Fish Gelatin from Cold Water Fish Species: Physical and rheological characterisation of fish gelatin and mixtures of fish gelatin and kappa-carrageenan
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This study comprises physical and rheological characterisation of fish gelatin (FG) from cold water fish species, and the mixtures of fish gelatin and kappa-carrageenan (CG). The low content of proline and hydroxyproline in fish gelatin gives gels with a considerably lower storage modulus, G’, and gelling and melting temperature compared to mammalian gelatin. The degree of regenerated “collagen structure” at the gelling temperature of fish gelatin and mammalian gelatin was, however, found to be almost identical. This clearly shows the correlation between the content of imino acids and the coil-helix transition temperature. The modulus of the FG gels was influenced by variations in ionic strength and pH, suggesting that electrostatic interactions are important in the stabilisation of the gelatin gel network. Fish gelatin is a possible alternative to, but not directly exchangeable with, mammalian gelatin, due to the lower gel modulus, and gelling and melting temperature. A possible approach to overcome these differences could be to mix fish gelatin and marine polysaccharides, like kappa-carrageenan, leading to systems with improved gel modulus, gelling and melting temperature. Mixtures of 1 % CG and 2 % FG gave turbid solutions and gels. At 60°C the turbidity is caused by the formation of insoluble complexes of FG and CG. The turbidity in the solidified state, on the other hand, is caused by the formation of a bi-continuous structure at the gelling temperature of CG, and by kinetically trapped complexes of FG and CG. At room temperature, the mixture of 1% CG and 2% FG with the addition of 20 mM KCl increases the gel modulus compared to pure CG gels. This is probably caused by an excluded volume effect, mainly due to the formation of the bi-continuous structure. Lowering the temperature to 4°C leads to a ripening of the bi-continuous structure and the trapped complexes, and to an improvement of gel properties. The increase in gel modulus at 4°C is probably caused by the contribution from the gelling of FG in the complexes and an excluded volume effect. The gelling and melting temperatures of the mixtures are slightly higher than those of pure CG. Mixtures of FG and CG gave solutions and gels with varying degree of turbidity, gel modulus, and gelling and melting temperature, depending on the concentration of biopolymers, pH, ionic strength and the nature of the added salt.