Abstract
Together with other so-called “bio-plastics”, Polyhydroxyalkanoates (PHAs) are expected to soon replace established polymers on the plastic market. As a prerequisite, optimized process design is needed to make PHAs attractive in terms of costs and quality. Nowadays, large-scale PHA production relies on discontinuous fed-batch cultivation in huge bioreactors. Such processes presuppose numerous shortcomings such as nonproductive time for reactor revamping, irregular product quality, limited possibility for supply of certain carbon substrates, and, most of all, insufficient productivity. Therefore, single- and multistage continuous PHA biosynthesis is increasingly investigated for production of different types of microbial PHAs; this goes for rather crystalline, thermoplastic PHA homopolyesters as well as for highly flexible PHA copolyesters, and even blocky-structured PHAs consisting of alternating soft and hard segments. Apart from enhanced productivity and constant product quality, chemostat processes can be used to elucidate kinetics of cell growth and PHA formation under constant process conditions. Furthermore, continuous enrichment processes constitute a tool to isolate novel powerful PHA-producing microbial strains adapted to special environmental conditions. The article discusses challenges, potential and case studies for continuous PHA production, and shows up new strategies to further enhance such processes economically by developing unsterile open continuous processes combined with the application of inexpensive carbon feedstocks.
Highlights
Continuous culture fermentation strategy has unquestioned advantages over established disand semi-continuous approaches, it is still anticipating its broad implementation at the industrial scale [1].On a laboratory- to pilot-scale, this technique has already been successfully tested with intact active microorganisms, either free or immobilized
At a D of 0.24 1/h, medium-chain-length PHA (mcl-PHA) copolyesters with constant shares of four different monomeric building blocks, namely 3HB, 3-hydroxyhexanoate (3HHx), 3-hydroxyoctanoate (3HO), and 3-hydroxydecanoate (3HD), was produced under nitrogen limited conditions; the intracellular mass fraction amounted to 0.13 g mcl-PHA per g cell dry mass (CDM)
Ps. oleovorans; here, mcl-PHA accumulation is maximal (mass fraction of 0.63 g per g CDM, volumetric productivity 1.06 g/(Lh)) when the cells are cultivated at a D of 0.22 1/h in the first stage, and 0.16 1/h in the second stage, whereas μmax. of this strain amounts to 0.48 1/h as reveled by single-stage chemostat cultures
Summary
Continuous culture fermentation strategy has unquestioned advantages over established disand semi-continuous approaches, it is still anticipating its broad implementation at the industrial scale [1]. For efficient large-scale production, PHA-producing organisms have to be cultivated under controlled conditions in closed bioreactors, where stability of process parameters (pH-value, temperature, oxygen supply, substrate concentration) can be warranted, and monoseptic conditions are guaranteed by excluding microbial competitors [27]. Apart from enhanced productivity and product quality (composition, molar mass and PDI of PHA), chemostat processes are suitable to elucidate the physiological background of bioprocesses; kinetics of cell growth and PHA formation under constant environmental conditions, and the impact of changing conditions on the kinetics and on product properties, can conveniently be investigated [39] This way, the optimization of nutritional media composition can be accomplished in chemostat processes by changing concentrations of single nutrients in the feed stream during steady-state, and monitoring the resulting reaction of microbial kinetics (growth rate, product formation rate, yields) [41].
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