## Results and Discussion
It is noted that in the shake-flask citric acid production experiments, the achieved productivities (calculated as previously stated) are, in general, much lower (0.07-0.25 g/L/h) [8, 19, 23, 40, 49, 53, 61, 62]. The completely different metabolic profile observed between the shake-flask and the batch bioreactor trial should be attributed to the different agitation/aeration regime between the two cultures and more specifically to the higher level of oxygenation that occurred in the bioreactor experiment; for all of the fermentation steps in which noticeable biosynthesis of polyols occurred in the shake-flask trials, DOT values in the flasks always ranged between 20% and 50% _w/v_. On the contrary, in the bioreactor experiment, with the exception of the very early fermentation step (\(t\) = 20 h) in which DOT \(\approx\) 20.0% _w/v_, rapidly afterwards, DOT level was always \(>\)40%, and at the end of the trial, when glycerol had been consumed and the maximum \(CA\) concentration was recorded (=42.4 g/L), the DOT value was \(>\)90% _w/v_ (kinetics of DOT evolution not presented). On the other hand, this differentiation in the metabolic responses between flask and bioreactor cultures did not seem to be seriously attributed to the different pH values observed between these two types of cultures; pH is certainly a crucial culture parameter that may influence the final spectrum of products for _Y. lipolytica_ cultures. It has been demonstrated that the very same strain cultivated under nitrogen limitation, can produce almost exclusively citric acid at medium pH values = 5.0-6.0, while metabolic shift and almost exclusive production of polyols may  
Figure 3: Kinetics of dry yeast cell mass production (\(X\), g/L) (a) and glycerol assimilation (\(Gly\), g/L) (b) in shake flask and batch bioreactor experiment of _Yarrowia lipolytica_ LMBF Y-46 during growth on pure glycerol, at initial glycerol concentration (\(Gly_{0}\)) \(\approx\) 100 g/L under nitrogen-limited conditions. Culture conditions as in Tables 1 and 3. Each experimental point is the mean value of two independent determinations (SE \(\leq\) 18%). occur when cultures are performed at pH values = 3.0-3.5 [23; 46; 48; 63]. In the current investigation, the differences in the pH values between the flask (pH mostly found within 4.8-5.8) and the reactor (pH = 6.0 \(\pm\) 0.1) experiments were not so high, thus we assume that pH was not the principal factor affecting the differences in the metabolism observed between the flask and the reactor trials. In a very similar physiological pattern with the strain LMBF Y-46, the wild-type food-derived strain ACA YC 5029, when cultivated in shake-flask trials (equally in pH > 4.5) produced mainly polyols (erythritl and mannitol). Minimal quantities of citric acid were synthesized [18]. In contrast, in well-agitated and aerated bioreactors (800 rpm, aeration between 0.1 and 2.0 vvm in order to always have a cascade of DOT \(\geq\) 10%, \(\psi\)/\(\psi\)--significantly higher DOT values than 10% \(\psi\)/\(\psi\) were finally recorded) mainly citric acid was synthesized, whereas simultaneously insignificant biosynthesis of polyols occurred [18]. In agreement with these results and according to the literature, \(CA\) biosynthesis is strongly influenced by the DOT values in the culture medium; [PERSON] et al. [64], performing culture optimization of the strains _Y. lipolytica_ W29 and CBS 2073, observed a clear positive effect of the oxygen mass transfer rate (OTR) upon the biosynthesis of \(CA\) for these yeast strains during glycerol fermentation in shake-flask experiments. Likewise, in batch bioreactor experiments with the strain W29 growing on glycerol, \(CA\) production was clearly favored in trials presenting higher oxygen volumetric mass transfer coefficients (\(k_{\text{La}}\)); therefore, increase in the initial \(k_{\text{La}}\) value from 7 h\({}^{-1}\) to 55 h\({}^{-1}\) led to a 7.8-fold increase in the final concentration of citric acid in this strain [65].