A sustainable approach for the downstream processing of bacterial polyhydroxyalkanoates: State-of-the-art and latest developments

Abstract

Bioplastics have emerged as a platform to reduce our dependence on fossil fuels. Polyhydroxyalkanoates (PHAs) are a family of biodegradable polyesters with large potential in consumer goods and medical applications. These polymers accumulate in prokaryotic microbes and their recovery is a challenging, often under explored, operation. In the past, oil-derived solvents and chemicals have been widely used as extracting agents, compromising the "environmentally-friendly" claim of bioplastics. Furthermore, the large amount of chemicals and solvents required at the industrial level would negatively impact the economics of the process. The present review presents the latest advances in the field of downstream operations for PHA recovery emphasizing those green technologies with scaling-up feasibility. As for the upstream and fermentation stages, the extraction process needs to be carefully optimized to accomplish a competitive production of PHAs.

Recovery of the PHA Copolymer P(HB-co-HHx) With Non-halogenated Solvents: Influences on Molecular Weight and HHx-Content

“…The recovery of PHAs accounts for a large portion of the process economics and overall ecological footprint (Pérez-Rivero et al, 2019). In addition, the PHA productivity, intracellular content, and yield from the substrate, as well as the carbon substrate price all contribute to the final production price (Choi and Lee, 1999).…”

“…Production of PHAs requires the isolation of the intracellular polymers from other biomass components. Several extraction strategies have been employed and comprehensively reviewed in the last years: solvent extraction (halogenated, non-halogenated), chemical disruption methods (hypochlorite, alkaline, surfactants) in combination with enzymes, mechanical disruption (bead mill, high pressure homogenization, ultrasonication), supercritical fluid extraction, aqueous two-phase systems, air classification (Koller et al, 2013b;Madkour et al, 2013;Pérez-Rivero et al, 2019), and biological approaches using insects and other animals (Murugan et al, 2016;Kunasundari et al, 2017;Ong et al, 2017;Zainab-L and Sudesh, 2019). However, solvent-based extraction is still the most explored PHA recovery method and allows the recovery of high purity PHA.…”

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

“…Moreover, appropriate downstream processing is a major factor for the ecological footprint of microbial bioplastics (12). Issues like the type and amount of extraction solvents, energy input, recyclability of solvents and other chemicals, and water requirement are factors determining the economic and environmental feasibility of a given PHA recovery process, and must be weighed against to obtained product quality and recovery yield (13). The review at hand provides an updated overview of established and emerging techniques for PHA recovery, with focus dedicated to trends and progress observed during the recent years years (for an overview, see Fig.…”

“…Briefly, solvent-based extraction methods and cell disruption by chemical, enzymatic, or mechanical techniques, or combinations thereof, are described, as it is broadly dealt with in many recent review articles. However, when carefully reading these publications, it becomes obvious that every one of these recovery methods has disadvantages, either economically, ecologically, for safety aspects, disappointing recovery yields, mediocre product purity, or inadequate scalability (13)(14)(15)(16). Notably, best-established PHA recovery methods, which generate superior recovery yields and maximum product purity, are based on extraction techniques using noxious halogenated solvents, predominately chloroform, in other words, materials that should not play a role in a sustainable production chain anymore.…”

Advances in Polyhydroxyalkanoate (PHA) Production, Volume 2

“…Still, experts in this field emphasize that sustainable and efficient PHA production requires the understanding and optimization of all individual process steps [2]. The holistic improvement of PHA production, applicable also on an industrial scale, inter alia calls for: optimized bioprocess engineering and adapted fermentation modes [3], consolidated knowledge about the enzymatic, metabolic, and genetic ongoings in PHA accumulating organisms in the context of "Next Generation Industrial Biotechnology" [4], the multi-facetted role of PHA granules in living cells and the impact of environmental stress factors on PHA formation [5], an in-depth understanding of the kinetics of the bioprocess [6], the selection of ethically clear, inexpensive feedstocks [7,8], tailoring the composition of PHA on the level of the monomeric constituents [9], and efficient and ecologically benign strategies for PHA recovery from biomass [10].…”