Physical Chemistry of Foods

whereas crystallizing in steps atT 2 andT 3 would give

a 3 
a 2

a 5 
a 2

þ

a 5 
a 3

a 5 
a 2

6

a 2 
a 1

a 4 
a 1

¼ 0 : 23 þ 0 : 7760 : 44 ¼ 0 : 57

By the same reasoning, slowly cooling to a low temperature yields less solid
fat upon melting than fast cooling to the same temperature.

4. Precooling to a lower temperature before bringing to the final
   temperature gives more solid fat than direct cooling to the latter. This can be
   reasoned by making a similar calculation as in point 3, and it is discussed
   below with respect to Figure 15.26b.


5. Unstable polymorphs have a (much) longer lifetime than those of
   pure triglycerides. Polymorphic transitions go along with changes in crystal
   composition.


6. Within one polymorph (usuallyb^0 ), changes in crystal composition
   can occur during storage at constant temperature (Figure 15.25). Such
   changes proceed (much) slower at a lower temperature.

Question

With reference to Figure 15.24, what would be the fraction solid when going fromT 1
toT 2 , and what when going toT 2 viaT 3?

Answer

0.24 and 0.37, respectively.

15.4.4 Nucleation, Growth, and Recrystallization

Now that we have learned about the various phenomena occurring during
fat crystallization, as well as about several factors affecting it, it may be
useful to discuss some aspects in the light of the knowledge gained.

Nucleation. This is the subject of Chapter 14, and nucleation of
triglyceride crystals is particularly discussed in Section 14.2.2. We may
conclude that nucleation will generally beheterogenousand occur in thea-
form. However, when an oil is cooled to, and kept for a while at, a
temperature between theaand theb^0 clear point, nucleation in theb^0 form
will generally occur. The nucleation rate is greatly dependent on
monoglyceride content, and these substances presumably form catalytic