The nutritive value of protein isolates and fibres from meat industry by-products

Protein isolates extracted from rumen, lung and plasma by alkaline solubilisation were spun into protein fibres and were evaluated for protein quality by rat feeding trials and by amino acid analyses. The net protein utilisation (NPU) of the fibres ranged from 53·0 to 76·9 for plasma and rumen fibres respectively, methionine plus cystine and valine being limiting amino acids. Lysine was not found to be a limiting amino acid in any sample and lysine availability was high in isolates and fell only slightly on spinning. There was a marked discrepancy between chemical score and NPU for rumen isolate and it is postulated that an anti-nutritional factor, possibly a trypsin inhibitor, normally present in bovine organs, could be active in the isolate by lowering methionine availability to the animal and decreasing NPU. Spinning the proteins, however, either destroys the inhibitor or decreases its concentration since NPU and chemical score become equal.