Physical Chemistry of Foods

inaw, a larger specific surface area of the material, and a higher diffusion

coefficient of water in the material. If water uptake occurs readily and if

it leads, moreover, to a perceptible change in properties of the material,

the latter is called hygroscopic. The changes may be of various kinds.

Biscuits and comparable foods lose their crispness upon water uptake;

this is discussed in Section 16.1.2. Most foods that primarily consist of

polymers become soft. Dry foods that contain a lot of water-soluble

components tend to become tacky upon water uptake: a concentrated

sugar solutionisvery tacky. If the food is a powder, the tackiness readily

causes caking of the powder particles. Further water uptake may cause

deliquescence.

Hygroscopicity is reflected in the shape of the sorption isotherm. If it

has a small slope (small dw/daw), the material is little hygroscopic; if the

slope is large, it may be strongly so. Looking at Figure 8.3b, we may

conclude that material 2 (dried apple) probably is quite hygroscopic for

most relative humidities; boiled sweets (material 3) will be little hygroscopic

for relative humidity<0.6, and greatly so if humidity is>0.7. Knowledge of

FIGURE8.6 Relation between water content (weight%) and water activity for
various situations. (a) Sucrose: in solution, amorphous and crystalline; recrystalliza-
tion takes a very long time (years) at low water content (say 3%) and is quite fast at
values over 6%). (b) Dried skim milk, with amorphous or crystalline lactose; see text.