Continuous Production Mode as a Viable Process-Engineering Tool for Efficient Poly(hydroxyalkanoate) (PHA) Bio-Production

Among all types of biopolymers occurring in living cells, poly(hydroxyalkanoates) (PHAs) display the highest potential for applications in diverse fields of the plastic market.1,2,3,4 PHAs are biobased, biocompatible and biodegradable polyesters of hydroxyalkanoic acids synthesized by prokaryotic microbes as intracellular carbon and energy storage compounds.5,6 Additionally, PHA synthesis by genetically modified plants harboring bacterial PHA synthesis genes is reported.7,8 Their material properties are similar to non-biodegradable, highly recalcitrant plastics of fossil origin. Therefore they are expected to substitute them to a significant extent in the not too distant future.9 In contrast to plastics of fossil origin, PHAs are produced starting from renewable resources, predominately carbohydrates, lipids or organic acids.10 Owing to their complete biodegradability, their ecological advantages are generally acknowledged.11,12,13,14,15 At the same time, production price of PHAs are still higher than for “classical” plastics, mainly due to the high cost of raw materials, downstream processing costs and low productivities caused by inappropriate process design.16,17,18,19 PHAs consist of almost 150 different compounds as monomeric building blocks. Based on the chain length of these building blocks, they can be categorized into short chain length PHA (scl-PHA) and medium chain length PHA (mcl-PHA).20 Monomers of scl-PHA consist of 3 to 5 carbon atoms and mainly constitute R-configured chiral 3-hydroxyalkanoates. Due to their physical characteristics, sclPHAs mainly exhibit properties of classical thermoplastics to be applied for packaging or everyday plastic commodities.19 Therefore, they compete on the market with poly(olefines) and, in the field of “green plastics”, also with biobased poly(lactate). Typically, scl-PHAs are accessible from sugars like glucose, fructose, sucrose, or lactose. Cupriavidus necator, a member of the bacterial group of Burkholderiaceae, constitutes the best investigated microbial scl-PHA producer.10,21 In contrast, mcl-PHAs Continuous Production Mode as a Viable Process-Engineering Tool for Efficient Poly(hydroxyalkanoate) (PHA) Bio-Production