that at a given temperature the viscosity will markedly decrease with
increasing water content. Such behavior is also observed for glucose, the
monomer of the starch polymer. This is primarily due to water molecules
being smaller, hence more mobile, than glucose molecules. It is further seen
that the values ofTgfor the monomer are by about 75 K lower than for the
polymer. Difference in mobility will, again, be the main cause. Generally,Tg
of polymers increases with increasing degree of polymerization,n, but a
plateau value tends to be reached forn&20.
In these examples, starch or sugar is the component responsible for
forming the glass and water is the diluent. Most foods contain several
substances that can be in the glassy state. A glass then is formed much more
readily—which means, in practice, at much slower cooling rates—than for a
pure system, because crystallization is hindered. A glass can be a true, albeit
noncrystalline, solid solution of many components, whereas crystallization
must go along with phase separation. Moreover, different components tend
to crystallize at different temperatures. Generally, some components will
hinder the crystallization of others; see Section 15.2.2. Finally, the viscosity
FIGURE16.3 Glass transitions in polymeric systems. (a) Melting temperatureTm
and glass transition temperatureTgof gelatinized potato starch as a function of mass
fraction of watercW. The values at very smallcWare extrapolated. ATgcurve of
glucose is also given. (b) Approximate relations between rheological properties of
polymeric systems and temperature; elastic shear modulusGfor a system where
crystallites melt atTm, and apparent viscosityZafor a system without a melting
transition.