Gelation of carrageenan: Effects of sugars and polyols

Abstract

The gelation of κ-carrageenan, a common polysaccharide for food gelling and thickening, is enhanced by the addition of sugars and polyols. Three different hypotheses have been proposed to explain this phenomenon: (i) the enhancement of the water structure around polysaccharides, and the concurrent change in polysaccharide hydration, induced by the cosolvent; (ii) exclusion of cosolvents from polysaccharide surfaces; (iii) binding between sugars/polyols and polysaccharides in the gel phase. To examine the above hypotheses, as well as to reveal the true driving forces responsible for gelation enhancement, we applied our recent statistical thermodynamic theory of gelation, derived from the rigorous Kirkwood–Buff theory. The hydration change is shown to be negligibly small relative to cosolvent exclusion and cosolvent-biopolymer binding, which can be rationalized by considering a sol–gel equilibrium in which (1) the exclusion of sugars/polyols from κ-carrageenan's surface in the sol phase, and (2) the binding of sugars and polyols in the gel phase, shift the sol–gel equilibrium of κ-carrageenan to the gel state. This novel picture is consistent with a wealth of experimental evidence and provides a mechanistic insight into how polyol and sugar cosolvents influence the gelation of κ-carrageenan on a molecular level.