Solvency Effects on Biopolymer Interactions in Pectin Gels

Low methoxyl pectin (LMP) is a biopolymer widely used in the food and pharmaceutical industries for its gelling properties mediated via divalent ions. Gel formation of LMP strongly depends on both the chemical characteristics of the polymer and the environmental condi- tions of gel preparation. Unlike high methoxyl pectin, LMP does not require co-solutes to induce gelation, i.e., sugars, allowing the possibil- ity of reducing the calorie content of foods requiring a gel texture. In addition, there is increased interest in LMP as fat substitute in con- fectionery products. Although some work has been done on highly fractionated LMP with Ca2+, virtually no detailed work seems to have been reported on commercial LMP + Ca2+ + high sugar con- centrations, despite the fact that work on fractionated LMP suggests that the presence of sucrose, for example, can have a significant ef- fect on LMP gelation. This thesis couples rheology and dynamic light scattering (DLS) to investigate the process of gelation of LMP samples in presence of a range of different sugars (at 60 wt. % ) in- cluding fructose, D-psicose, sorbitol and invert sugar (1:1 M fructose and glucose). In addition, to compare with the results of previous studies [1] [2], LMP gelation in the presence of 40 wt. % sorbitol was also investigated. Selected systems were also studied via com- plementary static scattering techniques including a novel rheospeckle set-up that allows simultaneous rheology and DLS analysis. Rheolog- ical characterization of both the sol and gel states of LMP prepared in water (i.e., without added sugars) show agreement with studies performed in the past. However, small angle light scattering revealed the presence of abnormally large structures (compared to conventional pectin molecule dimensions) that may coexist with smaller ones de- tected via static light scattering. These large pectin molecular struc- tures may explain some unexpected results obtained for LMP gels prepared in high concentrations of sugars. Thus, although some sug- ars tend to speed up gelation, at certain calcium concentrations the mixture of larger and smaller structures appears to get trapped in a non-equilibrium gel state, that can only be resolved be re-heating (gel melting) and re-cooling. In other words, the normal thermoreversibil- ity of LMP gels is disrupted. After the thermal cycling such gels
increase in strength. However, other sugars seem to disrupt gelation completely so that this effect is not seen. The effects of the different sugars are probably related to different specific sugar-sugar interac- tions that depend on their exact stereochemistry.