CITRIC-ACID PRODUCTION BY ASPERGILLUS-NIGER WITH BANANA EXTRACT

Experimental runs were carried out on citric acid bioproduction using banana extract as carbon source. The micro-organism used was a strain of Aspergillus niger. Different types of batch bioreactor were tested: bubble column (BCR) and stirred tank reactor (STR). With the STR the effects of two different impeller types were studied: Rushton six-bladed turbine and Lightnin A310 turbine, both at various rotational speeds. During the tests various parameters were measured: biochemical (biomass as w/v and mean mould pellet diameter), chemical (total sugar and citric acid concentration) and physical (dissolved oxygen tension, DOT; oxygen uptake rate, OUR; viscosity; gas/liquid volumetric mass transfer coefficient, K(L)a). The results showed that citric acid production is strictly connected to the morphology of the micro-organism; the mould must grow in pellet form, as occurs when a gentle agitation is provided such as in the BCR; otherwise, with mechanical agitation the pellets are broken, the biomass morphology becomes filamentous and the citric acid production markedly decreases. The fermentation results are discussed to achieve a better comprehension of the main influential parameters for process scale-up.