with moist air can thus adsorb water. The amount adsorbed increases with
increasing relative humidityðawÞof the air. Note that the amount adsorbed
per kg adsorbent would be proportional to the adsorbent’s specific surface
area, i.e., to the fineness of the powder.
Liquid or amorphous materials may (also) showabsorption. Here, the
absorbate can dissolve in the absorbent, or it can be seen as adsorbing onto
the surface of a great number of fine pores in the absorbent. Anyway, the
amount absorbed would be proportional to the mass of absorbent, other
things being equal. In most dry foods, it is unclear whether the mechanism is
adsorption or absorption; in liquid foods, it is always the latter. It is rarely
observed in a dry food that the equilibrium amount of water taken up
depends on the specific surface area of the food. Generally, the term
‘‘sorption’’ is used, leaving the mechanism involved out of consideration.
It is customary to constructsorption isothermsor vapor pressure
isotherms of foods, where water content (either as mass fraction or as mass
of water per unit mass of solids) is plotted againstaw. Figure 8.3a gives an
example of such an isotherm; here, the material is a powder below 10 and a
liquid above 30%water.
Figure 8.3a gives the whole range of water contents, but if the water
content is plotted on a linear scale, any differences, which are of greatest
importance for low water contents, are not shown in sufficient detail.
Consequently, one mostly plots only the part belowaw& 0 :9. Examples are
given in Figure 8.3b, and considerable variation among foods is seen. The
FIGURE8.3 Water vapor pressure or water sorption isotherms of foods. Given are
water content versus water activityðawÞ. (a) skim milk (powder). (b) Various foods:
meat (1), apple (2), boiled sweet (3), skim milk (4), and peanuts (5). (c) Caseinate
systems (water content expressed as g per g dry protein); pure caseinate (1), curd or
renneted milk (2), and cheese (3).