Traditional and new concepts in protein evaluation of feeds: a review

The purpose of protein evaluation of a diet is to determine its value for meeting amino acid requirements of animals. The early recognition that proteins can be utilized by different efficiency rates has promoted extended research to reveal quality differences and their reasons. A brief historical overview is given in this review about the development of concepts and theories in protein evaluation. Recent developments have focused attention on determination of available quantities of essential amino acids. In vitro chemical assays and dye-binding procedures are relatively fast and provide data without access to animals, however they do not always correlate well with biological estimates. Enzymic digestion methods and microbiological assays for prediction of availability of essential amino acids have the potential for serial determination in quality control, however they are constrained by a lack of information about the availability of peptides liberated and utilized during in vivo digestion of proteins. Animal growth assays and balance experiments can provide valuable estimates of available quantities of essential amino acids. However, the interpretation of data is complicated by the many factors affecting in vivo protein utilization, by the endogenous amino acid excretion and the effects of microflora of the alimentary tract. The search for reliable assays remains a worthwhile objective also in the future.