Production and characterization of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) copolymers from a pre- fermented hardwood hydrolysate.

Abstract

Economically viable production of poly(3-hydoxybutyrate-co-3-hydroxyvalerate) (PHBV) copolymers remains a challenge. The objective of this work was to produce low-cost PHBV copolymers from lignocellulose-derived mixed sugars without genetic engineering or addition of chemical precursors. A hardwood hydrolysate was first pre-fermented using the facultative anaerobe Propionibacterium acidipropionici, and the resulting propionate-rich effluent was used for subsequent PHBV biosynthesis in Paraburkholderia sacchari or Hydrogenophaga pseudoflava. P. acidipropionici displayed a high tolerance to the hardwood hydrolysate and produced up to 11 g L^-1 propionate (with varying amounts of lactate and acetate) under batch conditions. Propionic acid exerted significant toxicity toward P. sacchari and H. pseudoflava, so dilution of the pre-fermentation effluent was required prior to the PHBV production step. When P. sacchari and H. pseudoflava were grown on the pre-fermented mixture of glucose, xylose, lactate, acetate, and propionate (diluted to 15 mM propionate), the organic acids were consumed preferentially. H. pseudoflava accumulated up to 41.7 ± 7.0% cell dry mass (CDM) as PHBV that contained 13.7 ± 2.4 mol % 3-HV subunits. Meanwhile, P. sacchari accumulated up to 56.0 ± 5.8% CDM as PHA, but with lower 3-HV contents (1.2-5.1%). The PHBV copolymers resulting from this integrated process showed a desirable crystallinity, but the molecular weights were lower and the melt temperatures were higher than expected in all cases. Future work should focus on tuning the cultivation parameters to target higher molecular weight polymers while designing a feeding strategy of the pre-fermented stream that circumvents toxicity issues and allows a better control of the formation of random vs. block copolymers.