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

Citric acid production by Yarrowia lipolytica ATCC 20346 was studied i n a laboratory-scale fermenter and schematically subdivided into three different phases, appropriately characterised by unstructed kinetic m odels. As the sugar level in the growth medium was increased from 40 t o 108 g dm-3; a negative effect on the kinetic growth rate constant of the trophophase was obtained, where both the yield coefficients for g lucose and nitrogen were found to be approximately constant. By using a production medium devoid of nitrogen, a citrate lag phase was observ ed 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 concentra tion only. The yield factors for citric, isocitric and fumaric acids a ppeared to be independent of the initial concentration of glucose in t he production medium (S(op)). However, the yield factors for alpha-ket oglutaric, 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 ca pable of equalling the best mutant strains of Aspergillus niger used i ndustrially 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 cit ric and isocitric acid and 83.7% of the total acid.