Food Lipids: Chemistry, Nutrition, and Biotechnology

DSC alone depends on thermal history, and other techniques may be necessary to
sort out the complexity [22].
C. Milk Fat
At least 168 different triacylglycerol species have been identified in milk fat [29].
This complexity results in three distinct endotherms on heating of samples held at
26�C. As with many natural fats, the temperature at which crystallization occurs
influences milk fat firmness, crystalline conformation, and percentage of solid fat
[30]. Shear effects on milk fat crystallization have also been investigated [31]. Shear
rate was directly related to crystal growth rate at crystallization temperature of 15�C
and 20�C. At 30�C, shear was negatively related to growth rate, indicating the possibility
of different growth mechanisms. However, the lower amount of supercooling
at 30�C necessitates a larger crystal diameter for stability. Therefore, shear may
simply interrupt the crystal growth, preventing development of the critical crystal
size.
Fats may be thermally treated with a process referred to as tempering to produce
specific attributes in a finished product. Tempering normally results in the
formation of stable crystals having the proper size and in the proper amount [32,33].
Hardness variability in milk fat results from different thermal treatments. Less solid
fat results when dairy butter is cooled slowly [34]. This is better understood considering
the presence of three milk fat fractions with largely independent solid solutions
[35]. A DSC thermogram (Fig. 11) depicts these independent fractions. The fractions
are neatly defined as high-, middle-, and low-melting fractions (HMF, MMF, LMF,
respectively). LMF is liquid at ambient temperature. Stable polymorphs of MMF and
HMF were found to be a mixture of ��-2 � ��-3, and �-2, respectively. The role of
LMF in the polymorphic transformations in milk fat fractions was investigated [36].
Figure 11 Differential scanning calorimetry melting curve of milk fat showing independent
solid solutions.
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