Study On Cadmium Reduction Mechanism Of Rice Lactobacillus Fermentation And Its Application

Cadmium(Cd)is a kind of highly toxic heavy metal and one of the most easily accumulated poisons in the body.It was reported that the rice yield in Hunan province was 30.229 million tons in 2018,of which 2 million tons of early season rice were wasted because of excessive Cd.There was a great negative impact on Hunan's finance and farmers' enthusiasm.Currently,the main methods of rice Cd degrading in the industry are physical milling and chemical acid reduction,while the research and industrial application of Lactobacillus fermentation Cd reduction are rarely reported.In this paper,the technological conditions of eliminating cadmium in rice by Lactobacillus fermentation were studied,the mechanism of eliminating cadmium was discussed,and the effect of rice flour milling technology after eliminating cadmium on the processing quality of rice flour and its industrial application were analysed.The main contents and results are as follows:According to the rice fermentation technology in the rice product manufacturing process,response surface analysis methodology was adopted to test the conditions affecting the fermentation.Taking acidity of fermentation broth and Cd reduction rate of rice as evaluation indexes as well as taking into account the demand for large-scale production in the factory simultaneously,the optimum conditions for reducing Cd by fermentation were determined as water addition 120%,improved strains addition 0.08‰,fermentation temperature 32?.Other fermentation conditions included 2% fermentation broth,0.8% salt and fermentation time 22 h-28 h.The rice with Cd content of 0.6210 mg/kg and Cd content of 0.5380 mg/kg were separately used to verify the effect of fermentation Cd reduction.The Cd content can be reduced to 0.1651 mg/kg and 0.1421 mg/kg respectively,which is far below the national safety standard limit(0.2 mg/kg).To ensure the stable production required for large-scale technology application,the LAB fermentation and Cd reduction can be synergized and stabilized by increasing the nutrient substrate of the fermentation broth.Orthogonal test was utilized to determine the optimum supplementation conditions of nutrient substrates include: 3.5% of the fermentation broth addition,dextrin : sugar = 2:1,and 0.8% of the nutrient substrate addition.The rice with 0.3501 mg/kg Cd content was used for validation test.The optimum reduction rate of Cd was 80.83% in the pilot test and 73.1% in the pilot scale test.According to the distribution of Cd in fermentation broth after Cd reduction,it can be inferred that the Cd ions migrated from rice mainly existed in fermentation turbidity,with the content measured as 0.1578 mg/kg(excluding rice surface attachments),while the content in fermentation broth was only 0.02 mg/kg-0.03 mg/kg.By Fourier infrared spectroscopy,it was detected that the main functional groups for cadmium adsorption were polysaccharides and polypeptides such as R-NO2,-C-C-,-C-O-C-,O=CH-.In addition,the degraded Cd ions can also interact with proteins,which result in the enhancement of protein secondary structure disorder.By studying the effect of single strain on starch gel,it has been presented that M9 and Y11 have the best gelability,and when compounded with M9 : Y11 = 5:5,the surface of starch granules was the most complete.It has also proved that fermentation was more conducive to the protection of starch molecular structure as well as the increase of gel strength,so it could delay starch aging to a certain extent.DNA identification has exhibited that the main dominant strains in rice noodles fermentation broth were 3 kinds of Lactobacillus and 2 kinds of yeasts.Among them,Lactobacillus include: M7(Lactobacillus delbrueckiisubsp.bulgaricus),homology 97%;M8(Lactobacillus plantarum),homology 98%;M9(Lactobacillus fermentu fermentu),homology 97%.The above three Lactobacilli were used for fermentation Cd reduction test,and it was found that the contribution rate of M8 to rice Cd decreased is the highest which reaches 18.7%.The highest reduction rate of Cd could reach 69.1% by using this strain as a strengthening strain of mixed bacterial broth.This has illustrated that there are predominant strains in rice noodles fermentation broth,and this predominant strains have synergistic effect with other microorganism in fermentation broth for degrading Cd.Analysis of the physicochemical properties of rice after turbine crushing has demonstrated that the water content of ordinary early indica rice,high-quality indica rice H-you 518 and Yuchi rice after moistening for 18 hours was about 20%,and the damage starch rate was 2.6%,4.6% and 2.4%,respectively.The water content of de-cadmium rice after fermentation is more than 29% with damage starch rate being only 2.04%,whose starch integrity is the best.Additionally,the regeneration value of rice flour after de-cadmium fermentation was only 27% of that of the same kind of rice.By studying the effect of single strain on starch gel,it has been presented that M9 and Y11 have the best gel ability,and when mixed M9 lactic acid bacteria: Y11 yeast = 5:5,the surface of starch granules is the most complete,which is more conducive to the protection of starch molecular structureConsidering the effects of common mechanical grinding,cyclone grinding,colloidal grinding and ultra-fine grinding on the particle size distribution,broken starch content,starch-iodine-blue value and freeze-thaw stability of rice,the best grinding method of rice with Cd removal were selected.The order of the effect on starch breakage value of rice flour from strong to weak is:(1)Pretreatment method : no pretreatment > moistening rice > fermentation(with substrate)> fermentation(without substrate);(2)Milling methods: ultrafine grinding > ordinary mechanical grinding > cyclone or colloid grinding.After fermentation,rice flour with low starch breakage value can be obtained by cyclone milling,ultrafine grinding and colloidal milling.It was found that the damaged starch value of rice flour obtained by cyclone milling was 2.04%,which was close to that by colloid milling,and was second only to colloid mill(1.6%).The results of sol properties,iodine blue value and freeze-thaw stability of rice flour fermented by early indica rice 2 showed that rice flour with minimum retrogradation value,high gel strength,low iodine blue value and 9 freeze-thaw times could be obtained by fermentation.In conclusion,the results show that the best processing characteristics of rice flour,followed by moistened rice,with low Cd content,low retrogradation value,low starch breakage value,low iodine blue value,high viscosity,high elasticity and high freeze-thaw property can be obtained by colloidal mill and cyclone mill after fermentation of lactic acid bacteria.In the actual production process,according to the safety index of raw rice and the quality requirement of rice products,the different combinations of fermentation,wetting rice,colloid mill and cyclone mill can be reasonably selected to design the product processing scheme with high safety and low energy consumption.