Dynamic rheology of structure development in low-methoxyl pectin plus Ca2+ plus sugar gels

Storage (G') moduli of 1% low-methoxyl (LM) pectin containing different amount (%) of sugars (S): sucrose, glucose, fructose, and sorbitol, and % of CaCl2 (Ca2+, %) (0%S and 10%S-Ca2+ 0.10, 0.15, 0.20; 20%S-Ca2+ 0.10, 0.15, and 30%S-Ca2+ 0.10), 0.1 N NaCl, pH 4, were higher than (G ") over 16 h periods. Based on G' values, gels could be ranked as: sucrose > glucose > fructose > plain > sorbitol. G' values of gels were recorded at the same time intervals so that the influence of sugars and calcium concentrations on them could be examined quantitatively in terms of differences in G' values (Delta G'). At 0.1% Ca2+, Delta G(s)' values due 10% sugar showed major changes during the first 40 ks after which the differences were relatively constant. Negative values of Delta G(s)' were observed for gels containing sorbitol and fructose throughout the 16 h, with sorbitol gels showing more negative values than fructose gels. All the 10% sugar gels showed positive values of Delta G(c)' when calcium content was increased from 0.10 to 0.15% and 0.15 to 0.20%; however, at Ca2+ 0.20%, there was syneresis in sucrose gels. In general, increasing calcium content at a fixed sugar content increased G' more than when sugar was increased at a fixed calcium level. The apparent first-order structure development rate constant of gelation and initial structure development rate (SDR = dG'/dt) showed similar trends with respect to type of sugar. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.