| The demand of petroleum in the world is increasing daily due to the technological development and modernized life. Thus, the pressure of petroleum resource shortage on our life is so heavy that finding a substitute is very important. Starch is of a rich source, short harvest period, renewable, cheap and environmental friendly material. As an alternative to petrochemical products of the industrial raw materials, starch shows a broad prospect. At present, the researches of modified starch are carried out actively both at home and on abroad in order to explore its scope of application. For instance, starch was oxidized under certain conditions to become available for introducing some new functional groups, resulting in its improved performance. Now oxidized starch has been widely used in the fields of food, textile, paper manufactures. It can also complex calcium, magnesium and other metal ion to soften hard-water. It is significant to develop its phosphate detergent substitute to replace the STPP for reducing phosphorus in the water and diminishing water contaminants. However, starch granule usually has a crystalline structure with a tight crystalline outer layer and an amorphous inner core, generating a resistance for its combining water and other polar chemical reagents. Therefore, to meet the requirements of some specific applications, it is an extremely important research topic to develop an appropriate pretreatment method for changing starch granule structure and destroying starch crystallization in terms of obtaining a desirable oxidized product by improving its chemical reaction efficiency.In this paper, starch was first mechanically activated with a customized stirring-type ball mill, then, an oxidized starch was produced with hydrogen peroxide as the oxidant and copper sulfate as a catalyst by a dry method. Using the content of carboxyl as an evaluating parameter, the performance of the oxidized starch was evaluated by its content of carboxyl which was influenced by activation time, reaction time and temperature, amount of catalyst, oxidant concentration, pH, and water content. An orthogonal experiment was designed for optimizing the process of starch oxidation process. The structure of oxidized starch was characterized and analyzed by X-ray diffractormetry, differential scanning calorimetry, scanning election microscopy, fourier transform infrared spectroscopy and so on. The solubility, diaphaneity, freeze-thaw stabilization, retrogradation, hard-water softening ability and suspension property of the oxidized starch were also preliminarily investigated. Based upon the experimental results, some major conclusions can be drawn as follows:(1) Under the best optimize orthogonal experiment condition, for non-activated cassava and maize, the content of carboxyl was 0.27% and 0.25%, respectively, for 60 min-activated cassava and maize was 0.89% and 1.13%, respectively. The results showed that the mechanical activation pretreatment obviously enhanced the chemical reactivity of starch and the oxidation effectiveness from damaging the particle surface and the crystal structures of cassava, maize starch, reducing its crystallinity degree. Therefore, the reaction reagent more easily penetrated into its particles to oxidize starch.(2) The oxidation reaction of untreated starch mostly occurred in amorphous regions. After 60 min mechanical activation, both untreated starch and oxidized starch obtained an amorphous structure. However, the thermal stability of oxidized starch after activated was better with increasing of the carboxyl content.(3) The solubility, transparence, and freeze-thaw stability of oxidized starch were better than those of untreated and activated starches but retrogradation. The more content of carboxyl the oxidized starch has, the stronger calcium-complexing capability and fouling-suspension capability it shows. If content of carboxyl is the same, oxidized starch made from activated starch were stronger than the untreated starch.
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