Summary
The objective of the present study is to evaluate the capacity of four published three‐parameter equations in describing water sorption isotherms of various types of foods. Thirty‐nine food isotherms obtained from literature and relating to fruits, meats, milk products, proteins, starchy foods and vegetables were used to evaluate the fitting abilities of the various equations. The equations studied were: B.E.T. modified – Brunauer (1945), Chen (1971), Young & Nelson (1967) and Hailwood & Horrobin (1946).
The water uptake by several food materials that included animal and vegetable proteins and starch materials was studied. The amount q of water taken up by a sample powder during time t was described by the equation q = Qt/(B + t), where Q is the total water uptake of equilibrium and B is the time required to sorb Q/2. The rate of water uptake was proportional to the square of the amount of water that must still be absorbed to reach equilibrium. A specific rate constant for the process was calculated as (BQ)-'.
The water sorption isotherms of various nonsoluble food components (bio-polymers) as well as of model systems prepared by wet mixing of such constituents followed by freeze-drying, were measured and analyzed. An attempt to predict product isotherms from knowledge of component isotherms and weight fractions of components gave satisfactory results in various cases while in others the predicted product equilibrium moisture contents were significantly higher than the experimental values. This was attributed to interactions between constituents leading to a reduction of water-binding capacity.
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