The benefit of biodegradable “green plastics” over established synthetic plastics from petro-chemistry, namely their complete degradation and safe disposal, makes them attractive for use in various fields, including agriculture, food packaging, and the biomedical and pharmaceutical sector. In this context, microbial polyhydroxyalkanoates (PHA) are auspicious biodegradable plastic-like polyesters that are considered to exert less environmental burden if compared to polymers derived from fossil resources.
The question of environmental and economic superiority of bio-plastics has inspired innumerable scientists during the last decades. As a matter of fact, bio-plastics like PHA have inherent economic drawbacks compared to plastics from fossil resources; they typically have higher raw material costs, and the processes are of lower productivity and are often still in the infancy of their technical development. This explains that it is no trivial task to get down the advantage of fossil-based competitors on the plastic market. Therefore, the market success of biopolymers like PHA requires R&D progress at all stages of the production chain in order to compensate for this disadvantage, especially as long as fossil resources are still available at an ecologically unjustifiable price as it does today.
Ecological performance is, although a logical argument for biopolymers in general, not sufficient to make industry and the society switch from established plastics to bio-alternatives. On the one hand, the review highlights that there’s indeed an urgent necessity to switch to such alternatives; on the other hand, it demonstrates the individual stages of the production chain, which need to be addressed to make PHA competitive in economic, environmental, ethical, and performance-related terms. In addition, it is demonstrated how new, smart PHA-based materials can be designed, which meet the customer’s expectations when applied, e.g., in the biomedical or food packaging sector.
Polyhydroxyalkanoates (PHA), the only group of “bioplastics” sensu stricto, are accumulated by various prokaryotes as intracellular “carbonosomes”. When exposed to exogenous stress or starvation, presence of these microbial polyoxoesters of hydroxyalkanoates assists microbes to survive.
“Bioplastics” such as PHA must be competitive with petrochemically manufactured plastics both in terms of material quality and manufacturing economics. Cost-effectiveness calculations clearly show that PHA production costs, in addition to bioreactor equipment and downstream technology, are mainly due to raw material costs. The reason for this is PHA production on an industrial scale currently relying on expensive, nutritionally relevant “1st-generation feedstocks”, such as like glucose, starch or edible oils. As a way out, carbon-rich industrial waste streams (“2nd-generation feedstocks”) can be used that are not in competition with the supply of food; this strategy not only reduces PHA production costs, but can also make a significant contribution to safeguarding food supplies in various disadvantaged parts of the world. This approach increases the economics of PHA production, improves the sustainability of the entire lifecycle of these materials, and makes them unassailable from an ethical perspective.
In this context, our EU-funded projects ANIMPOL and WHEYPOL, carried out by collaborative consortia of academic and industrial partners, successfully developed PHA production processes, which resort to waste streams amply available in Europe. As real 2nd-generation feedstocks”, waste lipids and crude glycerol from animal-processing and biodiesel industry, and surplus whey from dairy and cheese making industry were used in these processes. Cost estimations made by our project partners determine PHA production prices below 3 € (WHEYPOL) and even less than 2 € (ANIMPOL), respectively, per kg; these values already reach the benchmark of economic feasibility.
The presented studies clearly show that the use of selected high-carbon waste streams of (agro)industrial origin contributes significantly to the cost-effectiveness and sustainability of PHA biopolyester production on an industrial scale.
Polyhydroxyalkanoates, microbial polyesters produced in vivo starting from renewable resources, are considered the future materials of choice to compete recalcitrant petro-chemical plastic on the polymer market. In order to make polyhydroxyalkanoates market-fit, (techno)economics of their production need to be improved. Among the multifarious factors affecting costs of polyhydroxyalkanoate production, increased volumetric productivity is of utmost importance. Improving microbial growth kinetics and increasing cell density are strategies leading to a high concentration of catalytically active biomass within a short time; after changing cultivation conditions, these cells can accumulate polyhydroxyalkanoates as intracellular products. The resulting increase of volumetric productivity for polyhydroxyalkanoates can be realized by supplying complex nitrogen sources to growing microbial cultures. In the present study, the impact of different expensive and inexpensive complex nitrogen sources, in particular whey retentate, on the growth and specific growth rates of Hydrogenophaga pseudoflava was tested.
Based on a detailed kinetic process analysis, the study demonstrates that especially whole (not hydrolyzed) whey retentate, an amply available surplus material from dairy industry, displays positive effects on cultivations of H. pseudoflava in defined media (increase of concentration of catalytically active biomass after 26.25 h of cultivation by about 50%, increase of specific growth rate μ from 0.28 to 0.41 1/h during exponential growth), while inhibiting effects (inhibition constant K i = 6.1 g/L) of acidically hydrolyzed whey retentate need to be overcome. Considering the huge amounts of surplus whey accruing especially in Europe, the combined utilization of whey permeate (carbon source) and whey retentate (complex nitrogen source) for biopolyester production can be considered a viable bioeconomic strategy for the next future.
Competitive polyhydroxyalkanoate (PHAs) production requires progress in microbial strain performance, feedstock selection, downstream processing, and more importantly according to the process design with process kinetics of the microbial growth phase and the phase of product formation. The multistage continuous production in a bioreactor cascade was described for the first time in a continuously operated, flexible five-stage bioreactor cascade that mimics the characteristics involved in the engineering process of tubular plug flow reactors. This process was developed and used for Cupriavidus necator-mediated PHA production at high volumetric and specific PHA productivity (up to 2.31 g/(Lh) and 0.105 g/(gh), respectively). Based on the experimental data, formal kinetic and high structured kinetic models were established, accompanied by footprint area analysis of binary imaged cells. As a result of the study, there has been an enhanced understanding of the long-term continuous PHA production under balanced, transient, and nutrient-deficient conditions that was achieved on both the micro and the macro kinetic level. It can also be concluded that there were novel insights into the complex metabolic occurrences that developed during the multistage- continuous production of PHA as a secondary metabolite. This development was essential in paving the way for further process improvement. At the same time, a new method of specific growth rate and specific production rate based on footprint area analysis was established by using the electron microscope.