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

The effect of cosolutes on the formation and properties of high-methoxy pec tin gels (pectin concentration 0.5 wt%; DE 70.3) has been investigated by s mall-deformation oscillatory measurements of storage and loss moduli (G' an d G ") and by compression testing at 5 degrees C. Solutions were prepared a t similar to 95 degrees C and pH was varied by addition of citric acid. Wit h 65 wt% sucrose as cosolute, a critically crosslinked network was formed o n cooling to 5 degrees C at pH 4.7. The changes in moduli observed for this composition during cooling were fully reversible on heating, with no therm al hysteresis. Progressive reduction in pH to below the pK(a) of the galact uronic acid residues caused a sharp, sigmoidal increase in moduli at 5 degr ees C, attributed to association of pectin chains into a pectinic acid netw ork. On heating to similar to 15 degrees C, the gel moduli decreased, follo wing the same temperature-course as the changes observed on cooling, but fu rther 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 temperat ure was decreased by reduction in sucrose concentration or increase in pH. Heating to 95 degrees 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 hig h and low ester content during thermal cycling, with the less highly esteri fied sequences giving stronger pectinic acid junctions at 5 degrees C. Repl acement 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) anticipat ed from compatibility with water structure and observed experimentally for the same sugars in combination with other biopolymers is attributed to inhi bition of intermolecular association by strong hydrogen-bonding of primary alcohol groups on the sugars (2 per residue in fructofuranose, 1.5 per resi due in sucrose, and 1 per residue in glucopyranose) to the carboxylic acid and methyl ester groups of pectin. (C) 2000 Elsevier Science Ltd. All right s reserved.