Calcium gelation of pectic polysaccharides isolated from unripe tomato fruit

Cell-wall material was prepared from unripe tomato fruit, and a pectic polysaccharide extracted with cyclohexanediaminetetraacetic acid. The structure of the purified pectic polysaccharide was examined by sugar and methylation analysis, and was typical of a rhamnogalacturonan from the primary cell wall. The physicochemical properties of the isolated polysaccharide were characterised by viscometry and size-exclusion chromatography. The polysaccharide was polydisperse, but of large molecular size as indicated by an intrinsic viscosity of 810 ml. g−1. At concentrations above ∼ 0.2–0.6% w/w, coil entanglement was observed as an increase in the dependency of viscosity on concentration. For these concentrated solutions, clear elastic gels were formed on addition of calcium ions. At concentrations in the range 0.6–2.8% w/w the shear modulus of the gel showed a c1.9 dependence on concentration. The modulus of the gel increased linearly with absolute temperature in a rubberlike way, enabling an estimate of cross-link density to be made.