Screening Of A Strain Of Degradable Protein In Potato Starch Juice And Analysis Of Fermentation Products

During the production of potato starch,a large amount of juice that containing protein and other organic components will be produced.If discharged directly,the protein in it will be decomposed to produce such as putrescine,putrescine,spermine,indole,sulfide and other odorous substances,which will cause air pollution.In this paper,by fermenting potato juice,it aims to screen a strain which could degrade the protein,and be effectively applied to field irrigation,without producing odor.The main research is as follows:(1)Odor Analysis of the odors of potato juice after natural fermentation:Potato starch juice was naturally fermented at 25?,and the components that might cause odors were determined.The results showed that the substances causing the odor of potato starch juice included cadaverine,putrescine,tryptamine,spermine,histamine,indole,hydrogen sulfide and 3-methylindole,etc.,and the content of the compounds was much higher than its odor threshold.(2)Strain selection and degradation experiment:According to the references,the casein selective medium was made with casein degrading as the index.Through the preliminary screening,50 strains that could degrade caseinstrains were obtained,and then 50 strains were inoculated into potato juiceby re-screening.3 strains were found to be able to degrade the protein in potato well through determination and analysis of volatile components,and no odor such as indole and hydrogen sulfide were detected.Although the presence of cadaverine,putrescine,spermidine and spermine can be detected,the content is very small and the odor of sensory evaluation is extremely low,which does not affect the ambient air.Therefore,these 3 strains were selected as the test bacteria for further fermentation.Finally,the strain which could maintain good fermentation activity at 20? and have a smaller difference with the the odor thresholds was selected as the final bacteria.The strain was further identified as Serratia marcescens and named nxys-s by bacteriology manual,physiological and biochemical identification and 16S rRNA identification.(3)Optimization of fermentation conditions of Serratia marcescens nxys-s:Due to the seasonality of potato starch production,it is mostly in autumn and winter.The temperature was generally low.After investigation,the temperature was about 15?.Therefore,taking the inoculum amount and pH value as variables,the single factor experiment and response surface optimization were carried out The optimal inoculum amount was 2.35%,and the pH value was 5.74.At this time,the final contents of cadaverine,putrescine and spermine wsa 5.41 mg/1,3.28 mg/l and 3.44 mg/1,respectively.Spermidine was not detected,indole,3-methyl and hydrogen sulfide was almost undetected,with the sensory score of 89.41,indicating that the odor acceptance was good.(4)Pilot scale up test of Serratia marcescens nxys-s:Serratia marcescens nxys-s was subjecte to three-stage expansion cultured,and the scale-up experiment was carried out in a 1000 L potato juice fermentor.The temperature was about 15?,the inoculation amount was 2.35%,and the pH value was 5.74.The results show that:in the pilot scale-up experiment,the protein degradation rate was 68.33%,and the remaining small amount of protein would not cause pollution.Only very low levels of cadaverine,putrescine and spermidine were detected,and the content of hydrogen sulfide was lower than its odor threshold.The sensory evaluation score was 80.74,which dose not affect the emission from returning to field.Therefore,Serratia marcescens nxys-s can be used for the fermentation of potato starch juice.