Physical Chemistry of Foods

Processes. To obtain a food in a glassy state, water generally has to
be removed without crystallization of solutes occurring. In some products
this is easy, in others difficult. The following methods can be distinguished.

Sugar boiling, as in the manufacture of boiled sweets. Evaporating

water from a solution of sucrose will lead to its crystallization. By

mixing sucrose with an equal amount (on a dry mass basis) of

glucose syrup, crystallization can be prevented.

Air drying at ambient temperature, or at a higher temperature

followed by cooling. This works well for many starchy foods, like

pasta. Toasting a slice of bread, if done slow enough to allow

evaporation of much of the water, also results in a glassy material

(mainly consisting of partly gelatinized starch and gluten). When

FIGURE16.5 Glass transition temperatureTgas a function of mass fraction of
watercW, for some substances (indicated near the curves; ‘‘starch’’ means native
wheat starch).