| Citric acid is an important organic acid with broad applications in beverage, pharmacy, cosmetics, agriculture and other areas. Currently, the industrial citric acid fermentation was achieved by Aspergillus niger, using starchy food as the substrate. Large amount of food crops is consumed as the carbon source for growth and fermentation in traditional citric acid production. Lignocellulose, such as corn stover, is the most abundant renewable carbohydrate resource. Therefore, the use of the abundant and cheap corn stover can significantly reduce the citric acid production cost. When A. niger is used in the relatively poor lignocellulosic environment, citric acid is not able to accumulate to a high concentration.In this thesis, firstly, three candidate strains A. niger SIIM M203, M288, M228were identified and screened for citric acid fermentation using corn stover feedstock. Then, the ability of the three citric acid fermenting strains in corn stover hydrolysate was compared. The results showed that the strain A. niger SIIM M288is the best one for producing highest citrate acid titer than the other two strains.Secondly, the tolerance of the strain A. niger SIIM M288against inhibitors in corn stover hydrolysate was detected. All the inhibitors negatively affected the citric acid fermentation performance of A. niger SIIM M288. Furfural even inhibited its growth. The result indicates that the pretreated corn stover can only be used after the inhibitors are removed completely (detoxicification).Finally, the fermentation conditions for the strain M288in the detoxicated corn stover hydrolysate prepared at25%(w/w) solids loading were optimized. The optimized condition were33℃, initial pH6.0,1.0g/L NH4Cl concentration, with no Mn2+addition.98.27g/L of citrate titer and77.20%of the conversion yield were obtained under the optimized condition.This study may be helpful for the large scale production of citric acid from corn stover in the future. |
