Sorption Isotherms and Water Activity

Labuza, T.P., Kaanane, A., and Chen, J.Y. 1985. Effect of temperature on the moisture sorption isotherms and water activity shift of two dehydrated foods../. Food Sci. 50 385. 

Wolf, W., Spiess, W.E.L., and Jung, G., Sorption Isotherms and Water Activity of Food Materials, Science and Technology Publishers, Hornchurch, Essex, England, 1985. 

Kapsalis, J.G. 1987. Influence of hysteresis and temperature on moisture sorption isotherms. In Water Activity Theory and Application to Foods (L.B. Rockland and L.R. Beuchat, eds), pp. 173-213. Dekker, New York. 

The moisture content of the material when it comes into equihbrium with drying air is a useful property included in most drying models. The relation between equilibrium material moisture content and the corresponding water activity for a given temperature is known as the sorption isotherm. The water activity at the pressures and temperatures that usually prevail during drying is equal to the relative humidity of air. 

Gal, S., Recent developments in techniques of obtaining complete sorption isotherms, in Water Activity Influence on Food Quality, Rockland, L.B. and Steward, G.E (eds.). Academic Press, New York, 1981, pp. 89-111. 

Roos (1995) has used a combined sorption isotherm and state diagram to obtain critical water activity and water content values that result in depressing Tg to below ambient temperature (Figure 1-25). This type of plot can be used to evaluate the stability of low-moisture foods under different storage conditions. When the Tg is decreased to below ambient temperature, molecules are mobilized because of plasticization and reaction rates increase because of increased diffusion, which in turn may lead to deterioration. Roos and Himberg (1994) and Roos et al. (1996) have described how glass transition temperatures influence nonenzymatic browning in model systems. This deteriorative reaction... 

The equilibrium relationship between water activity and moisture content is expressed by sorption isotherms and may be used to specify local moisture contents or to calculate average moisture content as outlined by Karel (1988)... 

A humidity-sensitive conducting polymer actuator made up of PEDOT doped with poly(4-styrenesulfonate) (PEDOT/PSS) was fabricated [47]. Water vapor sorption and electro-active actuating behavior of free-standing PEDOT/PSS films were investigated by sorption isotherm and electromechanical analyses. The non-porous PEDOT/PSS film, with a specific surface area of 0.13 m g sorbed water vapor of 1,080 cm (STP)g , corresponding to 87 wt%, at relative water... 

The relationship between equilibrium moisture content and water activity, known as the sorption isotherm, is an important characteristic that influences many aspects of dehydration and storage. It can be constructed graphically or derived mathematically. The shape of the isotherm generally determines the storage stability of the dehydrated product. This concept is used to establish product specifications for the effective drying, packaging, and storage of foods. 

The relationship between water content and water activity is indicated by the sorption isotherm of a food (Fig. 0.3). 

The relationship between water content in food (or equilibrium relative humidity of ambient air) and water activity is not simple. It is best described by the sorption isotherm of a particular food, which is the dependence of its water content on water activity. 

The most fundamental manner of demonstrating the relationship between sorbed water vapor and a solid is the water sorption-desorption isotherm. The water sorption-desorption isotherm describes the relationship between the equilibrium amount of water vapor sorbed to a solid (usually expressed as amount per unit mass or per unit surface area of solid) and the thermodynamic quantity, water activity (aw), at constant temperature and pressure. At equilibrium the chemical potential of water sorbed to the solid must equal the chemical potential of water in the vapor phase. Water activity in the vapor phase is related to chemical potential by... 

FIG. 15 A comprehensive stability map illustrating the general relationship between the occurrence of various reactions important in foods, as a function of water activity, superimposed on a sorption isotherm. M, mold Y, yeast B, bacteria. The isotherm is plotted as moisture content (left y axis) versus water activity, whereas all other curves are plotted as relative reaction rate (right y axis) versus water activity. Additional information corresponding to regions I, II, and III is given in Table IV. 

Typically, sorption isotherms are constructed for a single food ingredient or food system. An alternative approach is to plot the moisture content versus water activity (or relative vapor pressure) values for a variety of as is food ingredients and food systems. The result is a composite food isotherm (Figure 17). The composite isotherm fits the typical shape observed for a sorption isotherm for an individual food system, with a few products falling above or below the isotherm curve (chewing gum, honey, raisins, bread, and colby and cheddar cheeses). Slade and Levine (1991) were the first to construct such a plot using moisture content and aw values from van den... 

---