Maximization of poly(3-hydroxybutyrate) production in fed-batch cultures of A. vinelandii based on the variation of the agitation rate

1. Introduction The poly-hydroxybutyrate (PHB) is a polyethylene-like bioplastic naturally synthetized by several classes of microorganisms, as a source of energy and carbon unit [1]. This material, which is a promising candidate for the replacement of fossil-based plastic, is already present at an industrial scale. However, the maximization of its production process is the goal to achieve in order to reduce production costs. Azotobacter vinelandii is strictly aerobic gram-negative soil bacteria, able to produce PHB in a carbon substrate-rich environment and under oxygen-limitation conditions [2]. The production of this polymer is particularly interesting in this species, because it can accumulate up to 85% of its dried biomass as PHB [3]. It is known that agitation rate affects the PHB production by A. vinelandii [2]. In this work, the PHB productivity under fed-batch cultivation was evaluated to different volumetric oxygen transfer coefficient (kLa). Thus, it was possible to determine an adequate kLa range for scale-up the production of PHB. 2. Methods Batch cultures of Azotobacter vinelandii OP were performed in a 3 L bioreactor (30 g L-1 sucrose as carbon source) at 600 rpm during 30 h. After of this time, the cultures were fed by a single medium feeding pulse of the carbon source and the agitation speed was varied between 400 and 1000 rpm. The biomass evolution and the PHB production were evaluated. The kLa was estimated at different agitation rate using dynamic method. 3. Results and discussion The maximal accumulation of biomass was 13.3 g L-1, obtained at an agitation speed of 800 rpm (Table 1). The highest agitation speed increased the specific growth rate, reaching a value of 0.069 h-1 to 1000 rpm. During fed-batch cultivation, a change in the agitation speed affected the PHB accumulation, obtaining the highest value (79.1 % w w-1) to 600 rpm.