Effect Of Lactic Acid Bacteria Fermentation And Enzyme Treatment On The Quality Of Rice Starch And It's Product With Excessive Cadmium

Cadmium(Cd)is a toxic heavy metal in nature.Due to the mining of minerals,more and more cadmium is exposed to the fields.Cd can accumulate in human's body through agricultural products,and causes serious damages to our health.Among crops,rice is most likely to accumulate Cd,which combined with protein in rice.At present,Cd pollution still exist in some main rice producing areas,such as Hunan,Jiangxi and Guangxi province.Therefore,it's significant to reduce the excessive Cd content in rice.In this subject,firstly,two kinds of indica rice with different Cd content were used as raw materials.After reducing Cd with lactic acid bacteria fermentation,the changes of structure,physical and chemical properties of rice white starch and rice yellow starch were evaluated.Secondly,the suitable process conditions for extracting rice white starch with acid protease were selected,and the indica rice with cadmium content of 0.73 mg/kg was used as experimental raw material.Then,the extracted rice white starch was used as raw material,and the pullulan enzyme was used to extract rice debranched starch to study the effects of debranching enzymes on removing Cd.The structures and properties of rice-starch were also researched to evaluate the effects of debranching enzymes on the structure and properties of rice debranched starch.Finally,to expand the application of Cd-free rice,food-grade reagents were used to extract debranched starch to study its improving efffect on semi-dry rice noodles.The main conclusions of this study are as follows:(1)To explore the influence of LAB fermentation on reducing Cd in rice starch,the changes in structures,physical and chemical properties of rice starch were researched.The microstructure image of rice starch showed that,more proteins were attached to yellow starch granules.After fermentation,the content of small starch granules increased,and the grooves,small holes and cracks on the surface increased,but the overall structure of rice starch remained unchanged.Analysis of diffraction patterns and crystallinity showed that,there were no significant changes in the crystal form of white and yellow starch in rice,while the crystallinity slightly decreased.Analysis of FTIR showed that,during this fermentation process,neither rice white starch nor rice yellow starch produced new chemical bonds and groups.The results of thermal characteristics indicated that,the gelatinization temperature between rice white starch and rice yellow starch was different.After fermenting,the thermal characteristics of Cd-excessive rice starch were closer to normal indica rice,and it wasn't easy to gelatinize.The analysis of gelatinization characteristics showed that fermentation caused rice white and yellow starch resist gelatinization,increased the starch stability,and slightly improved its eating quality.The solubility and swelling capacities showed that,the solubility and swelling capacities of rice white and yellow starches increased.(2)To explore the effects of protease and debranching enzyme on removing Cd.Firstly,we used rice with a cadmium content of 0.73 mg/kg as raw material.The Cd content and purity of rice white starch were used as index to select the suitable extraction condition:enzyme addition 1%,temperature 55?,liquid-to-material ratio 8:1,pH 3.5,and reacting for 8 h.The extraction rate of rice white starch was 64%-73%,and the rate of Cd reduction was 77.95%.Then,rice white starch with a Cd content of 0.082 mg/kg was used as raw material,and the amylose content of rice debranched starch extracted by pullulan enzyme was used as an evaluation index.The results showed that the suitable process conditions were:pasting 25 min,enzyme addition 30 U/g,debranching temperature 55?,debranching pH 4.5,debranching for 5 h,and starch concentration was 12%.The extraction rate of debranching starch was 20%-40%,and cadmium was undetected after debranching,indicating debranching had significant effects on the Cd reduction.(3)After debranching,the changes of structure and physical and chemical properties of rice starch were explored.The microstructure observation results showed that,the overall volume of rice starch became larger,and a rough and dense structure formed.There were dents,corrosion marks and layered scale structure on the surface of starch granules,and many small shell-like fragments were scattered.The crystallinity analysis showed that,after debranching,the crystal form of rice starch changed from type A to type V,and the crystallinity significantly reduced.The infrared spectroscopy analysis showed that,after rice starch debranching,new chemical bonds and groups were not produced.The results of thermal characteristics showed that,after debranching,the stability of starch granules decreased,and starch was easier to be gelatinized.The gelatinization results showed that,after debranching,the shear resistance of starch became strong,and it was easy to regenerate,gelatinize,and absorb water.The results of rheological properties showed that,after debranching,rice starch formed a dense solid gel,but the shear resistance was weakened,and the viscosity was decreased.The results of solubility and swelling force showed that,after debranching,the solubility of rice starch was increased,but the swelling force was slightly reduced.(4)To explore the improving effect of debranched starch on semi-dry rice noodle.After adding debranched starch to semi-dry rice noodle,cooking experiments showed that,rice-noodle's cooking time was increased,the stripping rate and the cooking loss were reduced,and the cooking properties of semi-dry rice noodle significantly improved.Analysis of texture properties showed that,hardness and chewiness of semi-dry rice noodle were increased,and the viscosity of semi-dry rice noodle was decreased.Analysis of thermal characteristics and gelatinization characteristics showed that,semi-dry rice flour was less easy to gelatinize and regenerate,the viscosity was decreased,and it was more resistant to stretching and shearing.Analysis of rheological properties showed that,when the addition amount of debranched starch was 6%,the elastic modulus and viscosity modulus semi-dry rice noodle were the highest,the shear stress was the biggest,and the apparent viscosity was decreased.The sensory analysis showed that,when addition amount of debranched starch was about 6%,the brightness and whiteness were the highest,and the overall appearance was bright yellow;in the sensory evaluation,the semi-dry rice noodle with debranched starch had a higher sensory score.