Methods for purifying polyhydroxy alkanoates

Abstract

A method is provided for isolating and purifying PHA from microbial or plant biomass that contains PHA. The method includes the step of extracting PHA from the biomass using at least one solvent while simultaneously subjecting the biomass to a filtration process to remove cells. In a preferred embodiment of the method, an aqueous slurry of the biomass is directly extracted by diafiltration using an organic solvent. In a preferred diafiltration process, an aqueous slurry of microbial cells comprising PHA is recirculated through a filtration membrane, wherein the membrane has a pore size sufficiently small to reject individual cells or such aggregates of cells as may exist in the slurry. As liquid is progressively removed from the biomass slurry (by flowing out from the filtration membrane), an organic solvent, preferably a water-miscible solvent that also is a solvent for the PHA, is added to the biomass slurry at a rate which approximates the rate of liquid permeation through the filter, thereby maintaining the volume of the biomass slurry. Impurities which are insoluble in water become dissolved in the solvent-water mixture and pass through the filter membrane, and when the organic solvent concentration reaches a certain level, the PHA becomes soluble and flows through the filtration membrane and can be desolventized to recover the polymer.