Apparent inhibition of Pacific whiting surimi-associated protease by whey protein concentrate

Graduation date: 1994Surimi is a seafood product which is used to manufacture restructured products\ud such as artificial crab and lobster. Surimi is produced from fish fillets by washing to\ud remove sarcoplasmic proteins and increase the concentration of myofibrillar proteins, and\ud mixing with cryoprotectants. A valuable attribute of surimi is its ability to form an elastic\ud gel, the gel network being formed by the myofibrillar proteins of fish muscle. It is\ud generally accepted that the quality of surimi gels is influenced by the activity of\ud endogenous protease which acts on the myofibrillar proteins. The proteases in Pacific\ud whiting surimi (Merluccius productus) are particularly problematic due to their high\ud catalytic activity on muscle myosin. The addition of whey protein concentrate (WPC) to\ud Pacific whiting surimi has been shown to enhance the gel strength of the corresponding\ud products produced from this surimi. The mechanism through which WPC enhances the\ud gel strength of Pacific whiting surimi has not been determined, but it has been suggested\ud that WPC acts to inhibit surimi autoproteolysis. The objective of this study was to\ud determine whether the incorporation of WPC into Pacific whiting surimi inhibits\ud autoproteolysis and/or protects the myosin fraction from proteolytic degradation.\ud The effect of supplementing surimi with WPC, beef plasma protein (BPP) and\ud bovine serum albumin (BSA) on its apparent autoproteolysis activity was determined. Three WPC preparations were tested, WPC 34, 34% protein; WPC 80, 80% protein; and\ud WPC 95, 95% protein. Each of the additives was incorporated at the 1, 2, 3 or 4% level.\ud Proteolysis of surimi and supplemented surimi samples was allowed to occur at 55°C.\ud Proteolytic reaction mixtures were terminated by the addition of trichloroacetic acid\ud (TCA). Proteolytic activity was estimated by measuring the difference in TCA-soluble\ud peptides present in reaction mixtures of paired (identical) samples, one having been\ud incubated at 55°C while the paired sample was kept on ice. Peptides were quantified by\ud the bicinchoninic acid, Lowry, dye-binding and trinitrobenzenesulfonic acid methods.\ud Results based on the different peptide assays were compared in order to asses the reliance\ud of results on specific assay methods.\ud BPP was found to have the most inhibitory activity in the autoproteolysis assays,\ud followed by the WPC preparations and then BSA. Autoproteolysis was completely\ud inhibited by the incorporation of 1% BPP, 3% WPC 80 and 2% WPC 95. The extent of\ud inhibition by the WPC preparations was related to their protein content, the higher the\ud protein content the greater the extent of inhibition per unit weight added to surimi. BSA\ud was not an inhibitor of autoproteolysis under the conditions used in this study. The\ud relative extents of inhibition observed for the different additives were independent of the\ud method used to quantify the soluble peptide products.\ud Each of the additives was also tested for their ability to protect the myosin\ud component of surimi from proteolytic degradation. These experiments were done as\ud described above for the autoproteolysis assays with the exception that following the\ud incubation period a portion of the sample, either surimi or a surimi/additive mixture, was\ud completely solubilized in detergent solution, subjected to SDS-PAGE electrophoresis and\ud visualized by protein staining. In these experiments the additives were incorporated at the\ud 4% level. No apparent degradation of myosin could be detected over a 60 min reaction\ud period for surimi samples that were supplemented with BPP, WPC 80 and WPC 95. In\ud contrast, surimi samples incubated without additive clearly showed a loss of myosin after 15 min reaction period. Some myosin degradation was apparent following the 60 min\ud incubation period for the WPC 34-supplemented surimi.\ud A further experiment was conducted to determine the mechanism through which\ud WPC protects myosin and inhibits autoproteolysis. In this experiment WPC 95 and BPP\ud were separately incubated at 55°C with a crude fish protease preparation, i.e. the reaction\ud mixture approximates that used in the autoproteolysis assays except that it contains no\ud surimi. The results indicate that BPP and WPC 95 behave in a similar manner. However,\ud the results were inconclusive with regard to explaining the additive's mechanism of action.\ud Plausible mechanisms which are consistent with the results are discussed