DOI: 10.14457/tu.the.2017.1758
title: Gel-forming characteristics of rohu (Labeo rohita) and their changes during frozen storage

description: The objectives of this study were to investigate the gel-forming characteristics and endogenous protease type of rohu minces, to investigate the effect of egg white powder (EW), whey protein concentrate (WPC), and potato starch (PS) on gel-forming ability and autolytic inhibition of rohu gels, and to investigate the changes in biochemical properties of myofibrillar protein (MF) extracted from frozen surimi and the gel characteristics of frozen surimi. The gel-forming ability of rohu minces was studied to identify the optimum temperatures at which strong and weak gels are produced from unwashed and washed minces. In both unwashed and washed gels, the strongest gel was produced by heating at 40ºC for 30 min, followed by 90ºC for 20 min. The weakest gels were obtained at 65ºC for the unwashed gel and 60ºC for the washed gel. The optimum protease activity in both gels was found after setting for 120 min. Four selected protease inhibitors, soybean trypsin inhibitor, leupeptin, E64, and EDTA, were investigated to identify the protease groups responsible for gel degradation in both unwashed and washed minces. The results suggested that cysteine protease is the major endogenous protease involved in the autolysis degradation of unwashed gel, whereas serine protease plays an important role in the autolysis degradation of washed gel. To address the softening of modori gel and improve the gel-forming ability in kamaboko gel from both unwashed and washed minces, EW, WPC, and PS were added. The addition of EW and WPC was investigated at three concentrations (1%, 2%, and 3%), and PS at four concentrations (1%, 2%, 3%, and 4%). In modori gel from unwashed mince, the addition of 3% EW, 3% WPC, and 2% PS improved gel-forming ability, while the addition of EW, WPC, and PS at 2% improved autolytic inhibition. In kamaboko gel from unwashed mince, the gel-forming ability was improved by the addition of EW at 1%, WPC at 2%, and PS at 3%, while the autolytic inhibition was improved by the addition of EW at 1% and WPC at 2%. No marked change in autolytic inhibition was found after the addition of PS. The whiteness of modori gel was increased by the addition of EW at 3% and WPC at 1%, and that of kamaboko by the addition of EW and WPC at 3%. In modori gel from washed mince, the addition of 2% EW, 1% WPC, and 4% PS improved the gel-forming ability, while the addition of 1% EW and 3% WPC improved autolytic inhibition. The addition of PS had no marked effect on autolytic inhibition. In kamaboko gel from washed mince, the gel-forming ability was improved by the addition of EW at 2%, WPC at 1%, and PS at 1%, while autolytic inhibition was improved by the addition of EW at 2% and WPC at 3%. No effect on autolytic inhibition was observed after the addition of PS. The addition of EW, WPC, or PS had no effect on the whiteness of either modori or kamaboko gels from washed mince. In the microstructure, the addition of EW, WPC, and PS to both unwashed and washed gels resulted in smaller cavities and a more compact fibrous network. The change in the biochemical properties of myofibrillar protein (MF) extracted from frozen surimi and the gel characteristics of surimi during frozen storage at -18ºC for six months were investigated. The changes in biochemical properties of MF showed that extended frozen storage was shown to adversely affect myosin. This was reflected in the Ca2+-ATPase activity, which declined by approximately 20%, the reactive SH, which declined by approximately 20% in both surimi, and the total SH, which declined 15% in the washed surimi and 3% in the unwashed over the six months of storage, compared with those of the initial values after 12 h of frozen storage. These results were supported by the reduced intensity of the myosin heavy chain (MHC). The changes in the MF biochemical properties led to a change in gel characteristics. By the end of the sixth month, the gel strength had decreased by approximately 10% in washed surimi and 21% in unwashed surimi, and the water holding capacity by 18% in washed surimi and 11% in the unwashed surimi, compared with those of the initial values after 12 h of frozen storage. These results were supported by the SDS-PAGE patterns, which showed MHC degradation, and the aggregation of protein clusters in the microstructure. The whiteness of both unwashed and washed gels increased as the frozen storage time extended.