The effect of pH and NaCl content on starch gel ageing. A study by differential scanning calorimetry and rheology
1989
Russell, P.L. | Oliver, G.
Rheological (large deformation Instron measurements) and thermal (differential scanning calorimetry, DSC) techniques have been applied to study the ageing of starch gels (volume fraction of water v1 0.61, 51%) in the range of pH 4.4-9.4 (ionic strength, I less than or equal to 0.1) or salt content O-4.43% (NaCl, I = 0-1.48) for 20 days at 21 degrees C. All gels exhibited increases in 'elastic modulus' and peak stress with storage time, while DSC showed the development of a 'staling endotherm' S, associated with re-ordering of amylopectin. The data were analysed by fitting Avrami equations to each data-set via the method of non-linear least squares. The addition of buffers to control (no-salt) gels significantly affected the kinetics of rheological and thermal changes during ageing, but differences in pH had little further effect. Generally, increased salt concentration reduced the rate constants for development of both rheological parameters during gel ageing. For both buffered and control gels the developing S endotherm (range 41.2-71.4 degrees C) was monophasic. Gels with salt showed an additional peak at higher temperatures whose limiting size was proportional to salt content. AL values and rate constants, for both first peak and the combined endotherm, were inversely related to salt content, indicating that salt inhibits the re-ordering of the amylopectin fraction of starch gels during ageing. The observation of a biphasic endotherm suggests that salt induces the separation of domains within this fraction.
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