Answer
Provided that some conditions are fulfilled. The first is that the polymers are not
intermingled on a scale much smaller than the polymer length, since it is difficult to
envisage how small parts of a molecule could be glassy and adjacent small parts not.
This means that they should be phase-separated (see Section 6.5.2), which is the most
likely situation at such high concentrations. On the other hand, at low water content,
phase separation will not occur spontaneously at a reasonable rate, hence the water
content must have been much higher than 30%, or the temperature very much higher
thanTg, at some stage. The water can move faster, even close toTg, which implies
that the water activity will soon be the same everywhere. If now the two phases, if
taken apart, have clearly different values ofTg, one of them will be glassy and the
other rubbery in the range between the twoTgvalues. Such relations have been
observed, e.g., for casein and gluten. The situation becomes even more complicated if
one or both of the polymers forms crystallites.
16.1.2 Applications
Glassy Foods. Several dry foods are wholly or partly in a glassy
state. A simple example is ahigh-boiled sweet. It looks and feels like a piece
of glass. It consists of a mixture of sugars (sucrose and oligomers of glucose)
and 2–3% water. Some other sugar-based confectionary is also in this
category.
Somewhat similar isdried skim milk(skim milk powder). Its main
component is lactose (> 50 %), and it has some 4%water. Figure 16.5 gives
theTgof lactose (a mixture ofa- andb-lactose) as a function of water
content; the curve for dried skim milk is almost identical (at lowcW). The
powder particles consist of a glass of lactose and some other low-molar-
mass substances, like salts (almost fully in associated form), in which casein
micelles and globular proteins are embedded. Figure 16.5 gives curves for
lysozyme, a typical globular protein, and gluten, a mixture of various,
mostly nonglobular, proteins. It is seen that these can readily form a glassy
state.
In most dry foods, starch is the main component responsible for a
glassy state. This concerns several drycereal products, like hard biscuits and
various breakfast cereals. Figure 16.5 givesTgfor wheat starch, but if low-
molar-mass components, generally sugars, are also present, the curve is
shifted to lower temperatures, somewhere between those for starch and
those for lactose. Another example ispasta, in the dry form in which it is
commonly sold, which consists for the most part of wheat starch. Pasta has
an impressive shelf life in the glassy state, and it readily takes up water upon
cooking, to attain a soft rubbery texture.