Protein nutrition chemical score

Protein hydrolysis in 6N HCl and subsequent analysis to determine amino acids (except tryptophan, which is acid labile) chemically present is a first step in protein quality evaluation. The chemical score and the EAA index represent attempts to use this information to chemically estimate nutritional quality of protein their obvious limitation is their disregard for amino acid availability. The chemical score is obtained by evaluating the percent of the limiting amino acid in comparison to that amino acid in whole egg protein ). The EAA index is the geometric mean of the ratios of each of the essential amino acids to those amino acids occurring in whole egg (4). 

Values for a test protein will be lower than for the reference protein. The reliability of results for chemical scoring methods depends upon the accuracy of the amino acid determinations that form the basis for these assays. Chemical scoring and animal bioassays tend to provide similar relative rankings of protein quality however, the actual values may be different. Chemical scoring methods should not replace a bioassay for testing the quality of a food protein for which there is very little nutritional information. 

The chemical score is determined as follows. The contents of the indispensable amino adds of egg albumin (ovalbumin), which is used as a reference protein, and the food protein in question arc determined. Egg albumin is considered to be ideal and nutritionally complete The test requires several steps. The protein must be purified, hydrolyzed to the constituent amino acids, and then subjected to analysis using an amino acid analyzer. The values for each of the indispensable amino acids for ovalbumin and the test protein are then listed as shown in Table 8.14. The contents of the various amino acids in the two proteins are then compared. The amino acid in the test protein that is present in the low cst level, on a percentage basis, can be found by examining the table. This amino acid is called the limiting amino acid of the protein. The value of the percentage is the chemical score. Table S.14 indicates that the quantity of lysine in oat protein is 51% that in egg protein. Hence, the chemical score of oat protein is 51,... 

This method, proposed by Satterlee represents a modification of the chemical score technique. Amino acid analysis is used to determine the essential amino acid profile and an in-vitro proteolytic enzyme step estimates the overall digestibility of the test protein. The in-vitro protein digestibility data, together with the essential amino acid profile, are then used to predict the PER (c-PER). The results obtained so far indicate a good correspondence between the calculated PERs and the actual PERs in the rat assay. The c-PER value is thus an estimate of nutritive value, with correction of the chemical score for digestibility, but it is not a measure of amino acid availability. 

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