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

The effects of varying concentrations of pectin (4.5–6.5%, w/w), sucrose (40–60%, 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″, δ and η*) of pectin–sucrose–calcium dispersion were determined at 1.5% strain from 90–20°C at a cooling rate of 3°C min−1. Plots of G′ and G″ against frequency (rad s−1) showed G″>G′ throughout the frequency range with no occurrence of crossover for most of the pectin dispersions. In addition both storage (G′) and loss (G″) moduli of the dispersions increase on cooling. Increasing pectin, sucrose and calcium concentrations increased G′ and G″ with pectin having the greatest effect. Interactions amongst the three factors were also studied. At lower pectin concentrations, addition of Ca2+ increased G′ at all temperatures. This effect was also observed at higher pectin concentrations at 20°C but not at 90°C. The opposite effect was observed with the addition of sucrose, i.e. addition of sucrose at a higher pectin concentration increased G′ whereas at a lower pectin concentration no effect was observed. Interaction between calcium and sucrose gave rise to an increase in G′ when Ca2+ was added at high sucrose concentrations, but a decrease in G′ was evident at low sucrose concentrations. Dispersions of pectin alone or in combination with sucrose exhibited a more liquid-like behaviour with G″>G′. However, in the presence of Ca2+, mechanical spectra of G′>G″ were obtained.