made to agglomerate—a process applied to enhance the dispersibility of
powders in water—by shaking the powder for a short while at a temperature
where it is sticky and then cooling or drying it.
Stability. In the glassy state,molecular mobilityis greatly reduced,
hence the food will have greater stability (shelf life). The dependence of
various reaction rates on water content is discussed in Section 8.4.
An example iscrystallizationof sucrose in a system with low water
content. Figure 8.6 shows that this starts, albeit very sluggishly (at room
temperature), above a water content of 3%. At that value,Tgequals about
258 C. Skim milk powder with too high a water content will allow
crystallization ofa-lactose hydrate; according to the curve for lactose in
Figure 16.5, this will occur above 30 8 C for 5% water, which fits the
experimental results. Another example is stalingof systems containing
gelatinized starch, which is due to the formation of crystallites; see Figure
6.27a. It has been shown that the slow rate of staling at a temperature of,
say, 208 C is due to the system being near the glassy state, where the
molecular mobility of amylopectin molecules is already greatly reduced. In
glassy systems with a high water content, which generally means that the
glass is for the most part made up of polymers, freezing of water can occur a
few kelvins aboveTg.
Reaction rates in foods as a function of water content have been
discussed in Section 8.4.2. We may add that even below the glass transition,
small molecules usually have finite molecular mobility, allowing some
chemical reactions, such as lipid oxidation, to proceed, albeit it slowly.
Microbial growth (Section 8.4.3) generally stops at water contents far above
Tg.
Question 1
It was mentioned above that the crystallization of lactose can occur at a critical water
content, just above the glass transition. It was further (implicitly) assumed that this
would happen at the same mass fraction of water cWin skim milk powder.
Experiments show that this is not precisely correct but that the critical conditions for
crystallization are at the same water activity. Does this imply that the glass transition
is determined byaWrather thancW?
Answer
When making lactose water mixtures,cWcan be precisely known. This is not the
case for the lactose in skim milk powder. However, ifaWis the same in both systems