Study Of Xylose Utilization From Defatted Rice Bran For Nisin Production By Lactococcus Lactis

Nisin,as a safe and natural preservation,has been widely licensed in food industry,in the future it will have a broader prospects.However,large amounts of expensive refined sugars and nitrogen sources are applied to ensure Lactococcus lactis growth and nisin harvest,which are not economical.This substantially increases the cost of the fermentation process and nisin production.Therefore,the use of agricultural wastes as cheaper substrates for L.lactis fermentation is considered as an important strategy for reducing the production costs.Defatted rice bran(DRB)contains a lot of cellulosic polysaccharide,starchy and soluble proteinsremains which can be chosed and treated as substrates for L.lactis fermentation.To retain the nutrients especially protein fractions in defatted rice bran,thermal pretreatment followed by enzymatic hydrolysis without washing step was applied for saccharification.Thermal-preatment at 120 ℃ for 1 h followed by subsequent enzymatic hydrolysis(cellulose 30 FPU/g,xylanase 100 XU/g for 45 h then mixed glucoamylase 150 IU/g for 3 h)was applied for defatted rice bran saccharification.A maximum of 45.64 g reducing sugar from 100 g defatted rice bran was attained in hydrolysates of defatted rice bran.Xylose is the second most abundant sugar in hydrolysates,which contains 14.38% of total sugar.Results showed that the capacity of L.lactis strains to metabolize xylose is limited.Overexpression of xylR(xylose transcriptional regulator)can improve the consumption of xylose from 4.15 g/L to 10.64 g/L.The results of qRT-PCR indicated that xylose and xylR overexpression had a synergistic effect on acitivating xylose assimilating genes.Adaptive evolution on xylose combined with xyl R overexpression was chosed to further improved xylose utilization,which could consume 1.15 times xylose than strain was screened.To reduce the cost of fermention medium,addition of hydrolysates to replce total carbon source and partical nitrogen source should be optimized.The best sugar concentration in hydrolysates medium was 20 g/L leading to the maximum biomass and proteins in this amount of hydrolysates could compensate for 33.3% peptone.Shake-flask fermentation in hydrolysates medium was carried out by L.lactis FEXR,the maximum nisin titer was 3824.53 IU/ml which was 1.14 times higher than that in commercial fermentation medium and 1.37 times higher than L.lactis F44,1.79 yuan/L of medium was saved.This study showed that defatted rice bran could be used as substitutes for L.lactis fermention and nisin production,which was effective and economical.In addition,the process of defatted rice bran saccharification and hydrolysates medium preparation could be used for the fermentation of other industrial strains.xylR overexpression followed by adaptive evolution was a simple and effective method,which had the potential to be applied in other industrial microorganisms for enhancing xylose utilization.