Oxidation of powdered milk products is predominantly associated with unsaturated fats present in milk fat. Oxidation of unsaturated fats results in aldehydes and ketones, which are subsequently converted into alcohols. Fat oxidation occurs in the presence of O2 and moisture and can be catalyzed by light. The O2 atmosphere inside the package, the presence of antioxidants and the temperature all influence the rate of oxidation. 

Powders containing a high percentage of fats, particularly unsaturated fats, are susceptible to sensory effects, collectively called oxidative rancidity, and changes in flavor. Saturated fatty acids oxidize slowly compared with unsaturated fatty acids (Brown and Williams, 2003). The presence of unsaturated bonds in fat will increase oxidation. In general, the higher the level of unsaturation, the greater the chance of fat oxidation. 

It is therefore not surprising that to prevent oxidation of milk powder, the packaging should provide a high-level O2 barrier and be able to retain that barrier during the anticipated shelf life. Gas flushing with a chemically inert gas such as N2 may be essential to replace O2 present in the package before closing. This is particularly true for WMP, where the shelf life is governed to a large extent by the rate of oxidation of the unsaturated fats and the consequent development of objectionable flavors. Most of the common spoilage bacteria and fungi require O2 for growth. Therefore, to increase the shelf life of foods, the internal package atmosphere should contain a minimum concentration of residual O2. 

In addition to fat oxidation, atmospheric O2 and light are the prime factors influencing the stability of vitamins A and D. These factors, in combination with environmental factors such as temperature and moisture, infl uence the rate of reduction in the vitamin content.