Thiol Amino Acids in Quality Control and Nutritive Value of Fishery Products

Abstract

The quality of fish products is dependent upon the raw material and the process employed for their reproduction. The availability of the amino acids present in the protein component determine the value of the product as a food supplement. Amino acid availqbility can be determined by chemical, biochemical and biological methods. Most of the procedures are rather lengthy. A widely used chemical procedure, the available lysine method, is based on determining the quality of total lysine having free or reactable epsilon amino groups. This research has been directed toward the development of a procedure for determining the quantity of reactable -SH and -SS- groups remaining in the protein, as these groups are more reactive than the epsilon amino groups of lysine and should therefore provide a more critical index of change. Available procedures are not readily adaptable to determining -SH and -SS- groups in intact insoluble proteins. Normally the thiol amino acids cysteine and cystine are determined by acid digestion of the proteins and reported as 1/2 cystine. The procedure developed essentially involves a carefully controlled fish protein solubilization with a selected enzyme, followed by titration with AgN03 using a Ag/S specific-ion electrode to determine the quantity of titratable -SH groups. By reducing -SS- groups prior to titration the combined -SH and -SS- level can be determined. Subtracting the previous -SH value from the total gives the -SS- level. The results obtained when this procedure was used to determine the thiol amino acids content of a series of specially prepared and selected fish products, were considerably lower than those obtained by the use of an amino acid autoanalyzer but exhibited a high degree ' of ' correlation. The results also showed a high degree of correlation with those obtained by pepsin digestion, another procedure for determining protein quality. When the specially prepared and selected fish products were biologically assayed, using one week old White Rock chicks, differences in response were observed that indicated a trend but a high degree of correlation was not established. The results of the biological assay suggest that the procedure was not sufficiently sensitive to detect the small difference in the thiol amino acids present in the diet. The effect of selected process conditions on the level of thiol amino acids in the end products was determined. The results strongly suggest that two factors contribute to the loss of thiol groups; cooking in the wet reduction