on the fractional melting of milk fat and the properties of the fractions

$: On the fractional melting of milk fat and the properties of the fractions$
J . Sci. Food Agric. 1985,36,421-432

On the Fractional Melting of Milk Fat and the Properties of the Fractions

William Banks, John L. Clapperton and Anne K. Girdler

Hannah Research Institute, Ayr KA6 5 H L

(Manuscript received 26 July 1984)

Standard methods of fractionating milk fat from the melt produce solid fractions that exhibit melting at temperatures considerably lower than that used for the fractionation. The use of high concentrations of aqueous detergent, in the presence of ammonia as an aid to centrifugation, has resulted in this solid material being sub-fractionated into a semi-solid, plastic material and a crystalline solid. The technique may be applied directly to milk fat to produce three fractions at a given fractionation temperature. Fatty acid compositions and separations according to triglyceride number are recorded for the parent fats and their fractions. Melting fingerprints, obtained by differential scanning calorimetry, are shown for the various samples, and the heats of melting are discussed.

Keywords: Milk fat; fractional melting; fatty acid composition; triglyceride; melting fingerprints.

1. Introduction

Fractional crystallisation of milk fat from the melt was employed by Norris et d.,' who fractionated three samples at 25-26"C, using aqueous detergent as an aid to centrifugal separation. Liquid fractions contained no material melting above the fractionation temperature, but the corresponding solid fractions contained a considerable amount of low-melting material. Although the fatty acid compositions of the solid and liquid fractions were distinct, the differences were not marked. Similar conclusions regarding the comparatively small changes in fatty acid composition consequent upon fractionation have been reported more recently by de Man and Finoro' and by Badings et ~ l . , ~ using filtration to separate solid and liquid phases. Sherbon et ~ 1 . ~ reported that attempts to improve the properties of the solid fraction by subsequent re-fractionation were successful only if the second fractionation was carried out at a temperature higher than the first. However, the extent of even this improvement was limited (e.g. the melting temperature of the major peak in the solid fraction increased only from 39 to

Badings et ~ 1 . ~ ascribed the rather poor properties of the solid fraction to the presence of contaminating liquid fraction, which they claimed could account for up to 70% of the total weight. However, it is difficult to reconcile this explanation with the observation4 that re-fractionation of the solid component at the same temperature as used in the initial separation failed to achieve any significant further fractionation.

Methods were therefore sought for re-fractionating the solid material that did not involve changing the original separation temperature. This report describes the successful technique and the fatty acid and triglyceride compositions and thermal properties of the fractions resulting from its application.

43°C).

422 W. Banks e? al.

2. Experimental

2.1. Milk fats Milk fats of very different rheological properties were obtained by dietary manipulation, using two groups of four cows. Group A was housed indoors and received a hay-based diet and 7 kg day-' supplementary concentrates, the latter containing 0.5 kg day-' palm oil, as previously described.' Group B was composed of cows at pasture, but, after the afternoon milking, 2.5 kg of molassed sugar beet pulp containing 0.5 kg soya oil were offered daily.

Milks from each group were collected separately and butters ~ r e p a r e d . ~ The nature of the dietary manipulation was such that milk fat A (i.e. from group A cows) had an increased proportion of high-melting triglyceride,* whereas in milk fat B there was an increase in the proportion of low-melting triglycerides.' It is more convenient to refer to the fats as hard and soft, respectively.

Anhydrous milk fats were prepared from the butters by melting, washing with hot water and drying in vucuo at 50°C.

Butters were stored at -18°C and anhydrous milk fat at 4°C.

2.2. Fractionation The fractionation technique used was basically that of Poot et al.' Butter A was fractionated at 21°C and butter B at 16"C, these being the temperatures at which the dominant peaks of the melting thermograms, obtained using the differential scanning calorimeter (DSC), are just completely melted. Butter (ca 20g) was mixed thoroughly with glass beads in a proprietary detergent solution (50 ml, 0.4% Byprox). Liquid was drained from the beads, which were washed thoroughly with aqueous ammonia (SO ml, 25% ammonia) and the washings added to the fat-rich liquid. The dispersion was then centrifuged (25 OOOxg, 15 min), resulting in three layers, namely oil, water and solid fat. The oil layer was removed by suction, the aqueous layer decanted and the residual solid material washed with detergent solution and the separation procedure repeated. Generally, this second washing produced no further separation. All the above operations were carried out at the temperature of fractionation. The two fractions, oil and solid fat, were washed repeatedly with water and ethanol and then dried. No further separation of the solid material could be achieved by repeating the entire process at the fractionation temperature. However, a 10-fold increase in the concentration of the detergent did cause further fractionation of the solid material. In this case, the procedure yielded, after centrifugation, three layers-a semi-solid fraction at the top of the tube, an aqueous layer and a crystalline solid. The semi-solid, plastic fraction was removed by suction, the aqueous layer decanted and the crystalline solid removed by scraping. The fractions were again washed repeatedly with water and ethanol, and finally dried.

When the fractionation procedure involving the higher concentration of detergent was applied directly to butter, four layers resulted on centrifugation-oil, semi-solid material, an aqueous layer and a crystalline solid. The oil and semi-solid material were removed successively by suction, and the aqueous layer decanted from the crystalline solid. The triglyceride fractions were then washed and dried as above.

2.3. Fatty acid analysis Fatty acids were analysed as the methyl esters by g.l.c., using the methods previously described.

2.4. Triglyceride analysis Triglycerides were separated on a Pye Unicam PU 4500 GLC fitted with a Chrompack On-column injector (Chrompack Ltd, London) and an FID detector. Butterfat was dissolved in heptane (ca 2.5 g fat litre-') and 1 pl of the solution was injected directly on to the WCOT silica column (12 mx0.25 mm i.d.) coated with 0.13 pm chemically-bonded CP-SiI-8 (also Chrompack Ltd). Hydrogen was used as the carrier gas, at a flow rate of 13mlmin-'. An injection

Melting of milk fat 423

temperature of approximately 50°C was used, with an oven temperature of 60°C and a detector temperature of 350°C. Immediately after injection, the oven was heated rapidly to 25OoC, held at this temperature for 4 min, then the heating continued at 4°C min-' until a value of 340°C was reached, at which it was maintained for 17 min. The oven and injector were then cooled down to the starting temperature. The total time required for the analysis of each sample was approximately 80 min.

2.5. Thermal analysis Differential scanning calorimetry was carried out on anhydrous milk fat and its fractions 3s before,8 except that sealed pans were used. Calibration for obtaining heats of melting (AH) was performed with indium and stearic acid. Fats were sampled in the molten state.

3. Results and discussion 3.1. Fractionation of milk fat B Fatty acid compositions of the parent butterfat B and its solid and oil fractions obtained at 16°C using low concentrations of detergent are shown in Table 1. The corresponding melting profiles are shown in Figure 1.

Fractionation into oil and solid fractions at 16°C gave rise to changes in fatty acid composition very similar to those previously reported in the literature, i.e. a fairly limited preponderance of acids of low melting point (4:&12:0 and 18:l cis) accumulating in the oil fraction. Similarly, while the oil fraction contained only insignificant amounts of material melting above the fractionation temperature, the melting curve for the solid fraction showed there to be a considerable amount of low melting material present (see Figure 1). Subjecting the solid material to the fractionation process at 16°C failed to achieve any further separation.

Thus these observations are in complete accord with literature reports'-3 on the fractionation of milk fat from the melt.

When the concentration of detergent used in the fractionation procedure was increased by a factor of 10 (4% Byprox), again at a temperature of 16"C, two fractions were obtained, namely a plastic, semi-solid material and a crystalline fraction. The fatty acid compositions and melting behaviour of these two fractions are shown in Table 2 and Figure 2, respectively.

The crystalline fraction contained a greater proportion of the high-melting fatty acids (16 : 0 and 18 : 0) than did the semi-solid material, but less of the low-melting 18 :1 cis. This differznce in

Table 1. Fatty acid compositions (wt %) of milk fat B and its initial fractions (oil and solid). Temperature of

fractionation=lh"C: low level of detergent

Parent Oil Solid Fatty acid milk fat fraction fraction

4:O 2.9 2.3 2.1 6:0 1.8 1.5 1.2 8:O 1.0 1.2 0.9

1o:o 1.4 2.2 1.5 12:o 2.4 2.8 2.4 14:O 10.1 10.9 10.4 14: 1 1.9 1.8 1.9 16:O 26.2 25.0 29.9 16: 1 0.9 0.9 0.7 18:O 18.0 14.4 21.9 18:l ck 24.6 27.5 18.6 18:l tram 8.0 8.4 7.9 18:2+" 0.9 1.2 0.6 Proportion (5%) 100 so 50

a18:2+ =sum of CI8 polyunsaturated fatty acids.

W. Banks et al.

0.500 coI g-' O C - '

C)

- - 20 10 0 10 20 30 40

Temperature ("C) Figure 1. Melting profile of (a) milk fat B; (b) its oil fraction and (c) its solid fraction. Arrows indicate the temperature of

fractionation (16°C).

Table 2. Fatty acid compositions (wt %) of the semi-solid and crystalline materials obtained on sub-fractionation of the solid fraction of milk fat €3 (see Table 1). Temperature

of fractionation=lh"C; high level of detergent

Semi-solid Crystalline Fatty acid fraction fraction

4:0 2.6 1 .s 6:O 1.2 1.2 8:O 1.3 1.1

lo:o 2.1 1.4 12:o 2.7 2.1 14:0 11.4 11.9 14: 1 1.8 2.8 16:0 28.5 34.0 16: 1 0.7 0.6 18:O 18.0 24.1 18:l cis 20.9 11.2 18:l trans 8.1 7.5 18:2+" 0.7 0.6 Proportion (wt %)b 35 15

"See Table 1. bProportion is given relative to parent milk fat.


T 0.500 cal g-'"C-'

Temperature (OC)

Figure 2. Melting profile of (a) the semi-solid and (b) crystalline material obtained on sub-fractionation of the solid material from milk fat B. Arrows indicate the temperature of fractionation (16°C).

fatty acid profile was reflected in the melting behaviour of the two fractions (Figure 2). Thus the crystalline fraction possessed only a small amount of material melting below the fractionation temperature and exhibited a major melting peak at 46°C. On the other hand, the semi-solid fraction was characterised by a considerable proportion of the fat melting below 16°C and very little remaining solid at 40°C.

The use of high levels of detergent therefore allows the solid material to be successfully fractionated. However, no conditions could be devised that allowed further fractionation (at 16°C) of either the semi-solid or the crystalline materials.

When milk fat B was fractionated at 16°C using a high concentration of detergent, the oil, semi-solid and crystalline fractions were obtained directly. In terms of both fatty acid composition and thermal properties, these fractions were essentially identical to their counter- parts detailed above.

426 W. Banks el ~ 1 .

Table 3. Fatty acid compositions (wt %) of milk fat A, and of its oil, semi-solid and crystalline fractions. Temperature of fractionation=2l0C; high level of detergent

Parent Oil Semi-solid Crystalline Fatty acid milk fat fraction fraction fraction

~~~~ ~

4:0 6:0 8:0

1o:o 12:o 14:O 14:1 16:O 16: 1 18:O 18:l cis 18:l trans 18:2+" Proportion (wt %)

3.6 2.1 0.8 1.5 1.7 7.8 1.3

38.2 2.2

10.9 25.0 2.7 2.3

100

3.9 2.4 1 .0 1.7 1.9 7.8 1.3

36.0 2.4 8.9

27.8 2.7 2.4

55

2.9 2.1 0.8 1.5 1.8 8.4 1.2

40.8 2.0

12.5 22.0 2.7 1.3

30

1.9 1.7 0.6 1.3 1.9 9.4 1.1

45.6 1.5

15.5 16.2 2.6 0.7

15

a See Table 1

3.2. Fractionation of milk fat A The technique of using a high concentration of detergent was applied directly to milk fat A, a fat rich in C16 fatty acids (milk fat B was rich in CIR fatty acids), using a fractionation temperature of 21°C. Fatty acid compositions of the parent fat and its fractions are shown in Table 3; the corresponding melting profiles are shown in Figure 3.

Comparison of the data in Table 3 for the oil and crystalline fractions demonstrates how the fatty acids partition according to melting point. The oil fraction contained higher proportions of 4:0, 6:0, 8:0, 10:0, 14:1, 16:1, 18:l cis and 18:2+ and lower proportions of 14:0, 16:0, and 18 : 0, with 12 : 0 and 18 :1 trans being virtually constant. The smooth change in the proportions of the shorter chain fatty acids implies that one acid may well be equally divided between the oil and crystalline fractions, and in this particular case 12: 0 is the crossover point. (In milk fat B, the crossover point occurs between 12:O and 14:C-see Tables 1 and 2). While it is possible in this fashion to rationalise the distribution of 12:0, the position with respect to 18:l trans is more perplexing. As a long-chain acid of high melting point, 18:l trans might reasonably be expected to accumulate in the crystalline fraction, but such is not the case. The amount of 18:l trans in milk fat A was not very high, but even when the value reaches 8% (as in milk fat B), fractionation does not increase the proportion in the crystalline material (see Tables 1 and 2 ) . The ambivalent position occupied by 18:l trans in respect to its distribution on fractionation is, however, consistent with the earlier observation' that two milk fats having similar proportions of short-chain fatty acids and long-chain, high-melting fatty acids (16: 0+18 : 0+18:1 trans) could nevertheless have very different physical and rheological properties.

The fatty acid composition of the semi-solid material was rather similar to that of the parent fat, but the melting profiles of the two were quite distinct. The same conclusion may be drawn in the case of milk fat B-compare the fatty acid compositions for the parent fat (Table 1) and the semi-solid fraction (Table 2) and their respective melting curves (Figures 1 and 2).

The melting curve for the crystalline material exhibited a major peak at 47°C and only a comparatively small proportion of the fat was liquid at the fractionation temperature. In certain respects, the melting curve of the crystalline fraction was somewhat similar to that of the so-called high-melting glyceride fraction (HMGF) obtained by fractionation of butterfat from acetone solution.'." However, the major melting peak of the HMGF occurred some 9°C higher, at 56°C. An attempt was therefore made to produce a higher-melting crystalline fraction from


- 3 0 - 20 - 10 0 10 30

- 30 - 20 0 10 20

0 10 20 30 40

Temperature ( " C ) Figure 3. Melting profile of (a) milk fat A, and (b) the oil, (c) semi-solid and (d) crystalline materials obtained on

fractionation. Arrows indicate the temperature of fractionation (21°C).

milk fat A by fractionating at 3WC, the temperature corresponding to approximately the middle of the melting plateau region of the parent milk fat.

Fractionation in the presence of 4% detergent yielded the usual three fractions-oil (65%), semi solid (27%) and crystalline material (8%)-but only the properties of the last-named are considered here. The melting curve is shown in Figure 4; the fatty acid composition of the fraction was not significantly different from that recorded for the crystalline fraction in Table 3.

428 W. Banks et al.

_ -

I I I I I I I 0 10 2 0 30 40 50 60

Temperature ( "C )

Figure 4. Melting profile of the crystalline material obtalned from milk fat A by fractionation at 30°C (indicated by the arrow).

The use of the higher fractionation temperature has moved the melting curve of the crystalline fraction to higher temperatures. In particular, the major melting peak now occurs at 54"C, very little different from the 56°C for the authentic HMGF obtained using solvent fractionation. However, the melting curve shown in Figure 4 is still rather wider than that recorded in the literature for HMGFl and the use of a higher fractionation temperature has not narrowed the melting range of the crystalline fraction (compare Figures 3d and 4). Also, the crystalline material contained measurable amounts of short-chain fatty acids that are absent in authentic

Thus, although the present technique may yield a high-melting crystalline fraction, there are differences between that fraction and the HMGF. Solvent fractionation, which has been used to obtain HMGF, is inherently superior to fractionation from the melt in it5 ability to separate the complex triglyceride structures found in milk fat.

3.3. Triglyceride composition of the parent fats and their fractions The triglyceride compositions of the parent fats and their fractions are recorded in Table 4 (milk fat A) and Table 5 (milk fat B).

In the case of the parent fats, there was some degree of similarity in that both exhibited bimodal distributions of triglycerides. In milk fat A, peaks occurred at triglycerides with carbon

HMGF. l1


Table 4. Triglyceride compositions (wt %) of milk fat A and its fractions

Triglyceride Parent Oil Semi-solid Crystalline

30 0.3 0.5 0.7 0.2 32 1.1 1.5 1.7 0.6 34 4.4 5.0 5.5 2.0 36 12.8 14.9 15.6 5.3 38 18.4 18.7 19.1 8.4 40 9.8 10.5 10.0 4.8 42 5.1 5.8 5.5 4.6 44 4.6 4.0 3.9 5.9 46 6.0 4.4 4.1 11.6 48 9.9 7.7 7.1 15.5 50 14.8 11.9 12.3 24.0 52 11.0 11.9 11.4 14.3

CNnb 43.2 42.8 42.6 46.2

Carbon number (CN)" milk fat fraction fraction fraction ~- -

- 54 1.9 3.2 3.1 2.8

"Carbon number IS defined as the number of fatty acid carbon atoms in the triglyceride 'CN, = number-averdge value of CN

numbers (CN), i.e. 'the total number of fatty acid carbon atoms in the triglyceride, of 38 and SO, whereas in milk fat B, they occurred at CN of 40 and 52. The displacement to higher values in the case of milk fat B reflects the fact that it is comparatively rich in C18 fatty acids, while milk fat A is rich in Clh acids. Although the bimodal distribution of triglycerides was obvious in both milk fats, the comparative weighting of the two peaks differs from fat to fat. Thus in milk fat A , the lower peak was slightly dominant, in B the higher peak was heavily dominant.

In milk fat A , the oil and semi-solid fractions had very similar triglyceride distributions. Relative to the parent fat, they tended to have a greater proportion of triglycerides with CN 3 W 2 , with a corresponding decrease in the higher triglycerides. The crystalline fraction, on the other hand, had low proportions of triglycerides with C N 3 W 2 and increased contents of those with CN 44-52,

With milk fat B, the parent and each of the fractions had quite distinct triglyceride distributions. In going from the oil to the semi-solid to the crystalline fraction, there was a gradual decrease in the proportions of triglycerides with CN 3G42 and 54 and a corresponding increase in those with CN 44-52.

The number average molecular sizes of the triglycerides of the parent fats and their fractions,

Table 5. Triglyceride compositions (wt 9%) of milk fat B and its fractions

carbon number (CN)" milk fat fraction fraction fraction Triglyceride Parent Oil Semi-solid Crystalline

30 0.1 0.3 0.1 0 32 0.4 0.6 0.3 0 34 2.0 2.1 1.3 0.2 36 4.5 6.1 3.8 1.0 38 8.8 10.8 6.7 1.9 40 9.1 11.4 7.4 2.0 42 4.4 4.6 3.8 2.7 44 3.6 3.7 4.0 4.5 46 5.6 4.5 6.5 9.4 48 8.7 7.2 10.7 15.4 50 17.8 13.0 18.7 23.7 52 18.3 18.1 21.3 25.8 54 15.7 17.6 15.4 13.4

__ ~. ~- ___ __

CN.6 46.9 46.4 47.7 49.3

".bSee Table 4.

430

Table 6. Values for the heat of melting (AH) for the parent milk fats and their fractions

W. Banks et al.

AH (cal g-I)

Milk fat Milk fat A B

Parent fat 11.4 15.3 Oil fraction 16.6 14.1

17.3 Solid fraction -

Semi-solid fraction 14.9 12.7 Crystalline fraction 20.6 19.8 High-melting fraction 22.3 -

quoted as m,, reflect these changes in distribution. In milk fat A, the oil and semi-solid fractions had effectively the same mean value of m,, which was less than that of the parent fat, while the highest m, was observed in the crystalline fraction. With milk fat B, m, increased in the order oil, parent, semi-solid and crystalline fraction.

Thus, although it is possible to generalise to the extent that the oil fraction would be rich in triglycerides with CN 30-42 and 54 and the crystalline fraction correspondingly rich in those of CN 44-52, prediction of the distribution for the semi-solid fraction is more problematical-in one case the distribution was very similar to that of the oil fraction whereas in the other it was intermediate between those of the oil and crystalline fractions.

3.4. Heats of melting of the fractions The values for AH, the heat of melting, for the parent milk fats and their fractions, are recorded in Table 6. Each value is the mean of at least four determinations carried out by DSC.

Timrnsl2 obtained a value for AH of 17.6calg-' for whole butterfat; the values for the fractions, obtained by successive crystallisations from acetone, were in the region 15.9-27.7 cal g-', AH increasing with melting point. Although other DSC measurements on butterfat and its fractions are recorded in the literature, AH is seldom quoted. However, in many cases sufficient data are available for AH to be derived and Table 7 contains values calculated from melting thermograms.

The values of AH for whole butterfats lie in the region 14-23calg-' which is perhaps a surprisingly wide range. However, all the values derived from the literature and in the present

Table 7. Values for the heat of melting (AH) of milk fat and its fractions derived from published melting thermograms

AH Material (cal g--') Reference __ __ -

Whole butterfats 20. cL20.6 13 Whole butterfat 22.1 14

High molecular wt fraction 18.1 Medium molecular wt fraction 16.2 Low molecular wt fraction 18.2



Whole butterfat 16.7 1s

Whole, linoleate-rich butterfat 14.2 15

Whole butterfats 14.>18.0 16


work refer to samples that have been crystallised in the DSC. Thus, only limited tempering is possible, and hence the rather arbitrary crystallising conditions may contribute to the rather wide variations in H.

TimmsI2 noted that AH for his fractions increased progressively with melting point. In the present work, AH values for the oil, crystalline and high-melting fractions follow the same general trend. By contrast, when milk fat glycerides are fractionated according to molecular weight rather than melting point, there appears to be no specific trend in terms of AH.I4,l5 In the present work, the change in molecular weight is rather limited-in milk fat A, the number- average fatty acid carbon atom content of the triglycerides (m,) changes only from 42.8 in the oil to 46.2 in the crystalline fraction, and in milk fat B the corresponding values are 46.4 and 49.3, respectively. Thus molecular weight is of secondary importance to melting point in determining the value of AH.

In this work, the lowest values of AH were associated with the intermediate, semi-solid fractions (Table 6). These low values infer that the degree of crystallinity was also low. The nature of this intermediate material is obviously worthy of further study.

4. Conclusions Contrary to previous reports, it does appear possible to obtain reasonable fractionation of milk fat from the melt. The use of suitable levels of detergent in the fractionation procedure enables the solid material to be separated into a crystalline and a semi-solid fraction. The nature and properties of the latter fraction demonstrate the complex crystals actually formed in butter. From the results presented here, it is difficult to sustain the claim5 that the solid phase obtained on fractionation from the melt is composed merely of crystals with a high proportion of contaminating oil phase.

References 1.

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13. 14.

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16.

Norris, R.; Gray, I. K.; McDowell, A . K. R.; Dolby, R. M. The chemical composition and physical properties of fractions of milk fat obtained by a commercial fractionation process. J. Dairy Res. 1971. 38, 179-191. dc Man, J. M.; Finoro. M. Characteristics of milk fat fractionated by crystallisation from the melt. Can. Imr. Food Sci. Techno[. J . 1980, 13, 167-173. Badings, H. T . ; Schaap, J. E.; de Jong, C.; Hagendoorn, H . G. An analytical study of fractions obtained by stepwise cooling of melted milk fat. 2. Results. Milchwissemchafr 1983, 38(3), 15G156. Sherbon, J. W . ; Dolby, R. M . ; Russell, R. W. The melting properties of milk fat fractions obtained by double fractionation using a commercial process. J. Dairy Res. 1972, 39, 325-333. Badings, H. T.: Schaap, J. E.; de Jong, C.; Hagedoorn, H. G. An analytical study of fractions obtained by stepwise cooling of melted milk fat. 1. Methodology. Milchwirsenschaft 1983, 38(2), 95-97. Banks, W.; Clapperton, J. L.: Ferrie, M. E.; Wilson, A. G . Effect of feeding fat to dairy cows receiving a fat-deficient basal diet. I. Milk yield and composition. J . Dairy Res. 1976, 43, 213-218. Banks, W.; Clapperton, J . L.; Kelly, M. E.; Wilson, A. G.; Crawford, R. J . M. The yield, fatty acid composition and physical properties of milk fat obtained by feeding soya oil to dairy cows. J. Sci. Food Agric. 1980, 31, 368-374. Banks, W.; Clapperton, J . L.; Ferrie, M. E. The physical properties of milk fats of different chemical compositions. J . Soc. Dairy Technol. 1976, 29(2), 86-90. Poot, C.; Dijkshoorn, W.; Haighton, A. J . ; Verburg, C. C. Laboratory separation of crystals from plastic fats using detergent solution. J. Am. Oil Chem. Soc. 1975, 52, 69-72. Banks, W.; Clapperton, J. L.; Ferrie, M. E. Effect of feeding fat to dairy cows receiving a fat-deficient basal diet. 11. Fatty acid composition of the milk fat. 1. Dairy Res. 1976, 43, 219-227. Chen, P. C.; de Man, J. M. Composition of milk fat fractions obtained by fractional crystallisation from acetone. J . Dairy Sci. 1966, 49, 612-616. Timms, R. E. The phase behaviour and polymorphism of milk fat, milk fat fractions and fully hardened milk fat. Ausr. J. Dairy Technol. 1980, 35, 47-53. Taylor, M. W.; Norris, R. The physical properties of dairy spreads. N . Z . J . Dairy Sci. Technol. 1977, 12, 166-170. Taylor, M. W.; Norris, G. E.; Hawke, J . C. The thermal properties of bovine milk triacylglycerols. N . Z . J. Dairy Sci. Technol. 1978, 13, 236-241. Morrison, I . M.; Hawke, J . C. Influence of elevated levels of linoleic acid on the thermal propertics of bovine milk fat. Lipids 1979, 14, 391-394. Banks, W.; Clapperton, J . L.; Kelly, M. E. Effect of oil-enriched diets on the milk yield and composition, and on the composition and physical properties of the milk fat of dairy cows receiving a basal ration of grass silage. J . Dairy Res. 1980,47, 277-285.

on the fractional melting of milk fat and the properties of the fractions

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