Stabilization of gelatin gels by sugars and polyols

Abstract

Gelatin gels are known to be stabilized by sugars and polyols. We have made a quantitative study of this effect with the aim of determining the molecular mechanisms involved. The sugars and polyols added were sucrose, glucose, fructose, sorbitol, glycerol and ethylene glycol, at concentrations within the range 0 – 500 g/kg. With the exception of ethylene glycol, their effects were almost identical — at 400 g/kg of additive the shear modulus of the gels increased by a factor of 2.5 and the rate of gelation increased by a factor of 2. Ethylene glycol produced smaller changes. Sucrose at 500 g/kg increased the melting temperature by 3.9°C. The increased rate of gelation could be simply explained as due to increased melting temperature. The junction zones in gelatin gels are regions of collagen triple helix structure and added sugar or polyol increased the proportion of protein reverting to this conformation. The contribution per amino acid residue to the free energy of formation of junction zones changed from –0.34 kJ/mol in water to –3.44 kJ/mol with sucrose at 500 g/kg. The junction zones became smaller (averaging 15 turns of triple helix in water and 0.8 turns in sucrose at 500 g/kg) but at the same time more numerous. This results in a more extensive gel network and consequently greater rigidity. Stabilization appears to be due to preferential hydration of the unfolded polypeptide in the presence of sugar or polyol — rather than a direct effect of the additive on the intermolecular forces stabilizing the collagen triple helix.