| Kojic acid is a secondary metabolite produced by some aerobic microorganisms with sugars. Given its antimicrobial activity, antioxidant activity and inhibiting tyrosinase, the acid is used in preservatives, pesticides, and antibacterial agents. Therefore, kojic acid is widely applied in medicine, agriculture, food, cosmetic and many other fields.In this study, Aspergillus oryzae, a kojic acid-producing fungal strain was obtained. Effects of mycelium morphology and culture conditions on kojic acid production were studied. The optimization experiment was conducted in a 5 L fermentor, and the types and concentrations of by-product organic acids produced during kojic acid fermentation were analyzed. Finally, the extraction process of kojic acid was optimized. The main results are as follows:（1） A high kojic acid-producing fungal strain was screened from soil. And a mutant, named A. oryzae QN-21, was obtained and kojic acid production was improved by 47.11% compared with the wild type strain. In addition, the mutant had a good genetic stability.（2） Effects of different culture conditions on mycelium morphology were studied. Furthermore, the relationship between mycelium morphology and the kojic acid yield was investigated. The concentration of carbon and nitrogen sources and inoculation size exerted significant effects on the mycelium morphology of A.oryzae QN-21. Results indicated that kojic acid yield was closely correlated with pellet size, and the optimal pellet diameter range for higher yield of kojic acid per dry cell weight was 0.25 to 0.35 mm（0.78 g·g-1）.（3） The process conditions of kojic acid production through A. oryzae QN-21 fermentation were studied in terms of the following aspects: medium and culture conditions. Based on the single factor experiment and orthogonal test, the optimal conditions for acid production were glucose 240 g·L-1, yeast extract 5.0 g·L-1, bean cake powder 4.0 g·L-1, KH2PO4 2.0 g·L-1, MgSO4·7H2O 0.5 g·L-1, initial pH 6.0, fermentation temperature 30℃, and inoculation size 8%. About 37.27 g·L-1 kojic acid may be produced after optimization.（4） Experiments on initial glucose concentration, stirring rate, and feeding strategies were conducted in a 5 L fermentor, and the relationship between factors and kojic acid production was analyzed. The optimal fermentation conditions were 120 g·L-1 and 700 r·min-1. When the remaining concentration in the reactor was reduced to 10 g·L-1, it required feeding glucose to maintain the concentration glucose at 10 g·L-1. The total amount of glucose was 140 g per liter of initial fermentation. Under the optimized conditions, 69.77 g·L-1 kojic acid production was achieved.（5） By-product organic acids produced during kojic acid fermentation were investigated using the HPLC method. Fumaric acid and pyruvic acid were mainly by-product organic acids. But their concentrations were low in the fermentation broth. A. oryzae QN-21 was then demonstrated to be an excellent strain for producing kojic acid.（6） Single factor and orthogonal experiments were designed to optimize the conditions for concentration of fermentation broth, cation exchange column decoloration and active carbon decoloration. The optimal extraction conditions were as follows: concentrating 3 times of fermentation broth, decoloring flow rate 4.5 mL·min-1, 6% activated carbon and temperature 80°C, pH 3.5, decoloring time 45 min. Based on the optimum conditions, the recovery rate, decolorization rate and the purity of kojic acid reached 76.63%, 95.15% and 98.34%, respectively. |
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