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In vivo and in vitro gastric emptying of protein fractions of three milk replacers containing either milk protein (control), a mixture (50:50 on a CP basis) of milk protein and native whey protein concentrate, or a mixture (50:50 on a CP basis) of milk protein and heated whey protein concentrate was studied. In vivo gastric emptying was measured in three preruminant calves fitted with reentrant duodenal cannulas and used in a 3 x 3 Latin square design. In vitro gastric emptying was determined after enzymatic digestion in an artificial stomach. In vivo and in vitro flow rates of protein N (12% TCA-insoluble N) and total N were higher for milk replacers containing whey proteins than for control. Gastric emptying of NPN (12% TCA-soluble N) was slightly higher for diets containing whey proteins than for that containing milk proteins. Gastric emptying of all protein fractions was similar for the two milk replacers containing whey proteins. In vivo and in vitro results were significantly correlated, suggesting that the in vitro method reproduced conditions for proteolysis and could be used to predict gastric digestion of protein fractions.

The digestibility of proteins and individual amino acids of nineteen selected foods was determined by an in vitro assay. Samples were hydrolysed with pepsin for 30 minutes in an acidic medium; the pH was then raised to 7.5 and the mixture poured into the dialysis bag (molecular weight cut-off 1000) of a digestion cell with pancreatin. Digestion products, mixtures of free amino acids and low molecular weight peptides which pass through the dialysis membrane, were collected for 6 hours by sodium phosphate buffer circulation. All proteins from animal sources displayed a digestibility similar to casein, except for breakfast sausage. Vegetable proteins showed intermediate digestibility, except for cereals (lower) or peanut butter (higher). Target amino acids of enzymes were generally more readily hydrolysed. However, compared to other animal proteins, glycine in milk products, valine, isoleucine, methionine and lysine in breakfast sausage and hot dog, and histidine in tuna were more easily released. Overheating of non-fat dried milk not only reduced the lysine digestibility, but also that of methionine, phenylalanine, histidine and cystine. Among vegetable proteins, wheat products were characterized by a relatively greater release of threonine, isoleucine and histidine, and peas by a lower digestibility of methionine and lysine. Proline of soy isolate and isoleucine of pinto bean were resistant to hydrolysis while arginine of pinto beans and of rice-wheat-gluten was easily released.

No abstract

The impact of various endopeptidases on the nature of protein digestion products was measured with the digestion cell technique. After a 30 min pepsin pre-digestion, casein and rapeseed concentrates were hydrolyzed with various amounts of pancreatin, trypsin and/or chymotrypsin. This hydrolysis was performed in a dialysis tube (molecular weight cut-off 1000) with continuous collection of the digested material. The addition of pure trypsin or chymotrypsin to pancreatin (Enzyme:Substrate 1:25) did not change the digestibility of casein. Only a higher pancreatin level (Enzyme:Substrate 1:12.5) increased the total protein digestibility without affecting the amino acid spectra. Rapeseed digestibility was markedly increased by the addition of pure trypsin to pancreatin. Lysine and arginine, target amino acids of trypsin, were favored at the expense of chymotrypsin and elastase target amino acids. Supplementation of pancreatin with chymotrypsin enhanced rapeseed digestibility without affecting the relative amino acid digestibility. The impact of a higher pancreatin ratio (Enzyme:Substrate 1:12.5) was similar to that of enriched pancreatin but the rate of amino acid release was modified. The differences between protein sources were mainly attributed to protein structure.