what are the structural changes in potato starch when we add sodium hydroxide (5-10 mM) with heating between 50-90C. I am not getting any reference regarding this.
If we added sodium hydroxide, starch would be swell but it is reversible. In high concentration of sodium hydroxide, it would be gelatinized. Starch would be gelatinized to if heated above the temperature of gelatinization.
the function of sodium hydroxide in your question are more like an accelerator for gelatinization
I will discuss your question at (1) the granular level and (2) the molecular level.
(1) Granular level. Potato starch granules start to be gelatinized in excess water at ca 60 C. In moderately concentrated aqueous alkali (0.1-1M) this process already occurs at room temperature. So it is expected that addition of small amounts of alkali enhances the effects of heating in water. I am not aware of any research on this topic, so the best thing is to perform some simple experiments on your own samples and evaluate the treated systems by microscopy and viscosity measurements. A basic question is whether true molecular solutions are obtained by such treatment. This can be estimated by a high speed centrifugation test (> 10,000 g) and measuring the supernatant carbohydrate content. If some molecular degradation is allowed, we prepared molecular potato starch solutions in a Waring Blendor by adding a calculated amount of alkali to the agitated aqueous starch suspension and diluting it with water (or acid) as soon as possible.
(2) Molecular level. It has been reported that alkali may cause oxidative degradation resulting in a decrease of molecular weight. [Molecular degradation may affect also granule structure.] The older literature has been summarized in W. Banks, C.T. Greenwood: Starch and its Components, Edinburgh University Press (1975). I am not aware of any more recent studies on this topic. Most work relates to moderate alkali concentrations (0.1-1M). It is expected that oxidative processes are retarded at lower alkali concentration and promoted at higher temperature. In order to prevent molecular degradation, Banks and Greenwood recommend to work close to 0 C and under a nitrogen atmosphere when preparing alkaline amylose solutions.
Dear Deependra,
Native starch start to gelatinize at about 60C (potato starch), It's mean that granules become swollen due to break intramolecular hydrogen bonds due to interaction with water. When NaOH is added hydroxyl group (Starch-OH) transfer into Starch-O-Na+ (and additionally if the concentration of sodium cation is quite high there can occur repulsion of the same charge faciliting swelling or dissolution), leading to decrease of gelatinization temperature. Meybe this article will be useful: http://www.sciencedirect.com/science/article/pii/S0144861702000073
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