Studies of Heated Milk. IV. Observations on Browning

Summary A method of rendering the brown pigment of evaporated milk soluble in milk serum by tryptic digestion of the proteins proved a satisfactory preliminary step to photoelectric measurement of color. Using this principle, the optical densities of 11 commercially produced evaporated milks were evaluated when fresh and at monthly intervals during storage for 1 yr. at room temperature. Discoloration was evident in all fresh samples. Color increased rather rapidly during the first 3 mo. of storage, after which time further color development was quite gradual. More color develops in samples held at higher storage temperature and storage at 4° C. will substantially inhibit color development. One vacuum-packed sample discolored more readily at all storage temperatures than did an air-packed sample of corresponding initial color. The investigation has demonstrated that "browning" is still a significant problem in the evaporated milk industry. A number of factors involved in the heat-induced browning of skimmilk also were studied. These included time and temperature of heating, preheating, pH, the roles of certain milk components, particularly casein and the significance of certain compounds of importance in other browning systems. Casein and lactose were observed to be the principal reactants in production of the color. The temperature range 100 to 120° C. appeared critical in the browning of skimmilk. Heating for a period of 1 hr. at 100° C. produced no appreciable discoloration, whereas browning was readily evident in samples heated for 7.5min. at 120° C. The tendency to discolor as a result of autoclaving was observed in decreasing order in skimmilk, rennet whey and "protein-free" rennet whey. The comparative resistance of the latter material to browning suggests that inorganic phosphates are not of direct importance in the heat-induced browning of milk. The region of pH 6.0 to 7.6 was found to be of critical importance in the browning of milk. A change of as little as 0.1 of a pH unit in this range produced a noticeable difference in color. Studies of the effect of preheating skimmilk on its tendency to brown on reheating suggests that browning in the case of milk is stepwise, at least in part. Although preheated skimmilks showed no difference in color, small differences proportional to the extent of preheating were observed following a second heat treatment. The additions of hydroxymethylfurfural, methyl glyoxal or acetaldehyde in quantities up to 0.5g. per liter and glycine up to 1g. per liter had no significant effect on heat-induced browning of skimmilk. Further study of the contribution of amino acids to browning in milk systems, established that milk subjected to tryptic digestion browns less readily than undigested milk and that either enzymatically or acid-digested casein browns less readily in the presence of lactose than does intact casein. In discussion of the phenomenon, it is suggested that intact casein may act in the manner of a stronger base than casein hydrolysates. Supporting evidence for this hypothesis is drawn from earlier work in which compounds isolated from heated milk to date also have been reported as products of the alkaline degradation of sugars.