Chapter 8 Fractionation of fat KANES K. RAJAH, Welsh Institute of Rural Studies, University of Wales, Aberyst- wyth, Penglais Campus, Aberystwyth, Dyfed SY23 3DD 8.1 INTRODUCTION Edible fats are derived from animal, marine or plant sources. They are often not available in edible form and in forms whereby they could be used readily in food preparations. The recovery of fats from their source is usually through rendering (animal, marine), crushing (seed oils), cold expression (speciality seed oils) and churning (as in butter from dairy cream). In most instances animal, marine and vegetable fats are processed further through degumming, bleaching, neutralisation and alkali refining or physical refining, and finally deodorisation before the fat is declared satisfactory for edible use - i.e. as a bland, odourless, pale and clear fat with good shelf-life stability. However, before this final stage is reached the fat would have undergone some form of separation during one or several stages of process, starting initially with filtration to remove impurities. A number of these edible fats are treated further, through hydrogenation, interesterification and fractionation to improve their oxidative stability, nutritional and functional value and processing properties (Moran and Rajah, 1994; Rastoin, 1985). For instance, stability is improved by light hydrogenation, fractionation can be used to re- move a proportion of high melting components (more saturated triacylglycerols) for use as pastry fats (e.g. milk fat' stearin), and the sandy texture arising from using non- modified lard in margarine can be improved by interesterification (Hannewijk, 1972). Some hydrogenated or interesterified fats are themselves in turn fractionated to re- cover the appropriate fraction(s) for specific food applications. Hydrogenation and interesterification reactions are mostly followed by separation processes, to remove, for instance, nickel catalyst from the hardened fat. Fractionation, however, is primarily a separation process, where fat is first nucleated and crystallised and then separated from the liquid phase using one of several techniques. This chapter will deal mainly with the fractionation of edible fat. The products from fractionation and their application in food are outside the scope of this book but this subject has benefited from a comprehensive review in a recent publication (Rajah, 1994). 208 K. K. Rajah The fractionation of edible oils and fats was practised as early as the mid-nineteenth century when oleomargarine was made from fractionated bovine tallow. This early manufacture of margarine was based on the invention of the French chemist, Hippolyte Mege Mouries (in 1869). He first obtained fresh tallow by careful rendering. The purified fat was then submitted to a slow crystallisation process at about 25-30°C (Andersen and Williams, 1965). The grainy coarse product which resulted was hydraulically pressed and yielded about 60% of a soft semi-fluid yellow fraction, oleo- margarine, and about 40% of a hard white fat, oleo-stearine. The softer fraction had approximately the same melting point as milk fat and could be easily plasticised. Mouries also believed that the soft part consisted of margarine and olein, the acylglycerols of margaric and oleic acids respectively, and the crystalline material mainly of the acylglycerols of stearic acid. Hence the name oleomargarine for his new butter-like product. The advantages of fractionation were first appreciated in Europe by the importers of coconut oil from Sri Lanka (Rossell, 1985). Warm fluid oil which was filled into long wooden barrels called ‘Ceylon Pipes’ cooled slowly as it sailed towards the cooler European climate and, perhaps aided also by the gentle agitation of the ship’s movement, crystallised and separated into fractions. This partly crystallised fat was evalu- ated by the recipient fat companies who found that the stearin fraction could be used to advantage in the couveture and coatings industry. When commercial scale fractionation first commenced, the process of cooling took place in large wooden vats, agitation being a manual operation using paddles. The crystalline suspension was separated by filtration through cloth. The stearin was then collected, wrapped in cloth, and squeezed in tower presses to increase the olein yield. However, the fractionation of fats soon declined and in the years following World War I it virtually ceased. Meanwhile, although the consumption of margarine rose and with it the demand for the hard base stock, this need was satisfied by the then fast developing hydrogenation industry using the process invented by Senderens and Sabatier in 1902. Hardened, or hydrogenated fats, mixed with liquid vegetable oils and non-fractionated bovine tallow enabled the formulation of the base stock for margarine, and remains so as we know it even today. During that period the small quantities, i.e. 2-5%, of wax or stearin recovered from the winterisation of salad oils such as sunflower oil and cottonseed oil were also processed into the margarine oil blend. The revival of fat fractionation finally came during the mid-l960s, following the remarkable upsurge in palm oil production, particularly in Malaysia. It provided the impetus to many to review the principles, processes and techniques on the subject. It also aroused the interest of the international dairy industry, and they too studied the technol- ogy to seek new opportunities for milk fat. The principle of the fractionation process can be described schematically as shown in Fig. 8.1. Three major commercial processes are available for the fractionation of fats. These combine the crystallisation (Saxer and Fischer, 1983) and separation processes: (1) (2) Dry fractionation. The crystallisation stage can be either rapid or slow and crystals are separated through direct filtration i.e. without the use of additives. Detergent fractionation. Crystallisation is generally rapid and an aqueous solution Fractionation of fat 209 Feed oildfats CRYSTALLISATION e.g. palm oil cooling butteroil/AMF * Nuclei formation hydrogenated fish oil stirring (nucleation) lightly hydrogenated soyabean oil, etc. Further cooling and gentle stirring I Crystal growth (morphology and polymorphism) Crystal separation 1-’ (filtration) Olein (soft fraction) Stearin (hard fraction) Fig. 8.1. The fractionation process. containing detergent is used to facilitate the separation of the crystals from olein (Lipofrac) by centrifugation. Solvent fractionation. The crystallisation is carried out in solvents followed by filtration. This process is not used widely due to its high operating costs except for the production of high value products such as cocoa butter replacer. It will therefore only receive a brief treatment in this chapter. (3) 8.1.1 Crystallisation: nuclei formation and crystal growth The controlled cooling of molten fat slows the thermal motion of the molecules, drawing them closer together through intermolecular forces, whilst simultaneously enabling parallel ordering of the fatty acid chains to take place. As a consequence nuclei form and crystallisation commences. Here, if the probability of a molecule being absorbed exceeds that of a molecule being liberated, then these molecular aggregates will grow into real crystals. Cooling aids the absorption of molecules by lowering the potential energy. The nucleation rate increases until a maximum is reached (Tammann, 1903) but further cool- ing contributes to a reduction in nucleation rate because the viscosity of the melt is increased, which as a consequence reduces the rate of diffusion. Mortensen (1983) reported that when formed, milk fat crystal nuclei grow through the deposition of successive single layers of molecules on an already ordered crystal surface. The probability of the incorporation of these molecules into the crystal lattice as well as the material density and the temperature, which influences the rate of diffusion, all play a primary role in the rate of growth of the crystals. It is also evident from studies using milk fat that for a given reactor vessel with a fixed rate of agitation (stirring), the cooling rate, i.e. rate of temperature drop, determines final crystal composition (Rajah, 1988): (1) Rapid cooling rates resulted in high yields of crystals with low N20 values (solid fat index values at 20°C) due to entrapped olein. (2) Moderate cooling rates promoted the development of crystals with primarily high melting triacylglycerols and high N~o values, although the yields were somewhat reduced. 210 K. K. Rajah (3) Slow cooling rates resulted in high yields of crystals, but the solid fraction had reduced N20 values. This is interpreted as being due to the development of nuclei which allow growth of crystals comprising high and medium melting triacylglycerols. Deffense (1985) observed that in factory operations filtration units cannot compensate for poor quality crystals, the latter being formed as a result of rapid cooling; the crystals group together and form clumps within which part of the liquid phase is occluded. Hence there is a decrease in olein yield of up to 10%. Deroanne (1975) reported in his dissertation that a low yield could also result from intersolubility and the formation of mixed crystals. 8.1.2 Polymorphism Crystals can exist in three main forms, a-, p- and p- (Chapman et al., 1971) in order of increasing stabilities and melting points. A metastable a-form results upon rapid cooling and is produced reversibly from the liquid phase. Hence rapid supercooling can result in a mass of very small crystals. In general the oil crystallises into the unstable a-form and then rapidly transforms into the more stable p-form, and much more slowly into the p-form. However, Hoerr (1960) stressed that less pure samples and triacylglycerols with a more complex composition may exhibit intermediate forms which are difficult to identify. Deffense and Tirtiaux (1982) reported that when crystals are in the p-form they are firm and of uniform spherical size and hence are easy to separate from the olein phase. For palm oil these p-crystals should be near 0.1 mm in size (Deroanne, 1976). p-crystals are formed readily when the oil is free from crystal inhibitors such as gums, carbohydrates, soap, mineral acids and monoglycerides. 8.1.3 Quality of edible oils When edible oils are freshly recovered from their source they are generally referred to as crude oils. There are some notable exceptions, e.g. dairy cream, olive oil, which is preferred in its untreated form so that its delicate flavour is retained, and even palm oil is consumed in the crude form in some parts of Africa. Most crude oils, however, contain impurities which need to be removed to make them palatable. The major processes used in removing these impurities are degumming (if the crude oil is of plant origin), chemical and physical refining (to reduce free fatty acids), bleaching (to remove much of the colour) and deodorisation (to remove flavour taints and other volatile material contribut- ing to odour). Removal of impurities from the feed facilitates the fractionation process, particularly filtration throughput. For comparison, during fractionation, a Tirtiaux Florentine filter type FLO 1000 will have a throughput of 2 tonnes/h on crude palm oil, 6 tonnes/h on refined palm oil, 8 tonnes/h on olein and 11 tonnes/h on beef tallow (Tirtiaux, 1980). The impurities also affect some laboratory analyses. For instance it has been reported (Haraldsson, 1978) that when crude palm kernel oil was used in the Lipofrac process to produce hard butters, the ensuing dilatation curves showed the re- fined stearin to have higher dilatation values, and a higher melting point, when compared to the crude stearin, Fig. 8.2. He attributed this to the removal of the free fatty acids Fractionation of fat 21 1 during refining, as the free fatty acids in a way function as a solvent on some high- melting acylglycerols. Similar behaviour was noted for the olein fraction. 80 - 70- ----- 50 - Temperature (“C) Fig. 8.2. Influence of refining on dilatation of PKO fractions (Haraldsson, 1978). 8.2 DRY FRACTIONATION Dewaxing and winterisation (Thomas 111, 1985) are two limited forms of dry fractionation used for the removal of waxes and high melting triacylglycerols respectively from liquid vegetable oils. For instance, sunflower oil and corn oil contain a small proportion of waxes which give them a cloudy appearance at refrigerated temperatures. While others, such as cottonseed oil, contain triacylglycerols which are rich in saturated fatty acids. Dewaxing and winterisation, respectively, make these oils suit- able for use as salad oils and for use in emulsions like mayonnaise. In the case of the latter, without this treatment, the high melting triacylglycerols would crystallise and separate during storage, causing the emulsion to break. Cottonseed oil, however, has substantially more palmitic fatty acid, in the range 17- 29%, compared with other liquid vegetable oils. This saturated fatty acid which, present as palmitic-linoleic-palmitic (PLP) triacylglycerol in cottonseed oil, crystallises out at normal ambient temperatures. This was recognised very early on in the United States, where cottonseed oil is widely available, and was as a consequence the first oil to be winterised. If the stearin is not removed, the cottonseed oil partially solidifies when stored at temperatures below 10-15OC. Nowadays, winterisation of cottonseed oil is carried out at refrigeration temperatures. The amount of stearin formed can be large. Although yields can be well in excess of 20%, filtration is still quite rapid. In view of this, during dry fractionation-winterisation processes, such as that offered by CMB Bernadini of Italy, the crystallisation is carried out in several large horizontal crystallisers, Fig. 8.3, so that, when ready, enough feed is 212 K. K. Rajah Fig. 8.3. Winterisation of cottonseed oil using horizontal crystallisers (courtesy of CMB Bernadini, Italy). available for continuous filtration. If suitably prepared, the melting points of these stearins can be in the range 20-25°C (Rossell, 1994). Cottonseed oil stearins can be an important source of zero trans fats which can substitute for hydrogenated fats, the latter being the subject of some concern in relation to trans fatty acids in the diet (Applewhite, 1994). They may find application in a variety of food formulations including margarines, soups and sauces. With annual world consumption of cottonseed oil currently at about 3.5 M tonnes, this potentially large source of zero trans fat is not being fully exploited. Ironically, much of the stearin from cottonseed oil goes into blends with soyabean oil which is then hydrogenated into hard stock for margarine and shortening manufacture. Since it is the hydrogenation reaction which is the main cause of trans fatty acids in processed fats, fractionation could well gain further importance as a means of generating zero trans hardstock, such as stearins from palm oil fractionation with melting points typically in the range 40-50°C (Rossell, 1994). Fats which contain a large proportion of higher melting triacylglycerols, e.g. milk fat, palm oil and tallow, are treated to full fractionation where both fractions, i.e. stearins and oleins, are recovered in large amounts, typically 20-30% stearin and 70-80% olein. Although manufacturers of fractionation equipment offer complete systems incorporating both the crystallisation tanks as well as the filtration units, the processes are generally referred to by the filtration system selected. Three major filtration routes are available: (a) flat-bed vacuum band filter; (b) rotary drum vacuum filter; (c) membrane, positive pressure filter. Hot water I > I I1 Y system / Heating it 214 K. K. Rajah Bogota in 1969. Subsequently other plants were installed world-wide to fractionate a variety of oils including milk fat, lard, hardened (hydrogenated) soyabean and fish oils. Yields of oleins are typically in the range 67-72%, and Deffense (1991) has suggested a further 8% increase is achievable if a membrane filter is used. The Florentine filter can be used to separate high levels of solids, up to the 60-70% range if required. Vacuband batch filter The Miller Vacuband filter (Miller, 1980; Kehse, 1979) which is a stationary bed vacuum filter (see Fig. 8.5; Rajah, 1988), was used in semi-commercial scale production of milk fat fractions. This system, offered by CJC (Oakmere, Cheshire, UK), shows important advantages over the ‘open’ vacuum systems. Fat slurry crystal - Vacuum Pump Fig. 8.5. Vacuband filter arrangement (Miller Filter Company Ltd, Overath, Germany and Chris James Consultants, Oakmere, Cheshire, UK). Crystallisation of milk fat was carried out in a jacketed stainless steel vessel of virtually identical design to that of a batch stirred tank reactor (BSTR). Nominal working capacity of the vessel was approximately 400 kg charge of anhydrous milk fat (AMF) feedstock. To achieve good heat transfer characteristics, the vessel was fitted with a variable speed, full sweep, anchor-type agitator arranged to prohibit mass rotation. Agita- tor speeds were possible within the range 4-30 r.p.m. although the optimum range was found to be 7-10 r.p.m., i.e. 0.36-0.52 m s-’. The vessel was additionally rated at 3.3 bar, for positive pressure nitrogen blanketing of product. During crystallisation, the head space was purged to establish a nitrogen blanket. The temperature difference between the oil and water jacket was maintained at a maximum of 5°C. Separation of milk fat crystals was carried out on the novel, stationary-bed, vacuum band filter, the Vacuband, surface area 1 m2, Fig. 8.5. The novelty lies in being able to filter and separate the liquid from the solid phase, under vacuum, within an enclosed upper chamber. This solid-liquid separation system is being used in a variety of liquid processing industries and in the edible oil industry during bleaching earth filtration, winterisation, and hydrogenation catalyst filtration. The unit comprised an indexing, horizontal rolled stored filter medium (paper), arranged over a static lower vacuum chamber and with a second upper movable (vertically) vacuum/feed chamber in opposition. The standard design utilised the upper Fractionation of fat 215 chamber to recreate a self-feeding system using upper chamber vacuum level, and on completion of each filtration cycle the vacuum in each chamber was released, and the upper chamber opened by lifting up, allowing the band to be indexed forward to its discharge. A stainless steel wire, fixed along the width of the band, ensured that the cake was dislodged from the filter paper and dropped into the heated trough in front of the filter. When the cake liquefied it was transferred via a butterfly valve at the base for packaging or texturisation for food use. The filtrate (olein) drawn under vacuum during filtration, was transferred via an intermediate vacuum tank, filtrate receiver, into the filtrate storage tank before being drummed. The most suitable filtration medium was found to be Paper/Binzer Type 67/N, 80 g, roll, 0.108 m in width and approximately 200 m in length, of bleached crepe quality. The yields for milk fat were typically 76-80%, Table 8.1, compared to 67-72% (Deffense, 1985) for the Florentine filter. This is attributed to the improved efficiency achieved by using the integral, vacuum sealed, upper chamber. The fastest crystallisation rate was established as 6°C h-', cooling down to 28°C for satisfactory filtration. Laboratory analyses carried out on milk fat fractions from vacuband filtration are given in Table 8.1 (Rajah, 1988). Comparative results on products using rotary drum and membrane press filter are given in Table 8.2 (Kokken, 1992). (Note: The 'Drop point' is a measure of the melting point of the oil or fat relating to the temperature at which an oil drop falls freely when a solidified sample is warmed in a cup with a small hole.) The multi-step fractionation of milk fat was also carried out using the vacuband filter. In this type of process the oleins from successive fractionations are used as feed for further fractionation. Typically, the quantity and size of crystals is maximised when oleins are cooled to temperatures of between 2 and 5OC below their melting point. Using this route, two-, three- and four-step fractionations have been completed satisfactorily, Table 8.3 (Rajah, 1988). In low temperature fractionations it is important to ensure that environmental temperatures are carefully controlled and that all contact surfaces for the crystal slurry are held at the temperature of separation. Low melting point milk fat oleins can be used in food applications where only liquid oils are normally used, e.g. mayonnaise (Rajah et al., 1984). 8.2.2 Rotary drum filter De Smet supply complete fractionation plants incorporating the 'Stockdale' type rotary drum filter. The crystallisation step is quite rapid, an average maximum of a 6 h cooling cycle is common. However, in order to ensure efficient and effective crystallisation the design of the crystallisation tank has to include a large cooling surface with good agitation facility. Typically, industrial crystallisers capable of holding up to 25 m3 product are presently available with these features. To achieve homogeneous supersaturation of the oil during cooling and even temperature throughout the mass of the oil, the distance between each crystal and the cooling surface must be minimised to enable the efficient dissipation of the heat of crystallisation. For this reason the use of a two-speed motor, with variable- speed gearbox, or if possible a continuous variable-speed motor, is proposed to drive the agitator. At the start of the process when the oil is in the molten state, at higher temperature (65-7OoC), maximum agitation increases heat transfer and consequently Y- s5 cc, s $5 4-8 '3 Y 2- g &? 8e ez 9- 2E s Y 5 E y- h 03- cnc C -3 2 corn N .. 2 3 Y a aJ cu 3 >w 0 .I 3 h b L c 0 e L; 5 zy w s >" C td .I Y z32 - .e v M C '3 Y - c- .A Y Y e -0 D 3 0 9 a- $5 on C 3 C 0 .I m .- Y td C C 0 F? % & td CE- dl 2 .I u '3 5 z .I Y 0 .- Y .e - - + Y *u T! 3 00 @J G 3 I I$ I li N 2 ; --T : om 3 - Q 2 0 N zq 3 % 2: 2 60 - r79 a- In d-Q b CPI cc -- c?: 2 $2 $ mN m d-N ": 32 2 z$ m Nb d m Nb %% Qb PN co P b. - cc, c 0 .I C .- 0 Y Ym .e $9 a= ;g 3 x3x.s- g 22 0 sg c ss c Y g m .- ,s '5 LLg 53; m h L 03; =g 5-c 5 2s 0 2 3 E .s '8 - ZESu h + v c! Fractionation of fat 217 Table 8.2. Milk fat fractionation using (1) rotary drum filter and (2) membrane press filter, at a filtration temperature of 25°C (Kokken, 1992) Melting point (closed SFC Drop capillary Yield point method 10°C 20°C 30°C 4OoC (%I ("C) ("C) IV (%) (%) (%) (%) (A) Feedstock - 33.3 35 32.72 32.72 16.12 4.4 0.22 (B) 6 hours cry stallisation time Rotary drum filter S25 45.6 38.3 39.8 29.06 50.74 30.93 13.72 0.11 L25 54.4 21.5 23.5 34.25 28.81 2.58 0.13 0.05 Press filter S25 32.1 41.2 42.0 27.23 61.17 41.88 22.51 4.3 L25 67.9 21.2 22.8 34.91 29.06 1.85 0.15 0.11 shortens the cooling time. Once crystallisation commences, the agitation rate has to be decreased to avoid secondary nucleation caused by the fragments of crystals fractured by excessive agitation. If the latter happens control over the crystallisation is lost, since this leads to the formation of a very large number of small crystals which consequently lead to gel formation. In the crystallisation of palm oil De Smet have also observed (Kokken, 1988) that too low an agitation rate leads to local overheating from the exothermic crystallisation reaction. This leads to dissolution of nuclei which are not sufficiently close to the cooling surface. It also leads to a high degree of supersaturation in the vicinity of the cooling surface, thereby leading to the inclusion of olein in the crystal structure which results in weak crystals. The specially designed and patented oil cooler-crystallisers enable crystallisation to be completed in less than half the time taken with the typical Tirtiaux process (Figs. 8.6, 8.7). The rotary drum vacuum filter is essentially a multi-compartment filter, consisting of a drum rotating about a horizontal axis and so arranged that the drum is partially sub- merged in the trough holding the oil slurry (Fig. 8.8; Filtration Services Ltd, Macclesfield, England). The periphery of the drum is divided into compartments, each of which is provided with a number of drain lines; these pass through the inside of the drum and terminate as a ring of ports covered by a rotary valve. It is by way of this valve that vacuum is applied. The surface of the drum is covered with a filter fabric and the drum is arranged to rotate at low speed, usually in the range 0.1-0.25 r.p.m. but up to approxi- mately 3 r.p.m. for very free-filtering materials. a Y 5 2 c s d, a 2 v) h ‘0 h e Y $ h + v I- 2 Y cd cd -0 cd 0 - .- Y $A 2% 3 Y i %- ;E5 32 1 .9 3% >g 5s - r;: ‘3 e, ‘5 Wctr E 1 .C m u 2 3 ‘2 0 E k 0 .- U 03 E 0 0 cd .e Y ct: Y e 5 CA .- 3 Y t4 00 a4 E ?-?Or‘: OZV?$? m b- bm ZN N NbS;O -?(..!(..! co-m ,$ Y Y e 2 ‘G 09?\4 ?c? Oc???? owe\ zp m\,wbg L5 n 36 c. 9 LE (..!Or‘?? cT\zl-*- .9 aa e 272 zz ‘S 22: zg;zz e b mi cci E a ???rC?? --wm* 6 d+ bmm 3N CY e Y m & 2 .s e pz 22 09a\\qoq\q ZZWrnoc d N3 N h e; .4 aa 5. ZL?% yzz y?c?zz 6g 3-- mm3 c.l m C c ’C .5” g mln- ?QI.L?901-?-? 09c?OO\q m-*B 60vi chg~g--bo 26 *m .e L?L?viq Z?:g \0-*31 mw- odk+ig 3 6g bm Wdm N m mm- 0 0 m .e Y & E ea z6 *-- mN3 $ b n 3 .:g 00-0 qqq 09F?(..!9L? OWPZW3 c??g?z v Y E h E E- - & e, .9 E L 2 -3 & 8 YL *G E m Y Q.SG ‘0 ,x 000-3 u,oB $Ezzzzzzz g 3-31 h a v) wa Y c>DD u 0 .f - 0 .- o,~~$~g Y 2\ijwww 2ao- -- Fractionation of fat 219 Steam 7 - Cold Water \ j - Chilled Water [) 4 Cold Water 4 Chilled Water Ill Cyrtallisd Fat - Fig. 8.6. Oil cooler crystalliser (outside view, courtesy of De Smet Rosedowns, Belgium). t x i $ ‘fi c Fig. 8.7. Oil cooler crystalliser (inside view, courtesy of De Smet Rosedowns, Belgium). 220 K. K. Rajah Fig. 8.8. Stockdale belt discharge vacuum filter (courtesy of Filtration Services Ltd, Macclesfield, UK). As the drum rotates, each compartment undergoes the same cycle of operations, the duration of each of these being determined by the drum speed, the submergence of the drum and the arrangement of the valve. The normal cycle of operations consists of filtration, drying and discharge. The rotary drum vacuum filter shown in Fig. 8.9 is the ‘belt discharge’ unit. The filter fabric leaves the drum carrying solids and is washed after the cake is discharged and is then returned to the drum. In the De Smet system the oil is continuously filtered through a nylon cloth, and stearin is discharged by compressed air blowing through the nylon cloth, combined with scraper action. The capacity and suction pressure of the vacuum pump is selected by consideration of the size of the filter and the permeability of the cake. For many fractionation operations this is about 0.6 m3 min-’ m-* at -0.6 bar G (or 0.4 bar absolute). The diameter of the vessel or receiver is calculated to give a low vapour velocity, i.e. 1 m s-*, to allow the liquid to dis-entrain. The vessel height is determined by the volume of oil to be retained, which in turn is influenced by the filtrate pump capacity and its control (Bosley, 1994). More than 60 of the ‘Stockdale’ filters are in operation world-wide (current suppliers, Filtration Services Ltd). They are being used for the dry fractionation of palm oil, and other vegetable oils and fish and animal fats. Fractionation of fat 221 Fig. 8.9. Stockdale rotary drum vacuum filter (courtesy of Filtration Services Ltd, Macclesfield, UK). A rotary drum vacuum filter system by Nivdba is understood to be in operation in France for the production of milk fat fractions. This design makes use of smaller crystal- lisation tanks but with dimple plate internal walls for increased surface area to volume ratio. A similar filtration system was used by Schaap and van Beresteyn (1970) in their pilot scale studies on milk fat. In the CMB (Costruzioni Meccaniche Bernadini, Pomezia-Rome, Italy) dry fractiona- tion process the feed oil is first crystallised in batch crystallisers, each fitted with three thermo-regulated water tanks held at different temperatures. Cooling commences when water from each tank is circulated in sequence for a pre-determined time period to facilitate fast cooling of the oil. Palm oil, for instance, is thermoregulated from 35°C down to 16°C before crystals are ready for separation. The filter system comprises a slowly rotating horizontal shaft from which separate vacuum filter elements fan out in a radial arrangement. The unit is dipped in a chamber containing the free slurry, which is held at constant level. As the elements become submerged in the oil slurry, olein is sucked in and a coating of stearin forms on the surface which is expelled by compressed air. 222 K. K. Rajah 8.2.3 Membrane filters Low pressure The crystallisation stage can be either rapid or slow and along similar lines to that described for flat-bed or rotary drum vacuum filtrations. The membrane filter is manufac- tured by a number of companies, including Hoesch, Tirtiaux and De Smet, and the main principle of operation remains the same. The Hoesch membrane filter was the first to be tested commercially during 1982-84, for palm oil fractionation in Malaysia. It is a tech- nological advance on the time-tried chamber filter press, an external distinguishing fea- ture being air-supply hoses to every other filter plate. The principle of the operation is shown in Fig. 8.10. The plates are equipped on either side with a flexible diaphragm or membrane. When compressed air is introduced between the plate and diaphragms it exerts static pressure on the filter cake enveloped within the filter cloth and squeezes out the olein against a flat plate on the opposite side of the cloth. Initially, when the cloth is clean, filtration takes place at 0.1-1.0 bar. As fouling of the cloth through repeated filtration becomes more significant a much higher pressure is required, in excess of 2.0 bar, up to about 5 bar. A typical membrane would be made of oil-resistant and food- compatible natural rubber combined with neoprene rubber on account of its flexibility. The membrane is not rigidly connected to the filter plate. Instead it is attached inside a dovetailed groove to the plate and can therefore be easily replaced. This also means that any accidental addition of compressed air, when the press is open, would not cause it to tear but merely loosen it from its fixture. Tirtiaux addressed this problem by fitting a control mechanism which ensures that squeezing only takes place when all the chambers are filled with slurry. However, Hoesch claim that their membrane is capable of such Recessed plate Feed Olein Air supply Olein Feedcore I II I 1 I I Mem brane-plate Olein Membrane Stearin cakes Filtration Squeezing Discharge Filter J" cloth Fig. 8.10. Low-pressure membrane filtration: principle of operation (courtesy of sa. Fractionne- ment Tirtiaux). Fractionation of fat 223 high degree of elasticity that it can be inflated to stretch across to the opposite side of the chamber without any risk of damage. This is a significant improvement over the earlier polypropylene membranes which were damaged easily when exposed to such extreme stress, i.e. even when the chamber is only half-filled with slurry. Consequently, Hoesch membrane filters are able to operate with only one membrane per chamber. The operation of the filter press is fully automated. When draining the filter press, the various chambers are automatically opened in succession. Squeezing the stearin slurry causes the fat crystals to cake such that it detaches itself in the form of a solid slab from a specially treated cloth. This is in effect, to some extent, automatic cleaning of the filter cloth. This therefore removes the need to clean the filter cloth after each filter charge, and cleaning can instead be limited to two or three times per week. The energy consumption for membrane filtration is low compared to vacuum filters. De Smet's figures (Kokken, 1992) give the electricity consumption for the filtration of palm oil containing 50% crystals as 0.2 kW tonne-' for the membrane press and 5 kW tonne-' for the rotary drum filter. This large difference is attributed to the high energy requirement of the vacuum pump in the rotary drum filter. Plonis (1985) reported on a study carried out during the early 1980s comparing the Hoesch filter with a conven- tional vacuum filter. The evaluation was carried out when a 100 tonne d-' membrane filter was installed in an edible oil refinery in Malaysia. Initially, the refinery used the membrane filter for half its filtration requirement, while retaining the vacuum filter in service for the remaining quantity of feed slurry. This therefore provided the ideal opportunity to compare results of products from the same crystalliser charge (Table 8.4). [Note that the 'cloud point' is the temperature at which the oil begins to cloud, following crystallisation under controlled cooling conditions. It is related to the unsaturation of the oil, and decreases as unsaturation of the oil increases.] A statistical analysis of the results also confirmed that the mean values obtained indicated a high degree of reliability and reproducibility. The olein yield, the iodine value and higher melting point of the stearin were statistically accurate and compatible with the iodine value and cloud points of the olein. Kokken (1988) reported on the production of super oleins i.e. palm oleins at cloud points below 8"C, as well as on the fractionation of animal and vegetable oils using the membrane filter (Kokken, 1990). Table 8.4. Comparative fractionation of palm oil using (a) a membrane filter press (Hoesch), and (b) a vacuum filter (Plonis, 1985) (a) Membrane filter press (b) Vacuum filter Olein Yield (%) 78.0 66.4 iodine value 56.95 57.07 cloud point ("C) 8.87 8.86 Stearin iodine value 34.32 41.70 melting point ("C) 53.86 49.99 Factory scale production (100 tonnes/day) using 34 charges from the same crystalliser. 224 K. K. Rajah Milk fat fractionation using low pressure membrane filtration has been investigated in several countries including New Zealand, (Table 8.5; Illingworth, 1990) and England (Rajah, 1987), but the first commercial plant was installed in Belgium by De Smet in 1986. Soon after, New Zealand commenced commercial manufacture of fractionated milk fat products using membrane technology. Typical products range, relevant technical data, and their function in various food applications are summarised in Table 8.6. Recently, multi-step fractionation of milk fat has become the subject of much interest and close study (Deffense, 1993). Typical results for oleins recovered from the third step are given in Table 8.7 (Kokken, 1992). Table 8.5. Milk fat stearins from membrane filtration (Illingworth, 1990) Separation Drop Solid fat content (%) temperature point ("C) ("C) NO NlO N20 N30 N40 28 46.6 87.7 84.5 69.5 51.3 23.0 25 44.7 85.7 81.2 61.1 41.0 14.1 High pressure The process for high-pressure filtration of fat was pioneered by Krupp Maschinentechnik GmbH in the late 1980s, with commercial scale evaluation commencing in 1990 (Willner et al., 1992). In principle, the Krupp Statofrac@ process is descriptive of highly selective fractiona- tion, where an efficient high pressure filter, the Hydrofilter Press (HFP), is used to remove significantly more olein from the fat crystals than has been possible with the already impressive performance of the low-pressure membrane filters. In comparative terms, the Statofrac@ process is claimed to be capable of olein yields above 82% while the latter are known to yield up to 80% (compared to 70% yields for the continuous vacuum filters). This process would therefore be of particular interest to those involved in the production of speciality fats such as cocoa butter replacers (CBRs), while due to the variable pressure control facility of the HFP, the process is suitable for application gener- ally to all fats. The evidence for the 1990s therefore points towards the replacement of detergent fractionation, which is used mainly in CBR production, with dry membrane fractionation. The flow diagram for the Statofrac@ process, Fig. 8.11, describes a two-part crystalli- sation stage, comprising pre-cry stallisation and static cry stallisation. After melting and precrystallisation in the stirred vessel (B Ol), the feedstock is transferred into the maturation section (MS 02), where the crystals grow under slow cooling and low or no (1.e. static) movement. This crystallisation procedure is necessary to ensure the production of crystals which can withstand high-pressure filtration. Such crystals, when squeezed dry, resemble those produced using solvents in terms of their solid fat content (Fig. 8.12; Willner, 1993). The crystal slurry which develops upon maturation is - 1 F05 = - c 0 .- 42 Y e 2 E - i; 9 2 OD c 2 e, .- v1 'DE 0 3 'De '- Y 0 gv) E Y z 2 Y k rn c 0 a 0 a .- i .- - 3 LE -0 0 00 4 Q) 5 0'D ui % cg cb Grnni3 c - I; 92 0 C&?< s *ob 0ai bn e,$% ucrrcx 0;;;83 .- c9d5g8 bn 'lsz;2w -uZ'D Ze,= 2.22 se,gs 3 4 8 2.z 2: 2 em ss cd UZC0 E UhZ -0.55 &3;gQa4E * e, cdc '- C QQ.2 E wg� , 38s- S0.-"3,ccr0E $hs5a ;.ssg g 6-1 ,g gz 2,gz a' 2 5.2 &dbXSS 022.g @Ek cdcd =lJ-Je,,a 2% kAe,C> %#'Do cgsg &+zB%$g 0 'D g .2 .e 0z Lm C2233S% EOSL H.r,S> .- cdcr Rzt Y 0 2 s d 2 0 r- I I 2 a h N In In N I I c\I t: 2 2 z I + g2 N 2 cc\I 2 2 N ez Tt % d 0 rn+ gu W I 10 I I eEol oz r- m m m m 10 In 10 I m I I a I s 0 \o I \o \o T 10 10 e:" W 10 I -0 'D 2% $ .- 0 .y zt ct: 0 .- k, 2 0 .z Y m CY r/. LL do d .c 'B E z ZZ E'S .- 'D 2El" 32-c E '3 ,s 8 .ZZ .- CJ-qE 2 cd UT= g OD c N h h W -e" r.4 2 N N I r- I r- N N m '- .y + p-! VN 0 $ Y 9 9 % :% z - .- 4 Y 3 .e oa C s 3 3 e, C 3 .e Y w 8 13 $3 8 m% 58 3 G 0 v) -Qv) 2 ai cj & 4 -0 0 CE: .3 cv: *s g,z .- Y 0y 2 @ 3 h e %8 + 33- c 3- .E:$ 3 $2 &T - u .m 2 E 2" r" j M c.r* .C c -en 5.gg 2 A 2 MQ .- 23 3 A ca 3v: 2 a3.2a.h 8gZ .3 .3 z%sdc32; e L 2,, 2 bel+= L 6 3 ," $'" 9 3bZE ga z.2 G0.b D%ZcG %$+e,r;;y- 37g ai 2.2 Ma 2 2 2.2 0 A'"" g 85 2 2 o g g $= 0 0s L c e! O-,ZD CL 0 0 aEi55G-3 22 $ g20vrAo e cds* E 8.8 $-0 - 2 or- $& s ;3; $" N I d 01 N 3 e d m In I I Q $2 d Q r- I 0 ?In Y r- z 4 r- '0 2 4 .g .- .YE$ 92ze2 me 0-0 35M Y .s:,;gg .3 %& v:+ tkDOrrcdd0 E2.9 00 m I i g d b P 8.. 8G 2-3 0.; Y Y 2 o* .- Y Y 9 I$? 22 =a UD avi \d 228 K. K. Rajah Table 8.7. Three-step fractionation of milk fat using membrane filtration, and typical results on the olein of the third step (Kokken, 1991) Yield Iodine Cloud point Solid fat content (SFC) ooc 5°C 10°C 20-25% 4748% 0.5-l.O°C 15-18% 5-8% 0 Fig. 8.13. Laboratory scale pilot plant: Hydro Filter Press HFP03 (courtesy of Krupp, Germany). normally associated with designs which have corners. This is important because the applied pressure can reach up to 50 bar. Each press has several chambers, and a block of chamber plates is enclosed and held together by means of a hydraulic closing unit. The crystal slurry or suspension is filled into each chamber, and when full the chamber feed line is closed. On the front facing side of each chamber is a filter sieve which is plane and made of stainless steel, of very fine mesh, held vertical for good cake release when the chambers are opened after filtration. Opposite, facing the rear, a flexible Fractionation of fat 229 Fig. 8.14. Pilot plant: Hydro Filter Press HFP04 (courtesy of Krupp, Germany). mpermeable membrane is attached to surround and cover that side completely. When filtration commences, the suspension is pressed against the filter sieve by the flexible membrane. The filtrate (olein) passes through the sieve and drains to the bottom outside the press where it is collected. The crystals (stearin) remain within the chamber and are pressed into a cake, the press cake, as the olein is squeezed out. The filtration pressure exerted by the membrane is generated by the hydraulic liquid. The application of a liquid in preference to a gas reduces the risk of explosions when working at very high pressures. It is feasible to use the filtrate as the hydraulic liquid, the advantage being that in case of membrane damage the fractionated products are not contaminated with inedible or toxic material. During the filtration cycle, the filtration pressure is increased slowly up to the end pressure, a recommended maximum of 50 bar. The rate of pressure increase is program- mable and the whole filtration process is automated. The design of a 25 tonnesd-' hydrofilter press is shown in Fig. 8.15. The fractionation of palm kernel oil and palm oil has been investigated in detail, and some work has also been completed on milk fat (butterfat). Some of the results are reported here (Willner, 1993). Figures 8.16 (a, b) show the schematic and process conditions for the production of palm kernel stearins, particularly PKS IV 7 suitable for hardening into cocoa butter substitute (CBS) manufacture. The StatofracB process is also shown to be suitable for the production of palm mid- fraction (PMF) which is used in the manufacture of cocoa butter equivalents (CBEs) (Fig. 8.17 (a-c)). The yield of palm olein increases with filtration pressure (Fig. 8.18), and this corre- lates with the reduction in iodine value of the stearin fraction. 230 K. K. Rajah Fig. 8.15. A production scale 25 tonnes d-' Hydro Filter Press HFPO2 (courtesy of Krupp, Germany). Palm kernel oil fractionation: Improvement of product quality by increasing filtration pressure Palm kernel oil fractionation fL, HPKS (a) 32 I 28 PKOL 26 Filtration pressure bar) (b) Fig. 8.16. (a) Single-stage dry fractionation of palm kernel oil for the production of PKS IV 7 suitable for hardening for high-quality CBS production; (b) increase in pressure during HFP filtration squeezes out more olein from the crystals, resulting in the production of very low IV stearins. The fractionation of milk fat yields stearins of solid fat content (SFC) values in the region of N,o = 85% (Figs. 8.19 (a, b)). This is indicative of more efficient removal of the olein phase from the crystals than those reported for vacuum filtration by Ricci-Rossi and Deffense (1984). 70 65 s 60- - 54 HLPOL 52 - 50 - 48 - 2 46- - - 0 0 Mid POL I I I I 34 - e ” 0 232 K. K. Rajah Palm oil fractionation: Improvement of product yield by increasing filtration pressure A 83 82 - 81 - 8 - 80- x > 1 ,: 79- .- 8 78- 77 76 75 I I I I - - 0 10 20 30 40 50 r - Filtration pressure (bar) Fig. 8 18. High-pressure filtration, using the Hydro Filter Press, increases the olein yield. 8.3 DETERGENT FRACTIONATION 8.3.1 The Lipofrac process The principles of the detergent process were first described by Fratelli Lanza in 1905 and it consequently also became known as the Lanza process. More recently Alfa Laval (now Tetra-Laval, based in Tumba, Sweden) developed a commercial process (Fjaervoll, 1969 and 1970) and called it Lipofrac fractionation. In principle, fractional crystallisation (Saxer and Fischer, 1983) forms the first stage. When the crystals have formed, water containing an aqueous detergent (sodium lauryl sulphate) and an electrolyte (magnesium sulphate or sodium sulphate) is added and the crystals become dispersed in it; the electro- lyte facilitates the agglomeration of the oil droplets in the succeeding mixing process. Separation of the crystals is completed by centrifugation. 8.3.2 Crystallisation Since the Lipofrac process was first introduced a number of improvements have been made to increase the yield of olein and to maximise on plant efficiency and capacity. The most notable improvement was made in the mid-1980s when a substantially modified procedure for fat crystallisation was introduced (Bauren, 1986). Until then crystallisation of fat was being carried out in 14 or 28 m3 batch crystallisers fitted with agitators capable of scraping the walls, the walls also forming the cooling surface for the crystalliser. This low efficiency design combined low agitation rates with a limited low-temperature sur- face area and low-temperature water-cooling regimes. Alfa Laval modified this operation and achieved shorter crystallisation times while making the whole process continuous (Fig. 8.20). The cooling of the fat was transferred 100 90 80 70 60 9 - 50- 6 Y v) 40 30 20 10 0 - Attempt K18 - - - - - - - - I I I h 3 3 m^ E VY P G B c” - 3 g ‘il 2 u v * $ L 3 Lg 8 e $5 z e- s 0 .- 8 .- .- FEZ- h 5- 1 BEE U- u B cl 0 0 s- 2 - 8- na 9 5- 1 g- f b 2 x- 3 $5- 2 u- Q- 128 -8 s3g $0 2s + LL - 0 Fractionation of fat 235 to efficient external plate heat exchangers, thus making it possible to use inexpensively constructed tanks for the required holding time. For palm oil fractionation, crude palm oil from the storage tank is first cooled. in pre- cooler 1, to 35"C, using water from a cooling tower as coolant. Some detergent is then usually added before the second precooler where the crude palm oil is cooled further to between 25-27"C, again by water cooling. At this point the oil is transferred to crystalliser 1 for a holding time of about 30 min. No cooling takes place in the crystalliser but some increase in temperature follows due to the heat of crystallisation. The mixture, compris- ing liquid oil, crystals and detergent, is pumped over to crystalliser 2 through a plate heat exchanger which cools it by a further 3-5OC. After holding the mixture in crystalliser 2 for a time it is pumped into crystalliser 3 through the second heat exchanger, which cools it down to the final fractionation temperature or lower. After a further holding time the mixture is ready for separation. An additional crystalliser and a third heat exchanger would be required for feed quantities exceeding 300 tonnes per 24 h. The temperature of the cooling water circulating in the plate heat exchangers is in the region of 3-10°C below the temperature of the oil. To avoid fat crystallising on the contact surfaces the temperature of the coolant has to be controlled closely. Nevertheless some crystallisation does take place, particularly in the corner areas of the plate heat exchangers where the flow is very slow moving or stagnant. When this happens the oil flow is diverted to by-pass the heat exchanger and hot water is introduced into the circulation to melt any crystals. It is reported (Bauren, 1986) that the revised crystallisation system enables uniform cooling of the whole feedstock, thus ensuring consistent quality feed slurry for separa- tion. It is also maintained that crystal size distribution is narrower than those achieved on the earlier crystallisers and that the average crystal size is also larger, which therefore reduces the amount of wetting agent required. The energy losses from the plate heat exchangers are also less than those suffered from the earlier crystallisers. The scheme described above is designed for the crystallisation of palm oil for the production of RBD (refined, bleached and deodorised) palm olein with a cloud point of 8°C. The system is, however, flexible, and with some minor modifications the same equipment can be used for other raw materials, e.g. fatty acids, palm kernel oil etc. Separation of the mixture is preceded by the addition of more detergent solution and heavy mechanical working to disintegrate any crystal agglomerates which could trap liquid oil and thereby prevent the wetting of some of the crystals. Even more addition of detergent follows and the mixture is then held under gentle agitation to enable the oil droplets to form a continuous phase. When ready, the mixture is fed to the centrifugal separator, where the olein is removed as the light phase and the suspension of crystals in the detergent solution is the heavy phase. The crystal suspension is then heated to melt the fat crystals, the stearin, which then becomes the light phase and is removed in a second separator. The detergent solution, now the heavy phase, is returned to the deter- gent tank and reused. It is reported that 8-10 ppm of surface active agent remains in the fractions (Bernadini, 1973). The yield of palm olein from Lipofrac fractionation is usu- ally about 80% at iodine values of 52, 57 and 30 for crude palm oil, palm olein, and palm stearin respectively. The critical factor determining the yield is the amount of olein remaining in the stearin suspension as droplets. These droplets are so fine that they are 236 K. K. Rajah not separated as part of the olein phase. If the formation of small droplets can be reduced, the yield can be increased. To get a measure of the potential theoretical increase in yield laboratory studies were carried out on palm oil (Bauren, 1986). Palm stearin suspensions from three pilot plant fractionations were first washed to remove all remaining olein. The iodine values (IV) were determined and found to be 26.7, 23.8 and 19.7. The spread was rather wide, attributed mainly to difficulties in sampling and analysis, but it was never- theless possible to conclude that the maximum possible yield should be somewhat better than 85%. The separation of saturated and unsaturated fatty acids has been discussed by Haraldsson (1984). He also earlier (1978) reviewed the production of hard butters from palm oil fats, Another typical process employing aqueous detergent fractionation is the Henkel process (Stein and Hartman, 1957). Jebson and Lochore (1975) compared the efficiency of the Henkel filtering centrifuge with that of the Alfa Laval equipment for the fractionation of milk fat. The Henkel centrifugal filter, normally a punched or wire mesh screen mounted on a rotating cone, yielded a maximum of 60% crystals, compared with 70% using the Alfa Laval process, on fat crystallised at 25°C. The New Zealand Dairy Research Institute (Norris et al., 1971) published a report on the fractionation of milk fat using a detergent process. However, due to world-wide resistance from those within the industry and consumers alike against any treatment of milk fat with additives even if only as a processing aid, such processes have never been scaled up to industrial manufacture. More recently Glassner (1987) studied the separation mechanism for the detergent fractionation of beef tallow and its relation to key process variables. Crystallisation conditions, the amount of surfactant (SDS), the electrolyte (sodium citrate) concentration, the weight ratio of detergent solution to partially crystallised tallow and the viscosity of the dispersion all formed important process con- siderations. He cited the example of beef tallow separated at 40°C, where all these variables affected the level of separation. He found, for instance, that over-dosing with surfactant caused the olein to emulsify while too low an inclusion resulted in incomplete separation. Apparently the selection of the correct dispersion had a significant effect in maximising the olein yield (Glassner and Grulke, 1987). 8.4 SOLVENT FRACTIONATION During solvent-aided fractionation either apolar (hexane) or polar (isopropyl alcohol, acetone or 2-nitropropane) solvents may be used. Typically, during commercial fractionation using the Bernadini process, hexane is used in a 1: 1 proportion for palm-oil fractionation. Alternatively, in a Unilever patented process, 3 parts of acetone are mixed with 1 part of palm oil. In the HLS process, the polar isopropyl alcohol is mixed with palm oil in a 1:l ratio (Hoffman, 1989). Fractionation by solvents is important when fats (containing a high proportion of triacylglycerols comprising long-chain fatty acids) remain highly viscous or even solid at temperatures normally associated with fractionation. It is based on the underlying principle that fractional cry stallisation from dilute solutions is more efficient with respect to separations than when solvents are not used. Consequently yields of olein are higher and stearin purity is much improved, while processing time is also reduced. Although these benefits are significant they are partially Fractionation of fat 237 off-set by the relative high cost of the initial capital investment as well as the high operational costs involved, particularly in relation to energy requirements and solvent loss estimated at 3-10 kg tonne-'. During solvent fractionation (Fig. 8.21) it is necessary to cool not only the oil, but the entire quantity of solvent as well, which adds to the cost. Three major solvent processes are understood to be in commercial operation. These are the Bernadini CMB process (Bernadini and Bernadini, 1971, 1975; Kreulen, 1976), the E.merso1 pracess (Muckerheide, 1960; Zilch, 1967) and the Zsndek psocess (Zondek, 1978). Other processes are also available and have been reviewed (Thomas 111, 1985). Most solvent processes can also be used for winterisation of oils. Solvent j. 1- Molten fat Crystalliser J. Filter (usually rotary drum vacuum filter) Olein -j b Stearin Recovered solvent Recovered solvent for recirculation for recirculation Fig. 8.21, Schematic diagram describing the general principles of solvent fractionation. The crystallisation temperatures are in the range from 20°C down to 0°C or less (Rossell, 1985). Filtration is often through a rotary drum filter which is enclosed to prevent solvent loss, Fig. 8.22. Jebsen et al. (1975) have outlined a process for solvent fractionation of milk fat using a batch plant designed to process one tonne of milk fat per day. It is reported that to produce good quality stearins, i.e. with a minimum amount of the liquid phase entrapped in the crystal structure, lower coolant temperature and faster stirring rates were critical factors (Munro et al., 1976). However, flavour problems from solvent residues in fractional products make solvent fractionation of milk fat impracticable (Cant et al., 1975). The use of carbon dioxide as a solvent in supercritical extraction (SCE) of fat fractions is a relatively recent process. This is discussed more comprehensively in Chapter 2. It is understood that the most recent activity in this area is being undertaken at the University of Wisconsin, where SCE is used to fractionate milk fat and, in a second stage process, the fractions are then modified using lipase enzymes to yield specific end-product fractions for a variety of food uses (Dairy Foods, 1989). Solvent fractionation is now being challenged by high pressure membrane filtration. The evidence to date suggests that the quality of the fractions produced by the latter is 238 K. K. Rajah Fig. 8.22. Rotary vacuum filter fully enclosed for use in solvent fractionation (courtesy of Filtration Services Ltd, Macclesfield, UK). similar to that from the former. 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