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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