In order to identify an optimal aeration strategy for intensifying bio-fuel ethanol production in fermentation processes where growth and production have to be managed simultaneously, we quantified the effect of aeration conditions--oxygen limited vs non limited culture (micro-aerobic vs aerobic culture)--on the dynamic behaviour of Saccharomyces cerevisiae cultivated in very high ethanol performance fed-batch cultures. Fermentation parameters and kinetics were established within a range of ethanol concentrations (up to 147 g l(-1)), which very few studies have addressed. Higher ethanol titres (147 vs 131 g l(-1) in 45 h) and average productivity (3.3 vs 2.6 g l(-1) h(-1)) were obtained in cultures without oxygen limitation. Compared to micro-aerobic culture, full aeration led to a 23% increase in the viable cell mass as a result of the concomitant increase in growth rate and yield, with lower ethanol inhibition. The second beneficial effect of aeration was better management of by-product production, with production of glycerol, the main by-product, being strongly reduced from 12 to 4 g l(-1). We demonstrate that aeration strategy is as much a determining factor as vitamin feeding (Alfenore et al. 2002) in very high ethanol performance (147 g l(-1) in 45 h) in order to achieve a highly competitive dynamic process.

Yarrowia lipolytica, a non-conventional yeast with a promising biotechnological potential, is able to undergo metabolic and morphological changes in response to environmental conditions. The effect of pH perturbations of different types (pulses, Heaviside) on the dynamic behavior of Y. lipolytica W29 strain was characterized under two modes of culture: batch and continuous. In batch cultures, different pH (4.5, 5.6 (optimal condition), and 7) were investigated in order to identify the pH inducing a stress response (metabolic and/or morphologic) in Y. lipolytica. Macroscopic behavior (kinetic parameters, yields, viability) of the yeast was slightly affected by pH. However, contrary to the culture at pH 5.6, a filamentous growth was induced in batch experiments at pH 4.5 and 7. Proportions of the filamentous subpopulation reached 84 and 93 % (v/v) under acidic and neutral conditions, respectively. Given the significant impact of neutral pH on morphology, pH perturbations from 5.6 to 7 were subsequently assayed in batch and continuous bioreactors. For both process modes, the growth dynamics remained fundamentally unaltered during exposure to stress. Nevertheless, morphological behavior of the yeast was dependent on the culture mode. Specifically, in batch bioreactors where cells proliferated at their maximum growth rate, mycelia were mainly formed. Whereas, in continuous cultures at controlled growth rates (from 0.03 to 0.20 h) even closed to the maximum growth rate of the stain (0.24 h), yeast-like forms predominated. This pointed out differences in the kinetic behavior of filamentous and yeast subpopulations, cell age distribution, and pH adaptive mechanisms between both modes of culture.