Recovery and properties of muscle proteins extracted from tilapia (Oreochromis niloticus) light muscle by pH shift processing

Functional proteins can be extracted from fish muscle using acid- or alkali-aided solubilization and recovered with isoelectric precipitation. It was of interest to evaluate acid- and alkali-aided solubilization/precipitation on muscle material from the warm-water fish tilapia. Higher levels of proteins were extracted with high compared with low pH (P < 0.05) because of higher protein solubility at high pH. Protein extraction was not influenced by the specific low or high pH tested or homogenization time. Similar protein types were extracted for the low and high pH solubilization, while proteolysis was observed at low pH. Viscosity was significantly higher at pH 2.3 to 2.9 compared with pH 10.8 to 11.4 (P < 0.05) and varied greatly at low pH indicating a more unstable system. Two different low (pH 2.5 and 2.9) and high (pH 11 and 11.2) solubilization pH values were selected and protein recovery investigated in the pH range 5.1 to 5.7. The alkali-extracted proteins had more solubility at pH 5.1 to 5.7 (P < 0.05) and thus less protein (P < 0.05) was precipitated compared with the acid-aided process. More protein was recovered as pH increased from 5.1 to 5.7 (P < 0.05). More protein types were found in the supernatant after precipitation for the alkali-aided treatment, and soluble proteins were similar to those of untreated tilapia muscle homogenate. Viscosities of the acid- and alkali-extracted proteins at pH 5.1 to 5.7 were significantly higher than the viscosity of native proteins at that pH (P < 0.05). While this study demonstrated significant differences in protein extractability and precipitation of the 2 processes, no statistical difference (P > 0.05) was found for overall protein recovery (61% to 68% for alkali-aided process and 56% to 61% for acid-aided process) of the 2 processes.