HEMICELLULOSE AS A POTENTIAL SUBSTRATE FOR PRODUCTION OF POLY(BETA-HYDROXYALKANOATES)

Pseudomonas cepacia was evaluated for its ability to utilize xylose, a major hemicellulosic sugar of hardwoods, for the production of the bi odegradable, thermoplastic poly(beta-hydroxybutyrate) (PHB). This cult ure produced 2.6 g . L(-1) of biomass containing 60% (w/w) PHB when gr own in shake flasks on an ammonium-limited, mineral salts medium conta ining 10 g . L(-1) of xylose. Batch fermentation data showed that grow th and PHB production kinetics on xylose were similar to previously pu blished results for the same microorganism on fructose. On xylose, the maximum specific growth rate, the maximum specific PHB production rat e (based on total biomass minus PHB biomass), the overall yield of bio mass produced from substrate consumed, the yield of PHB produced from substrate consumed (Y-PHB/S), and the percentage of PHB were 0.22 h(-1 ), 0.072 g . g(-1). h(-1), 0.29 g . g(-1), 0.11 g . g(-1) and 45% (w/w ), respectively. A high maintenance energy (0.119 g of xylose . g of b iomass(-1). h(-1)) is probably responsible for the low overall yield. However, the product yield, Y-PHB/S, was Still the highest reported fo r any microorganism grown on pentosic sugars. Using the Y-PHB/S of 0.1 1 g . g(-1), it was estimated that the substrate cost (in terms of hyd rolyzed hemicellulose) for PHB production would be similar to that of cane molasses and half that of bulk glucose.