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3. 20%. Find the intersection of the 45°C and 75%C lines and follow the sloping RH line upward to read the % RH.

4. 36°C. Find the intersection of the 55°C and 30% RH lines and follow the wet-bulb line left until the RH reaches 100%.

5. 50-100%. Find the intersection of the 39°C wet-bulb and the 50°C dry-bulb temperatures and follow the horizontal line to the intersection with the 86°C dry-bulb line; read the sloping RH line at each intersection (this represents the changes that take place when air is heated before being blown over food.

6. 10-70%. Find the intersection of the 35°C wet-bulb and 70°C dry-bulb temperatures and follow the wet-bulb line left until the intersection with the 40°C dry-bulb line; read sloping RH line at each intersection (this represents the changes taking place as the air is used to dry food; the air is cooled and becomes more humid as it picks up moisture from the food).

Water Activity

Deterioration of foods by microorganisms can take place rapidly, whereas enzymatic and chemical reactions take place more slowly during storage. In either case, water is the single most important factor controlling the rate of deterioration. The moisture content of foods can be expressed either on a wet-weight basis:

_ mass of water mass of sample mass of water mass of water + solids x 100

x 100

mass of water mass of solids

The dry-weight basis is more commonly used for processing calculations, whereas the wet-weight basis is frequently quoted in food composition tables. It is important, however, to note which system is used when expressing a result. Dry-weight basis is used throughout this article unless otherwise stated.

A knowledge of the moisture content alone is not sufficient to predict the stability of foods. Some foods are unstable at a low moisture content (eg, peanut oil deteriorates if the moisture content exceeds 0.6%), whereas other foods are stable at relatively high moisture contents (eg, potato starch is stable at 20% moisture) (2). It is the availability of water for microbial, enzymatic, or chemical activity that determines the shelf life of a food, and this is measured by the water activity (Aw) of a food. Examples of unit operations that reduce the availability of water in foods include those that physically remove water (dehydration, evaporation, and freeze-drying or freeze concentration) and those that immobilize water in the food (eg, by the use of humectants in intermediate-moisture foods and by formation of ice crystals in freezing). Examples of the moisture content and Aw of foods are shown in Table 1. The effect of reduced Aw on food stability is shown in Table 2.

Water in food exerts a vapor pressure. The size of the vapor pressure depends on

1. The amount of water present.

2. The temperature.

3. The concentration of dissolved solutes (particularly salts and sugars) in the water.

Water activity is defined as the ratio of the vapor pressure of water in a food to the saturated vapor pressure of water at the same temperature:

where P (Pa) is vapor pressure of the food and P0 (Pa) the vapor pressure of pure water at the same temperature. Aw is related to the moisture content by the Brunauer-Emmett-Teller (BET) equation

where Aw is the water activity, M the moisture as percentage dry weight, Mx the moisture (dryweight basis) of a monomolecular layer, and C a constant (2).

A proportion of the total water in a food is strongly bound to specific sites (eg, hydroxyl groups of polysaccharides, carbonyl and amino groups of proteins, the hydrogen bonding). When all sites are (statistically) occupied by adsorbed water the moisture content is termed the BET monolayer value. Typical examples include gelatin (11%), starch (11%), amorphous lactose (6%), and whole spray-dried milk (3%). The BET monolayer value therefore represents the moisture content at which the food is most stable. At moisture contents below this level, there is a higher rate of lipid oxidation and, at higher moisture contents, Maillard browning and then enzymatic and microbiological activities are promoted.

The movement of water vapor from a food to the surrounding air depends on both the moisture content and composition of the food and the temperature and humidity of the air. At a constant temperature the moisture content of food changes until it comes into equilibrium with water vapor in the surrounding air. The food then neither gains nor loses weight on storage under those conditions. This is called the equilibrium moisture content of the food, and the relative humidity of the storage atmosphere is known as the equilibrium relative humidity. When different values of relative humidity versus equilibrium moisture content are plotted, a curve known as a water sorption isotherm is obtained (Fig. 2).

Each food has a unique set of sorption isotherms at different temperatures. The precise shape of the sorption isotherm is caused by differences in the physical structure, chemical composition, and extent of water binding within the food, but all sorption isotherms have a characteristic shape, similar to that shown in Figure 2. The first part of

Table 1. Moisture Content and Water Activity of Foods

Food

Moisture content (%)

Water activity

Degree of protection required

Ice (0°C)

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