Nisin Fermentation Using Wheat Straw As Raw Material By Biorefinery

As a kind of microbial antimicrobial peptides,Nisin which is a class of small molecule cationic hydrophobic polypeptide compound produced by fermentation of Streptococcus lactis or Lactococcus lactis.It was considered to be the safest green food preservatives,because of it with non-toxic,no drug resistance.In recent years,it is expected to be as a substitute for antibiotics in the pharmaceutical industry.As an inexhaustible,infinitely renewable biomass resources,wheat straw with wide source,low price and renewable could cause serious environmental pollution in misconduct.A variety of valuable products can be obtained by biorefining of biomass raw materials.In thesis,various factors of hydrolysis and fermentation of wheat straw were studied.The main research contents and experimental results were as follows:(1)The factors affecting the pretreatment of wheat straw were explored,and the conditions of its were optimized by orthogonal experiment.The experimental results showed that the optimum conditions were temperature of 50?,ratio of solid to liquid 1:40,alkali concentration 3.5%,reaction time 30 h,the yield of reducing sugar was 34.35%.The effect of alkali concentration and time on the pretreatment was significant,most of the lignin and a small part of the hemicellulose in the straw can removed by scanning electron microscopy,X-ray diffraction and infrared spectroscopy,so that the cellulose inflated result in cracks and fissures on the surface of wheat straw,the contact area between the enzyme and the cellulose is expanded to promoting the enzymatic reaction.(2)Based on the single factor experimental,the response surface model was constructed by the Box-Behnken experiments to optimize conditions of enzymolysis.The experimental results showed that the addition amount of composite enzyme was 32.5 mg/g,the reaction time was 23.87 h,the pH value was 4.57,the temperature was 50.9? and the ratio of material to liquid was 1:28.6,under this condition,reducing sugar yield to reach 51.22%,and the reducing sugar content 37 g/L.Accuracy rate of the response surface modelreach 91.24% by experimental verification.(3)The types and addition amount of components in wheat straw hydrolyzate medium were studied and the fermentation conditions of Nisin by Streptococcus thermophilus 6032 were optimized by orthogonal experiment.The results showed that the addition of corn syrup had a significant effect on nisin production,and yeast extract of 13 g/L,dipotassium hydrogen phosphate 4.5 g/L,sodium acetate 5 g/L,diammonium citrate 2 g/L,magnesium sulfate 0.58 g/L,manganese sulfate 0.24 g/L,corn steep liquor 32%,reducing sugar content 37g/L of wheat straw hydrolysate,the fermentation titer unit of the Nisin reached5752 IU/mL.(4)Based on the single factor experiment,fermentation conditions were optimized using analysis of the response surface.The experiment results showed that the pH value of the fermentation broth is the key factor for the production of nisin in fermentation,the effect of rotational speed on the production of nisin is the second,and the effect of temperature and initial pH on fermentation is small,The optimal fermentation conditions were as follows: the initial pH 7,the temperature of 30?,the pH value of fermentation broth was 5.6,and the speed of 49 r/min,the titer unit of Nisin could reached 6172 IU/mL.The response surface model is verified by experiments,and the recurrence rate was 96.36%.