Mechanism for texture modifying effects of hydrodynamically active biopolymers in composite surimi gel

The rheological and structural properties of many formulated composite foods are governed primarily by the behavior of incorporated biopolymeric ingredients. This study attempted to elucidate the mechanism for the texture modifying effects of added biopolymers (potato, waxy maize and potato-tapioca blend starches, and $\iota$-carrageenan), in composite surimi gels. The rheological and hydrodynamic properties of dispersed biopolymers were altered by varying their solid content. Effects of unactivated and preactivated biopolymers were compared at various volume fractions as well as at the different moisture levels in the continuous phase. Expressible moisture, viscosity, creamability, initial compressive force, relaxation rate, texture and thermal properties were measured to evaluate rheological and hydrodynamic properties. The volume fraction of added preactivated starches negatively correlated with both composite sol viscosity and gel strength. Starches which produced a rheologically stronger preactivated paste (potato $\u3e$ potato-tapioca blend $\u3e$ waxy maize), yielded a stronger composite gel. For a given biopolymer, increased rheological strength with ascending order of solid contents, significantly reduced composite sol viscosity and gel strength. In contrast to a negative relationship between expressible moisture and gel strength which is commonly observed in most gel systems, a positive one was observed for the gels containing preactivated starches. As the rheological strength of preactivated starches increased, protein unfolding and subsequent gel network formation were retarded, as evidenced by the reduction in total enthalpy. The rheological properties of preactivated biopolymers were responsible for the dispersion pattern during mixing. Sticky potato starch was dispersed in small globules and strands, while short-textured waxy maize and potato-tapioca blend were dispersed in significantly larger globules of irregular shape. Preactivated $\iota$-carrageenan displayed a dispersion pattern similar to the latter. The rheological manifestation of dispersed biopolymers in composite surimi gel depended not only upon their rheological and hydrodynamic properties at the active state but also upon the form of dispersion (strand or globule). The strands of preactivated biopolymers may have structurally interfered with a crosslinking required for gel network formation. Such interference may be one of the causes for gel weakening with a preactivated biopolymer and possibly explain the mechanism for texture modification in composite surimi gel