Biochemical Engineering Journal 18 (2004) 21–31 Production of poly(3-hydroxybutyrate) in an airlift bioreactor by Ralstonia eutropha Lana Zanetti Tavares, Elda Sabino da Silva, José Geraldo da Cruz Pradella ∗ São Paulo Institute for Technological Research, Biotechnology Group, Division of Chemistry, Avenida Professor Almeida Prado 532, CEP 05508-901 São Paulo, SP, Brazil Received 11 April 2002; received in revised form 11 April 2003; accepted 16 April 2003 Abstract The influence of different aeration rates (Q air ), 12, 20, 30, 35, 40, and 50 l min -1 , on the production of poly(3-hydroxybutyric acid) (PHB) by Ralstonia eutropha DSM 545 during the accumulation phase, was investigated in an airlift bioreactor and the results were compared to the ones obtained in a stirred tank bioreactor. The time constants for mixing, oxygen transfer, oxygen consumption, power consumption, and kinetic parameters were the tools used to compare the systems. The results showed that, for a superficial gas velocity (V s ) greater than or equal to 10 m s -1 , the PHB productivity reached 0.6 g l -1 h -1 at 50% of PHB cell content, although, for the velocity of 0.10 m s -1 the observed value of dissolved oxygen concentration in fermentation medium was zero. The analysis of time constants calculated in the accumulation phase showed that, for V s under 0.11 m s -1 , the rate of oxygen consumption was larger than the rate of oxygen transfer indicating that this was the rate-limiting step. In the conventional stirred tank bioreactor, the PHB productivity achieved 0.82 g l -1 h -1 at 50% of PHB. However, the latter demand higher power consumption than the airlift bioreactor, indicating that, for these bacteria, the low supply of oxygen in airlift leads to better performances on PHB production with the advantage of lower demand of energy. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Airlift bioreactor; Poly(3-hydroxybutyrate); Ralstonia eutropha; Time constants 1. Introduction Poly(3-hydroxybutyric acid) (PHB) is an intracellular car- bon and energy storage material accumulated by many mi- croorganisms under unfavorable growth conditions such as limitation of N, P, S, Mg, or O 2 and excess of carbon source [1,2]. PHB is a biodegradable thermoplastic polyester that can be applied similarly to many conventional petrochemi- cal derived plastics currently in use [3,4]. Ralstonia eutropha has been the most widely used mi- croorganism for the production of PHB because it is easy to grow, it accumulates large amounts of PHB (up to 80% of dry cell weight) in a simple culture medium and its physi- ology and biochemistry leading to PHB synthesis are well understood [5,6]. The synthesis of PHB usually occurs be- cause the key enzyme for PHB production (-ketothiolase, Fig. 1) starts to act due to the starvation of nutrients in the medium leading to the accumulation of free Co-A [7]. Ac- cording to the metabolic pathway of the bacteria (Fig. 1), a low concentration of O 2 in the medium also leads to an ∗ Corresponding author. Tel.: +55-11-37674315; fax: +55-11-36674055. E-mail address: pradella@ipt.br (J.G. da Cruz Pradella). excess of reduced coenzymes (NADH and NADPH) and a higher carbon flux could be directed towards PHB synthe- sis for reoxidation of these coenzymes [8] (see step 7 in Fig. 1). However, a very severe limitation of oxygen causes formation of intermediates of the Krebs Cycle and even of the PHB biosynthetic pathway, harming or even making un- feasible the formation of PHB [9]. It is clear that oxygen limitation could enhance PHB biosynthesis in recombinant E. coli by decreasing cell growth rate [10]. This also shows that the timing of PHB biosynthesis can be artificially con- trolled in recombinant E. coli, as the timing of nitrogen limi- tation controls PHB biosynthesis in Ralstonia eutropha [11]. In other bacteria PHB producers, the same effect of oxygen limitation could be shown. According to Ward et al. [12] PHB accumulation is induced during oxygen limitation in Azotobacter beijerinckii cultures. Airlift bioreactors are a special class of pneumatic con- tactors that do not have any mechanical components, like impellers and seals. Their main characteristics are low shear stress, simplicity of design and construction [13] and low energy requirements for transport rates, besides a better def- inition of internal flow [14,15], and a good aseptic control [16]. The performance of airlift reactors becomes limited when the cell used in the process consumes high amounts 1369-703X/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S1369-703X(03)00117-7