Food Lipids: Chemistry, Nutrition, and Biotechnology

simply crystallized in the sub-� form, sometimes referred to as ��2 because of its
orthorhombic perpendicular subcell arrangement [43]. This form results only at very
low temperatures. The sub-� form of cottonseed and peanut oils transformed into a
stable ��-2 form. Four other oils (corn, safflower, sunflower, and soybean) showed
1
polymorphism similar to that of peanut and cottonseed, but these four fats developed
a stable �-2 form.
G. Hydrogenated Fats
Complete hydrogenation eliminates the asymmetry, often leading to �� stable polymorphs.
Therefore, soybean, peanut, sunflower, corn, and sesame oils, having a large
composition of C18 unsaturated fatty acids, are converted to hydrogenated fats having
stearoyl esters and consequently show the stable �-2 form. More highly saturated
fats, such as cottonseed, olive, palm, and cocoa, are converted to fats containing
1,3-dipalmitoyl-2-stearoyl-sn-glycerol (PStP), 1,3-distearoyl-2-palmitoyl-sn-glycerol
(StPSt), or 1,-palmitoyl-2,3-distearoyl-sn-glycerol (PStSt) [42,44]. The rearrangement
of PStP into a stable � form is hindered by misalignment of the methyl end plane
of the �� unit cell [13], and a fat rich in this triacylglycerol will stay in the �� form.
Hydrogenated fats rich in StPSt can transform into a stable � form, since the interlamellar
methyl end plane can rearrange more easily. The high PStSt fats have
equally stable �� and � forms, and any transformation to the � form occurs over a
long period of time.
H. Cocoa Butter
Two reviews of the composition of cocoa butter provide a good understanding of
the heterogeneity of this natural fat [45,46]. More than 98% of cocoa butter is simple
lipid. More than 95% of this is triacylglycerol. Three triacylglycerols predominate:
1-palmitoyl-2-oleoyl-3-stearoyl-sn-glycerol (POS) composes about 40%, 1,3-distearoyl-2-oleoyl-sn-glycerol
(SOS) makes up 27.5%, and 1,3-dipalmitoyl-2-oleoyl-snglycerol
(POP) makes up about 15%. At least 15 minor triacylglycerols have also
been identified. Free fatty acid values are reported to be 1.5%, and the concentration
of mono- and diacylglycerol is about 2% of the simple lipid fraction. The concentration
of phospholipid and glycolipids in cocoa butter varies by climate and method
of analysis. These complex lipids constitute about 1% of original cocoa butter.
1. Cocoa Butter Polymorph Nomenclature
More is known about the phase behavior of cocoa butter than that of any other fat
[28]. X-ray diffraction techniques have been used to define six polymorphs, each
having a distinctive melting point [47]. Interestingly, refined cocoa butter was used
in this work, and the possible effects of the compounds removed by refining appear
to have been discounted. The existence of six polymorphs in a study with 12 different
cocoa butters was confirmed [48]. Thermal analysis alone was used to evaluate the
polymorphism of cocoa butter and mixtures of cocoa butter with other fats [49]. That
study led to some of the confusion with terminology regarding cocoa butter polymorphs
in that the previously defined [47] designations (from lowest, I, to highest,
VI, melting point) were reversed regarding melting temperature. Also, six cocoa
butter polymorphs were found in work combining microscopy and thermal analyses
[50]. Table 3 lists the nomenclature and melting points of the cocoa butter poly-
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.