Yam bean starch: a novel substrate for citric acid production by the protease-negative mutant strain of Aspergillus niger

Abstract

Selection of protease-negative mutant strains of Aspergillus niger in semi-solid culture was carried out in order to enhance citric acid production from yam bean. The protease-negative mutants were obtained by UV-irradiation of the parental strain Yang no. 2. Using a halo-selection medium, a number of mutants with decreased extracellular protease activity were selected. Citric acid productivity by the selected mutant strains was tested on a modified starch–methyl red agar plate. The best mutant strain YW-112 was obtained and produced 106 g/l of citric acid, whereas the parental strain Yang no. 2 produced 58 g/l, from 140 g/l of soluble starch in semi-solid culture at 5 days of cultivation time. During the whole period of cultivation with soluble starch, the extracellular glucoamylase activity of YW-112 was higher than that of the parental strain Yang no. 2. In addition, when 17 g/plate of rod-shaped yam bean was used instead of soluble starch, the protease-negative mutant strain YW-112 produced 490 g/l of citric acid, which is approximately 1.5 times as much citric acid as Yang no. 2 produced and showed enhanced extracellular glucoamylase production.