Milk-fat TAG

The uniqueness of milk fat is not limited to its fatty acid profile. If the 400 fatty acids of milk fat were distributed randomly in the milk fat TAGs, the total theoretical number of glycerides would be 64 x 106 (Jensen, 2002) ... 

Minor components (non-triacylglycerol species) were removed from anhydrous milk-fat (AMF) to obtain purified milk-fat TAG (MF-TAG) by column chromatography using Florisil as the stationary phase (2). As previously described, the crystallization behavior of the original AMF, the MF-TAG, and MF-TAG to which 0.1% milk-fat diacylglycerol was added (MF-DAG) was studied by pNMR and turbidimetry (2). Although crystallization was studied between 5.0 and 27.5°C, we will concentrate only on data collected at 22.5°C. 

Among the biological lipids, few exceed bovine milk fat in the complexity of fatty acids present and triacylglycerol (TAG) structure. This, together with its importance commercially as a human food, has generated very large data bases on the synthesis and composition of milk fat. In spite of this, Jensen (2002) lamented the paucity of new information on the content of trace fatty acids and complex lipids in milk fat. 

More than 95% of Ci8 and longer-chain fatty acids in milk fat are derived from the blood TAG-rich lipoproteins. Non-esterified fatty acids are... 

The principal determinant of butter consistency is the ratio of solid to liquid fat (Rohm and Weidinger, 1993). Therefore, the extent of crystallization is critical to the texture of butter. Milk fat is composed of literally hundreds of unique and varied triacylglycerol (TAG) species (Jensen et al, 1991). This results in milk fat having complicated crystallization, melting, and rheological behaviour (Mulder, 1953 Hannewijk and Haighton, 1957). 

The rate of crystal growth is determined by the degree of supersaturation, the rate of molecular diffusion to the crystal surface, and the time required for TAG molecules to fit into the growing crystal lattice (Mulder and Walstra, 1974 Walstra, 1987). Compared to nucleation, the driving force required for crystal growth is relatively low (Sato et al, 1989). However, in a multicomponent fat, the supersaturation for each TAG is small (Walstra, 1998). This fact, combined with competition between similar molecules for the same sites in a crystal lattice, means that milk fat crystallization is especially slow (Skoda and van den Tempel, 1967 Knoester et al, 1968 Grail and Hartel, 1992). 

When TAGs in the liquid state are mixed, no changes in heat or volume are observed (Walstra et al., 1994). However, ideal behavior is not observed in the solid phase of milk fat (Timms, 1984 Walstra et al., 1994). As a result, the melting curve of milk fat does not equal the sum of its component TAGs (Walstra et al., 1994). Mulder (1953) proposed the theory of mixed crystal formation to explain the complex crystallization behavior of milk fat. Mixed or compound crystals contain more than one molecular species (Rossell, 1967 Mulder and Walstra, 1974). Mixed crystals form in natural fats, like milk fat, which are complex mixtures of TAGs (Mulder, 1953 Sherbon 1974 Walstra and van Beresteyn, 1975b Timms, 1980 ... 

Several studies have explored the phase behavior of milk fat and its fractions (Mulder, 1953 Timms, 1980, 1984 Marangoni and Lencki, 1998). Milk fat composition is often discussed in terms of groups or fractions of TAGs, which are chemically and physically distinct (Timms, 1980 Bornaz et al., 1993 Marangoni and Lencki, 1998). For example, saturated and monounsaturated TAGs account for 65 mol% of the TAGs in milk fat... 

Changes in milk fat composition can be brought about by altering the original FA and TAG composition by fractionation, hydrogenation, interesterification or blending. 

Figure 8.8. Effect of chemical interesterification on the relative proportion (w/w) of milk fat triacylglycerols as a function of carbon number (CN). TAG = triacylglycerol. Noninteresterified milk fat (O-O), interesterified milk fat-15 min ( - ), 30 min ( - ), 60 min ( - ), 90 min (A-A), and 120 min (A-A)- (Reproduced with permission from Rousseau et al., 1996a).

Milk fat contains a number of different lipids, but is predominately made up of triacylglycerols (TAG) (98%). The remaining lipids are diacylglycerols (DAG), monoacylglycerols (MAG), phospholipids, free fatty acids (FFA) and sterols. Milk fat contains over 250 different fatty acids, but 15 of these make up approximately 95% of the total (Banks, 1991) the most important are shown in Table 19.1. The unique aspect of bovine, ovine and caprine milk fat, in comparison to vegetable oils, is the presence of high levels of short-chain volatile FFAs (SCFFA), which have a major impact on the flavor/aroma of dairy products. Most cheeses are produced from either bovine, ovine or caprine milk and the differences of their FFA profile are responsible for the characteristic flavor of cheeses produced from such milks (Ha and Lindsay, 1991). 

The crystallization rate constant k) is a combination of nucleation and growth rate constants, and is a strong function of temperature (47). The numerical value of k is directly related to the half time of crystallization, ti/2, and therefore, the overall rate of crystallization (50). For example, Herrera et al. (21) analyzed crystallization of milkfat, pure TAG fraction of milkfat, and blends of high- and low-melting milk-fat fractions at temperatures from 10°C to 30°C using the Avrami equation. The n values were found to fall between 2.8 and 3. 0 regardless of the temperature and type of fat used. For temperatures above 25°C, a finite induction time for crystallization was observed, whereas for temperatures below 25°C, no induction time was... 

The relevance of the molecular structural diversity of the TAG to practical application may be understood by taking some examples of natural fats cocoa butter with major TAG of POP (l,3-dipalmitoyl-2-oleoyl-jn-glycerol), POS (1,3-palmitoyl-stearoyl,2-oleoyl-rac-glycerol) and SOS (l,3-distearoyl-2-oleoyl-jn-glycerol), milk fats whose major TAG are saturated-unsaturated mixed-acid TAG, and mixed-acid TAG with saturated fatty acids having different chainlengths (2). In these natural fats, few mono-acid TAG are present as major TAG components, and the major fats are composed of the mixed-acid TAG. Therefore, it is required to elucidate for the polymorphic structures of the mixed-acid TAG. 

AMF, anhydrous milk-fat MF-TAG, milk-fat triacylglycerol MF-DAG, milk-fat triacylglycerol with 0.1% miik-fat diacylglycerol. See Table 1 for other abbreviation. 

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