Biochemical and nutritional changes in fish proteins during drying

Abstract

Biochemical and nutritional changes in the muscle proteins of a lean marine fish Nempiterus japonicus during drying at 50, 60 and 70°C were investigated. Solubility of proteins in water, 0.6 M NaCl, 1.5 M urea, 8 M urea and 10 g litre−1 sodium dodecyl sulphate (SDS) decreased as drying progressed at all three temperatures; most of the decrease occurring in the initial 4 h of drying SDS-polyacrylamide gel electrophoresis of 1.5 M urea, 8 M urea and SDS extracts showed that higher molecular weight (MW) protein fractions were more sensitive to drying and disappeared much earlier from electropherograms than the lower MW protein fractions. Residual solubility of proteins near the pH range of 4–6 was found to increase during drying, but solubility at acid and alkaline pH was adversely affected. Decrease of solubility by drying was more affected at acid pH, especially at higher temperatures than at alkaline pH. Sulphydryl groups registered a regular and sharp decrease with drying except at 50°C, where initially an increase was observed. Apart from disulphide and hydrophobic bonds, free amino groups also appear to be involved in denaturation reactions during drying. Pepsin digestibility of fish muscle decreased slightly during drying but a clear relationship with drying temperature was not evident. Highly significant differences in proteins between protein efficiency ratio (PER), net protein utilisation (NPU) and biological value were observed between the drying temperatures. The PER and NPU of fish dried at 60°C were significantly higher than those dried at 50 or 70°C.