Production Of Single Cell Protein From Mixed Sugars Using Two Yeasts

With the increasing shortage of feed protein resources in China,the conversion of lignocellulosic raw materials into single cell protein(SCP)has great application prospects.The hydrolysate of lignocellulosic material mainly contains glucose and xylose.According to the characteristic that the ratio of glucose to xylose in the enzymatic hydrolysate of xylooligosaccharide waste residue was 4:1,SCP was prepared by simulated mixed sugar fermentation.In order to solve the problem of low sugar utilization and conversion rate of yeast in shaking flask fermentation with mixed sugar as carbon source to produce SCP.In this thesis,the optimization of fermentation conditions of Candida utilis in fermenter system was studied.On this basis,the effects of the co-fermentation and step-by-step fermentation of Pichia stipitis and C.utilis,and step-bystep fermentation of Saccharomyces cerevisiae and C.utilis on SCP production were studied.The main results were as follows:(1)On the basis of shaking flask fermentation,the mixed sugar concentration and fermentation conditions of C.utilis were optimized in the 3 L fermenter.The optimum conditions were as follows: 50 g/L of mixed sugar,10 % of inoculum,2 L/(L·min)of ventilation,300 r/min of stirring rate,controlled pH=7.0,and NaOH as neutralizer.The highest dry cell weight of C.utilis reached 18.74 g/L after 96 h of fermentation.Compared with 14.55 g/L after 60 h of shaking flask fermentation,the dry cell weight(DCW)of C.utilis increased by 28.79 %.However,the utilization rate of xylose of C.utilis was low,and the utilization rate of mixed sugar was only 79.11 %.(2)In order to solve the problem of low utilization rate of total sugar,50 g/L of mixed sugar as carbon source,on the basis of SCP production by P.stipitis and C.utilis in shaking flask system,to expand the experiment in the 3 L fermenter system.The results showed that the highest DCW was reached 22.83 g/L and the utilization rate of mixed sugar was reached 100 % after 96 h of two yeasts fermentation with step-by-step in fermenter system.Compared with C.utilis fermentation in fermenter,the highest DCW was increased 21.76 %.The crude protein and crude fat content of the single cell protein were 37.12 % and 9.18 %,respectively,and the total amino acid content reached 32.54 g/kg.In order to further increase the yield of SCP per unit volume,it was necessary to increase the concentration of mixed sugar,and adopt the method of step-by-step fermentation of P.stipitis and C.utilis,which will lead to produce high ethanol concentration by P.stipitis in the first stage.Excessive ethanol concentration will result in inhibition of the fermentation of C.utilis in the second stage.(3)In order to solve the problem that SCP produced by C.utilis fermentation was inhibited under high sugar concentration,on the basis of SCP produced by S.cerevisiae and C.utilis in shaking flask system,to expand the experiment in the 3 L fermenter system.The results showed that 125 g/L of mixed sugar as carbon source,the highest DCW of bacterial could reach 28.19 g/L and 27.68 g/L,respectively under shaking flask system and fermenter system.The crude protein and crude fat content of the prepared single cell protein were 39.74 % and 7.91 %,respectively,and the total amino acid content reached 75.37 g/kg in the fermenter system.