Inactivating enzymes reduces chance of deteriorating product quality.

Enzymatic activity can be responsible for the loss of quality in foods in a number of ways: by causing discoloration, the formation of off-flavors and changes in texture. Moisture is a critical factor that influences enzymatic reactions. As such, controlling moisture levels can help you limit the chance that undesirable changes will occur in foods.

One way that water in foods can influence enzymatic activity is by serving as a reactant in hydrolysis reactions. Dehydration processes or the addition of humectants can reduce the amount of water available for the reaction and consequently reduce its rate of activity.

In addition, water can act as a solvent, solubilizing reactants and co-factors, which would influence enzyme activity. Researchers at Auburn University found that dehydrated model systems containing sucrose and invertase revealed this to be the case. Here, inversion occurred at a faster pace above a critical water activity. This was caused by greater sucrose dissolution.

Water also influences the mobility of reactants within a food by its plasticization capability. Reactants must diffuse into the active site of an enzyme, which itself must be in the appropriate configuration for the reaction to occur. Efforts aimed at increasing the glass transition temperature of the food, such as by reducing its moisture content, can restrict the mobility of the enzyme and substrate, and possibly lower enzymatic activity.

Water also influences enzyme stability. Enzymes are more susceptible to denaturation and inactivation at higher moisture content. For example, the thermal denaturation of lysozyme occurs at progressively lower temperatures as its moisture content increases. Enzyme inactivation would greatly reduce the quality changes associated with enzymatic reactions. So, understanding the various roles of water with respect to enzyme stability and activity can help control enzymatic reactions and the subsequent deterioration of a product's quality.

Further information. Leonard Bell, Department of Nutrition and Food Science, Auburn University, 328 Spidle Hall, Auburn, AL 36849; phone: [phone]; fax: [phone]; email: [e-mail].