Use this URL to cite or link to this record in EThOS: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.519127
Title: Effect of cosolutes on polysaccharides gelation
Author: Tsoga, Areti K.
ISNI:       0000 0001 3537 2822
Awarding Body: Cranfield University
Current Institution: Cranfield University
Date of Award: 2001
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Abstract:
In the first stage of the investigation, the effect of high levels of sugars (mixture of 50% sucrose with 35% glucose syrup) on agarose (0.7 wt%) was characterized by low amplitude oscillatory measurements of storage modulus (G'), loss modulus (G") and loss tangent (tan δ) as well as large deformation techniques. Samples were prepared at 90°C, and measured immediately, or after storage at 5°C. The combined Williams-Landel-Ferry (WLF)/free volume theory was used to derive the glass transition temperature, the fractional free volume, and the thermal expansion coefficient of the glass. Solution of high concentrations of sucrose crystallizes, but addition of the polymer encourages intermolecular interactions, which transform the mixture into a high viscosity glass. The mechanical properties of glucose syrup follow WLF behavior in the glass transition region and revert to an Arrhenius type prediction in the glassy state. Measurements on sugar samples and agarose-sugar mixtures were resolved into a basic function of temperature alone and a basic function of frequency (time) alone. The former traces the energetic cost of vitrification, which increases sharply with decreasing temperature. The latter, at long time scales, is governed by the infinite molecular weight of the agarose network. In the region of short times, the effect of free volume is active regardless of the sample composition. In a continuation of investigating the significance of polymer-cosolute interactions, the effect of sucrose, glucose, fructose, sorbitol, xylitol, glycerol and ethan-1,2-diol 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%, (ii) the mixtures prepared at pH 4 and subsequently acidified to pH 3, rather than being prepared at neutral pH, (iii) the cosolute concentration was varying from 50 to 65 wt% 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 90° 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 thermal transitions in all cases, one over the temperature-range of the initial gelation process on cooling and the other coincident with the increase in modulus on heating in the presence of solid cosolutes. The absence of any detectable increase in modulus on heating with the liquid cosolutes is atttributed to accumulation of cosolute around the polymer chains promoting hydrophobic association between methyl ester groups on the same chain, or within clusters of chains, with, therefore no contribution to network structure. At high concentrations of the solid cosolutes, the increase in modulus on heating was followed by a decrease at higher temperature; this was attributed to excessive aggregation, and was reflected in lower moduli on subsequent re-cooling to 5°C, in contrast to the enhancement in gel strength after heating and cooling observed at lower concentration of the same cosoutes. In the presence of fructose as cosolute, calorimetric studies showed an intense endotherm followed immediately by an intense exotherm on heating. These transitions occurred over approximately the same temperature-range as initial gelation on cooling and increased in magnitude with increasing concentration of the sugar. The displacement of both transitions to progressively higher temperature as the rate of heating was increased was much greater than anticipated from a simple thermal lag, indicating that the undelying structural changes are slow. The proposed interpretation is that fructose is capable of site-binding to pectin in both the ordered and disordered state.
Supervisor: Kasapis, S. ; Morris, E. R. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.519127  DOI: Not available
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