Bioprocess parameters and oxygen transfer effects in the growth of Pseudomonas dacunhae for L-alanine production

Because alanine produced by most micro-organisms through fermentation is of the DL-racemate form and because it is too difficult to obtain L-alanine by fermentation, industrial production of L-alanine is accomplished by using L-aspartic acid as the reactant and L-aspartate beta-decarboxylase (ADL) enzyme of resting Pseudomonas dacunhae cells as the biocatalyst. This work reports on the optimum growth conditions, i.e. optimum pH, temperature, agitation rate, substrate concentration and growth kinetics, of P. docunhae investigated in shake flask cultures, as well as oxygen transfer kinetics studied in laboratory-scale bioreactors. Values of pH 7.0-7.5 and T = 229 K were optimal for both the growth and the enzyme activity. Although the specific growth rate CL increased with increasing agitation rate between 50 and 300 min(-1), the enzyme activity showed a maximum at an agitation rate of 150 min(-1). Substrate inhibition was observed for high glutamic acid concentrations, i.e. C-Go > 10 kg m(-3), which is used as the carbon source for the cultivation. The oxygen transfer kinetics were investigated under optimum conditions in a bioreactor of working volume 4 x 10(-3) m(3) with temperature, pH, dissolved oxygen, stirring rate and foam control. The micro-organism concentration reached a plateau and the enzyme activity reached a maximum at a residence time of t = 15 h. The oxygen transfer coefficient K(L)a and oxygen uptake rate r(o), were maximum at residence times of t = 10 h and t = 7 h respectively. Increases in both the agitation rate and the oxygen feed rate increased the K(L)a values, with the agitation rate increase being much more effective. (C) 1997 Elsevier Science S.A.