Effect of pH, sugar type and thermal annealing on high-methoxy pectin gels

Abstract

The effect of cosolutes on the formation and properties of high-methoxy pectin gels (pectin concentration 0.5 wt%; DE 70.3) has been investigated by small-deformation oscillatory measurements of storage and loss moduli ( G′ and G″) and by compression testing at 5°C. Solutions were prepared at ∼95°C and pH was varied by addition of citric acid. With 65 wt% sucrose as cosolute, a critically crosslinked network was formed on cooling to 5°C at pH 4.7. The changes in moduli observed for this composition during cooling were fully reversible on heating, with no thermal hysteresis. Progressive reduction in pH to below the p K a of the galacturonic acid residues caused a sharp, sigmoidal increase in moduli at 5°C, attributed to association of pectin chains into a pectinic acid network. On heating to ∼15°C, the gel moduli decreased, following the same temperature-course as the changes observed on cooling, but further increase in temperature caused large increases in moduli, consistent with hydrophobic association of methyl ester substituents. These increases became more pronounced as the strength of the gel structure at low temperature was decreased by reduction in sucrose concentration or increase in pH. Heating to 95°C and re-cooling caused a substantial increase in gel strength ( G′ from small-deformation measurements and breaking stress under compression), which is attributed to segregation of chain sequences of high and low ester content during thermal cycling, with the less highly esterified sequences giving stronger pectinic acid junctions at 5°C. Replacement of sucrose by glucose or fructose caused large changes in gelation temperature, in the order: fructose<sucrose<glucose. The departure from the normal order of effectiveness (fructose<glucose<sucrose) anticipated from compatibility with water structure and observed experimentally for the same sugars in combination with other biopolymers is attributed to inhibition of intermolecular association by strong hydrogen-bonding of primary alcohol groups on the sugars (2 per residue in fructofuranose, 1.5 per residue in sucrose, and 1 per residue in glucopyranose) to the carboxylic acid and methyl ester groups of pectin.