[Intermediate moisture foods and water activity determination].

In order to define the concept of Intermediate Moisture Foods (IMF) some ideas concerning the water bonding state in foods are reminded: water molecules properties, polar groups of hydrophilic macromolecules, activity of water (aw); sorption and desorption isothermal curves. Between dried products (solids with friable and breakable texture) anv very hydrated products (meat or fresh fruits for example), the IMF are characterized by their high aw (from 0.65 to 0.90) and their generally soft texture. Some typical examples of traditional and new IMF are then given. The IMF alterations are induced by chemical (oxydation) or biochemical (enzyme), biological (micro-organisms) or biophysical (water migration) causes, each of them being controlled by aw, temperature, pH, and other parameters. IMF technology consists generally to formulate products, the aw of which are less than 0.9, with aw depressors. Precise measurements of aw are then one of the most important mean for controlling fabrication, the direct computation being generally impossible. A manometric reference method for determination of aw is described; it consists to measure directly the equilibrium water vapor pressure. This method is preferable to a dew point determination. Some practical methods are reported, in connection with their accuracy and facilities of use.