Melting and Mixed Crystal Formation

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  

Walstra et al., 1994 Breitschuh and Windhab, 1998 Marangoni and Lencki, 1998). Mixed crystals have a lower density and a lower enthalpy of fusion than pure crystals of the same polymorph. They also tend to rearrange slowly into purer crystals (Walstra, 1987). 

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 

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