Physical Chemistry of Foods

If one of the chains is an oleic acid residue, a bended (L3)

conformation is often assumed for these chains, as illustrated for

SOS.

For several mixed triglycerides, especially those containing unsatu-

rated chains, or chains that all differ in length, other forms of

molecular packing can occur.

The distances between the bilayers orlong spacingscan be determined by
small-angle x-ray diffraction (SAX).
A wide variety of polymorphs has been observed. Often two forms of
b^0 and ofbcan be observed, generally differing in tilt angle (apart from the
differences between the L2 and L3 forms). These are, e.g., designatedb 1 and
b 2 , the former being more stable than the latter. Another variety is
designatedgor sub-aor pre-b^0. It tends to have an orthorhombic subcell,
whereas the molecular packing is like that in thea-form (no tilt). Some data
of the three main forms of tristearate are given in Table 15.3. See Table 15.2
for melting points of variousaandbcrystals.

Transitions. Table 15.3 gives the molar entropy of fusion of the
three polymorphs of SSS. It is seen that the values increase when going from
atob^0 tob, which presumably means that the conformational freedom of
the chains decreases in that order (probably being about zero for theb-
form). For triglycerides containing unsaturated chains and for mixed
triglycerides, the variation inDSfamong polymorphs is even greater. All this
means thatacrystals tend to form easiest (at least if the temperature is

TABLE15.3 Some Properties of the Three Main Polymorphic

Modifications of Tristearin

Modification

Property ab^0 b

Crystal unit cell hexagonal orthorhombic triclinic

Main short spacings, nm 0.415 0.38; 0.42 0.46

Melting point, 8 C556373

Enthalpy of fusion, kJ?mol
^1110150189

Entropy of fusion, J?mol
^1 ?K
^1335446546

Melting dilatation, cm^3 ?kg
^1119167