New alternative processes for the continuous production of gluconic acid by Aureobasidium pullulans
, using biomass retention by cell immobilization or cross over filtration, are described in the present work. 315 g/l gluconic acid was continuously produced in chemostat cultures at 21 hrs residence time without any biomass retention. 260 g/l gluconic acid was produced in fluidized bed reactor at 21 hrs residence time. The support carrier was overgrown resulting in limitations of oxygen transfer towards the inner layers of immobilized biomass. 375 g/l gluconic acid was produced under continuous cultivation at 22 hrs of residence time with a formation rate for the generic product of 17 g/(l x h) and a specific gluconic acid productivity of only 0.74 g/(g x h), using biomass retention by cross over filtration. 370 g/l were obtained at 19 hrs RT and 100% conversion with 25 g/l biomass and a formation rate of 19 g/(l x h). At 100% conversion, a selectivity of only 78% was determined at 22 hrs and of 77% at 19 hrs RT, because of the very high biomass concentration. Biomass retention makes it possible to break the existing link between growth and residence time.