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 tage is reached the fat would have undergone some form of separation during one 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- over 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)
Chapter 8 Fractionation of fat KANES K. RAJAH, Welsh Institute of Rural Studies, University of Wales, Aberystwyth, 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 remove 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 nonmodified lard in margarine can be improved by interesterification (Hannewijk, 1972). Some hydrogenated or interesterified fats are themselves in turn fractionated to recover 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 ch. This early manufacture of margarine was based on the invention of the French mist, 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 abo 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 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 place in large wooden vats, agitation being a manual operation using paddle m 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 ye know it even today. During that period the small quantities, i. e. 2-5%o, 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-1960s, 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 oused 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) 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 (2) Detergent fractionation. Crystallisation is generally rapid and an aqueous solution
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, oleomargarine, 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 evaluated 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 technology 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 oils/fats CRYSTALLISATION cooling utteroilAMF stirring Nuclei formation ightly hydrogenate soyabean oil, etc Further cooling Crystal growth (filtration) containing detergent is used to facilitate the separation of the crystals from olein (Lipofrac) by centrifugation (3) Solvent fractionation. The crystallisation is carried out in solvents followed by iltration. 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. 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 ng contributes to a reduction in nucleation rate because the viscosity of the melt is ncreased, which as a consequence reduces the rate of diffusion Mortensen(1983)reported that when formed, milk fat crystal nuclei grow thre position of successive single layers of molecules on an already ordered crystal The probability of the incorporation of these molecules into the crystal lattice 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 lues at20°C) due to (2) Moderate cooling rates promoted the development of crystals with primarily high melting triacylglycerols and high N2o values, although the yields were somewhat
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 Crystal I growth (morphology and polymorphism) Crystal 1-’ separation (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 cooling 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. Rajal (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, B-and B-( Chapman et al., 1971)in order of increasing stabilities and melting points. A metastable a-form results upon rapid cooling ind 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 B-form, and much more slowly into the B-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 B-form they are firm and of uniform spherical size and hence are easy to separate from the ole phase. For palm oil these B-crystals should be near 0. 1 mm in size(Deroanne, 1976) B-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 onsumed 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
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 contributing 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 refined 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 211 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 Crude stearin Crude olein Fig8.2. Influence of refining on dilatation of PKo fractions(Haraldsson, 1978 8.2 DRY FRACTIONATION Dewaxing and winterisation (Thomas Il, 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 whie 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 10-15.C 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%0, 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
Fractionation of fat 21 1 during refining, as the free fatty acids in a way function as a solvent on some highmelting 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 suitable 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 20-25C(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 40-50C(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 vell 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)
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
Fractionation of fat 213 8.2.1 Flat-bed vacuum band filter Florentine continuous filter The Tirtiaux process(Tirtiaux, 1980; Ricci-Rossi and Deffense, 1984)for fractionation of fats by gradual and selective cooling followed by filtration on their patented, continuous belt 'Florentine, vacuum filter is probably the most widely used flat-bed system. Crystallisation is normally carried out in two stages(Kreulen, 1976). The plant layout includes a recrystallisation stage where the feed is first cooled slightly to reach nucleation. It is then pumped into crystallisation tanks, The vertical tanks are jacketed and fitted with an agitator or a coil or both, depending on their size, varying between 12 and 50 tonnes. The agitator provides convection movements without scraping the wall during the whole cooling stage for efficient crystallisation. The oil is cooled under con trolled conditions, whereby it is the temperature of the oil that actually controls the rate of cooling. This slow crystallisation enables control of the latent heat of crystallisation and avoids supercooling. When crystallisation is complete and the filtration temperature is reached, the slurry is filtered on the Florentine continuous-belt filter(Fig. 8. 4) Air conditioning Scraping Recycling Fig.8.4. Florentine filter(courtesy of Tirtiaux, Belgium). The Florentine is a horizontal type, flat-bed filter. The filtration takes place on a continuous perforated stainless steel belt operating at a vacuum of 50-200 mbar. The filter is fitted with a recycling device which enables the filtrate from the first filter section to be recycled. This facilitates filtration on a preformed stearin cake and increases the quality of the filtrate. The filter is self-cleaning, and the filtration area is enclosed and air conditioned. The latter helps to maintain the feed slurry at the temperature of fractiona- tion until separation of the fractions is completed. Filtration is possible at temperatures as high as 45C(tallow)or as low as 2 C (lightly hardened soyabean oil The Tirtiaux process was first developed in Europe for the fractionation of beef tallow. This was later extended to palm oil when they installed the first commercial plant in
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 ariety 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 lter(see Fig 8.5; Rajah, 1988), was used in semi-commercial scale production of milk fat fractions system, offered by CJC (Oakmere, Cheshire, UK), shows important advantages over the ' open vacuum systems Fiiter Pressure (stearin) pper chambe Filtrate chamber 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 5C. Separation of milk fat was carried out on the novel, stationary-bed, vacuum band filter, the Vacuband, surface area l m, 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
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. Agitator 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 ischarge. 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%o, 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 6Ch-l, 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 membrane press filter are given in Table 8. 2(Kokken, 1992). (Note: The ' Drop pointis 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 5C 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 olei 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 he crystallisation tank has to include a large cooling surface with good agitation facilit 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 he 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(65-70.C), maxim m agitation increases heat transfer and
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 variablespeed 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
Table 8.1.Milk fat fractionation using Vacuband filtration Filtration temperature: 28C)(Rajah, 1988) Solid fat content(SFC) Melting lodine Yield 20°C30°C (%) ) (%) (%) (1) Fast crystallisation Feed(anhydrous 34.7 5.6 Olein 45.2 l0.7 Stearin 42.3 25.6 23.7 (2)Slow crystallisation Feed(anhydrous 17.8 39.9 164 milk fat) Olein 399 Stearin 20 4.4 26.4 61.1 40.6
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