Abstract:
A novel method for isolating fish proteins by shifting pH to high acid or high
alkali pH was the focus of the study. Biochemical and physicochemical properties of
various pH-treated soluble fish proteins as a function of ionic strength were
determined. Effect of ionic strength and various storage conditions on gelation
properties and stabilization of fish protein isolate (FPI) were also elucidated.
At low ionic strength (IS 10 mM NaCl), the solubility of Pacific whiting (PW)
proteins was low between pH 5 and 10, but increased significantly as the pH was
shifted to either acidic or alkaline pH. The isoelectric point (pi) shifted toward
acidic direction as IS increased to 600 mM. High IS (600 mM NaCl) resulted in
protein aggregation at low pH but improved myosin heavy chain (MHC) solubility at
pH 6 - 10. Changes in total sulfhydryl (SH) content and surface hydrophobicity (S [subscript o])
were associated with the different molecular weight distributions of the soluble proteins. At pH 4 and IS 10-100 mM, MHC was soluble but degraded. At pH 10,
the formation of high MW polymers was observed at IS [greater than or equal to] 150 mM.
Gels obtained from FPI prepared at pHl1/IS150 and conventional surimi (CS)
were superior to FPI prepared at pH 3 and/or other IS levels. There was no correlation
between protein solubility and gel properties of FPI. Gelation mechanisms of acid and
alkali-treated FPI were identical under the same IS condition. FPI prepared at pH
3 or 11 could be partly refolded at pH 7.
No significant difference in texture was observed between alkali-treated
protein isolates (AKPI, pH 11) kept frozen at pH 5.5 and 7.0. Strongest gel was found
for AKPI with cryoprotectants (C) and without freeze/thaw (FT) cycles at both pH
storage (5C & 7C), while poor gel was obtained from AKPI without cryoprotectants
(NC) and with FT (5NC-F & 7NC-F). 5NC-F & 7NC-F demonstrated the lowest S [subscript o]
and total SH probably suggesting that proteins were more aggregated as a result of
hydrophobic interactions and disulfide bonds.
Scanning electron microscope (SEM) revealed the most discontinuity of gels
from AKPI without cryoprotectants and with FT and showed less protein stability
when stored at pH 5.5 than at neutral pH. Raman spectral analysis demonstrated that
refolding of AKPI by pH adjustment to 7.0 was achieved, but not identical to the
native protein. CS contained higher α-helix content (~50%) than AKPI (~20-30%).
Frozen storage induced a decrease and an increase in the α-helix of CS and AKPI
samples, respectively. Alkali-treated proteins were slightly less stable than CS during
frozen storage.
