Alpha and beta mechanical dispersions in high sugar/acyl gellan mixtures.
The real (G') and imaginary (G") components of the complex modulus have been measured between 0.1 and 100 rad/s in the temperature range of 70--55 degrees C for a mixture of 1% high acyl gellan with 79% glucose syrup, and 79% glucose syrup. The method of reduced variables gave superposed curves of G' and G" as a function of timescale of measurement, which matched the thermal profiles of shear modulus obtained by scanning at the constant rate of 1 degrees C/min. Data of the gellan/co-solute mixture could be analysed in terms of two distinct mechanisms. For the alpha dispersion, G' and G" superposed with the horizontal reduction factor a(T) whose temperature dependence followed an equation of the Williams-Landel-Ferry form. Mechanical spectra of the beta dispersion also superposed with the factor a(T) whose temperature dependence, however, corresponded to a constant energy of activation. Relaxation spectra have been calculated for both dispersions. Those for the alpha mechanism were attributed to the chain backbone motions and the friction coefficient per tetrasaccharide repeat unit in backbone motion was calculated from the extended Rouse theory. When the contribution of the solvent alone was studied, no spectra for the beta dispersion were observed supporting the hypothesis of the dispersion being attributed to the side-chain motions of the acyl groups. The spectra of the beta mechanism were relatively broader than for the alpha dispersion. The relative location of the beta dispersion on the time scale or temperature range was found to be between the alpha dispersion (glass transition region) and the glassy state.
Kasapis
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Department of Food Science and Nutrition, College of Agriculture, Sultan Qaboos University, PO Box 34, Al-Khod 123, Oman. stefan@squ.edu.om
Al-Marhobi
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IM
Sworn
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D002236:Carbohydrate Conformation; D005947:Glucose / Q000737:chemistry*; D008956:Models, Chemical; D011134:Polysaccharides / Q000737:chemistry*; D011135:Polysaccharides, Bacterial / Q000737:chemistry*; D013696:Temperature; D013816:Thermodynamics; D013997:Time Factors
D011134:Polysaccharides; D011135:Polysaccharides, Bacterial; C048288:gellan gum; D005947:Glucose