The influence of water content and water activity on chemical changes in foods of low moisture content under packaging aspects
Deteriorative chemical reactions in low moisture foods limiting shelf-life are highly influenced by water activity or water content. While oxidation of lipids is promoted in the very low moisture range, and also creates problems in the intermediate moisture interval, the rate of enzymatic changes and ascorbic acid oxidation is increased by increasing the water content, which has a bearing on consecutive reactions impairing the sensory quality of foods. On the other hand, the Maillard reaction, causing a decrease of nutritive value and undesirable sensory changes, in a certain moisture interval has a maximum of reaction rate. Despite these countercurrent influences of water content on the stability of low moisture foods, there is an optimum range of stability depending on the binding characteristics of water molecules according to the sorption isotherm. This optimum moisture interval must be reached in the drying process and maintained by proper packaging using a packaging material which will ensure that the water content does not reach the 'danger point' on the sorption isotherm during the scheduled turnover time. The corresponding critical water content or water activity may cause changes in texture, colour or flavour, limiting shelf-life. Knowing the water vapour permeability of the package, the water activity of the ambient atmosphere as well as the initial and critical water content of the packaged food, it is possible to calculate the shelf-life to be expected under these conditions. If a distinct shelf-life is postulated, a packaging material having a water vapour permeability low enough to secure this shelf-life may be selected. If, on the other hand, oxidative changes are limiting shelf-life, the permeability of the packaging material against oxygen may be of primary interest. If shelf-life is estimated under this aspect, the rate of oxygen uptake bythe packaged food, dependent on oxygen partial pressure and the critical (shelf-life limiting) amount of absorbed oxygen, must be known.