Established and advanced approaches for recovery of microbial polyhydroxyalkanoate (PHA) biopolyesters from surrounding microbial biomass

Downstream processing for recovery of microbial polyhydroxyalkanoate (PHA) biopolyesters from biomass constitutes an integral part of the entire PHA production chain; beside the feedstocks used for cultivation of PHA-production strains, this process is currently considered the major cost factor for PHA production. Besides economic aspects, PHA recovery techniques need to be sustainable by avoiding excessive use of (often precarious!) solvents, other hazardous chemicals, non-recyclable compounds, and energy. Moreover, the applied PHA recovery method is decisive for the molecular mass and purity of the obtained product, and the achievable recovery yield. In addition to the applied method, also the PHA content in biomass is decisive for the feasibility of a selected technique. Further, not all investigated recovery techniques are applicable for all types of PHA (crystalline versus amorphous PHA) and all PHA-producing microorganisms (robust versus fragile cell structures). The present review shines a light on benefits and shortcomings of established solvent-based, chemical, enzymatic, and mechanical methods for PHA recovery. Focus is dedicated on innovative, novel recovery strategies, encompassing the use of "green" solvents, application of classical "PHA anti-solvents" under pressurized conditions, ionic liquids, supercritical solvents, hypotonic cell disintegration for release of PHA granules, switchable anionic surfactants, and even digestion of non-PHA biomass by animals. The different established and novel techniques are compared in terms of PHA recovery yield, product purity, impact on PHA molar mass, scalability to industrial plants, and demand for chemicals, energy, and time.