Evaluation of protein quality: Methodological Considerations

The enormous efforts that have gone into protein evaluation in the intervening quarter century suggest a need to reiterate and emphasize points made at that meeting (Bender, 1958) namely, that all measures of protein quality are a function of the limiting amino acid (AA), that the tests do not yield any information about other AA and that there is no 'true' value with which to compare new methods.Protein efficiency ratio (PER) has been used for too long as the touchstone of protein quality evaluation and it is time to abandon it.The fundamental error underlying all this work is the attempt to express protein quality by a single figure.Such an attempt might have been justified when biological measures were first developed because at that time it was not known that the value was dependent on the AA in the food-in fact, the major methods were in use before the last of the essential AA, threonine, was discovered.The term NDpE is used in which both quality (actually the amount of (unspecified) limiting AA) and quantity are incorporated.NDpE does not indicate whether there is insufficient protein in the diet, whether more food of the same type would be beneficial or whether the quality of the protein is low and supplementation is the solution, Evaluation of protein qualityThe term 'bioassay' in relation to protein quality is somewhat misleading.This term is usually applied to assays in which there is dose-related response, such as in pharmacological assays and in many assays using micro-organisms.The validity of such tests is often demonstrated by the slope of the line.The classical protein quality evaluations (PQE), however, despite occasional statements to the contrary, are not dose-related but measure simply how much of the protein is retained by the animal, whether estimated from N retention or