| Corn starch is widely used in food, medical products, beverage, paper making, packing, textile and petroleum industries. The purity of starch has a significant influence on its various physicochemical properties and application, so it is important to improve its productivity and purity. Constantly technological improvements are necessary for increasing the yield and purity of commercial starch. But the basis of technological improvements depends on knowing the binding mechanism between starch and protein in corn kernels. At present, a relatively clear binding mechanism is disulfide-bond which links to protein matrix. However other binding mechanisms are rarely reported. Therefore, a part of protein is still difficult to be separated from starch products by current corn starch production process, and there is a portion of starch which cannot be extracted in corn gluten. This indicates that besides disulfide-bond linkage, there are still some other bonding forces between starch and protein that hinder the extraction and separation of starch, of which the mechanism is not elucidated. All of these limit the improvement of the extraction and separation methods of starch and the presentation of new methods. Therefore, it is of significant value to study the combined structure and separation mechanism between starch and protein in endosmpern, which can put the light on increasing productivity and improving purity of corn starch. This article is based on dent corn as material. The separation degree and structure variation of corn endosperm and different wet-milling productions (corn gluten and starch products) treated by different reagents were studied. The adsorption of starch and protein in the process of wet-milling was also studied to analyze the combined structure and separation mechanism in order to provide the theoretical foundation new extraction methods. The major conclusions were listed below:The morphology of corn endosmpern was observed using optical microscope and scanning electron microscope. The micrograph showed that the starch was still tightly wrapped with protein in corn endosmpern after water treatment. According to the composition and characteristic of endosmpern protein, some reagents (sodium hydroxide, sulphurous acid, L-cysteine, ethanol, sour liqulid) were used to soaking the endosmpern. After determining the variation of micro-structure, free-starch content, soluble protein content and combining the mechanism of reagent soaking, it can be inferred that the main reason inducing the difficulties of starch extraction is the disulfide-bond linkage and hydrogen bond in glutelin. In addition, there was still a portion of protein on free-starch granule which couldn’t be separated completely under light microscope and scanning electron microscope observing. This showed that the starch and protein in endosmpern were not only wrapped together, but combined with each other.The Bonding forces leading to the tightly wrapped starch and protein in corn gluten were analyzed by chemical reagents (urea, sodium chloride, pH, SDS, L-cysteine). The content of free-starch was determined and the mechanism of reagent destruction method was analyzed. The results were indicated that the bonding forces leading to the difficulties of separation between starch and protein were electrostatic forces, disulfide-bond and hydrophobic interaction. The variation of micro-structure of corn gluten showed that the proteins of untreated corn gluten gathered as a black and starche granules were wrapped in it. The proteins of corn gluten were influenced a lot and their bonding strength was decreased after breaking these bonding forces. The block was broken into small pieces or fraction and the starch granules were released. The SDS-PAGE results of corn gluten protein indicated that the aggregate of prolamine were broken, the number of 13.5kDa protein subunit of glutelin was increased and 24.5kDa protein subunit was decreased.The surface and internal proteins of starch granules were extracted with different reagents. SDS-PAGE was used to determine the variation of protein content, in order to analyze the combination form of starch and protein. The results showed that the surface proteins of starch particles were combined or absorbed with hydrophobic interaction and electrostatic force while the internal ones with non-covalent bond, including hydrophobic interaction and hydrogen bond. The internal combined proteins could be extracted only after starch gelatinization. The starch granule proteins were also hydrolyzed by protease. The results showed that alkaline proteinase could only hydrolyze the surface proteins while neutrase could hydrolyze both surface and internal proteins.The proteis combined with starch granules had different influences on starch properties. The content of amyloses in starch granules was decreased after the extraction of combined protein. The results of X-ray showed that the crystal of starch was not changed, and the peak intensity of crystallization was increased. The micropores on starch granules surface were bigger and coarser than that of control group and an increasing number of microscopes were observed by scanning electron microscope. The RVA results indicated that the expansion velocity and gelatinization degree were increased. All these results showed that the protein maybe combined with amyloses and fat or absorbed on the surface of starch granules, which can inhibit entry of water during gelatinization, resulting in impediment of starch granules expansion and the decrease of the expansion velocity and degree.The adsorption condition and adsorption isotherm of corn storage protein on starch granules were studied. The optimized adsorption time was 1h. The adsorption volume was increased with heating. With the increase of pH, the adsorption volume increased first and then decreased from pH 3-9. A maximum adsorption was obtained at pH 6. With the increasing ionic strength, the adsorption volume was decreased. There was a significant promotion effect of fructose and mannan on the adsorption. The addition of sour liqulid inhibited protein adsorption. The adsorption volume was lowest at the sour liqulid addition of 50%. The number and the size of the pore on starch granules increased the adsorption volume of protein. In this article, the adsorption isotherm of the corn protein combined with starch granules belonged to II type. From the mechanism of adsorption isotherm and adsorption condition, it can be inferred that the adsorption forms of corn protein combined with starchy granules were electrostatic forces and hydrogen bond. |
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