coalescence of aqueous droplets in butter and margarine, and partial
coalescence in some O–W emulsions (see Section 13.5).
Visual appearance, such as the appearance of bloom on chocolate (tiny
white fat crystals appearing on the surface); gloss of chocolate and
margarine; and turbidity in oils.All of these properties can change during storage.
Fat crystallization is a highlycomplex phenomenon. Before a more
detailed discussion, we mention
Triglycerides are large and highly anisometric molecules that can
assume many conformations. This gives rise topolymorphism; the
main polymorphs are calleda,b^0 , andb, in order of increasing
melting point and stability.
Natural fats are always mixtures of a (great) number of different
triglycerides and therefore have amelting range, generally spanning
several times 10 K.
Several of the different triglycerides are nevertheless quite similar in
molecular structure. This leads to the extensive formation of
compound crystals(solid solutions).The first two causes mentioned are also responsible for theslownessof fat
crystallization. All of them contribute to thenonequilibriumstate that is
common in nearly all fats.
Nucleation, including that of fat crystals, has been discussed in
Chapter 14, nucleation of emulsified fat in Section 14.3. Growth rate is
discussed in the present chapter, especially in relation to Figures 15.8 and
15.11. Mechanical properties will be discussed in Section 17.3, proceeding
from the discussion in the present section.
15.4.1 Melting Range
Natural fats always contain several different fatty acid residues. If the
number of these is given byn, the possible number of different triglyceride
molecules (including stereoisomers) equalsn^3. Since several natural fats
contain well over a hundred different residues (especially milk fat and fish
oils), the number of different triglycerides can be enormous, but most of
these are present in trace quantities. More realistically, the number of
triglycerides in significant quantities will mostly be 10–100. In most fats, the
distribution of fatty acid residues over the molecules is neither fully random
nor fully ordered.