The Effect of Heat Treatment on the Nutritive Value of Milk Proteins III. The Effect of Heat on Casein, Lactalbumin, and Their Lactose-Induced Derivatives, with Special Reference to Digestibility and Rate of Release of Lysine, Methionine and Tryptophan

Abstract

The effects of heat or lactose or both on casein and several types of lactalbumins have been investigated. These milk proteins have been shown to bind lactose in amounts ranging from 2 to 8% under different experimental conditions. Casein, refluxed at 95°C. for 24 hours in a solution containing a ratio of protein to lactose of 1:1, bound twice as much lactose as when heated at 52°C. for the same length of time in a solution in which the ratio of protein to lactose was 1:0.3 or 3.5:5. The two lactose-free lactalbumins used, one prepared commercially by an unknown process and the other a laboratory-prepared, ethanol-precipitated sample, combined with no more lactose when refluxed at 95°C. than when heated at 52°C. if the ratio of protein to lactose was more than 1:1. If the ratio of protein to lactose were decreased to 3.5:5 or 1:2, more lactose was bound by the protein at 52°C. That free amino groups are essential for reaction with lactose was established by acetylation experiments. Acetylated lactalbumin reacted with no lactose present. Acetylated casein reacted with considerably less lactose than did the original protein. Mild lactose treatment (52°C. for 24 hours) of these milk proteins rendered them somewhat resistant to enzymatic digestion as measured by release of α-amino nitrogen, lysine, methionine, and tryptophan. Severe lactose treatment (refluxing at 95°C. for 24 hours) effected a further decrease in digestibility and availability of the same amino acids. Heat alone had no appreciable effect on the total in vitro digestibility or availability of amino acids of A (soluble) lactalbumin but led to a decreased digestibility of casein and ethanol-precipitated lactalbumin. Heat subsequent to reaction of the proteins with lactose greatly intensified the decrease in nutritive value of both casein and lactalbumin. Heating of a commercial lactose-containing lactalbumin, B(15–42), had the same deleterious effects as heating the pure proteins after they had been reacted with lactose. The various amino acids were released at different rates following heat or lactose treatment or both. It was suggested that this difference in the rate of release of certain essential amino acids may be sufficient to prevent their mutual supplementation in in vivo experiments. Arguments were presented in support of the view that browning of milk proteins in the presence of lactose takes place in several steps and may lead to the formation of new cross-links, thereby accounting for the resistance of the protein derivatives to enzymatic attack.