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Hydrolyzing a protein in acid for a single hydrolysis interval, normally 24 h, will lead to inaccurate estimates of the amino acid composition of that protein due to an effect of the time of hydrolysis on peptide bond cleavage and amino acid degradation. The simultaneous yield and decay of amino acids during the hydrolysis of a protein can be described by a compartmental model with parameters for the hydrolysis and loss rates specific to each amino acid in a protein. The amino acid composition of the protein prior to hydrolysis can be determined by nonlinear regression of data derived from multiple hydrolysis intervals. In the present study egg-white lysozyme was hydrolyzed in 6MHCl using 18 hydrolysis intervals (range, 2–141 h) using the conventional duplicate hydrolyses/interval system. Hydrolysis and loss rates were determined for each amino acid. Increasing the number of hydrolysis intervals prior to the maximum point on the hydrolysis curve, and including an hydrolysis interval greater than 100 h increased the accuracy with which the hydrolysis and loss rates were estimated. Most of the amino acids underwent some degree of loss during hydrolysis. Of particular note was the loss rate for cysteic acid, which was greater than that found for serine which is commonly regarded as an acid-labile amino acid. The determined amino acid composition of the protein, based on the nonlinear regression of the data from four different series of hydrolysis intervals, was compared with the known amino acid composition (sequencing). Using the routine duplicate sampling system, a nonlinear regression including 10 hydrolysis intervals (2, 6, 10, 14, 18, 22, 26, 30, 60, and 141 h) resulted in a mean amino acid recovery of 100% (range, 94–110%) and provided an acceptable compromise between accuracy and the cost of analysis.

Summary: The piglet was evaluated as a model animal for studying the digestion of high-quality proteins in human infants. Three-week-old male piglets (n = 6) and 3-month-old male human infants (n = 6) were fed a bovine-milk-based formula over a 17-day experimental period comprising 7 days' adaptation followed by a 10-day fecal collection period. The piglets and infants were given 345-g liquid formula/kg body weight/day and 170-g liquid formula/kg body weight/day, respectively, which equated to similar dry matter intakes per unit stomach volume (0.923 g dry matter/cm3/day). Both the piglets and infants were individually bottle-fed the reconstituted milk formula (12.2 dry matter) at similar meal frequencies. Small but statistically significant differences (p < 0.01) were found for the apparent fecal digestibility (mean ± overall SE) of dietary dry matter (98.8 versus 97.4 ± 0.13), organic matter (99.0 versus 97.7 ± 0.12), and total nitrogen (97.5 versus 94.5 ± 0.36) between the piglets and infants. The fecal digestibilities for most of the amino acids were not significantly different (p > 0.05) between the species. The digestion of protein appeared to be similar in the two species. The study provides support for using the piglet as a model animal for studying protein digestion in human infants.

Summary: The piglet was evaluated as a model animal for studying the digestion of high-quality proteins in human infants. Three-week-old male piglets (n = 6) and 3-month-old male human infants (n = 6) were fed a bovine-milk-based formula over a 17-day experimental period comprising 7 days' adaptation followed by a 10-day fecal collection period. The piglets and infants were given 345-g liquid formula/kg body weight/day and 170-g liquid formula/kg body weight/day, respectively, which equated to similar dry matter intakes per unit stomach volume (0.923 g dry matter/cm3/day). Both the piglets and infants were individually bottle-fed the reconstituted milk formula (12.2% dry matter) at similar meal frequencies. Small but statistically significant differences (p < 0.01) were found for the apparent fecal digestibility (mean ± overall SE) of dietary dry matter (98.8 versus 97.4% ± 0.13%), organic matter (99.0 versus 97.7% ± 0.12%), and total nitrogen (97.5 versus 94.5% ± 0.36%) between the piglets and infants. The fecal digestibilities for most of the amino acids were not significantly different (p > 0.05) between the species. The digestion of protein appeared to be similar in the two species. The study provides support for using the piglet as a model animal for studying protein digestion in human infants.