Chemical Changes

During the processing and storage of foods, several chemical changes occur that involve the internal food components and the external environmental factors. These changes may cause food deterioration and reduce the shelf life. The most important chemical changes are associated with enzymic action, oxidative  reactions, particularly lipid oxidation that alters the flavor of many  lipid containing foods, and non-enzymic browning that causes changes in appearance.
 
Fruits upon cutting tend to brown rapidly at room temperature due to the reaction of phenolase with the cell constituents that are released upon cutting of the tissue in presence of oxygen. Enzymes such as lipoxygenase, if not denatured during the blanching process, can influence food quality even at sub-freezing temperatures. In addition to the temperature, other environmental factors such as oxygen, water and pH induce deleterious changes in foods that are catalyzed by enzymes.
 
The presence of oil and fats containing unsaturated fatty acids is a prime reason for the development of rancidity in foods during storage as long as oxygen is available. Development of off-flavours which is markedly noticeable in rancid foods is the result of autoxidation of unsaturated fatty acids.  The generation of free radicals during the autocatalytic process leads to other undesirable reactions, for example, loss of vitamins, alteration of colour, and degradation of proteins. In addition  to lipid oxidation, there are other chemical reactions that are induced by light such as loss of vitamins, and browning of meats.

Non-enzymic browning is a major cause of quality change and degradation of nutritional content in many foods. This type of browning reaction occurs due to the interaction between reducing sugars and amino acids. These reactions result in the loss of protein solubility, darkening of lightly coloured  dry products and the development of bitter flavours. Environmental factors such as temperature, water activity and pH have an influence on non-enzymic browning.