Appl Microbiol Biotechnol (1990) 32:526-532 Applied Microbiology Biotechnology © Springer-Verlag 1990 The effects of pressure on the growth of Aureobasidium pullulans and the synthesis of pullulan Robert Dufresne, Jules Thibault, Anh Leduy, and Robert Lencki Department of Chemical Engineering, Laval University, Sainte-Foy, Quebec, Canada, G1K 7P4 Received 30 May 1989/Accepted 1 September 1989 Summary. The feasibility of pressurized culture was explored for the growth of Aureobasidiumpul- lulans and the synthesis of pullulan. For all volu- metric flow rates of air, the production of biomass increased with pressure up to a critical value ranging from 0.50 to 0.75 MPa, at which point a drastic decrease in biomass production and a change in cellular morphology was observed. For pullulan synthesis, the same dramatic decrease was observed at approximately the same critical pressure. In the pressure range 0.1.-0.65 MPa, the synthesis of pullulan was subject to what is be- lieved to be the competing effects of oxygen avail- ability and pressure inhibition. Introduction The overall rate of oxygen transfer to a liquid in a fermentor is given by the volumetric oxygen trans- fer coefficient multiplied by the mean concentra- tion driving force. This can be expressed by the following equation: dC~ -- = kLa (C* - CL) (1) dt where a = specific gas-liquid interfacial area (m2/ m3); CL=dissolved oxygen concentration (tool/ I); C* =saturated dissolved oxygen concentration (mol/1); kL=mass transfer coefficient (Ix/s); kLa=volumetric mass transfer coefficient (s-l); t =time (s). The C* is approximately 8 ppm under the con- ditions normally found in a typical fermentor. An Offprint requests to: J. Thibault actively respiring yeast population consumes oxy- gen at a rate which is of the order of 750 times the oxygen saturation concentration per hour (Bailey and Ollis 1986). It is, therefore, necessary to trans- fer this amount of oxygen from the gas to the li- quid phase to satisfy the demands of the microor- ganisms. This is not an easy task in view of the relatively low oxygen solubility, which renders the concentration driving force extremely small. Oxygen transfer rates may be improved by in- creasing the oxygen partial pressure in the gas phase by using pure oxygen in lieu of air or by increasing the operating pressure of the fermen- tor. As a result, C* and, thus, the concentration driving force will increase. For practical and eco- nomical reasons, however, the use of pure oxygen is not feasible in large-scale fermentors. On the other hand, it is possible, even for large fermen- tots, to sustain a mild pressure increase to pro- duce a higher saturated dissolved oxygen concen- tration. For these reasons, this paper will only consider the influence of pressure. High pressure fermentation is not only of in- terest in fermentors where higher pressure can be imposed but also for fermentations taking place in large fermentors where high hydrostatic pres- sure exists, such as the 22 m-long tubular loop reactor developed by Ziegler et al. (1980), or at the bottom of deep aeration tanks, such as the ICI deep-shaft type. In these reactors, ceils grow un- der a hydraulic pressure of 0.5-1.0 MPa and dis- solved oxygen concentrations can increase by a factor of two or three (Sato et al. 1984; Takamatsu et al. 1981). By increasing the pressure in the headspace of the fermentor, the oxygen partial pressure will in- crease. However, a simultaneous increase in the partial pressure of carbon dioxide will also take place, which has been shown in the literature to