Fractionation of fat 21 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 (closed l0°C20°C30°C (%)(°C)(°C) ⅣV(%)(%)(%) (A) Feedstock 33.335 32.7232.7216.12440.22 crystallisation time Rotary drum filter S25 45.638339.8290650.7430.9313.720.11 54421.523.5 34.2528812580.130.05 Press filter S25 32.141.2420 272361.1741.8822.514.3 67921.2228 349129.061.850.150.11 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- nerged 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
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 submerged 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 approximately 3 r.p.m. for very free-filtering materials
Table 8.3. Multi-step fractionation of milk fat using the Vacuband filter. Analytical data for (1)two",(2)three-and(3)four-step fractionations(Rajah, 1988) 1)Two-step fractionation (2)Three-step (3)Four-step Olein Olein Stearin Olein Stearin Stearin (feed) (feed) (feed) Carotene(ppm) 9.5 9.6 10.8 8.4 odine value 41.5 46 48.7 41.7 44.8 41.2 Melting point 119 13.7 13.2 °C) Solid fat content 50.8 37.2 21.2 45.6 25.0 14.7 45.9 29.6 54.9 29.6 39.2 7.3 37.5 17.3 47.5 17.6 0.6 31.0 0.4 5.6 0.2 Fatty acids(%) 86 10.8 16:0 21.2 21.8 16 21.4 190 21.7 11.0 18: 1 trans 4.0 3.6 18: 1 cis 27.1 244 314 32.1 279
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