Study On Dry Heating Treatment Of Starch And Xanthan/Pectin Blends

Dry heating treatment is heating of starch under dry condition (<10% moisture) at a temperature range from 120 to 200 , thus properties of starch could be changed. As a new method of physical modification by dry process, it has a lot of advantages including no use of chemical reagents, safety, non-pollution, simple process, high yield, etc. It is a promising method for preperation of modified starch.In this thesis, cassava, normal maize and waxy maize starches were used as raw materials, then they were dry heated after blended with xanthan or pection. Pasting properties, morphological properties and crystalline structures of these modified starches were studied, and the reaction mechanism was investigated as well.Xanthan was blended with cassava, normal maize and waxy maize starches respectively, and pectin was blended with cassava starch, then they were dry heated. It was found that dry heating of three kinds of starches blended with xanthan/pectin has the same effect on pasting properties. Dry heating of acidic blend leaded to starch degradation resulting in viscosity reduction; alkaline blend leaded to crosslinking reaction between starch and xanthan/pection, thus starch gelatinization was inhibited. If the pH was more than 11, the strong alkaline condition caused starch gelatinization. Crosslinking degree rised with the increasing of dry heating temperature, time, and concentration of xanthan/pectin. When dry heating temperature was higher than 170 , starch proned to browning. After dry heating at 150 for 16h, crosslinking degree reached the maximum.There is no significant increase of crosslinking degree, when dry heating time continued to incease.Properties of these three kinds of starch dry heated with xanthan/pectin were studied, the results indicated that properties of products were similar to crosslinked starch. Products of cassava, normal maize and waxy maize starches had similar properties, and effect of xanthan and pectin on perperties of products were basically the same. Dry heating with xanthan/pectin did not change the granular shapes of cassava starch, normal maize and waxy maize starches, the surface of modified starches was not as smooth as that of native starches, pits, adhesion and fragments of xanthan/pectin were observed on the surfaces of modified starches. In all modified starches, birefringence decreased after dry heating with xanthan/pectin, birefringence in the center of the granules disappeared, whereas birefringence retained at the periphery. Solubility, swelling power, and paste clarity significantly decreased. Except dry heated maize starch and pectin blend that rised the peak viscosity, peak viscosity of other starches were all reduced; all these products exhibited decrease in breakdown, final viscosity and setback. These indicated that starch and xanthan/pectin were crosslinked, gelatinization of starch was inhibited, while starch became more likely to retrogradate. Cassava starch evidenced more profound changes in paste clarity and viscosity than others, followed by maize starch, then waxy maize starch, indicating cassava starch was more susceptible to dry heating. Crystalline type of all starches and xanthan/pectin blends before and after dry heating showed A type, dry heating did not change the crystalline structure.