The effect of K+, Ca2+ and their mixtures on low acyl gellan gels

V Evageliou1, M Karantoni1, I Mandala1 and M Komaitis1

1Department of Food Science and Technology, Agricultural University of Athens, 75 Iera Odos, 11855, Athens, Greece

Low acyl gellan is a water soluble polysaccharide which forms hard and brittle gels in the presence of cations. The three dimensional network is developed upon cooling, where three fold double-helix structures are formed and which are further aggregated through the cations Monovalent cations contribute in indirect double helices interactions whereas divalent cations in direct interactions. In the present study, the effect of a monovalent (K+), a divalent (Ca2+) cation and their mixtures on the properties of low acyl gellan gels was investigated by large deformation compression experiments. Gellan concentration was kept constant at 0.5 wt% whereas that of the salts varied from 20-80 mM for KCl and 5-80 mM for CaCl2. The effect of mixtures of the two salts at a total concentration of 80 mM was also explored. Mixtures of KCl: CaCl2 were prepared at molar ratios of 20:60, 40:40 and 60:20. Samples prior to compression were refrigerated at 5°C for 24 hrs. Young’s modulus, true stress at failure and true strain at failure were the parameters calculated from the compression curves. Stress and strain at the break point are indicative of the network’s strength and elasticity, respectively, and Young’s modulus is related to the network firmness. For a given gellan concentration, the gel strength increases with cation concentration up to a certain value (critical cation level) above which the strength is decreasing. In our study, this behaviour was seen in its total for potassium whereas calcium exhibited only the descending part of the curve, indicating that the maximum network strength is achieved at a concentration of 5 mM or lower. The corresponding critical cation concentration for potassium was 80 mM. For both cations, firmness shared similar profile as strength. Experimental data of stress at the break point and Young’s modulus over concentration, for gels prepared in the presence of calcium, were fitted with an exponential decay model, presenting high regression coefficients. The values of strain did not, generally, exhibit great differences over concentration, for both cations studied. In the case of mixtures, the replacement of CaCl2 with KCl had no significant effect on the strength of the network, arguing that the divalent cation defines the final strength. However, the monovalent cation determined the remaining properties as its progressive addition led to a progressive increase of both elasticity and firmness.This behaviour was attributed to the replacement of direct with indirect interactions in the double helices of gellan.