Digestion of Heat-damaged Egg Albumen by the Rat

Abstract

The influence of heat treatment of egg-white protein in the presence or absence of glucose on the intestinal digestion and absorption of the protein in rats has been studied. Lysine-labeled egg albumen was produced by injecting laying hens with ¹⁴C-l-lysine·HCl. The radioactive egg white was dialyzed to remove traces of glucose, lyophilized and autoclaved at 120° for 60 minutes, either in the presence or absence of glucose to obtain a Maillard reaction protein and a heat-damaged carbohydrate-free protein, respectively. The two heat-treated proteins were fed to weanling male rats which were killed at different time intervals to recover the contents of different segments of the gastrointestinal tract. The distributions of radioactivity and nitrogen were determined both in the total and the trichloroacetic acid-soluble fractions. Rats fed with the Maillard egg albumen excreted three times more radio-activity in the urine than those fed with glucose-free ovalbumen, whereas the ¹⁴CO₂ recovery in the expired air was three times higher in the latter. This is possibly due to the absorption of an epsilon-N-lysine compound produced through the Maillard reaction which cannot be utilized and is therefore excreted in the urine. In one experiment where conventional rats and rats treated with an antibiotic diet (2% phthalylsulfathiazole plus 0.1% penicillin) were fed the above protein, a significantly (P < 0.05) greater recovery of radioactivity was obtained from the antibiotic-treated animals. This is possibly due to a greater bacterial proteolysis in the conventional rat and contributes support for the concept that the intestinal microflora may increase the apparent digestibility of heat-damaged proteins. The percentage recovery of lysine radioactivity in the gut 16 hours post cibum (found mainly in the cecum and large intestine) and the total radioactivity recovered in the feces 72 hours after feeding were exactly the same, indicating that under the conditions of this experiment there was no detectable absorption of lysine or its degradation products from the undigestible residue in the cecum and colon. A digestibility trial with conventional and coprophagy-prevented rats, using autoclaved ovalbumen and Maillard protein, indicated that under the vigorous heat treatment to which these proteins were subjected, lysine was prevented from absorption to the same degree as the rest of the protein. Coprophagy prevention reduced the digestibility of Maillard protein to a greater extent than that of the “glucose-free egg albumen.”