The Combined Effect of Transglutaminase and Protease Inhibitors on the Thermal Gelation of Actomyosin Sol from Carp and Salmon Muscles

Freshwater fish and some marine fish species are general ly less desirable as a raw material for surimi-based products because of their poor gelling ability in compari son with walleye pollack surimi. In particular, carp and salmon are typical fish of poor gelation which results primarily from the lack of setting response and, in addi tion, from modori (gel weakening).1-3) Much of the effort has been devoted to understanding the mechanisms of ther mal gelation as well as to alterations of gel textural proper ties by the regulation of suwari (setting) and modori to produce desirable product texture.1) Several studies have demonstrated that setting effect is correlated with cross linking reaction of protein mediated by endogenous TGase and results in more elastic and rigid gels after cooking at high temperatures.4-6 Salmon muscle contains low activity of endogenous TGase2) and relatively high protease activi ty.7,8) Furthermore, anserine, which inhibits TGase activi ty, exists in large quantities in the muscle.3) Therefore, the repeated washings are required to remove proteases and an serine as well as removing plasma proteins to concentrate myofibrillar proteins. In the case of carp muscle, the en dogenous TGase activity level is rather high as reported by Araki and Seki,9) though the rate of enzyme-catalyzed cross-linking of carp myosin heavy chain is limited by the conformation of substrate molecule. On the other hand, we found that MTGase added into carp actomyosin sol cross-linked more extensively myosin heavy chains, con nectin and actin than carp endogenous TGase did.10) There fore, the first objective of this study was to determine possible contribution of MTGase-induced setting to the reinforcement of thermal gelation of carp and salmon ac tomyosin sols. Two possible mechanisms of modori have been postulat ed that one is of non-proteolytic and the other is a proteo lytic process. The former is not well understood, though it has been suggested that the modori results from the dissoci ation and depolymerization of actin filament during ther mal gelation of actomyosin.11,12) The proteolytic modori is