Microbial growth is directly linked to water activity. In general water activity a w represents the water which is made available for the microbial action. Many of the chemical and biological processes that cause deterioration of foods, and ultimately spoilage, are water dependent. For example, if honey (a w ≈ 0.6) is exposed to humid air (a w ≈ 0.7) the honey will absorb water from the air. Water migrates from areas of high a w to areas of low a w. Bacteria usually require a w of at least 0.91, and fungi at least 0.7. Higher a w substances tend to support more microorganisms. Where, P is the vapor pressure of water in the substance, and P 0 is the vapor pressure of pure water at the same temperature.Īs the temperature increases, a w typically increases, except in some product with crystalline salt or sugar. It is defined as the vapour pressure of a liquid divided by that of pure water at the same temperature therefore, pure distilled water has a water activity of exactly one. ” Beyond Water Activity: Recent Advances Based on an Alternative Approach to the Assessment of Food Quality and Safety.” Critical Reviews in Food Science and Nutrition 30.2-3 (1991): 115-360.Water activity or a w is a measurement of water content.

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How does water activity impact shelf life? Intrinsic or inherent food properties, such as water activity and pH, can be controlled to assure food safety, as well as to inhibit microbiological spoilage and to protect product quality.Īlong with thermal technologies, salting, drying, acidification, and fermentation, the reduction of water activity in food enables bakers to extend shelf life and become more efficient.Īs a rule of thumb, the higher the a w, the shorter the shelf life of the food product (the less stable it becomes).

Water activity and shelf-life of food products This is because syrups have much higher amounts of sugars and salts added to them, causing water in such systems to be “more bound” or “less free” than in bread matrices.

This rule of course applies when water is the only component that changes, meaning that solids are concentrated.ĭespite being liquid and having a much higher moisture content, syrups have lower a w values than solid food products, such as cakes or bread. It can be concluded from the table that the lower the moisture content, the lower the activity. Water activity values of selected food products: Bound water simply means the water molecules which are linked to sugars, organic acids, salts and other small molecules by polar or hydrogen bonding. On the other hand, water activity is rather a qualitative measure of the “free water” content of a food sample. Moisture content of foods is a quantitative measure of how much total water, both “bound” and “free”, a food product contains. Water activity should not be confused with moisture. 1 Difference between moisture content and water activity In modern laboratories, a w determinations are made quickly and accurately by the instrumental determination of dew point depressions. Because ERH readings are limited to the range of 0% to 100%, a w readings will range from 0 to 1.0. The a w of a food is simply its ERH / 100. Nowaday, this activity can be accurately measured by using reliable instruments that measure the equilibrium relative humidity (ERH) of food samples.

Of course, solid food products, such as bread or cakes, do not only contain solutes but also plasticized polysaccharides and proteins which confer on baked goods their unique texture. Ketchup, for example, can be seen as a water solution in which solutes are diluted or dissolved. Solutes in food can be sugars (both mono- and disaccharides), salt and organic acids. 1įor example, the addition of 1 mole of sugar to 1 L (55.5 moles) of water would give a 1.0 molal solution with an ideal a w value of 55.5 / (1 + 55.5) = 0.982. The ideal a w of a solution or food, with the solvent being merely water, can be determined by dividing the moles of solvent (n 2) by the sum of the moles of solvent and the moles of solute (n 1). The earliest measurements of a w involved manometric determinations of vapor pressures and dividing the vapor pressure of a solution or food product (p 1) by the vapor pressure of pure water (p 0).