Pectin-sucrose-Ca2+ interactions: effects on rheological properties

The effects of varying concentrations of pectin (4.5-6.5%, w/w), sucrose (4 0-60%, w/w) and calcium (20-60 mg/g pectin) on the viscoelastic properties of pectin dispersions at pH 3.0 were investigated. Pectin samples used were extracted from pomelo fruit peels (Citrus grandis) grown in Malaysia. The dynamic rheological parameters (G ', G ", delta and eta*) of pectin-sucrose -calcium dispersion were determined at 1.5% strain from 90-20 degreesC at a cooling rate of 3 degreesC min(-1). Plots of G ' and G " against frequency (rad s(-1)) showed G " > G ' throughout the frequency range with no occurr ence of crossover for most of the pectin dispersions. In addition both stor age (G ') and loss (G ") moduli of the dispersions increase on cooling. Inc reasing pectin, sucrose and calcium concentrations increased G ' and G " wi th pectin having the greatest effect. Interactions amongst the three factor s were also studied. At lower pectin concentrations, addition of Ca2+ incre ased G ' at all temperatures. This effect was also observed at higher pecti n concentrations at 20 degreesC but not at 90 degreesC. The opposite effect was observed with the addition of sucrose, i.e. addition of sucrose at a h igher pectin concentration increased G ' whereas at a lower pectin concentr ation no effect was observed. Interaction between calcium and sucrose gave rise to an increase in G ' when Ca2+ was added at high sucrose concentratio ns, but a decrease in G ' was evident at low sucrose concentrations. Disper sions of pectin alone or in combination with sucrose exhibited a more liqui d-like behaviour with G " > G '. However, in the presence of Ca2+ mechanica l spectra of G' > G" were obtained. (C) 2001 Elsevier Science Ltd. All righ ts reserved.