Incorporation of Nicandra physalodes (Linn.) Gaertn. pectin as a way to improve the textural properties of fish gelatin gels

Abstract

In this study, Nicandra physalodes (Linn.) Gaertn. pectin (NPGP) was incorporated in fish gelatin (FG) solutions to improve the gelling capacity of FG. Mixed FG/NPGP gels were prepared at a fixed FG concentration (5.0%, w/v ) and varying NPGP concentrations (0–1.2%, w/v ) by adopting three fabrication approaches, which aims to regulate the network formation process based on the design principle of mixed food gels. Visual observation showed that all gels exhibited macroscopic phase separation when the NPGP concentration was greater than 0.2%, irrespective of the gel fabrication approaches. The underlying reason was ascribed to the associative electrostatic interaction between NPGP and FG, which can occur below and above the electric point of the FG (pI ∼ 8.8), leading to a much higher turbidity for the mixed solution than the NPGP and FG alone. At low concentrations of NPGP (<0.2%), direct mixing NPGP with FG at natural pH conditions (pH 5.55–5.96) led to the formation of the FG-dominant gel network augmented by associative interaction between NPGP and FG via electrostatic attraction (approach 1), which gave an enhancement of 2 °C in gelation and gel-melting temperatures of the FG, and up to 27% of improvement in gel hardness. In addition, a small amount of calcium was added in the mixed FG/NPGP solutions in the hope of further improving the gelling capacity of the FG (approach 2). It was found that the addition of calcium led to the formation of calcium-mediated NPGP crosslink network, which greatly increased the gel melting temperature by ∼20 °C, and the gel hardness increased by ∼20%. Moreover, in the mixed polymer systems, a complete formation of NPGP-Ca 2+ crosslink network in preference to FG gelation (approach 3) did not significantly improve the gelling properties of FG solutions but further increased the gel-melting temperature due to the higher thermal stability the NPGP-Ca 2+ crosslinks. Therefore, approach 1 was considered to be more feasible to modify the gelling capacity of FG. Eventually, a schematic model was proposed to illustrate the progressive network formation of the mixed FG/NPGP solutions by three approaches. • NPGP addition significantly improved the gelling capacity of fish gelatin (FG). • Higher concentrations of NPGP addition led to associative phase separation. • Mixing NPGP with FG increased the gel hardness of FG by ∼27%, increased the T gel and T melt by ∼2 °C. • Mixed FG/NPGP gels were dominated by FG network structures augmented either by NPGP molecules (approach 1) or by NPGP-Ca 2+ crosslinks (approach 2&3).