Water activity and its estimation in food systems

One of man's earliest discoveries for food preservation must have been the fact that fresh foods become much less perishable when their water content is drastically reduced. It is general knowledge that not all biological products show the same behavior in this respect. The various biological products as well as their components show great differences in water-binding proper­ ties. For example, potato starch containing 24% moisture on dry basis is perfectly stable, whereas crystalline white sugar (sucrose) may spoil, albeit slowly, even at only 4% moisture. These and comparable phenomena in the area of food deteriora­ tion have been poorly understood for many decades until it was realized that not the actual water content, but some other factor related to the "nature" or "state" of the constituent water, de­ termined eventual spoilage. It was a problem to find a convenient expression for this state of the water or better the "availability" of water for deterioration reactions in foods.

Water activity and its estimation in food systems: theoretical aspects

Summary
The purpose of the present work is to present a review of literature on equations for fitting water sorption isotherms of foods and food products. Twenty-three equations, which have been proposed in the literature for correlating equilibrium moisture content in food systems, have been compiled and analysed. Their origin, range of applicability (both to type of food and water activity) and use are discussed. It is hoped that this critical compilation may be a useful guide for those researchers interested in the mathematical description of the water sorption isotherms of foods.

Equations for fitting water sorption isotherms of foods: Part 1 — a review

Summary

The objective of this study was to evaluate the capacity of eight published two-parameter equations in describing water sorption isotherms of various types of foods. Thirty-nine food isotherms obtained from literature and corresponding to fruits, meats, milk products, proteins, starchy foods and vegetables, were used to evaluate the fitting abilities of the various equations.



The results obtained made it possible to propose the best equations to fit the experimental sorption data for each group of foods.

Equations for fitting water sorption isotherms of foods

Most analytical formulas used to model moisture retention in unsaturated porous media have been developed for the wet range and are unsuitable for applications in which low water contents are important. We have developed two models that fit the entire range from saturation to oven dryness in a practical and physically realistic way with smooth, continuous functions that have few parameters. Both models incorporate a power law and a logarithmic dependence of water content on suction, differing in how these two components are combined. In one model, functions are added together (model “sum”); in the other they are joined smoothly together at a discrete point (model “junction”). Both models also incorporate recent developments that assure a continuous derivative and force the function to reach zero water content at a finite value of suction that corresponds to oven dryness. The models have been tested with seven sets of water retention data that each cover nearly the entire range. The three-parameter sum model fits all data well and is useful for extrapolation into the dry range when data for it are unavailable. The two-parameter junction model fits most data sets almost as well as the sum model and has the advantage of being analytically integrable for convenient use with capillary-bundle models to obtain the unsaturated hydraulic conductivity.

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R. Stevenson Bradley

Papers

318. Polymolecular adsorbed films. Part I. The adsorption of argon on salt crystals at low temperatures, and the determination of surface fields

400. Polymolecular adsorbed films. Part II. The general theory of the condensation of vapours on finely divided solids

420. The rate of reactions in solution