Optimization of fermentation process of removal of cadmium in rice powder using lactic acid bacteria

Abstract: In recent years, along with our country cadmium pollution in rice is increasingly serious, the safety and quality of rice in China has been under serious threat. Contamination of rice with cadmium also draws great attention because of its serious threat to food safety and its potential harm to human health. Thus, it's very important and necessary for food industry to find a method to decrease the degree of cadmium pollution in polished rice. Cadmium in rice has been demonstrated that it mainly bonded with protein and little cadmium accumulated in starch and other nutritional ingredients. And processing of soaking, heating, washing can reduce the content of cadmium in rice powder, but the effect of cadmium removal is not obvious. Hence, it also was proved that the amount of heavy metals in rice dropped significantly after fermentation using lactic acid bacteria. Therefore, in order to resolve the problem of "excessive cadmium" polished rice utilization, this paper tried to reduce the content of cadmium in "excessive cadmium" polished rice(cadmium content of 0.6479mg/kg) by lactic acid bacteria and produce safe fermented rice products, and the fermentation conditions were studied and optimized.The mixed strains of Lactobacillus plantarum and Pediococcus pentosaceus(2:1,v/v) were selected as the suitable fermentation strain by comparative test. The effects of rice powder size, fermentation time, fermentation temperature and inoculum size of mixed strains on the removal rate of cadmium in polished rice also were evaluated by single-factor experiment. The results showed that fermentation time, fermentation temperature and inoculum size of mixed strains could significantly affect the removal rate of cadmium(P<0.05). However, the effect of rice powder size(20-100 mesh) on the removal rate of cadmium was not significant(P>0.05). Then, fermentation time, fermentation temperature and inoculum size of mixed strains were further researched using a three-variable, three level Box-Benhnken design to optimize the fermentation conditions. The results indicated that the interaction effects of fermentation temperature and fermentation time, fermentation temperature and inoculum size of mixed strains on the removal rate of cadmium were extremely significant (P<0.01). Moreover, the significant degree of three factors was in order as fermentation temperature (A, P<0.01)>fermentation time (B, P<0.01)>inoculum size of mixed strains (C, P<0.01). When rice powder size was selected as 40 mesh, the optimum fermentation conditions were obtained with fermentation temperature of 40.8℃, fermentation time of 23.4 h, and the inoculum size of 3%. In this optimal conditions, the verification test demonstrated that the removal rate of cadmium in polished rice powder was 85.73%, and the content of cadmium in fermented rice noodle was 0.0925 mg/kg and met the national food sanitation standard (0.2 mg/kg). In addition, the verification test showed that there was a good fit between the experimental and the predicted values, relative standard deviation was 0.918. Therefore, the results indicated that this method could efficiently remove cadmium in polished rice and provide a reference for industrial production of fermented rice noodle. The results of research also can provide a theoretical basis for further study on the mechanism on cadmium removal in rice by lactic acid bacteria.