Effect of cosolutes on polysaccharides gelation.

Abstract

In the first stage of the investigation, the effect of high levels of sugars (mixture of 50% sucrose with 351/'o glucose syrup) on agarose (0.7 w-L °'o) was characterized by low amplitude oscillatory measurements of storage modulus (G'), loss modulus (G") and loss tangent (tan 6" as well as large deformation techniques. Samples were prepared at 90°:, and measured immediately, or after Storage at 5°C. The combined Williams-T andel-Ferry W F; /free volume theory was used to derive the glass transition temperature, the fractional free volume, and the thermal expansion coefficierit of the glass. Solution of high concentrations of sucrose Crystallizes, but addition of the polymer encourages Bitei- olecular interactions, which transfoý the mixture into a high v iscosi Ly glass. The mechanical properties of glucose syrup follow ýýý. iý behavior in the glass transition region and revert to an Arrhenius type prediction in the CrIa-SSy State. feasurements on sugar Samples and aýarose-Sugar ii 1x üres were resolved into a bas iiriCuuu Gi temperature alone and a basic function of equency (time) alone. T ie former traces the energetic cost of vitrification, which increases sharply with decreasing tempera ure. The layer, at long time scales, is goes erned by the mtinite molecular weight of the äcräiGSZ ne ork. In the region of S1 O1 tImes, tie effect of Ze vole ie is active rega dless of In a continuation of, investigating the significance of polymer-cosolute interactions, the effect Gi sucrose, glucose, fructose, sorbitol, hylitol, glycerol and ethan-1,2-dial on gelation of high methoxy pectin was studied under different experimental conditions. The main changes in procedure in comparison with the work on agarose were: (i) the polymer concentration was increased from 0.7 to 1.0 wt °'o, (ii) the mixtures prepared at pH AT and subsequently acidified to pH 3, rather than beinte prepared at neutral pH, (i11) the cosolute concentration was varying from 50 to 65 w IL °'o and (vi) the mixtures were studied through rheology, calorimetry and optical rotation. The samples were prepared at 95°C and changes in storage modulus (G') and loss modulus (G") during cooling to 5°C, heating to 900 and re-cooling to 5°C, at 1°C/min, were measured at 1 rad s"1 and 0.5% strain. In all cases, the onset temperature for gelation during cooling and the moduli recorded at 5'C increased With increasing concentration of cosolute. However, both values were substantially lower for the liquid cosolutes than for mixtures with solid cosolutes at the same concentrations. The difference is attributed to inhibition of pectin-pectin interactions by pectin-cosolute interactions, which in turn are inhibited by cosolute-cosolute interactions. On heating there was an initial reduction in modulus, with the same temperature-course as the increase on cooling; for the solids, this was followed by an increase attributable to hydrophobic association of methyl ester substituents. No SUCH increase was seen with the liquid cosolutes, but DSC studies showed two reversible I. thermal ti ansitions in all cases, one over the temperature-range of the initial gelation process ý nfý r 'A f+ .ýýn on cooling and t he oLlleICoincidenf t with the increase in modulus ulon hea ing in the presence o1- ý solid COSO1UteS. The absence of any detectable increase in modulus on heating with the liquid COSOtUteS is a trlbuted to accumulation of cosolute around tiie polymer chains promoting hydrophobic association between methyl ester groups on the same Chain; Or within Cl sters o chains, v with, therefore no contribution to network strucLarz. At high concentrations of the solid cosolutes, the increase in modulus on heating was followed by a decrease at higher ýempei atur e; this -was attributed to excessive aggregation, and was reflected in lower moduli on subsequent re-coolIný to 5'C, in coast to the enhancement in gel strength after heating aril cooling observed at lower concentration of the same COSül teS. in the presence of fructose as cosolute, calorimetric studies showed an intense endotherm follovýied immediately by an intense exotherm on hea ng. These transitions occurred over approximately the same teMperat'ure-rande as initial gelation on cooling and increased in mag liLude with increasing concentration of the sugar. The displacement of both transitions to progressively hid her temperature as the rate of heating was increased was much Greater than anticipated from a simple thermal lag, indicating that the undelying structural chancres are slow. The proposed interpretation is that fructose is capable of site-binding to pectin in both the ordered and disordered state