Comparison of egg-yolk protein hydrolysate and soyabean protein hydrolysate in terms of nitrogen utilization

Egg-yolk protein hydrolysate (YPp) is an alternative protein source in formulas for infants with intolerance to cow's milk or soyabean protein, or for patients with intestinal disorders. However, the nutritional value of YPp has never been investigated. YPp was prepared by enzymic hydrolysis of delipidated yolk protein, which led to an average peptide length of 2.6 residues. Three experiments were performed. In Expt 1, we compared the intestinal absorption rate of YPp and soyabean protein hydrolysate (SPp) in rats. YPp and SPp solutions were injected into the duodenum of anaesthetized rats and blood samples were taken from the portal vein at 7, 15, 30, 60, and 120 min. A higher amino acid concentration in the serum of the YPp group demonstrated that YPp was absorbed faster than SPp. In Expt 2, the effects of dietary YPp and SPp on body-weight gain, protein efficiency ratio (PER) and feed efficiency ratio (FER) were determined. At the end of the experiment, body weight had increased in both groups, while PER and FER were significantly higher in rats fed on YPp. In Expt 3, to investigate the effects of dietary YPp and SPp on N metabolism, we determined the biological value and net protein utilization. Yolk protein was the reference protein. Biological value and net protein utilization values were very similar between animals fed on yolk protein and YPp diets, and significantly higher than in rats fed on the SPp diet. The present findings demonstrate that there is no adverse effect of hydrolysis of yolk protein on N utilization, and that the nutritive value of YPp is similar to that of yolk protein and superior to that of SPp.