Preparation And Modification Of The Starch From Cadmium-contaminated Rice

Heavy metal cadmium is widely existed in the environment and migrated to the grain through soil and water.Among available cereals,rice absorbs more cadmium than other plants.In this study,cadmium-contaminated rice?cadmium content of 0.38mg/kg?and native rice?cadmium content of 0.013mg/kg?were used as raw materials to extract starch.The physicochemical properties and structure of starch prepared from cadmium-contaminated rice and native rice were analyzed,and the reduction of cadmium content during the procedure was discussed.Due to the limitation of the properties of native starch,the starch extracted from native rice and cadmium-contaminated rice were carboxymethylated respectively.The optimum technological conditions for preparing carboxymethyl starch were obtained by optimizing parameters,and then the physiochemical properties and structure were analyzed.The objective of this research is providing the new ideas and theory for the development and utilization of cadmium-contaminated rice.Firstly,the alkali extraction process of starch in cadmium-contaminated rice was optimized.The extraction rate of rice starch and the degree of cadmium reduction were used as indicators to study the four factors of alkali process,such as reaction temperature,solid-liquid ratio and reaction time,etc.Under the conditions of solid-liquid ratio of 1:11,reaction temperature of 35?,alkali concentration of 0.3%and reaction time of 3 h,the yield rate of starch could reach to 94.66%,and the cadmium content in rice starch was reduced to 0.014 mg/kg.Next,the starch extracted from native rice and cadmium contaminated rice was characterized respectively.Scanning electron microscopy?SEM?was used to observe the structure of rice flour and starch samples.The results showed that the structures of cadmium-contaminated rice starch and native rice starch were irregular polygons with a particle size of 4-7?m.The infrared spectroscopy of samples was performed by Fourier transform infrared spectroscopy?FT-IR?.The results showed that there was no significant difference between cadmium-contaminated rice starch and native rice starch.The X-ray diffraction pattern showed that there were strong peaks appeared at 15°,17°,18°and 23°,which indicated that they were typical type A starch.Differential scanning calorimeter?DSC?was used to examined the gelatinization properties of rice flours and starches,and the results showed that the gelatinization temperature of native rice starch and cadmium-contaminated rice starch was at 70°C.Rice starch was used in various industries such as food,medical and chemical industry for its good gelling and thickening functions.However,for the low water solubility,poor freeze-thaw stability and low gelatinization temperature,the native starch was still limited in many industries.Therefore,the cadmium-contaminated rice starch and native rice starch were modified respectively to improve those functions.Seven factors such as alkalization temperature,alkalization time,etherification time,etherification temperature,water content in solvent,molar ratio of sodium hydroxide to starch molecules(n_NaOH:n_AGU)and the molar ratio of chloroacetic acid to starch(n_MCA:n_AGU)affected the degree of substitution?DS?of carboxymethyl starch?CMS?were analyzed by single factor experiment.The results showed that the optimal conditions were that the alkalization time was 30 min,the alkalization temperature was 35°C,the etherification time was 90 min,the etherification temperature was 30°C,the water content in ethanol was 15%,the n_NaOH:n_AGU was 1.75 and the n_MCA:n_AGU was 1.0.Carboxymethyl starch with the DS of 0.56could be prepared under the optimal conditions.The carboxymethyl starch prepared under the optimal conditions was characterized by analytical apparatus.The infrared spectrum illustrated that the carboxymethyl starch showed a new absorption peak at 1650 cm^-1 and 1157 cm^-1.The SEM patterns showed that the starch structure was destroyed after carboxymethylation.The X-ray diffraction pattern showed that the CMS sample lost crystallinity which indicated that the carboxyl groups were introduced to the rice starch.The physiochemical properties and structure of the carboxymethyl starch from cadmium-contaminated rice and native rice showed no differences.The carboxymethyl starch prepared from cadmium-contaminated rice could replace the native carboxymethyl starch to serve for the food industries.