Effects of Direct Steam Heating and Vacuum Treatments on the Chemical Composition of Milk with Especial Reference to Substances Involved in Oxidized Flavor Development

Abstract

Summary Milk was heated with live steam to from 190 to 240° F. and subjected to vacuum treatments of approximately 9 in. of mercury in one vacuum chamber and 20 in. of mercury in a second vacuum chamber. The direct steam heating and vacuum equipment were used in conjunction with a H.T.S.T. pasteurizing system and a homogenizer. The combined steam heating and vacuum treatment was found to produce a significant denaturation of whey proteins, as measured by the increase in the activity of sulfhydryl groups and the decrease in whey protein nitrogen. The decrease in whey protein nitrogen was shown to be approximately that which would be expected from published data on the effect of indirect heating. However, the extent of activation of sulfhydryl groups was lower than had been expected. The possibility that the vacuum treatment removed some of the volatile sulfhydryls was discussed. Increased activation of sulfhydryl groups was observed upon homogenization, particularly after direct steam heating. It was suggested that sulfhydryl activation may be a factor contributing to the high resistance of homogenized milk to oxidized flavor development. The lack of correlation between the nitroprusside test and cooked flavor is believed to indicate that the nitroprusside test is less sensitive than flavor perception, or that cooked flavor is not necessarily due to nitroprusside reactive substances. A high retention of ascorbic acid was observed in milk subjected to direct steam heating and vacuum treatment. This effect, along with resistance to oxidized flavor development, was noted even in milks to which copper was added. The removal of oxygen by the vacuum treatment and the activation of sulfhydryl groups by the heat treatment were considered to be contributing factors. Homogenization was also shown to contribute to the stability of ascorbic acid and flavor. However, homogenization alone was less effective than the combination of homogenization with steam heating-vacuum treatment.