Oxalic acid production by Aspergillus niger

Abstract

Aspergillus niger is able to produce a quite high concentration of oxalic acid using sucrose as carbon and energy source. Operating at pH higher than 6 and an enriched N and P medium is necessary in order to conduct the fermentation towards oxalic acid production. A pH shift technique, operating at acid pH in the first two days and then setting pH to 6, allowed the productivity to slightly increase in shaking flasks cultures up to 3.0 kg/m 3 . d, with a final oxalic acid concentration of 29 kg/m 3 . When operating at more controlled conditions, in a stirred tank, both productivity and oxalic acid concentration were improved (4.1 kg/m 3 . d and 33.8 kg/m 3 , respectively). However the main drawback of this fermentation is the low yield attained (about 0.3 kg oxalic acid/kg sucrose) because most of glucose, resulting from the hydrolysis of sucrose by the extracellular enzymes secreted at the beginning of the fermentation, is very quickly oxidised to gluconic acid, a process which is favoured at a pH close to 6. Milk whey was proved to be a very good substrate as it allows oxalic acid to be produced with a similar productivity (2.5 kg/m 3 . d in shaking flasks) giving excellent yields of almost 0.6 kg oxalic acid/ kg lactose.