Immobilization Of Candida Tropicalis And Its Application In Straw Based Xylitol Fermentation

Biorefinery is a comprehensive use of traditional agricultural waste biomass,such as crop straw biomass,as raw material to produce high value-added products,reusing resources lost in agricultural production.The straw is rich in hemicellulose hydrolysed to release fermentable sugars before being converted into products with both economic and environmental benefits.Xylitol is a high value-added platform compound derived from biomass resources and is widely used in the food and pharmaceutical industries.Traditional catalytic production of xylose by hydrogenation has the disadvantages of being costly and environmentally unfriendly.The use of microbial metabolic processes to produce xylitol is a more economical,environmentally friendly and promising alternative to chemical synthesis.However,the inhibition produced during straw acid hydrolysis can have an inhibitory effect on microbial growth and fermentation,so reducing or removing this inhibition is a critical issue for our research.In addition to the detoxification of the hydrolysate,cell immobilization offers the advantages of both reusability and ease of separation to improve fermentation performance,while immobilized cells are more tolerant,thus reducing the impact of inhibitors in the hydrolysate on fermentation.Therefore immobilization of the cells enables to be a suitable solution.The aim of this study was to investigate the feasibility of immobilizing Pseudomonas tropicalis in sodium alginate/chitosan as a carrier and fermenting xylitol using quinoa straw hydrolysate as a carbon source.The main research is as follows:（1）The quinoa straw is used as a raw material to obtain a lignose-rich hemicellulose hydrolysate by dilute sulphuric acid hydrolysis.The optimum conditions for the hydrolysis of hemicellulose in quinoa straw were determined by optimising the concentration of dilute sulphuric acid,treatment temperature and time,and analysing the sugar and inhibitor production during the hydrolysis process;when the dilute sulphuric acid was 3%for 60 min at 120°C,the xylose content of the hydrolysate could reach 17.83 g/L and the hemicellulose conversion rate reached75.6%.The optimum detoxification conditions were determined by analysing the changes in the content of the main inhibitors in the hydrolysate before and after detoxification.The removal of formic acid,acetic acid,furfural and 5-HMF reached 66.52%,64.54%,88.31%and 89.44%,respectively,at 30°C for 40 min at 3%activated carbon addition.The percentage of decolourisation and xylose loss were 94.9%and 11.47%respectively.（2）The effect of immobilization with sodium alginate and sodium alginate/chitosan on the production of xylitol by Pseudomonas aeruginosa was investigated and the optimum carrier and its immobilization conditions were determined by relative mass transfer rate,mechanical strength,swelling rate and expansion rate.The results showed that xylitol yield of 14.01 g/L could be achieved by mixing 2%sodium alginate,1:2 bacterial gel ratio,with 2%Ca Cl₂for intercalation and then adding to 0.6%chitosan at p H 5.5 for coating and applying it to xylitol fermentation.（3）To examine the tolerance of immobilized Candida tropicalis to the main inhibitors in the saccharification solution.The fermentation and viability of immobilized and free cells were compared by adding different concentrations of formic acid,acetic acid,5-HMF and furfural to the fermentation medium respectively.The results showed that the immobilized cells did little differently from the free bacteria in terms of their ability to survive metabolism in formic acid,but in furfural and 5-HMF,the immobilized cells were better tolerated than the free cells.（4）To explore the xylose utilization and the ability of immobilized cells to ferment xylitol in a fermentation medium using quinoa straw hydrolysate as a carbon source by investigating different medium compositions and fermentation parameters,and to further optimize them through a response surface.The optimum conditions were obtained with an initial xylose concentration of 60 g/L,an inoculum of 9.05%,a nitrogen source of 15 g/L with a combination of ammonium sulphate and yeast powder,a medium p H of 6.01 and a temperature of 29.88°C.The xylitol content was then 19.36 g/L and the immobilized cells were still capable of producing xylitol on the 10th cycle.