Browsing by Author "Morris, E. R."
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Item Open Access Effect of cosolutes on polysaccharides gelation.(Cranfield University, 2001-06) Tsoga, Areti K.; Kasapis, S.; Morris, E. R.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 stateItem Open Access Gelation and melting of gelatin.(Cranfield University, 1990-03) Clegg, Stuart Mark; Morris, E. R.Chiroptical, rheological and thermodynamic studies have been undertaken to investigate temperature-induced changes in the ý molecular organisation of gelatin. From the results obtained, a unified model for gelation and melting has been developed, and tested using Monte Carlo computer simulation. The temperature at which gelatin gels are formed has a major influence on the properties of the resulting network, with higher curing temperatures conferring increased thermal stability. In particular, gels formed by sequential curing at two different temperatures show biphasic melting. This is explained in terms of a temperature-dependence of helix length within the junction zones of the gel, and quantified by considering end-effects in the thermodynamics of helix stability. Measurements of 'initial slope' kinetics, performed over a broad concentration range, showed first-order kinetics at low gelatin concentrations, while at higher concentrations a second-order process was also evident. The results are interpreted as triple-helix nucleation at metastable 'hairpin turns' in one chain (bringing two chain segments into close proximity) together with a third strand from either the same chain (first order) or a different chain (second order). From simple geometric considerations, the maximum length of intermolecular helices ( which contribute to the gel network) is greater than that of twasted 9 intramolecular structures, giving a qualitative explanation of the increased strength of gels formed by precuring at higher temperatures (where only long helices are stable) over those quenched directly to low temperature. Monte Carlo simulation incorporating an initial assumption that helix propagation is rapid and proceeds to geometric limits gave unrealistic helix lengths and simulated melting profiles, and was replaced by the assumption that cis-trans isomerisation of peptide bonds is the controlling factor in helix propagation. Using the latter assumption, most aspects of the observed behaviour were successfully reproduced using program variables set within realistic ranges or, where possible, fixed at experimentally-determined values. In particularg the co-operativity of the simulated melting process was critically dependent on the value of a parameter x (the number of triplet units within each helix incapable of participating in bonding, due to end-effects), with a value of x=1 giving the best fits with experiment (consistent with accepted bonding patterns for the collagen triple helix). Other key parameters were the midpoint temperature for melting of the parent collagen, which gave best agreement when set at 37-38"C, and t6e proportion of cis peptide residues present in disordered gelatin chains, with an optimum lower limit of 0.15. Using these values, the simulation reproduced, with excellent precision, the helix fraction and melting profile of gels formed over a wide range of quench temperatures, and gave an acceptable approximation to the form of reaction progress curves obtained for helix formation. The biphasic melting of samples held at intermediate temperature before final quenching was also modelled realistically.Item Open Access Hydrophobicity in polysaccharide gelation(Cranfield University, 1992-11) Jones, Amanda Kay; Morris, E. R.The role of hydrophobic substituents on the gelation mechanism of highly esterified pectin and the cellulose derivatives methylcellulose and hydroxypropylmethylcellulose (HPMC) has been explored by monitoring the behaviour of the amphiphilic polysaccharides in varying combinations of an ethylene glycoVwater solvent. The gelling ability (mechanical spectroscopy, visual observation) of very highly esterified (- 100%) pectin in high concentrations of ethylene glycol (>60%) is greatly reduced, however, the polymer still undergoes conformational ordering (CD, OR). A model for gel formation involving a two stage process has been proposed, comprising adoption of the ordered structure stabilised by hydrogen bonding between OH groups of contiguous polysaccharide chains, followed by (or coincident with) aggregation of the ordered sequences by 'hydrophobic' clustering of the fundamental structural subunits to form the three dimensional gel network. It has been found that ethylene glycol promotes the fIrst stage (ordering) but is antagonistic to the second (aggregation). The reversibility (mechanical spectroscopy) of the thermo-gelling cellulose derivatives can be largely abolished in the presence of ethylene glycol (40% for methylcellulose, 10% for HPMC) , attributed to solubilisation of the proposed ordered 'bundle' structure at low temperatures removing the enthalpic advantage (DSC) of gel melting. The increased sensitivity of HPMC to modification of the solvent environment is due to the presence of the polar hydroxypropyl substituent causing an inceptive destabilisation of the 'bundle' structure. It is suggested that gelation is driven by the entropic advantage of melting-out 'cages' of structured water surrounding the hydrophobic groups giving rise to intermolecular 'hydrophobic' aggregation.