Raisins moisture content

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... 

Two selected food materials are presented as an example in Figure 4.13. Potatoes exhibit a typical behavior. Equilibrium material moisture content is increased [173]. Raisins, on the other hand, exhibit an inverse temperature effect at large water activities [174], As shown in Figure 4.13, potatoes and raisins exhibit sorption isotherms of types II and III, respectively. 

FIGURE 4.13 Effect of air-water activity and temperature on equilibrium material moisture content for two foods. (Data for potatoes from Kiranoudis, C.T., Maroulis, Z.B., Tsami, E., and Marinos-Kouris, D., J. Food Eng., 20(1), 55,1992 and data for sultana raisins from Maroulis, Z.B., Tsami, E., Marinos-Kouris, D., and Saravacos, G.D., J. Food Eng., 7(1), 63,1988.)... 

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