Properties of gels from mixed agar and fish gelatin

Agar (A) and fish gelatin (Fg) are acceptable gelling agents in halal food. Agar gel is brittle, has high gelling and melting temperatures and high syneresis, while fish gelatin gel is soft with low gelling and melting temperature, so neither is alone practical. The main objectives of this study were to characterize the properties of mixed gels between fish gelatin and agar, and to understand sol-gel-sol behavior of these mixed gels, in order to provide an alternative gelling material particularly suited for halal food. The agar concentration was fixed at 1% (w/v), while the fish gelatin concentration was varied (0, 5, 10, 15, 20 and 25%, w/v). The key properties of mixed gels were determined: sol-gel-sol transitions during cooling (60 - 0°C) and heating (0 - 100°C) at the gelling temperature (Tgel) and melting temperature (Tm) were determined by oscillatory rheometry. Gel strength, springiness, and syneresis were also measured. It was found that the thermal transitions (sol-gel-sol) of agar gel were at comparatively high temperatures (37°/87°C). For the mixed gels these temperatures decreased dramatically between fish gelatin contents 5% and 10%; at contents over 10% the transition temperatures again increased slightly. Individual transitions of each mixture component were detectable in the storage modulus (G′) profile only for the A:Fg = 1:5 mixture, indicating a bicontinuous gel, with Tgel and Tm closest of all the blends to those of 1% agar. The mixtures with 1:15 to 1: 25 A:Fg had storage modulus trace behavior similar to a single component gelatin during cooling and heating, but the storage modulus values synergistically exceeded those of both fish gelatin and agar. This is a synergistic effect of two biopolymers in this specific mixing range. The Tgel and Tm were shifted toward those of Fg, and this shift should be carefully tuned for each specific application. Compared to agar gel, springiness and syneresis of these co-gels were improved. In these cases with high content of Fg, the fish gelatin acts as the dominant continuous phase, while the 1% agar component is a dispersed phase in the co-gel. Therefore, two modes of behavior were observed depending on the mixing ratio: bicontinuous gels without disperse phase, and gels with continuous and disperse components. In conclusion, the key gelling and gel properties for food applications of fish gelatin and agar mixtures are mostly between the extremes of pure components. Therefore mixtures enable food applications not possible with either component alone, but mixing ratio should be tuned for the specific application. Particularly in applications to halal food products these mixed gels may prove very useful.