Conformational changes and dynamic rheological properties of fish sarcoplasmic proteins treated at various pHs

The conformational changes and rheological properties of soluble sarcoplasmic proteins isolated from striped catfish (Pangasius hypophthalmus), treated at various pHs (2–12), were investigated. Isoelectric point of striped catfish sarcoplasmic proteins was determined to be pH 5. SDS–PAGE of sarcoplasmic proteins treated at various pHs, showed molecular masses ranging from 11 to 97 kDa. Most sarcoplasmic proteins, regardless of treated pHs, showed a molecular mass of 43 kDa. A decrease in total sulfhydryl content was observed when the pH was shifted away from 6, indicating disulfide formation at pH lower and higher than 6. Gradual increases of S₀-ANS and S₀-PRODAN were observed as pH increased from 6 to 12, indicating the unfolding of sarcoplasmic proteins during alkaline extraction. DSC thermograms of sarcoplasmic proteins treated at pH 5–9 exhibited an exothermic transition peak, probably due to disulfide bond formation, and/or hydrophobic interactions, which was highly related to the onset temperature of G′ rising. Gel network formation of sarcoplasmic proteins did not take place at extreme pHs (<4 or >9) where proteins were highly charged while the viscoelastic properties of sarcoplasmic proteins were observed at pH 5.5–9. The highest G′ value at 90 °C was observed at pH 5.5 and 8 (P ? 0.05). The gel point, a temperature at which G′ = G″, increased to higher temperature as pH was shifted away from 7.     

Comparative Study of Washing Treatments and Alkali Extraction on Gelation Characteristics of Striped Catfish (Pangasius hypophthalmus) Muscle Protein

ABSTRACT:  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 °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.     

Gelation characteristics of tropical surimi under water bath and ohmic heating

Gelation characteristics of tropical surimi, namely threadfin bream (TB), bigeye snapper (BS), goatfish (GF) and lizardfish (LF) prepared in the absence and presence of 10 g kg^?1 egg white proteins were evaluated using either ohmic (OH) or water bath (WB) heating. LF and GF surimi exhibited higher endogenous proteolytic activity than BS and TB. Ohmic heating markedly minimized proteolysis of LF and GF surimi as evidenced by a reduction of trichloroacetic acid (TCA)-soluble oligopeptide content of gels and more retention of myosin heavy chain (MHC). Ohmic heating increased breaking force and deformation of TB and BS surimi by 1.3 and 1.6 times, respectively, as compared to water bath heating. However, TB surimi gels heated by a higher applied voltage gradient of 16.7 V cm^?1 exhibited lower breaking force than those heated at 6.7 V cm^?1. Gels heated ohmically contained lower total sulfhydryl concentration, indicating the greater extent of disulfide bond formation as compared to gels heated in a 90 °C water bath. The rapid heating method with shorter heating time could improve water holding capacity and preserve color of tropical surimi gels when compared to water bath heating.