Gelation of food proteins is crucial to the formation of desired textural quality in many food systems. Restructured muscle (red meat or fish) products are typical foods in which protein gelation induce numerous levels of hardness, cohesiveness, springiness, gumminess, chewiness etc. which influences the quality of the end product (Bourne, 1978; Brennan, 1980). These textural attributes can be measured by a variety of different approaches and techniques including both instrumental and sensory methods (Cheng et al. 1979; Lyon and Hamm, 1984; Hamann, 1987).
Unlike terrestrial animal, fish muscle has higher levels of indigenous proteases which immediately start to break down the proteins after the fish are harvested and during processing or improper handling, storage, and cooking (Aksnes, 1989; Toyohara et al. , 1990; Morrissey et al., 1993). In the fishery area, investigations have focused on the proteolysis which affects several protein functionalities such as emulsion, gelling, and binding properties in muscle foods. For example, the quality of a cooked meat gel (kamaboko) is directly related to its elasticity, and if a gel of poor elasticity is produced, it often looses its commercial value (Su et al., 1981; Makinodan et al., 1985; Boye and Lanier, 1988). Indigenous proteases are active at both low or high temperatures and they are responsible for flakiness, non-adhesive attributes and weak gel forming ability of fish muscle. As a possible solution to high proteolysis, incorporation of a non-fish protein or binder and appropriate cooking processes has been investigated to produce a surimi type fish gel. But little is known about the relationships between the physicochemical properties of non-fish protein and textural properties of protein added to restructured fish products (Abide et al., 1992; Chung and Lee, 1990). It has been reported that incorporation of protease inhibitors might be necessary to maintain gelling ability of surimi products. Ingredients which have been suggested include egg white, wheat or potato starch, blood plasma with or without hydrolysis, whey proteins, soy protein isolates, and fats or oils (Lee, 1986; Aksnes, 1989; Hamann et al., 1990; Toyahora et al., 1990).
The tumbling process, which has been widely used in the red meat industry and is a physical operation in which meat pieces are subjected to physical forces to improve quality characteristics. It relays on gravitational impact and abrasion against other meat pieces to disrupt muscle fibers and extract the essential myofibrilar proteins that are necessary for binding meat sections together. Subsequently, molding or placing the meat fragments into a casing and cooking will produce the final tumbled and reformed meat product. There is a great demand for processed seafood products (Chung and Lee, 1990). However, there is little information on the indigenous proteases in restructured and formed fish products, the gelling properties of these products, and there is no study regarding tumbling and massaging of fish muscle for reforming purposes. Therefore, the objectives of this research were: to evaluate textural changes of fish muscle gels during processing, and to observe the effects of incorporating a non-fish protein prior to the tumbling technique on the gel forming and water holding properties of fish muscle.