If you place a sample of food in a sealed container, the moisture in the air-space will come to an equilibrium value. Water activity is defined as the equilibrium relative humidity (ERH) of this air divided by 100.
The water activity of a
food is important because it may affect the stability to
degradation by chemical reactions, moulds, yeasts, bacteria and
textural changes. Water activity may therefore give an indication
of shelf life.
Water activity is a function of moisture and temperature of the food. The graphical relationship between moisture and water activity at constant temperature is called an isotherm. Isotherms are generally drawn with water activity on the horizontal axis.
An isotherm allows changes in moisture level of the food to be used to predict changes in water activity. It also allows prediction of the equilibrium moisture level of a food stored in an environment of known RH and temperature.
The isotherms are based on the BET
and GAB models.
4. Add the product to the database
The isotherm generally has an
almost linear region at intermediate values of aw.
The program automatically calculates the extent of this region,
and the slope and intercept of the straight line of best fit. The
region is determined by calculating the widest aw
range, within which the average difference between the isotherm
curve and the straight line is <0.005 g water/g solid.
Two isotherm charts are shown, one
with moisture on a dry basis and one on a wet basis. The charts
show the data points, together with isotherms calculated by the
GAB equation (and BET if selected under set-up). The straight
line calculated for the linear region is also shown, if selected
Two isotherm charts are shown, one with moisture on a dry basis and one on a wet basis. The charts show the data points, together with isotherms calculated by the GAB equation (and BET if selected under set-up). The straight line calculated for the linear region is also shown, if selected under set-up.
You will be presented with a summary of the data and isotherm. You may choose to use Solver to try to improve on the fit if you wish. At least a marginal improvement usually occurs. Solver is constrained to keep 0.7<=k<=1 and c>=1.
the aw for a known
moisture, or the moisture for a known aw,
at any temperature.
The program contains a database of
products, with raw data points and calculated parameters.
If you choose to work with an existing product, you must choose a product/temperature from the list. The data for the chosen product will be entered into the model, and the linear region recalculated.
If you to add the GAB constants for a
product, you will be prompted for the following details: Product
name and temperature GAB constants
If you choose to add an overlay to your isotherm plot, you will be asked to pick any product/temperature from the database. The isotherm for this product will then be overlaid on your current isotherm.
It is water activity, rather than moisture levels, that determine whether a microorganism can live on or in a food or other substance. A low water activity indicates that the moisture is bound to the substance, therefore unavailable to the microorganism.
The water activity of many saturated salt solutions is accurately known, and published in the literature. The water activity / temperature relationship of a saturated salt solution generally follows the following model:
Click here for values of k1 and k2 for various salts
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