PUMPKIN PECTIN - GEL FORMATION AT UNUSUALLY LOW CONCENTRATION

The gel properties of high-methoxy pectin from pumpkins have been inve stigated to assess the potential of this material as a hard-currency e xport from the former Soviet Union. Comparison was made with commercia l slow-set, medium-set and rapid-set pectins from citrus peel. Gels we re formed by cooling pectin solutions (pH 3.0; 60% (w/w) sucrose; 5% ( w/w) corn syrup) from 95-degrees-C to 25-degrees-C, and the time-tempe rature course of network formation was monitored by small-deformation oscillatory measurements of storage modulus (G'). At concentrations ab ove approximately 1% (w/w) the pumpkin pectin gave weaker gels than th e other three samples, but its minimum critical gelling concentration (c0) was found to be much lower (by at least a factor of five). Compre ssion testing gave similar results, with pumpkin pectin giving useful breaking-stress ('hardness') at concentrations down to approximately 0 .5% (w/w), about a factor of two lower than for the citrus samples. It s gelation was also less rapid, giving G' values below those of the ot her three samples at temperatures down to approximately 60-degrees-C, but then setting sharply; this behaviour could be useful in avoiding ' pregelation' in commercial processing. The commercial slow-set pectin showed typical 'weak gel' properties in the solution state at 95-degre es-C, with systematic reduction in gel-like character with increasing ester content in the other samples. The rigidity of the final gels als o decreased systematically through the series: rapid-set < medium-set < slow-set. These observations are tentatively ascribed to stable asso ciation of unesterified galacturonate chain segments at low pH, where electrostatic repulsion is suppressed.