Oxalic acid production by Aspergillus niger Part II: Optimisation of fermentation with milk whey as carbon source J.T. Bohlmann, C. Cameselle, M.J. Nunez, J.M. Lema Abstract Oxalic acid is formed by Aspergillus niger at nearly neutral pH values. In this study the applicability of milk whey as a carbon source was investigated, both in shaking ¯ask experiments and in a stirred tank reactor. The in¯uence of pH on oxalic acid formation showed that the maximum production rate and higher concentration of the product are observed at pH 6. At pH 7 the same production rate was obtained although at a lower oxalic acid concentration. The process was shown to be inhibited by product from an oxalic acid concentration of about 10 kg/m 3 and its behaviour was ®tted by Luong's equation. In a 10-dm 3 strirred tank ferment the stirrer speed was varied in a range from 100 to 600 rpm. At values between 200 and 400 rpm, maximum production rates of oxalic acid of 6.8 kg/m 3 ád and 6.5 kg/m 3 ád were reached, respec- tively. A ®nal concentration of 41.4 kg oxalic acid/m 3 was reached operating at 400 rpm. List of symbols r OA,max kg/m 3 á d Maximum production rate of oxalic acid Y OA/TS Yield in oxalic acid related to total sugars consumed P kg/m 3 Initial concentration of oxalic acid (inhibition study) P m kg/m 3 Concentration of oxalic acid causing a complete inhibition n Inhibition coef®cient r OA,p  0 kg/m 3 ád Maximum production rate when no oxalic acid was added at the beginning (inhibition study) K L a s )1 Oxygen transfer coef®cient C O 2 kg/m 3 Concentration of oxygen in the fermenter C  O 2 kg/m 3 Saturation concentration of oxygen at the air-liquid interface r kg/m 3 ád Oxygen consumption rate X kg/m 3 Biomass concentration V aer m 3 /m 3 áh Aeration rate V ag rpm Agitation rate a,b,c Coef®cients in Eq. (4) S 0 kg/m 3 Initial concentration of substrate t d Fermentation time P f kg/m 3 Final oxalic acid concentration 1 Introduction In the ®rst part of this study [1] we analysed the pro- duction of oxalic acid by fermentation of sucrose by Aspergillus niger. Although a high concentration of oxalic acid was achieved, a signi®cant fraction of sugars was metabolised to gluconic acid. Strasser et al. [2] reported that organic wastes such as green corn syrup and molasses are not suitable for oxalic acid production, but lactose permeates (consisting of at least 50% of lactose) are quite suitable as a culture media. In shaking ¯ask experiments with milk whey as the carbon source [1] it was shown that the lactose containing media allowed oxalic acid concen- trations of about 23 kg/m 3 with yields of about 58% to be produced. There are several different reports on the optimum pH leading to high yields of oxalic acid. Some authors con- sider a neutral pH as the most adequate [3], while Kubicek et al. [4] reported an optimum pH of 6 using sucrose as the carbon source. Although the capability of A. niger to form oxalic acid is well known [5±7] there is little information on fermentation in reactors. The production of metabolites with ®lamentous fungi is largely in¯uenced by culture conditions, such as agi- tation and aeration [8±11]. The production rate is strongly affected by agitation in which, at low agitation rates, low oxygen transfer rate are achieved, causing a Bioprocess Engineering 19 (1998) 337±342 Ó Springer-Verlag 1998 337 Received: 27 October 1997 J.T. Bohlmann 1 , J.M. Lema Department of Chemical Engineering, Institute of Technology, University of Santiago de Compostela Av. das Ciencias s/n., E-15706 Santiago de Compostela, Spain e-mail: jmlema@usc.es C. Cameselle Department of Chemical Engineering, University of Vigo Present address: 1 Institut fu Èr Biotechnologie, Fachgebiet Bioverfahrenstechnik, Sekr. GG2, Technische Universita Èt Berlin, Seestrasse 13. D-13353 Berlin, Germany Correspondence to: J.M. Lema The stay of J.T. Bohlmann in the University of Santiago de Compostela was supported by a grant from the Deutsche Forschungsgemeinschaft.