Studies of polyol production by the yeast Yarrowia lipolytica growing on crude glycerol under stressful conditions

Crude glycerol, the principal by-product of biodiesel production process, was employed as substrate by three wild-type Yarrowia lipolytica strains (ACA-YC 5030, LMBF 20 and NRRL Y-323). Stressful conditions (low pH value = 2.0 ± 0.3, low incubation temperature T = 20 ± 1 °C, non-aseptic conditions) were employed. Interesting production of yeast biomass and polyols (viz. erythritol, mannitol and arabitol) was noted at pH = 2.0 ± 0.3 and T = 20 ± 1 °C. Strains failed to produce significant quantities of cellular lipid, while variable quantities of intra-cellular polysaccharides were produced. Fermentations under previously pasteurized media supported significant biomass and polyols production for most of the tested strains, while only one strain (NRRL Y-323), managed to produce polyols at media that were not previously thermally treated at all. The production of mannitol was favored at low initial glycerol (Glol₀) concentrations, whereas higher Glol₀ quantities favored the biosynthesis of erythritol. For the strain NRRL Y-323, highly aerated / agitated bioreactor trials showed different physiological profiles as compared to the respective flask experiments. Finally, in flask experiments with the strain NRRL Y-323 at high Glol₀ amounts (≈140 g/L) at low medium pH (=2.0 ± 0.3), a significant production of polyols (=84.2 g/L) with the corresponding remarkable conversion yield on glycerol consumed = 62 % w/w was achieved.

Practical application

Renewable and biodegradable fuels, such as biodiesel, are safer and environmentally friendlier than the conventional petroleum diesel. Glycerol is a cost-effective substrate obtained as the main side-product from biodiesel production process and is currently being employed in the realm of Industrial Microbiology and Biotechnology to produce metabolic products with added value. Current research focuses on using glycerol as a starting substrate for biotechnological conversions aiming at producing, amongst other compounds, polyols, microbial biomass, citric acid, etc. from selected strains of the Generally Recognized Αs Safe (GRAS) yeast Yarrowia lipolytica. In the current investigation therefore, we examined the capacity of new wild-type non-extensively studied strains of this yeast to grow and assimilate this inexpensive substrate. Specifically, we have performed the acclimatization of the mentioned strains to stressful environments (i.e., low pH, low incubation temperature, non-aseptic conditions, etc.) and remarkable quantities of the added-value compounds (polyols, yeast mass, citric acid) were produced.