Abstract
Kojic acid is a highly valuable compound, which is commonly produced by filamentous fungi and has wide applications in several fields due to its special biological functions. Because of its wide applications, there is an urgent need for the production of kojic acid on an industrial scale. This study concerns the development of fermentation process including strain improvement, medium design and fermentation operation for mass kojic acid production Strain improvement, combing induced mutation and protoplasting were applied to improve the wild type strain Aspergillus oryzae ATCC 22788 for high kojic acid production. From the three rounds of strain improvement, we obtained the best kojic acid hyper-production strain M3B9 that could produce kojic acid at level of 32 g/L in shake flask fermentation, which was 64-fold higher than productivity by the wild strain. Besides, wild type and improved strain showed significant differences in terms of byproducts, cell growth, and uptake of carbohydrates. However, gradual decay of specificity indicated that phenotype of M3B9 was not stabile when serially subcultured. After strain improvement, culture medium screening and optimization was carried out for promoting high kojic acid production. Sucrose and yeast extract were found to be the better candidates for use as carbon and nitrogen sources in the medium for high kojic acid productivity, cell morphology and cost. Through the use of response surface methodology (RSM), the optimal sucrose (100~130 g/L) and yeast extract (2~5 g/L) were identified and used in culture medium. The study also focused on the feasibility of large scale kojic acid production by using bioreactor. Similar productivity was shown in stirred tank fermentor as well in shake flask. Batch culture demonstrated that M3B9 has the potential for large scale kojic acid production. In order to improve the efficiency of kojic acid fermentation, fed-batch and repeated-batch cultures were performed. Despite accumulation of high concentration of kojic acid at the end of fed-batch culture, productivity decreased markedly during late fermentation period. Repeated-batch culture with 75 % medium replacement revealed the better productivity similar to batch culture. These results suggested that strain improvement, optimal medium design and suitable fermentation strategies can enhance the kojic acid production and can successfully applied for its industrial scale production .