Physical Chemistry of Foods

Answer

1. As shown in Figure 15.17, lactose needs a much higher supersaturation for
   reasonably fast nucleation to occur than sucrose.


2. When lactose crystallizes, a-lactose is depleted from the solution,
   decreasing the supersaturation. Thenb-lactose is converted intoa, to make up for
   part of the depletion, but this is a slow process, especially at low temperatures. In
   other words, the supersaturation ofa-lactose is on average smaller than the diagram
   leads us to expect.


3. The supersaturation ratio b as given in the figures is based on
   concentrations, whereas it should be based on activities. Figure 2.3 shows that
   near the saturation concentration of sucrose (about 65%by mass), the slope of the
   relation between activity and concentration is strongly increasing with increasing
   concentration. This means that the true supersaturation ratio for sucrose will be
   much higher than the concentration ratio, leading to much faster crystallization.
   Such a strong effect is not expected for lactose, since the saturation concentration is
   far smaller (about 20%), so that the difference between mole fraction ratio and
   activity ratio will be quite small.
   Incidentally, the greatly enhanced activity ratio of sucrose must also be the
   explanation of cause 1.

15.4 FAT CRYSTALLIZATION

Fat and oils are mixtures of triacylglycerols—or triglycerides, for short—
where oils are liquid at room temperature and fats are apparently solid. In
fact, a fat is always a mixture of triglyceride crystals and oil. Many foods
contain fat, and fat crystallization is an important phenomenon. It largely
determines the followingimportant properties.

Mechanical properties of ‘‘plastic’’ fats and of high-fat products

(butter, margarine, chocolate) during storage and handling. This

concerns especially yield stress—in relation to stand-up during

storage and firmness or spreadability during handling—and fracture

properties.

Eating properties. This may concern, again, yield or fracture proper-

ties, and also stickiness (observed when small fat crystals do not

melt in the mouth), and coolness (due to the heat of fusion

consumed during melting in the mouth).

Physical stabilityof some foods. This concerns, for instance, formation

and sedimentation of crystals in oil, oiling off in plastic fats and

related products (see the Question in Section 5.3.1), (prevention of)