RESUMO
Wild-type Aspergillus niger N402 and glucoamylase++ overproducing transformant A. niger N402[pAB6-10]B1 have grown in maltodextrin- and xylose-limited recycling culture at pH 4.5 on mineral medium. The only products formed were organic acids and proteins, among which glucoamylase. The production of organic acids by the fungus has been analyzed qualitatively and quantitatively using capillary electrophoresis. The only organic acids produced in these cultures were substantial amounts of citric acid. This is the first demonstration of abundant oxalic acid production and a very low citric acid production by submerged cultures of A. niger. In the maltodextrin-limited culture the oxalic acid production rate increased during the first 80 h of cultivation and decreased after that time. In xylose-limited recycling culture the oxalic acid production rate always increased in time and highest values were found in the last samples taken from the culture after about 140 h of cultivation. Oxalic acid production rates were highest by the wild-type strain grown on xylose as carbon source, i.e., when the lowest glucoamylase production rates were observed. A clear negative correlation was found between the oxalic acid production rate and the respiration quotient (RQ). An increase in the oxygen consumption rate, due to the production of strongly oxidized oxalic acid, caused the RQ to be lowest at those stages of recycling cultivation when highest oxalic acid production rates were observed.
