The Production of Polyhydroxyalkanoates Using an Oscillatory Baffled Bioreactor

Abstract

Biopolymers are biodegradable polymers made from renewable resources. They can be made from plants or by microorganisms. Pseudomonas putida is one such microorganism. Under nutrient limitation and carbon excess it produces polyhydroxyalkanoates (PHA), which are linear polyesters, to store carbon as energy resource. PHAs are biodegradable and are used in the production of bioplastics.The production of biopolymers from microorganisms is currently not cost effective, because of expensive substrates, low yields and complex downstream processing. Therefore, intensification of PHA production to achieve higher yields in smaller, more efficient equipment at lower capital costs would be an advantage.Oscillatory baffled reactors (OBRs) are a form of plug flow reactor that can be used for long residence time reactions, as the plug flow is not dependent on achievement of a certain net flow velocity in the reactor, leading to much more compact designs. Fermentations, such as the production of PHA by P. putida, show characteristically long residence times. Therefore, they could be intensified in an OBR. Since one major advantage of the OBR is its highly uniform, controllable mixing, which can be used to enhance gas-to-liquid mass transfer, it should be suitable for biological reactions. Fed-batch or continuous fermentation is a second advantage, as it should minimize substrate and product inhibition. Lower, more uniform shear rates than in a stirred tank reactor would be an additional advantage for processing shear-sensitive cells. Previous research has shown that the OBR can provide higher biomass concentrations and greater gas-liquid mass transfer than conventional fermenters. In this work, the production of PHA by P. putida KT2442 was studied. The parameters investigated here were agitation rate and temperature. It was demonstrated that PHA can be produced in the OBR. The fermentation temperature and agitation rate resulting in the highest OD and PHA yield in this parameter space were determined as 30 ± 1°C and an oscillatory Reynolds number (Reo) of 300, respectively. It was also demonstrated that using an OBR can lead to higher OD (10.8 vs 7.26) and dry cell weight (3.75 g/L vs 2.4 g/L) than in a comparable conventional stirred tank reactor.