EFFECT OF GLUCOSE-CONCENTRATION ON CITRIC-ACID PRODUCTION BY YARROWIA-LIPOLYTICA

Citric acid production by Yarrowia lipolytica ATCC 20346 was studied in a laboratory-scale fermenter and schematically subdivided into three different phases, appropriately characterised by unstructed kinetic models. As the sugar level in the growth medium was increased from 40 to 108 g dm-3; a negative effect on the kinetic growth rate constant of the trophophase was obtained, where both the yield coefficients for glucose and nitrogen were found to be approximately constant. By using a production medium devoid of nitrogen, a citrate lag phase was observed whilst the yeast reduced its intracellular nitrogen fraction from 7-8% (w/w) to a new equilibrium value as low as 2.3-4.4%. Finally, the idiophase was found to be a non-growth associated process, the citric acid formation rate being dependent on the cellular nitrogen concentration only. The yield factors for citric, isocitric and fumaric acids appeared to be independent of the initial concentration of glucose in the production medium (S(op)). However, the yield factors for alpha-ketoglutaric, malic, pyruvic and cis-aconitic acids exhibited a negative correlation with respect to S(op), being virtually equal to zero for S(op) values greater than 160-175 g dm-3. The strain in question was capable of equalling the best mutant strains of Aspergillus niger used industrially in citric acid productivity (ca. 1.05 g dm-3 h-1), but not in selectivity, since citric acid represented only 85.5% of total citric and isocitric acid and 83.7% of the total acid.