Factors Affecting the Gas Content of Milk

Abstract

The average gas content of mixed raw milk as received at a commercial milk plant over a period of several months was found to be 0.47 volume per cent oxygen, 1.29 volume per cent nitrogen and 4.45 volume per cent carbon dioxide. The effect of temperature and pressure on the gas content of the milk is shown to follow the well-known physical laws governing the solubility of gases in liquids. The oxygen content of hot milk is reduced about 50 per cent at the high temperatures prevailing in short-time pasteurization methods, however, upon subsequent cooling a substantial amount of oxygen is reabsorbed, the degree of reabsorption depending upon the prevailing conditions during cooling; oxygen may be reabsorbed to a higher level than prevailed in the unpasteurized milk if a large surface per unit volume is exposed to air during cooling. Milk may be deoxygenated by subjecting to an appropriate vacuum treatment, flushing with other gases, or a combination of these principles; efficiency of the particular method employed is increased at elevated temperatures. Ascorbic acid may be considered a reactive substance consuming oxygen dissolved in milk. It may be employed as a means of deoxygenating milk and conversely its stability or preservation in milk is dependent upon the absence of dissolved oxygen. In the absence of dissolved oxygen ascorbic acid in milk is stable to intense light for long periods, to the oxidizing effect of copper salts, and to high temperatures in excess of 212° F. as employed for complete sterilization. Light, heat and metallic contamination are considered as catalytic agents which accelerate the oxidation of ascorbic acid in the presence of dissolved oxygen, but are seemingly ineffective in the absence of oxygen. © 1941, American Dairy Science Association. All rights reserved.