Water activity in food

There are two types of water measurement:
*Water content
*Water activity

The water activity (aw) of a food is expressed as the ratio between the vapor pressure of the food itself, when in a completely undisturbed balance with the surrounding air media, and the vapor pressure of distilled water under identical conditions. It predicts whether water is likely to move from the food product into the cells of micro-organisms that may be present.

A water activity of 0.80 means the vapor pressure is 80 percent of that of pure water. The water activity increases with temperature. High water activity promotes the growth of microorganisms.

Products with a higher water activity are prone to degradation, and the development of mold and bacteria. Accuracy is very important when it comes to these products. At water activity 0.59 a product could be considered safe from the potential growth of bacteria and mold.

Controlling water activity in a powder product maintains proper product structure, texture, stability, density, and rehydration properties. Water activity is suitable measurement for predicting food safety and quality.
Water activity in food

Food drying effects on microorganisms

The main purpose of drying foods is to lower their moisture content to a particular level that will exclude the growth of microorganisms (bacteria, mould and yeasts).

Significant growth of microorganisms in a food product can be prevented by lowering is water content by an appropriate drying process.

The lower the water activity of a food, the less probable that microorganisms will grow. Water activity affects the growth and multiplication of microorganisms. When aw <0.9, growth of most molds is inhibited.

Generally, mold at lower water activities than yeasts and yeasts will grow at lower water activities than bacteria. For this reason, mold are opt to grow in dried foods than are yeasts or bacteria. In dried foods, the moisture content is lowered to the point which microorganisms will not grow and it is kept that way through packaging, which includes moisture.

Water activity is a vital parameter for food monitoring. Water activity (aw) is defined as the ratio of the water vapor pressure of a food (p) to that of pure water (p0) at the same temperature: aw = p/p0.

Water activity values are used extensively to predict the stability of food stuff with respect to microbial growth and enzymatic, chemical and physical changes.

Water activity also can be lowered by soluble components, such as sugar or salt. Thus, certain syrup and salted, partially dried foods (e.g. foods) are relatively stable as far as at the growth of microorganisms is concerned , although there may be conditions in which they become subject to the growth of yeasts or molds.

Most fresh foods such as fresh meat, vegetables and fruit have aw values that are close to the optimum growth level of most microorganisms (0.97-0.99).
Food drying effects on microorganisms