solid crystal network. This is regularly scraped off the surface, whereby it is
strongly worked and hence made soft. If now the fat leaves the heat
exchanger after crystallization is about complete, it will have a low yield
stress; upon storage it will set to some extent, about as shown in Figure
17.25b. If the fat is not fully crystallized when leaving the heat exchanger,
further crystallization will cause the network to become more homogeneous,
comparable to what is shown in Figure 17.23, and also sintering will
generally occur. Within an hour or so, the yield stress can reach a value
more than twice that of a fat that was fully crystallized when leaving the heat
exchanger, but it will not exhibit significant setting upon further storage.
In conclusion, many different processes occur during fat crystallization,
at widely varying rates, all depending on temperature (history) and on fat
composition. This makes it very difficult to predict the mechanical properties
of plastic fats. However, the various processes involved have been identified,
and their dependencies on composition and several external variables have
been established. This means that trends can often be predicted.
Question
Consider a fat that is fully liquid above 40 8 C and that contains numerous different
triglycerides. Upon cooling from 50 to 20 8 C a plastic fat is formed. The yield stress is
determined at 20 8 C under various conditions: (a) 1 hour or 1 day after cooling; (b)
after fast or slow cooling to 20 8 C; (c) cooling first to 4 8 C before bringing to 20, or
direct cooling to 20 8 C; (d) keeping a few hours at 30 8 C before bringing to 20, or
direct cooling to 20 8 C; (e) after one day at 20 8 C, keeping it a day at 10, then a day at
208 C; or keeping three days at 20 8 C; (f) after one day at 20 8 C, keeping it a day at 25,
then a day at 20 8 C; or keeping three days at 20 8 C; (g) measure the yield stress while
slowly or rapidly increasing the strain. For which of each pair is the measured yield
stress higher than the other and why? Tip: Also consult Section 15.4.
Answer
(a) After a day: then sintering will be strong and the fraction solid may be higher; (b)
fast cooling: smaller crystals generally give a firmer and more homogeneous network;
(c) precooling: for about the same reasons as given in b; moreover, the fraction solid
will be higher in a system forming compound crystals; (d) direct cooling: the same
arguments as given in c; (e) probably little difference; the increased firmness at 10 8 C
will be lost after warming again; (f) probably the temporary stay at 25 8 C: this gives
much sintering as the melted fat crystallizes again; (g) fast deformation: see Figure
17.6 and the explanation given for the effect.