Comparative study of washing treatments and alkali extraction on gelation characteristics of striped catfish (Pangasius hypophthalmus) muscle protein.

The effect of washing treatments (water and alkali washing) and alkali extraction/acid precipitation on the physicochemical and textural properties of striped catfish (Pangasius hypophthalmus) muscle protein was compared. Alkali extraction/acid precipitation process resulted in the highest protein recovery of 98.77% (dry weight basis) and lowest fat content of 0.98% (dry weight basis). The extractable proteins of fish protein isolates (FPI) at various ionic strengths (0 to 0.6 M NaCl) exhibited majority of protein at 38 kDa with trace amount of others. Unsaturated fatty acids, namely, oleic, linoleic, and docosahexaenoic acid, were greatly reduced in FPI. Water-washed mince (W) exhibited higher autolytic activity than mince (M), suggesting the presence of myofibril-bound proteinase(s). Autolytic activity was lowest in the FPI. Breaking force of striped catfish gels was greatly improved when set at 40 degrees C for 30 min, regardless of preparation treatment. The typical water washing process was effective to improve gel-forming ability of striped catfish. FPI gels contained the lower total sulfhydryl content, indicating the greater extent of disulfide bond formation as compared to that of washed mince. The FPI gels showed higher breaking force but lower deformation values than W and alkali-washed (AW) gels. Addition of NaCl improved deformation, but adversely affected breaking force of FPI gel. Whiteness of FPI gel increased with NaCl addition.