can occur, which will be a slow process. The original value ofsyis not
reached.
Especially a multicomponent fat exhibitssetting, i.e., it increases in
firmness with time, be it after working or without. This is because it is not in
equilibrium: upon storage slow transitions take place in polymorphic form
and in compound crystal composition. Since such recrystallization goes via
the liquid state, it can cause further sintering. However, if slow
recrystallization causes the formation ofb-crystals—which especially occurs
if nearly all fatty acid residues have 18 carbon atoms—quite large crystals
may form, and the firmness then markedly decreases. This phenomenon
occurs upon storage of some margarines. The transition to b can be
counteracted by the presence of diacylglycerols.
Crystallization Under Agitation. To obtain a firm fat and to
speed up operation, crystallization at a high supersaturation is desirable.
This implies that a space-filling network is rapidly formed, which greatly
hinders the removal by diffusion of the heat of fusion released, unless the
distance over which it has to occur is quite small. In practice, e.g., in
margarine making, a scraped-surface heat exchanger is used. The fat is
rapidly cooled at the cold surface, which soon results in the formation of a
FIGURE17.25 The yield stresssyof plastic fats. (a) Examples of the dependence
ofsyon the fraction solidj. (b) Example of work softening of a plastic fat and the
subsequent increase insywith timet.