Effects of sucrose, glucose and fructose on the large deformation behaviors of fish skin gelatin gels

Fish gelatin extracted from tilapia (Oreochromis niloticus) skins was used to investigate the large deformation behaviors of gelatin gels (2, 4 and 6.67 g/100 g H2O) under uniaxial compression as affected by sugars (0, 5, 10, 15 and 20 g/100 g H2O) including sucrose, glucose and fructose. The first order Ogden model was applied to understand the mathematical relationship between the true stress and true strain of tilapia skin gelatin (TSG) gels under large compression deformation (stretch ratio λ = 0.35–1). The results indicated that the addition of sucrose, glucose or fructose in a TSG gel led to a decrease in the gel strength, but the extent of this gel-weakening effect depended on the types and concentrations of sugar. The first order Ogden models fitted very well with the compression responses of TSG gels with or without sugar (R2 = 0.9949–0.9998). Significant difference (p < 0.05) in the initial shear modulus (μ) and/or the Ogden constant were found among TSG gels with a specific gelatin concentration but different levels of a sugar. The reduction of the μ value was as much as 11–22% for TSG-sugar gels compared to the corresponding gels without sugar. There were power law relationships between the μ values and gelatin concentrations (μ∝c1.49−1.65) for TSG-sugar gels. This study showed the great fit of the Ogden model for the large deformation responses of gelatin gels and the gel-weakening effects of sugars, which is of significant importance from both theoretical and application aspects.