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Aims: To study the biochemical response of Yarrowia lipolytica LGAM S(7)1 during growth on raw glycerol (the main by-product of bio-diesel production units) in order to produce metabolic products of industrial signi®cance. Methods and Results: Yarrowia lipolytica was cultivated on raw glycerol or glucose in¯asks. Although nitrogen-limited media were employed, growth was not followed by production of reserve lipid. Nitrogen limitation led to citric acid excretion. Growth and citric acid production parameters on glycerol were similar to those obtained on glucose. When high initial glycerol media were used, citric acid up to 35 g l )1 (yield 0á42±0á44 g acid g )1 glycerol consumed) was produced. Conclusions: Raw glycerol was an adequate substrate for Y. lipolytica. Growth was not followed by reserve lipid accumulation, but amounts of citric acid were produced. Signi®cance and Impact of the Study: Raw glycerol is an industrial feedstock appearing in increasing quantities as the main by-product of bio-diesel production facilities. The present study describes an alternative way of glycerol valorization, with the production of remarkable amounts of citric acid, in addition to its main valorization way (production of 1,3-propanediol by bacteria).

Financial support: The present work was partially financed by the international cooperation project in industrial research activities of development in precompetitive stage-2005 (ΟΔΣΒΕΠΡΟ-69) entitled "Exploitation of the natural microflora of the olive fruit for the production of virgin oil of high quality" (General Secretariat of Research and Development of the Greek Ministry of Development -E.U.).

Yarrowia lipolytica was cultivated on mixtures of saturated free fatty acids (an industrial derivative of animal fat called stearin), technical glycerol (the main by-product of bio-diesel production facilities), and glucose. The utilization of technical glycerol and stearin as co-substrates resulted in higher lipid synthesis and increased citric acid production than the combination of glucose and stearin. The lipids produced contained significant amounts of stearic acid (50-70%, wt/wt) and lower ones of palmitic (15-20%, wt/wt), oleic (7-20%, wt/wt), and linoleic (2-7%, wt/wt) acid. Single-cell oil having a composition similar to cocoa-butter up to 3.4 g/L was produced, whereas in some cases relatively increased citric acid quantities (up to 14 g/L) were excreted into the growth medium. The microorganism presented a high specificity for lauric, myristic, and palmitic acid, while a discrimination for the stearic acid was observed. As a conclusion, microbial metabolism could be directed by using mixtures of inexpensive saturated fats, glycerol, and glucose as co-substrates, in order to accumulate lipids with predetermined composition, e.g., cocoa-butter equivalents.

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In the present report, the effect of glucose and stearin (substrate composed by saturated free-fatty acids) on the production of biomass, reserve lipid, and citric acid by Yarrowia lipolytica ACA-DC 50109 was investigated in nitrogen-limited cultures. Numerical models that were used in order to quantify the kinetic behavior of the above Yarrowia lipolytica strain showed successful simulation, while the optimized parameter values were similar to those experimentally measured and the predictive ability of the models was satisfactory. In nitrogen-limited cultures in which glucose was used as the sole substrate, satisfactory growth and no glucose inhibition occurred, although in some cases the initial concentration of glucose was significantly high (150 g/l). Citric acid production was observed in all trials, which was in some cases notable (final concentration 42.9 g/l, yield 0.56 g per g of sugar consumed). The concentration of unsaturated cellular fatty acids was slightly lower when the quantity of sugar in the medium was elevated. In the cases in which stearin and glucose were used as co-substrates, in spite of the fact that the quantity of cellular lipid inside the yeast cells varied remarkably (from 0.3 to 2.0 g/l-4 to 20% wt/wt), de novo fatty acid biosynthesis was observed. This activity increased when the yeast cells assimilated higher sugar quantities. The citric acid produced was mainly derived from the catabolism of sugar. Nevertheless, citric acid yield on sugar consumed and citrate specific production rate, as evaluated by the numerical model, presented substantially higher values in the fermentation in which no fat was used as glucose co-substrate compared with the cultures with stearin used as co-substrate.

Strains of Zygomycetes, belonging to the genera Zygorhynchus, Mortierella, Rhizopus, Mucor, and Cunninghamella, when cultivated on glucose produced significant quantities of γ‐linolenic acid (GLA). After exhaustion of the nitrogen source from the culture medium, all strains accumulated cellular lipids in concentrations ranging from 10 to 28% (oil/dry mycelium). However, in some strains after the depletion of the carbon source (glucose) from the culture medium, a reconsumption of the accumulated oil and synthesis of fat‐free cell material was observed. Accumulation of large amounts of oil in the mycelium resulted in the production of oil with low GLA content. Rhizopus stolonifer strain LGAM (9)1, and Cunninghamella sp. strain LGAM (9)2 produced more than 30 mg GLA/g of dry cellular mass. Cunninghamella sp. accumulated 28.1% oil/dry cellular mass, which contained 11.9% GLA. The production of GLA was 260 mg/L of culture medium.

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