PUMPKIN PECTIN - GEL FORMATION AT UNUSUALLY LOW CONCENTRATION

The gel properties of high-methoxy pectin from pumpkins have been investigated to assess the potential of this material as a hard-currency export from the former Soviet Union. Comparison was made with commercial slow-set, medium-set and rapid-set pectins from citrus peel. Gels were 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-temperature course of network formation was monitored by small-deformation oscillatory measurements of storage modulus (G'). At concentrations above approximately 1% (w/w) the pumpkin pectin gave weaker gels than the other three samples, but its minimum critical gelling concentration (c0) was found to be much lower (by at least a factor of five). Compression 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. Its gelation was also less rapid, giving G' values below those of the other 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-degrees-C, with systematic reduction in gel-like character with increasing ester content in the other samples. The rigidity of the final gels also decreased systematically through the series: rapid-set < medium-set < slow-set. These observations are tentatively ascribed to stable association of unesterified galacturonate chain segments at low pH, where electrostatic repulsion is suppressed.