Physical Chemistry of Foods

cooling, crystallites form again (retrogradation). In many foods, starch is in
its gelatinized form. Figure 16.3a gives an example ofTgandTmvalues.
Native starch tends to have a higherTgvalue at the same water content;
compare Figures 16.3a and 16.5.
In the glassy state, the polymer backbone is considered to be almost
completelyimmobile, causing the material to be brittle. BetweenTgandTm
the system is not to be considered as a highly viscous liquid but as an elastic
material; the transition nearTgis called aglass–rubbertransition. The
crystallites act as cross-links between flexible stretches of the polymer
chains, causing the material to have a rubbery consistency, with an
appreciable elastic modulus. AboveTma viscous liquid is formed. Figure
16.3b illustrates the rheological relations.
For glassy polymer systems without crystallites or other cross-links, a
glass–liquid transition occurs. For systems with permanent, i.e., covalent,
cross-links, the elastic modulus keeps decreasing with increasing tempera-
ture until a plateau value is reached.

Mixtures. Figure 16.3a concerns mixtures of starch and water. In
such a case, the solvent (water) acts as aplasticizer. It is seen thatTg(andTm
as well) strongly decreases with increasing water content, which also means

TABLE16.1 Estimated Values of the Melting PointTmand the

Glass Transition TemperatureTgof Pure Substances of Various Molar

MassesM, As Well As the Special Glass Transition Parameters

(Tg^0 andcw^0 ) for Maximally Freeze-Concentrated Solutions

Compound Dry systems

Freeze

concentration

Name M,Da Tm, 8 C Tg, 8 C ðTg=TmÞa Tg^0 , 8 C cW^0

Water 18 0 
 137 0.50

Glycerol 92 18 
 93 0.62 
 65

Fructose 180 125 7 0.70 
42 0.14

Glucose 180 158 31 0.71 
43 0.16

Sucrose 342 192 70 0.74 
33 0.17

Maltose 342 87 
32 0.17

Lactose 342 214 101 0.77 
28 0.15

Maltohexose 991 134 
 14

Gelatin 106 25 b 
 12

Starchc > 107 (255)b (122) 0.8 
6 0.26

aAbsolute temperatures.

bOf crystallites.

cGelatinized.