Production of poly-β-hydroxybutyrate on lactic acid by Alcaligenes eutrophus H16 in a 3-l bioreactor

Abstract

Poly-β-hydroxybutyrate (PHB) was produced by Alcaligenes eutrophus H16 in a 2-l working volume Biostat® MD bioreactor in both batch and fed-batch mode, using lactic acid as the sole carbon source during the production phase. For the batch and 2-stage fed-batch fermentations, the organism was first precultivated on a fructose-based, nutrient-rich medium for 24 h, followed by another 24 h on medium in which fructose was omitted and ammonium sulfate supplied as the nitrogen source. In fed-batch fermentations, the dissolved oxygen level was automatically controlled at about 3 to 4.5%. In batch fermentation with 20 g l −1 of lactic acid as the carbon source and about 9 g (d.m.) of initial cell concentration in the production phase, the maximum PHB concentration obtained in 25 h was 5.8 g l −1, corresponding to 62% of cell dry matter. In 2-stage fed-batch fermentations with the same initial cell concentration and with a total of 20 g l −1 of lactic acid, the maximum PHB concentration increased to 7.5 g l −1, 81% of the dry biomass. In a l-stage fed-batch fermentation with both the growth and production phases carried out in the bioreactor, the maximum PHB concentration could be increased to 8.4 g l −1, 70% of dry biomass. In this case the total PHB production time was decreased by about 24 h as compared with the 2-stage batch and fed-batch processes. The polymer accumulation could be conveniently monitored by the consumption of acid as the PHB was synthesized.