Effects of acetic, malic, and citric acids on the large deformation behaviors of fish gelatin gels

This research was aimed to quantify the effects of acetic acid, malic acid, and citric acid (0, 0.5, 1.0, and 2.0 g/100 g H₂O) on the stress–strain responses of fish gelatin (FG) gels (2, 4, and 6.67 g/100 g H₂O) under uniaxial compression up to 68% of deformation. The first-order Ogden model fitted quite well for the compression responses of FG gels (R² = 0.9909–0.9997). Protons from the acids played a key role on weakening the FG gel structures (gel rigidity, μ, decreased 11%–27%), as the μ values and pH values of FG gels were linearly correlated (R² = 0.8240–0.9748), regardless of the acid type. The addition of an acid also resulted in a significant increase (p < .002) in the strain hardening capacity (α) of gels with 2 g FG/100 g H₂O. Both μ and α values of FG gels with higher gelatin concentrations were less affected by an acid partly due to their stronger buffering effects. The μ and α values of FG gels as affected by acids could not be fully explained based upon the pH changes, implying that the effects of acetate, malate, and citrate ions on the gel structure could not be ignored.