Upgrading wheat straw to HOMO and co-polyhydroxyalkanoates

Abstract

Polyhydroxyalkanoates (PHAs) are biodegradable and thus environmentally friendly thermoplastics that are synthesized by various microbial strains as intracellular storage materials. These polyesters present a broad range of properties varying from very crystalline to more elastomeric polymers and find applications from agriculture to medicine. Despite their versatility, they are still not competitive due to the high production costs, of which the C-source accounts for circa 30%. To decrease raw materials costs, lignocellulosic agro-industrial residues rich in cellulose and hemicelluloses can be used as the C-source after being processed to yield simple sugars. Wheat straw lignocellulosic hydrolysates (LCH) were prepared (biorefinery.de GmbH) by pre-treating this residual biomass using the AFEX process followed by enzymatic hydrolysis. A hydrolysate rich in glucose and xylose and with low titres of inhibitory compounds is produced that can be used as carbon source for PHA production. Burkholderia sacchari DSM 17165 was selected for its ability to use both hexoses and pentoses. Polymer production was optimized in fed-batch cultivations in stirred-tank reactors (STR). Polymer concentration, volumetric productivity and polymer cell content of respectively 84 g/L, 1.6 g L−1h−1 and 68 % (w/w) were attained [1]. Poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P3HB-co-4HB) copolymers exhibit attractive thermal and mechanical properties due to the 4HB monomer. Synthesis of this monomer was achieved upon the addition of gamma-butyrolactone (GBL) as co-substrate to fed-batch cultures. Using a DOstat feeding strategy for LCH and a continuous addition of GBL, the maximum attained P(3HB-co-4HB) productivity and 4HB molar % were 0.5 g/(L.h) and 5.0 molar %, respectively [2]. Extraction of P(3HB) from the cells usually involves the use of halogenated solvents to attain high recovery yields and purities. However, the use of these solvents causes health and environmental hazards. To lessen this drawback green solvents were tested and high recovery yields and purities were achieved. Lignocellulosic agricultural residues can thus be ugraded with high yields and productivities to value-added products using the biorefinery concept.