impurities. Monoglycerides may readily form inversed micelles in the
presence of a trace of water, and the micelles are possibly transferred into a
crystals. Whether micelles or crystals form the catalytic impurities is
unknown. Monoglycerides in fats arise for the most part from lipolysis, i.e.,
hydrolytic splitting off fatty acids from tri and diglycerides. The fatty acids
are often removed from industrial fats by washing with an alkaline solution,
but this leaves the monoglycerides.
An example involving nucleation is in Figure 15.26. In frame (a)
considerable hysteresis is observed between cooling and heating curves, and
the explanation must be the limited probability of a catalytic impurity being
present in a small droplet. Below 10 8 C, i.e., about 10 K below theaclear
point, significant nucleation occurs, and it strongly increases with decreasing
temperature. Results for bulk fat are in Figure 15.26b, and it is seen that the
melting curve is almost like that in frame (a) but the cooling curve greatly
differs. Apparently, 24 h keeping suffices to induce sufficient nucleation for
almost all of the crystallizable fat to crystallize. The hysteresis is very small
between 20 and 35 8 C, by about 1 K. This indicates an important fact,
namely that triglyceride crystals are very effective catalytic impurities for
FIGURE15.26 Cooling and heating curves of milk fat: mass fraction solid (cS)
versus temperature (T). Cooling at each temperature (5 K intervals) for 24 h; after
keeping for 24 h at 0 8 C, heating to the higher temperatures for 0.5 h. Theaclear
point of the fat was about 20 8 C. (a) Fat emulsified (droplet volume about 1mm^3 ); (b)
the same fat in bulk. (After results by P. Walstra, E. C. H. van Beresteyn. Neth. Milk
Dairy J. 29 (1975) 35.)