Physical Chemistry of Foods

1. Thenumber of carbon atoms. The higher it is, the higher areTm
   andDHf. The larger the molecule is, the greater the gain in conformational
   entropy upon melting, and the greater the melting enthalpy.


2. The presence and thenumber of double bonds. Crystallization of
   paraffinic compounds is difficult but occurs easiest if the chains can assume
   a linear zigzag conformation. A chain with a cis double bond cannot do this:
   it tends to form a bend (see Figure 15.19b, SOS). This makes crystallization
   more difficult. The effect will be greater for more double bonds. Cf. the
   series SSS–OOO–LiLiLi–LnLnLn.


3. Double bond configuration. A trans double bond allows the
   formation of an almost straight chain and hence causes a smaller difference
   in melting properties than a single cis bond. Cf. SSS, EEE, and OOO.


4. Position of the double bondsin the chain. The most important
   effect is that two conjugated cis double bonds ( 22 CH 55 CH 22 CH 55 CH 22 )
   can give a more nearly straight chain than two nonconjugated ones
   ( 22 CH 55 CH 22 CH 222 CH 55 CH 22 ), the common form in natural fats. Hence
   the former one crystallizes more readily (higher melting point).


5. Branchingof the chain tends to cause an appreciably lower melting
   point.
   Moreover, modified fatty acid residues occasionally occur, e.g.,
   containing an 22 OH group.
   Table 15.2 also gives data for somemixed triglycerides(not to be
   confused with a mixture of triglycerides). The melting point and the heat of
   fusion are generally smaller than the ‘‘average’’ of the corresponding
   uniform triglycerides. (Only if it concerns saturated chains that do not differ
   by more than two C-atoms in length, the results agree well with the average.)
   It is also seen that symmetric molecules, like POP, crystallize more readily
   than asymmetric ones, like PPO. Large differences in chain length (as in
   PPB) have a strong effect.
   It should finally be mentioned that the data on the enthalpy of fusion
   are subject to uncertainty; various authors give values that may differ by
   several percent.

Simple Mixtures. In natural fats, triglycerides do not crystallize
from the melt but from solution. The higher melting species are dissolved in
the lower melting ones. The solubility is given by the Hildebrand equation
(2.9), which is repeated here:

lnxs¼

DHf

R

1

Tm

1

T



ð 15 : 8 Þ

wherexsis the mole fraction soluble,DHfis the heat of fusion of the pure