water activity

Studying water in foods should start with an analytical
determination of water content and water activity for commercial and legal
reasons which are evident. The determination is one of the most popular and
frequent analysis in the laboratory of food industries.

Moisture content is the absolute amount of water (%) available in
foods or it can be defined as the per cent weight of water content in relation
to the dry weight of the sample.     
Basically, perishability of foods is related to moisture content. Concentration
and dehydration process decrease moisture content and decrease perishability.
However, measuring the moisture content alone is not a reliable indicator of
perishability because various foods with same moisture content differ significantly
in perishability. Some foods with low moisture content are unstable such as
peanut oils, which contain about 1% of moisture whereas some foods with
relatively high moisture content are stable such as potato starch at 20%
moisture content.

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Water activity (aw), is a measure of free water
available for microbial, chemical or enzymatic activity that determines food
perishability or food shelf life. In other word, it is also is a measure of how
efficiently the water present can take part in a chemical, physical or
microbiological reactions. It is calculated as the ratio of the water vapour
pressure in any food system to the water vapour pressure of pure water at the
same temperature.

There are two types of methods that can be used in the
determination of water content in food, which are direct and indirect methods. Direct
method is the used of physical separation techniques like distillation, drying
or chemical reaction that produced gases like H2 or C2H2,
which are then measured using specific techniques. For indirect methods, it
relies on the spectroscopic properties of water molecules. Some properties of
foods especially sugar solution and fruits juices allow obtaining of the dry
substance (concentration) in the medium. These properties are either optical
(refractometry, polarimetry), gravimetric (density) or electrical
(conductivity) and allow indirect determination of moisture content (as a

For the determination of water activity, the methods used are
different from those that have been mentioned above. Methods that usually used
are bithermal techniques of Stokes, monometry, electric hygrometry and
isopiestic equilibration technique. Water activity is an important parameter on
the growth of microorganisms that then will leads to food deterioration.
Therefore, it is important to keep the water activity of foods in the suitable
range. Deterioration of foods commonly due to microorganism, chemical reactions
and enzymatic reactions that probably will be encourage by the water activity
(availability of water). The availability of water can be reduced by physically
removed by dehydration or immobilized water using humectants.