Effect of konjac glucomannan deacetylation on the properties of gels formed from mixtures of kappa carrageenan and konjac glucomannan

Abstract

Mixtures of konjac glucomannan and kappa carrageeean at temperatures above and below the melting point of the kappa carrageenan helix were studied by rheometry and calorimetry. The objective of the work was to investigate the effect of alkali deacetylation of the konjac glucomannan on the properties of the mixed gels. It was shown that the gel storage modulus, at temperatures above the helix coil transition for kappa carrageenan, increases with increasing deacetylation of the konjac glucomannan. Values were much higher than would be expected for konjac glucomannan in the absence of kappa carrageenan. The ability of kappa carrageenan to promote alkali gel formation of konjac glucomannan was attributed to a segregative interaction between the two biopolymers. It is proposed that the system is single phase but gelation of konjac glucomannan is enhanced since this reduces thermodynamically unfavourable contacts between the two polymers. The coil to helix transition temperature for kappa carrageenan, monitored by calorimetry, generally increased in the presence of konjac glucomannan. An exception was at the highest degree of deacetylation when the major exotherm has a similar peak temperature to kappa carrageenan alone. This may be because at high degrees of deacetylation the konjac glucomannan chains are self-associated to such an extent that they are initially unable to promote the formation of kappa carrageenan helices. The increase in the modulus of mixed gels formed below the kappa carrageenan coil to helix transition temperature also increases with alkali deacetylation. It was postulated that this reflects the increase in strength and extent of these associations with increasing deacetylation. As deacetylation increases the gels become less elastic, as monitored by stress relaxation, which was a further consequence of increasing association.