water activity

Studying water in foods should start with an analyticaldetermination of water content and water activity for commercial and legalreasons which are evident.

The determination is one of the most popular andfrequent analysis in the laboratory of food industries. Moisture content is the absolute amount of water (%) available infoods or it can be defined as the per cent weight of water content in relationto the dry weight of the sample.      Basically, perishability of foods is related to moisture content. Concentrationand dehydration process decrease moisture content and decrease perishability.However, measuring the moisture content alone is not a reliable indicator ofperishability because various foods with same moisture content differ significantlyin perishability. Some foods with low moisture content are unstable such aspeanut oils, which contain about 1% of moisture whereas some foods withrelatively high moisture content are stable such as potato starch at 20%moisture content. Water activity (aw), is a measure of free wateravailable for microbial, chemical or enzymatic activity that determines foodperishability or food shelf life.

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In other word, it is also is a measure of howefficiently the water present can take part in a chemical, physical ormicrobiological reactions. It is calculated as the ratio of the water vapourpressure in any food system to the water vapour pressure of pure water at thesame temperature. There are two types of methods that can be used in thedetermination of water content in food, which are direct and indirect methods. Directmethod is the used of physical separation techniques like distillation, dryingor chemical reaction that produced gases like H2 or C2H2,which are then measured using specific techniques.

For indirect methods, itrelies on the spectroscopic properties of water molecules. Some properties offoods especially sugar solution and fruits juices allow obtaining of the drysubstance (concentration) in the medium. These properties are either optical(refractometry, polarimetry), gravimetric (density) or electrical(conductivity) and allow indirect determination of moisture content (as adifference).For the determination of water activity, the methods used aredifferent from those that have been mentioned above. Methods that usually usedare bithermal techniques of Stokes, monometry, electric hygrometry andisopiestic equilibration technique. Water activity is an important parameter onthe growth of microorganisms that then will leads to food deterioration.Therefore, it is important to keep the water activity of foods in the suitablerange. Deterioration of foods commonly due to microorganism, chemical reactionsand enzymatic reactions that probably will be encourage by the water activity(availability of water).

The availability of water can be reduced by physicallyremoved by dehydration or immobilized water using humectants.