The Research Of Fermentation Process Optimization And Properties Of Curdlan By Agrobacterium Sp.

Curdlan is synthesized by Agrobacterium sp.and Rhizobium sp.linked by ?-1,3-D-glucoside.Curdlan can be used as a gelling material to improve the texture,water retention and thermal stability of foods;it is widely used as a food additive in noodles,sauces,frozen foods and packaged meats.Modified curdlan derivatives with anti-tumor,anti-HIV activity has great potential in the pharmaceutical industry.In order to meet the market demand for curdlan,in the process of industrialization we need to screen out high-yielding bacteria,and further optimize its production process to improve its production.Firstly,the high-yield strains were screened out by shake flask,and the metabolic characteristics of high-yield strains were studied.Secondly,the fermentation process of high-yielding strains was optimized,and the properties of curdlan were explored.Finally,the effects of adding polysaccharides on synthesized curdlan were explored.In order to meet the wide applications of curdlan in the food industry and lay the foundation for the industrial production of curdlan.The main results are as follows:?1?Screening strains and studying on the metabolic characteristics of high yield strain in fermentor:The results showed that the yield of 29?strain was improved by 25.08%compared with the control,its gel strength was 873 g/cm² and it is higher than the control strain 815 g/cm².In the 10 L and 50 L fermentors,29#strains were divided into bacteria growth and production period,the first 12 h for bacteria growth period,until 12 h,the bacterial dry weight is 4.7 g/L.At this point the amino nitrogen has been fully consumed,the pH decreased rapidly to about 5.0.After 12 h into the production period,it used sucrose as a substrate to composite curdlan.The pH of the fermentation system was maintained between 5.1-5.3 by CaCO₃,after fermentation for 96 h,the yield of curdlan was 45 g/L.Electron micrographs show that the changes of 29#strains and curdlan morphological structure in the fermentation process:in bacteria growth period,the bacteria were single rod shape,a few bacterias procuced curdlan in 12 h,bacterias were significantly shorter and thicker in 24 h,the vast majority of bacterias produced curdlan in 48 h,the bacterias were tightly wound together,but there are a few bacterias without curdlan around in 96 h.?2?The optimization of curdlan fermentation process and the research of the nature for 29#strain.The fermentation medium ? was the best culture medium with the curlan yield and gel strength as the index.Compared with the control,the activity of ?-1,3-glucanase in the supernatant was increased after adding MnSO₄,the activity of ?-l,3-glucanase in the supernatant was inhibited after adding CoCl₂.FeSO₄ and ZnCl-2 had no significant effect on the activity of ?-1,3-glucanase.By adding MnSO₄,the weight average molecular weight of curdlan was reduced to 833KDa and the lowest gel strength was 708 g/cm2.While the weight average molecular weight increased to 892KDa and the highest gel strength was 902 g/cm2 by adding CoCl₂.Therefore,it was confirmed that the culture medium which 5.0 g/L of CoCl₂ was added alone was the optimum medium in the fermentation medium I.There was a positive correlation between the gel strength and the weight average molecular weight?Mw?,and the correlation coefficient was 0.85.?3?The establishment of the regression model of sensory evaluation and texture analysis:The toughness in sensory evaluation was strong positive correlation,with the three indexes of hardness,cohesion and chewing in TPA test,the relevant models:Y₁=123.574-0.033X₁-282.413X2+0.109X3.The brittleness was strong negative correlation,with the three indexes of hardness,cohesion and chewing in TPA test,the relevant models:Y₂=-159.201+0.044X₁+405.06X2-0.148X3.The results show that the three indexes of hardness,cohesion and chewiness in TPA test can partially replace sensory evaluation to reduce human error and facilitate the standardization of products.?4?The study on water-soluble polysaccharides for curdlan fermentation:The yield of curdlan with 0.1%?0.5%CMC-Na addition and with 0.05%? 0.15 xanthan gum addition was higher than the control.The addition of 0.3%soybean polysaccharide and 0.075%xanthan gum could increase the gel strength of curdlan.The reason is that the weight average molecular weight is also increased respectively from the control 1295KDa to 1682KDa and 1380KDa.The addition of 0.3%soy polysaccharide and xanthan gum after fermentation resulted in an increase in hardness and chewiness compared to the control.By adding CMC-Na,the rehydration and water retention of curdlan was significantly improved compared with the control.The addition of different water-soluble polysaccharide has a certain effect on the glass transition temperature?Tg?of curdlan.The microstructure of the curdlan by adding soluble polysaccharides was analyzed by electronic scanning electron microscopy.The results showed that these polysaccharides had different effects on its structure.