Water activity

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

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. Wet-weight basis is used throughout this text 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  (for example peanut  oil deteriorates if the moisture content  exceeds 0.6%),  whereas other foods are stable at relatively high moisture contents (for example potato starch is stable at 20% moisture). It is the availability of water for microbial, enzymic or chemical activity that determines the shelf life of a food, and this is measured by the water activity (aw) of a food, also known as the Relative Vapour Pressure (RVP).

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 immobilise water in the food (for example by the use of humectants in ‘intermediate-moisture’ foods and by formation of ice crystals in freezing).  \

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

•  the amount of water present
•  the temperature
•  the concentration of dissolved solutes (particularly salts and sugars) in the water.

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

aw= P/P๐

where P (Pa)   vapour pressure of the food, P๐ (Pa)   vapour pressure of pure water at the same temperature.

Effect of aw on foods

Almost all microbial activity is inhibited below aw   0.6, most fungi are inhibited below aw   0.7, most yeasts are inhibited below aw   0.8 and most bacteria below aw   0.9. The interaction of aw with temperature, pH, oxygen and carbon dioxide, or chemical preservatives has an important effect on the inhibition of microbial growth. When any one of the other environmental conditions is sub-optimal for a given micro-organism, the effect of reduced aw is enhanced. This permits the combination of several mild control mechanisms which result in the preservation of food without substantial loss of nutritional value or sensory characteristics

Enzymic activity virtually ceases at aw values below the BET monolayer value. This is due to the low substrate mobility and its inability to diffuse to the reactive site on the enzyme. Chemical changes are more complex. The two most important that occur in foods which have a low aw are Maillard browning and oxidation of lipids. The aw that causes the maximum rate of browning varies with different foods. However, in general, a low aw restricts the mobility of the reactants and browning is reduced. At a higher aw, browning reaches a maximum. Water is a product of the condensation reaction in browning and, at higher moisture levels, browning is slowed by ‘end product inhibition’. At high moisture contents, water dilutes the reactants and the rate of browning falls. Oxidation of lipids occurs at low aw values owing to the action of free radicals. Above the BET monolayer value, anti-oxidants and chelating agents (which sequester trace metal catalysts) become soluble and reduce the rate of oxidation. At higher aw values the catalytic activity of metals is reduced by hydration and the formation of insoluble hydroxides but, at high aw values, metal catalysts become soluble, and the structure of the food swells to expose more reactive sites.

Effect of water activity on microbial, enzymic and chemical changes to foods.