, as the process is essentially one of sieving through a screen coarser than the grain (to remove string, straw and larger objects such as stones and large grains of different species) and over a screen finer than the required grains, through which broken grains, small seeds and sand can pass. Specific gravity separations may be made on the selected material leaving the separator. The grains leave the machine as a curtain and this is subjected to an upward stream of forced air which lifts light material such as chaff, while the denser particles fall under gravity. The aspirated air can flow to an expansion chamber where solid particles are deposited; air can then be recycled. For more rigorous removal of stones special- ized dry stoning machines may be used. Stoners have an inclined vibrating deck through which a uniform current of air flows upward. Feed is directed on to the lower end of the deck and spreads and mixes in response to the vibration. On reaching the region of the bed where the fluidizing effect of air is experienced, the grains are lifted beyond the range of influence of the vibration. They fall under gravity and are dis- charged at the lower end of the deck. Stones are too dense to be lifted by the air stream and continue to the top of the deck where they are discharged. Unwanted species Elimination of grains of other cereals and weed seeds surviving the above treatments is achieved by disc separators (see Fig. 5.10). Discrimination is on the basis of length, the width being of no consequence to this operation. A series of discs are mounted on a single horizontal axle in a trough partly filled with grain. The axle is driven causing the discs to rotate through the bed of grain. Each disc has, on both surfaces, a series of indentations arranged concentrically. The indentations behave as pockets into which grains of the required depth or smaller can fit. They are thus lifted out of the grain mass. As pockets pass the high point of the rotation they fall out of the pockets into channels adjacent to the disc faces by which they are conducted into a discharge in common with channels from the other discs. Material failing to be picked up in pockets is driven by worm from the feed end to the discharge end of the machine. A gate at the discharge end can be adjusted to control the amount of time spent in the trough. The more impurities the shorter the time needed for sorting. By use of several machines, each with different sized pockets in its discs, a number of separations may be made in sequence, with the required 117STORAGE AND PRE-PROCESSING ledium clean grain ,eed A 8 I-I --" I / I I r Oats and barley I ~maLl seeds G- Small clean grain FiG 5.10 Carter-Day Disc Separator for length separation of impurities from wheat, rye and other cereals grains. Exterior view of machines, right; diagram of operation, left. (by courtesy of the Carter-Day Company, Minneapolis.) to the destoner. Stocks are stratified according to their specific gravity by the oscillating action of the sloping deck, the lighter stocks are conveyed on a current of air while the heavier material is directed by friction on the oscillating deck. The routes of the two fractions can thus be separately ordered. Dust is removed by the air flow that provides the air cushion. Aspiration The rate at which a particle falls in still air is the resultant of the speed imparted to it by the force of gravity, balanced by the resistance to free fall offered by the air .The rate of fall, or 'terminal velocity' , depends on the weight of the particle and its surface-area:volume ratio. Compact spherical or cubical particles thus have a higher terminal velocity than diffuse or flake-like particles. Instead of allowing the particles to fall in still air , it is more usual to employ an ascending air current into which the stock is introduced. The velocity of the air current can be regulated so that particles of high terminal velocity fall, while those of low terminal velocity are lifted. Utilizing this principle, cereal behaving as the selected material when eliminating larger impurities, or the rejected material when eliminating small impurities. In some machines several separations can be made within a single unit. The same principle of separation in pockets on a rotating device is used in trieur cylinders. In these the interior surface of the cylinder has the pockets in it. The capacity of disc machines is greater than that of cylinders of the same diameter but the selectivity of cylinders is said to be better . The loading of discs or cylinders can be reduced if concentrators are used upstream of them. Concentrators, otherwise known as gravity selectors or combinators (Fig. 5.11), serve to effect a prelim- inary streaming of stocks so that the individual streams can be subjected to treatment appropriate to the impurities they are most likely to contain. Thus a light fraction would be treated to remove small seeds and grains of unwanted small cereals by use of discs or cylinders. The denser stream would not need to pass through these machines but may be routed to the more appropriate destoner . The concentrator operates on a similar principle 118 TECHNOLOGY OF CEREALS -* 2 __- -- I -I *IC; FIG 5.11 Gravity selector. (Reproduced by courtesy of Satake UK Ltd, Stockport.) STORAGE AND PRE-PROCESSING 119 slots. The recess upends undersize grains directing them through the hole. Where size and shape are similar and only small differences in specific gravity may exist, sophisti- cated gravity separators are required. An example of this type of machine is the Paddy separator (Fig. 5.12). In this a table mounted on rubber tyres is made to pivot around its horizontal axis. Compartments on the table are formed by a series of zig-zags arranged at right angles to the direc- tion of motion. Grains accumulate and stratify within the channels and the motion of the table causes those light grains which rise to the top, to travel towards the upper side of the channel flanks, while the heavier stocks pass down the table. As might be expected with a separation based on small differences careful adjustment of variables is necessary. Gravity separations have recently been found capable of separating satisfactory grains of wheat from sprouting grains. The sprouting, having led to some mobilization of starch, has rendered the affected grains lighter while not changing their particles of chaff, straw, small seeds, etc., having terminal velocity lower than that of wheat, can be separated from wheat in an aspirator, in which an air current is directed through a thin falling curtain of stock. In a duo-separator the stock is aspirated twice, permitting a more critical separa- tion to be made. In this type of machine, the lifted particles are separated from the air current by a type of cyclone, and the cleaned air is re- cycled to the intake side of the machine. Such closed-circuit aspirators save energy and minimize effluent problems. The separations described thus far are related to the separation of the required species from other cereal species, weed seeds and other materials quite unlike the grains of the required species. There are occasions when it is required to make a separation of two types of grain of the required species. This usually means shrivelled grains or those that have been hollowed as a result of insect infestation. They are removed mainly by aspiration or other methods depending on specific gravity. In oats the following types are present: overall size. 1. Double (bosom) oats. 2. Pin oats. Cleaning Cleaning, in contrast to separating, is performed 3. Light oats. 4. Other types of oats. on scourers. As their name suggests, these subject In double oats the hull of the primary groat grains to an abrasive treatment designed to remove envelopes a normal groat, plus a second complete dirt adhering to the surface of grains. Surface grain. Both groats are usually of poor size. Pin layers such as beeswing (outer epidermis of the oats are thin, short and contain little or no groats, pericarp of wheat) may also be removed. Scourers light oats consist of a husk which encloses only propel grain within a chamber by means of rotors a rudimentary grain. Other types are twins, to which are fixed beaters or pins. The axis of discoloured green and hull-less. (Deane and the rotor may be horizontal or vertical according Commers, 1986). to type. In the horizontal type, beaters rotate While indented discs and/or cylinders separate within a cylindrical wire mesh. Grains enter at on the basis of length, it is sometimes required, one end and are cast against the mesh by the particularly with oats, to eliminate narrow grains inclined rotor beaters; cleaning is achieved by the by a separation on the basis of width. This friction of grains against each other or against the involves the use of screens with elongated but mesh. Dust removed passes through the screen narrow slots, through which only reject grains and falls into hoppers by which it is discharged. can pass. The screens are commonly in the form Aspiration following scouring removes particles of a perforated rotating cylinder through which the loosened but not removed from the grains. Some bulk flows horizontally. Undersize grains drop vertical machines work on a screen and beater through into a hopper. Small round holes recessed system but others combine a scouring action with into the cylinder walls are used as an alternative to aspiration. 120 TECHNOLOGY OF CEREALS FIG 5.12 Paddy separator. (Reproduced by courtesy of Satake UK Ltd. Stockport.) FIG 5.13 Schematic diagram of a possible flourmill screen room. STORAGE AND PRE-PROCESSING 121 ings removed stage by stage. As the total quantity of screenings removed frequently amounts to only 1-1.5% of the feed by wt, every machine in the screenroom flow must have a capacity (i.e. be able to deal with a rate of feed) practically equal to that of the first machine. Durum wheats are subjected to particularly vigorous treatment with beaters. In addition to cleaning, the beaters also eliminate much of the embryos, considerably facilitating the clean- ing of semolina produced and reducing its ash yield. Scourers have replaced washers in the cleaning programmes of wheat in most countries on account Of 1. Problems of pollution control concerning the 2. Problems with microbiological control (mainly 3. High costs of operating machinery and of Loop system The feed rate to many of the machines can discharge of effluent. be reduced, and the efficiency of separation increased, by an arrangement known as the loop bacterial). or by-pass system. In the loop system, the first machine is set to water. reject a large cut-off (say 10% of the feed) containing all the separable impurities, together Washing may still be practised in the former with a proportion of clean grain. The remaining Soviet Union, where ultrasonic vibrators were 90% or so is accepted as clean, requiring no used to assist the cleaning process (Demidov and further treatment. The cut-off is retreated to Kochetova, 1966). Wet cleaning is also in use in recover clean wheat. As the cut-off amounts to the United States for removing surface dirt from only a fraction of the total feed, the feed rate maize before milling (Alexander, 1987). in the retreatment machines can be much re- duced, and the efficiency of separation improved. The loop system is frequently applied in the operation of disc separators and trieur cylinders, and is a feature of the Twin-Lift Aspirator, which embodies main aspiration of the whole feed, and re-aspiration of a substantial cut-off to recover clean grain. The Buhler ‘concentrator’ operates on the loop system. A cut-off of about 25% by wt. is lifted by aspiration and specific gravity stratification: this contains various impurities but is stone-free, and goes to a gravity table or scourer for separa- tion of clean wheat from the impurities. The remaining 75% is free of impurities other than stones and needs only to be de-stoned on a dry stoner or gravity table. Individual components of a mixed grist to a flour mill (cf. p. 192) are cleaned separately before blending. Wheat types differ in grain size and shape, and in the types of impurities contained, and require individual treatment in the screen- room, particularly as regards choice of sieve sizes for the milling separator, and of indent sizes for the cylinders and discs. Screenroom operation In practice, the grain passes in succession through a series of machines working on the various principles described. No single machine can remove all the impurities, but all the machines, considered as a unit, remove practically all the impurities for the loss of very little of the required grain. A possible cleaning flow for wheat is shown in Fig. 5.13. The efficiency of operation of screenroom machinery depends on machine design, feed rate and proportion of cut-off (reject fraction). As the feed rate is reduced, interference between particles decreases, and efficiency of separation increases. As the proportion of cut-off increases, the rejection of separable impurities becomes more certain. In the conventional screenroom arrangement, it is customary to feed to each machine in succession, the entire feed, except for the screen- 122 TECHNOLOGY OF CEREALS Dust explosions Dust is released whenever grain is moved: the atmosphere in grain silos and mills therefore tends to be dusty, leading to conditions under which dust explosions may occur. A suspension of dust in air, within certain limiting concentra- tions, may explode if a source of ignition above a certain limiting temperature is present. A series of dust explosions in U.S. grain elevators in 1978 caused the death of at least fifty persons, and led to the initiation of extensive programmes of research into the causes and avoidance of dust explosions. Avoidance of dust explosions is directed to: washing and, as the water is absorbed by the grains, there is no effluent problem. Neither is bacterial infestation usually a problem if lying times are short. If problems do arise they can be minimized by chlorination to levels above that of the water supplied. The study of the effects of water on the physical properties of cereals, and hence their milling properties, is complex. The cells of the grain have several components, the physical properties of which are altered, each in its own way. Water also affects the adhesion between the components of the cells, the cells themselves and the various tissues of the grain. Although applied to almost all cereals, condition- ing is not a single process appropriate as a preparation for all milling treatments. Even different types of the same cereal undergoing the same treatment may receive different treatments or different degrees of the same treatment. Con- sequently each cereal type will be considered separately here. Common or bread wheat The effects of moisture content and distribu- tion have received more attention in relation to wheat than any other cereal. Variation in endo- sperm texture demands different treatments; in general the amount of water added and the length of lying time increase in proportion to endosperm hardness. The reasons for conditioning are: 1. to toughen the outer, non-starchy endosperm components of the grain so that large pieces survive the milling, and powdering of bran is avoided; 2. to mellow the endosperm to provide the required degree of fragmentation. The term ‘mellow’ is much used to describe a desirable state of endosperm but a useful definition of its meaning is difficult to find. The changes induced by water penetration have been better described by Glenn et al. (1991) who found that endosperm strength decreased. ‘Strength’ as used by these authors has its strict materials-science meaning, i.e. a measure of the stress required to 1. suppression of dust and 2. avoidance of sources of ignition. Devices that minimize dust formation include light damping with water (about 1% by wt), dust- free intake nozzles, dust suppressors, and the ‘Simporter’ system - a method of mechanical grain handling in which the grain is conveyed ‘sandwich fashion’ between two belts held together by air pressure. By the use of inclined belts operating on this principle, grain can even be conveyed vertically. Sources of ignition most frequently responsible for dust explosions in flour mills are welding and hand lamps. Other potential sources are flames, hot surfaces and bearings, spontaneous heating, electrical appliances, friction sparks, static elec- tricity, magnets, bins and bucket elevators. Damping Damping, as a pretreatment for milling grists, is a long established practice. It probably origin- ated as part of the cleaning process when washing featured as an important treatment. It was adopted as a treatment in its own right when the beneficial effects of washing followed by a period of lying were noted. In current practice the washing of most cereals for cleaning purposes has now been abandoned but conditioning - or tempering (both terms mean the controlled addition of moisture) remains an important stage of proces- sing. Conditioning requires much less water than STORAGE AND PRE-PROCESSING 123 bring about fracture. The toughness (senso stricto) Losses of between 1 and 2.5% have been of the endosperm (manifested as the energy recorded due to evaporation of water from stocks. needed to bring about complete fracture) was found by the same authors to decrease with Water penetration increasing moisture content, although this was less consistently observed than the increase Water is absorbed rapidly by capillarity into in strength. The hardness of the endosperm is the empty pericarp cells. They are capable of not the only factor determining the level of taking up 80% of their weight, equivalent to about water addition. Clearly the grain moisture has 8% (w/w) of total grain weight. Once wetted an effect on the fractions separated during mill- therefore, the bran can behave as a reservoir from ing. It influences their surface properties which which water can pass into the endosperm. The are particularly significant in relation to their rate of passage is restricted by the presence of behaviour on sieves if the covers are made from impermeable components of the testa and possibly absorbent fibres such as nylon or (particularly) the nucellar epidermis. Since these are thinnest in silk. Moisture can also be transferred to the fabric the non-adherent layers overlying the embryo this of the sieves themselves, possibly altering the is the area through which most water gains access frictional properties and even changing the to the inner components of the grain. It advances tension and the effective aperture size. The towards the distal end of the endosperm, leading ease with which starchy endosperm may be to near-uniform distribution within the whole separated from bran is also influenced by mois- grain. Whereas saturation of the pericarp takes ture content and distribution. Both increase only minutes, equilibration throughout the grain and decrease of difficulty of this separation takes hours. with increasing moisture content have been The exact time taken depends on several factors imputed (although an increase is the more including: frequently claimed) but direct evidence is scarce and measurements must be difficult to make. It is generally accepted that, for the milling of high extraction rate flour, conditioning to a lower than normal moisture level is appropriate. Another factor influencing the degree of damping and post-damping lying is the target moisture content of the final products. Storage life of flour declines with increasing moisture content and most speci- fications impose a maximum moisture content - mainly to avoid buying water at flour prices. The relative costs of transport and the require- ment to absorb adequate water in dough making also exert an influence however. In calculating the amount of water required to achieve the target moisture content of stocks, characteristics of the mill and its location have to be considered. These include: 1. the conveying system in use (pneumatic sys- tems incur more moisture loss than elevator systems); 1. the initial moisture content of the grain; damper wheats absorb more rapidly (Moss, 1977) as well as requiring less damping. 2. The degree of permeability of the testa - particularly in the embryo region. There is little information on this from conditioning studies, but studies on imbibition in relation to germination show this to vary (Wellington and Durham, 1961; King, 1984). 3. The mealiness of the endosperm - water permeates through mealy endosperm more quickly than through vitreous endosperm. The essential feature appears to be the greater proportion of air spaces in mealy tissues. As soft wheats are more mealy than hard, equilibration is achieved more rapidly in softer wheats (Stenvert and Kingswood, 1976). As the continuity of the protein matrix is influ- enced by total nitrogen content of the endo- sperm, moisture movement is slower in high nitrogen wheats than in low. 4. The temperature of the water - a 3-fold increase in penetration rate was recorded for 2. ambient temperature and humidity; 3. roll temperatures. 124 TECHNOLOGY OF CEREALS each 12°C rise, by Campbell and Jones (1955) within the range 20"-43"C, above 43°C smaller rate increases applied up to 60°C. 5. The uniformity of the water distribution among grains. Where less than 8% water (w/w) is added it is preferentially absorbed into the pericarp of the grains which it contacts first, leaving little for distribution to more remote grains. Equilibration among grains takes even longer than within a grain. It is possible to exploit the above to achieve more rapid conditioning. Thus: to achieve the optimum moisture content of a particular type of wheat clearly depends upon the initial moisture content of the wheat. Wheat that travels long distances by sea tends to be traded at lower moisture content than wheat bought locally (water is expensive to transport!). If more than 3-5% needs to be added, it must be done in more than one stage. The amount capable of addition in one stage depends on the method of addition. In preparing wheats for milling as a mixed grist, in wfich components are introduced into the mill at different moisture contents, the individual higher starting moisture content for each blended after lying. Although the time taken for water to penetrate into the grain is reduced addition; impermeable layers; conditioning at elevated temperatures has declined 3. damping cauSeS expansion of the endosperm, as a result of prohibitive costs and the introduction but this is reversed as equilibrium is achieved of alternative practices that allow reduced lying or if drying occurs. such changes induce times. Details of warm and hot conditioning increase effective mealiness; Lockwood, 1960; Smith, 1944). They will not be 4. warm (above ambient to 46"C), or hot (above described here* 46°C) conditioning may be practised; Principles underlying the design of modern 5. precision placement of water in a uniform conditioning systems are: fashion andor VigOrOUS mixing of grains during 1. addition of accurately metered quantities of and immediately following damping may be water; employed. 2. uniform distribution of moisture. Much research has been carried out on the length Accurate water addition is achieved by control- of time required to achieve equilibrium moisture ling the water addition and flow rate of wheat, conditions. With so many factors affecting the rate possibly using feed-forward and feed-back control of penetration it is not surprising that results are systems. Uniform distribution is ensured by variable. Estimates range from several hours to careful placement of water followed by rapid several days, with more recent work tending to mixing. Most milling engineers now produce favour the shorter periods. Such revision of ideas conditioning equipment with sophisticated control has influenced the manner in which conditioning systems. The H20-Kay (Satake UK Ltd) system, of wheat is carried out (Hook et al., 1984). consisting of the Kay-Ray controller and a Techno- vator mixer, employs moisture measurement by microwave attenuation meters. This is the only method by which the moisture of freshly damped Conditioning practice Both phases of conditioning (i.e. addition of wheat can be monitored, allowing corrections to water, and lying for a period of equilibration) be made to the levels of water addition on a feed- require specially designed equipment, the design back basis. The system has the additional facility of which has evolved to suit changing circum- for adding steam. A sampling stream is diverted stances and changing ideas about conditioning. from the main flow, through a cell in which its The amount of water required to be added moisture content is measured. Additionally, flow 1. multistage damping provides a progressively components are best conditioned separately and 2. abrasion of the grain surface can remove the as a function of temperature, the practice of stresses which cause minute Stress cracks that systems are provided in older textbooks (e*g* Capacitance moisture - measurement -- - - - - - - - - - I c- - ------- Microprocessor - Temperature measurement I I I Throughput measurement I1 I yjjy+------A I . *. ... .e.... . . . . . . 126 TECHNOLOGY OF CEREALS Rice Rice is milled at 14% moisture content with no damping immediately preceding dehulling. For rice subjected to parboiling however an elaborate heat/water treatment is involved. Paddy rice has a hull of adherent pales (Fig. 2.23, p. 43) and water penetrates very slowly. Parboiling breaks the tight seal between the two pales, thus removing the main obstacle to entry. This was the original purpose of parboiling, which is an ancient tradi- tion, originating in India and Pakistan. Other changes occur however, including an improve- ment in nutritional quality of the milled product. The hot water involved in treatment dissolves vitamins and minerals present in the hull and bran coats, and carries them into the endosperm. Conversely, rice oil migrates outwards during parboiling, reducing oil content of the endosperm and increasing it in the bran. Starch present in the endosperm becomes gelatinized, toughening the grain and reducing the amount of breakage during milling. Aleurone and scutellum adhere more closely so that more of each remains in the milled rice. Some discoloration of grains occurs but susceptibility to insect attack is reduced. .-.--------.-/-- Barley FIG 5.15 Manifold discharge from a First-in-first-out type conditioning bin. From: Screenroom Operations 1. Flour Milling Industry Correspondence Course. By courtesy of Incorporated National Association of British and Irish Millers Ltd. ing . It is particularly important for conditioned grain. It is usual for a bin to be filled by deposition from a central spout: grains delivered to the centre tumble towards the edges but the pile remains deepest at a point below the delivery spout and a smooth conical profile with an ‘angle of repose’ characteristic of the particular sample remains at the top. As the bin empties, the profile changes because the grains in the centre fall more quickly than those at the edges. This leads to a reversal of the original surface profile, the centre becoming the lowest point. Such a means of emptying is disorderly, it leads to a mixture of grains and a sequence of removal different from that of filling. The sequence of emptying may not Conditioning for pearling consists of adjusting the moismre cOntent Of&e grhs to 15vo, followed by a rest of 24 h. Maize The details of the conditioning protocol in preparation for degerming of maize depend upon the process in use: for the Beall degerminator, the moisture content is raised to 20-22%, while for entoleter or rolling processes an addition of only 2-3% to stored grain is appropriate. Follow- ing damping a rest of 1-2 h is usual but up to 24 h is possible. Conditioning bins In all storage bins used for cereal grains, sound design is important with regard to efficient empty- STORAGE AND PRE-PROCESSING 127 F. H. (Ed.) Amer. Assoc of Cereal Chemists Inc., St. Paul, MN. U.S.A. DEMIDOV, P. G. and KOCHETOVA, A. A. (1966) Effect of ultrasonic waves on the technological properties of grain. Izv. Vyssh. Ucheb. Zaved., Pishch Tekhnol. 1: 13. EVERY, D. (1987) A simple four-minute protein-solubility GLENN, G. M., YOUNCE, F. L. and PITTS, M. J. (1991) Fundamental physical properties characterizing hardness of wheat endosperm. J. Cereal sei. 13: 179-194. HOOK, S. C. W., BONE, G. T. and FEARN, T. (1984) The conditioning of wheat. A comparison of U.K. wheats milled at natural moisture content and after drying and conditioning to the same moisture content. 3. Sci. Food KING, R. W. (1984) Water uptake and pre-harvest sprouting damage in wheat: grain characteristics. Aust. J. Agric. Res. 35: 338- 345. Simon Ltd. Stockport, Cheshire. MOSS, R. (1977) The influence of endosperm structure, protein content and grain moisture on the rate of water penetration into wheat during conditioning. J. Fd. Techml. 12: 275-283. NELLIST, M. E. (1978) Safe Temperatures for Drying Grain. Report No. 29, Nat. Inst. Agric Engng. (To Home Grown Cereals Authority.) OSBORNE, B. G., FISHWICK, F. B., SCUDAMORE, K. A. and ROWLANDS, D. G. (1988) The Occurrence and Detection of Pesticide Residues in UK Grain. H-GCA Research Review No. 12. Home-Grown Cereals Authority, London. PIXTON, S. W. (1980) Changes in quality of wheat during 18 years storage. In: Controlled Atmosphere Storage of Grains, pp. 301-310, SHEJBAL, J. (Ed.) Elsevier Scientific Publ. Co. NY. U.S.A. POICHOTTE, J. L. (1980) Wheat storage. In: Wheat, pp. 82-84, HAFLIGER, E. (Ed.) Ciba Geigy Ltd, Basle, Switzer1and. POMERANZ, Y. (1974) Biochemical, functional, and nutritive changes during storage. In: Storage of Cereal Grains and their Products, pp. 56114y CHR1STENSEN2 c. M. (Ed.) ROBINSON, I. M. (1983) Modern Concepts of the Theory and Practice of Conditioning and its Influence on Milling. Gold medal thesis of Nat. Jt. Ind. Council for the Flour Milling Industry. Publishing Co. Ltd Liverpool. graphic demonstration of the penetration of water into wheat during tempering. Cereal Chem, 53: 141-149. STOREY, C. L. (1987) Effects and control of insects affecting 200, WATSON, S. A. and RAMSTAD, P. E. (Eds.) Amer. Assoc. of Cereal Chemists Inc., St. Paul, MN. U.S.A. wASSERMAN, T. and cALDERWOOD, D. L. (1972) Rough rice drying. In: Rice: Chemistry and Technology, pp. 140- 165, HOUSTON, D. F. (Ed.) Amer. Assoc. of Cereal Chemists. Inc. St. Paul, MN. U.S.A. WELLINGTON, P. S. and DURHAM, V. M. (1961) The effect DEANE, D. and COMMERS, E. (1986) Oat cleaning and of the covering layers on the uptake of water by the embryo be important in general storage considerations but in conditioned grain, where lying time is controlledy it is importan' that the grains intra- duced into the bin first are also the first to leave. The difficulties in achieving this are compounded the tendency to non-sequentid flow is exaggerated* Also the shortening of lying times associated with contemporary practice demands greater precision in controlling the lying period, emphasizing the need for sequential flow. The expedient by which improvements have been made is the deconcentration of outlets. In new bins a matrix of funnel+haped hoppers may that ducts to a common discharge hopper via a manifold (see Fig. 5.15). ~~i~~i~~ bins may be converted by imposing a cone-shaped baffle near the top of the discharge hopper. The cone may be point-up, surrounded by an annular space, directing flow to the periphery Of the biny Or point-down with an annu1ar space and an orifice in the centre also. In bins of square section pyramidal baffles replace the conical ones described for cylindrical bins. as damped grain flows less readily than dry and test for heat-damage in wheat. J. Cereal Sci. 6: 225-236. ~gn~. 35: 591-596. be provided at the base> each leading to spouting LOCKWOOD, J. F. (1960) Flour Milling. 4th edn. Henry References ALEXANDER, R. J. (1987) Corn dry milling: processes, products, and applications In. Corn: Chemistry and Tech- nology, pp. 351-376, WATioN; s. A. (Ed.) Amer. assoc. of Cereal Chemists Inc. St. Paul, MN. U.S.A. BOTHAST, R. J. (1978) Fungal deterioration and related Biology and Biotechnology, Ch. 8, pp. 210-243, HULTIN, H. o. and MILNER, M. (Eds.) Food and Nutrition Press Inc. Westport, Conn. U.S.A. BUSHUK, W. (1976) Rye, Production, Chemistty and Technology. phenomena in cere&, legumes md o&eeds. In: Postharvest Amer. Assoc of Cereal Chemists. St. Paul, MN. u.S.14. Amer. Assoc. of Cereal Chemists Inc, St, Paul, MN. SM1TH, L. (1944) F1ourMilling Techm1ogy. 3rd. edn. Northem U.S.A. functional changes in cereals: storage and germination. In: Postharoest Biology and Biotechmlogyy pp' 1-33, HULT1N7 Inc. Westport, Conn. U.S.A. CAMPBELL, J. D. and JONES7 c. R. (1955) The effect of temperature on the rate of penetration of moisture within damped wheat grains. Cereal Chem. 32: 132-139. CHRISTENSEN, C. M. (1974) Storage of Cereal Grains and Their Products. Amer. Assoc. of Cereal Chemists Inc. St Paul, MN. U.S.A. processing. In: Oats: chemistry and Technology, WEBSTER, BUSHUK, W. and LEE, J. W. (1978) Biochemical and STENVERT, N. L. and KINGSWOOD, K. (1976) An autoradio- H. 0. and MILNER, M. (Eds.) Food and Nutrition Press corn quality. In: Corn: Chemistry and Technology, pp. 185- of the wheat grain. Ann. Bot. 25: 185-196. 128 TECHNOLOGY OF CEREALS Further Reading ANON. (1987) Pests of Stored Wheat and Flour. Module 2 in Workbook Series, National Association of British and Irish Millers. H-GCA London. BAKKER-ARKEMA F. W., BROOK, R. C. and LEREW, L. E. (1978) Cereal grain drying. Adv. Cereal Sci. Technol. 2: 1-77. BHATTACHARYA, K. R. and ALI, S. Z. (1985) Changes in rice during parboiling, and properties of parboiled rice. Adv. Cereal Sci. Technol. 7: 105-159. CORNWELL, P. B. Pest Control in Buildings. A Guide to the Meaning of Terms. Rentokill Ltd. E. Grinstead. GORHAM, J. R. (Ed) (1991) Ecology and Management of Food- Industry Pests. FDA Technical Bulletin 4. Arlington VA. Assoc. of Official Analytical Chemists. HOME-GROWN CEREALS AUTHORITY (1992) Storage Crops SAUER, D. B. (Ed.) (1992) Storage of Cereal Grains and their Products. Amer. Assoc. of Cereal Chemists Inc. St. Paul MN. U.S.A. TKATCHUK, R., DEXTER, J. E. and TIPPLES, K. H. (1991) Removal of sprouted kernels from hard red spring wheat with a specific gravity table. Cereal Chem. 68: 390-395.