The effects of concentration and sodium hydroxide on the rheological properties of potato starch gelatinisation

The effects of starch concentration on the gelatinisation of starch in the presence of sodium hydroxide (NaOH) were studied in dispersions containing either fixed NaOH/water ratios or fixed NaOH/starch ratios. Microscopic studies revealed that addition of NaOH to a starch dispersion induced immediate and rapid swelling of the starch granules at room temperature. On heating, granule swelling was more pronounced in the presence of NaOH. At high temperature, the granule remnants were smaller and more disperse in the presence of NaOH. Dynamic rheological measurements revealed a change in gel properties as the ratio of NaOH/water was increased. The storage modulus (G′) showed a c2-concentration dependence (where c is the wt% of starch as a percentage of the total sample weight), at high starch concentrations, in the absence of NaOH. As the NaOH/water ratio was increased, G′ decreased for equivalent starch concentrations, probably due to the combination of effects of the size and strength of the filler particles, the influence of ionic strength on solvent quality and charge screening, and the hydrolytic effect of NaOH on the starch chains. In the presence of NaOH and at high starch concentrations, deviations from the c2-dependence were observed. These may be linked to incomplete chemical gelatinisation by the NaOH. In dispersions with a fixed NaOH/starch ratio of 5:95 measurements of the steady shear (η) and complex viscosities (η∗) were fitted to power law equations and the fitting parameters were then plotted versus starch concentration, c. These data can then be used in the prediction of the viscosity of starch dispersions.