The flavor of dairy products is largely determined by the fat component. Consequently, it is particularly important to restrict the development of oxidized off-flavors in the fat source before use. Oxidation is the chief mode of deterio ration of fats and a major factor in determining the shelf life of fat-containing foods (23). Unsaturated fatty acid esters react with oxygen to form peroxides. Although flavorless themselves, peroxides are unstable and readily decompose to yield flavorful carbonyl compounds. The latter are the source of the characteristic oxidized flavors that are detectable at low concentrations. The rate of oxidation depends on the concentration of dissolved oxygen, the temperature, the presence of prooxidants such as copper and iron, the degree of unsaturation of the fat, and the presence of antioxidants that may retard the onset of oxidation. Compared with many fats, milk fat has a good oxidative stability, because it is high in total saturates, low in polyunsaturates, and contains natural antioxidants, principally a-tocopherol.

The development of oxidative rancidity in milk fat is the major determinant of the stability of the fat on storage. Dissolved air in the milk fat can give dissolved oxygen levels of up to 40 ppm at 30°C. In practice, the dissolved oxygen level in the freshly processed milk fat would be about 5 ppm at 45°C, a level sufficient to permit the development of oxidative rancidity, but if the milk fat were allowed to equilibrate with the air, then this level could increase to 33 ppm with a consequent increase in the rate of development of oxidative rancidity.

The oxygen level in the milk fat may be limited by either active or passive actions. For passive control, processing procedures and plant design are established to minimize oxygen (air) exposure. Deaeration devices (24), the use of antioxidants, effective destruction of lipases, and nitrogen spanning of container headspace are examples of active control of product quality.

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