Effect of NaCl on gelation characteristics of acid- and alkali-treated pacific whiting fish protein isolates

The physicochemical properties of gels prepared from Pacific whiting protein isolated at pH 3 and 11 with three concentrations of NaCl were characterized after a pH readjustment to 7.0. The strongest gels were obtained from fish protein isolate (FPI) prepared at pH 11/150-mM NaCl and conventional surimi. The protein solubilities of FPI treatments did not contribute significantly to their gel properties. Surface hydrophobicity and differential scanning calorimetry demonstrated that, in addition to adjusting the pH to 3 or 11, salt addition during protein solubilization and subsequent recovery at the isoelectric point (pI) led to protein denaturation. Rheological study indicated that gelation mechanisms of FPI were identical with the same NaCl concentration regardless of pH. The FPI prepared at pH 3 or 11 with NaCl could be partly refolded at pH 7. Nevertheless, some myosin fragments and actin did not refold. PRACTICAL APPLICATIONS Fish protein isolate (FPI) processing is a new concept for protein recovery using a pH shift. While conventional surimi processing is performed by avoiding protein denaturation, this FPI process is performed chemically by unfolding proteins and subsequent refolding. Since FPI is made with the inclusion of sarcoplasmic proteins, which are removed at the conventional surimi process, it guarantees a significant yield increase. This study revealed that the strongest gels were made from conventional surimi and FPI prepared at pH 11 and 15-mM NaCl. It also confirmed that protein solubility does not support thegelation properties of FPI. Refolding of FPI prepared at extreme pHs (3 or 11) was noted, but in a full scale.