M. Koller and A. Mukherjee, Polyhydroxyalkanoates…, Chem. Biochem. Eng. Q., 34 (3) 115–129 (2020) 115 Polyhydroxyalkanoates – Linking Properties, Applications, This work is licensed under a and End-of-life Options Creative Commons Attribution 4.0 International License M. Kollera,b* and A. Mukherjeec aUniversity of Graz, Office of Research Management and Service, c/o Institute of Chemistry, NAWI Graz, Heinrichstrasse 28/IV, 8010 Graz; bARENA – Association for Resource Efficient and https://doi.org/10.15255/CABEQ.2020.1819 Sustainable Technologies, Inffeldgasse 21b, 8010 Graz, Austria Review cGlobal Organization for PHA (GO!PHA), Amsterdam, Received: May 6, 2020 The Netherlands Accepted: October 5, 2020 When it comes to “bioplastics”, we currently notice an immense complexity of this topic, and, most of all, a plethora of contradictory legislations, which confuses or even misleads insufficiently informed consumers. The present article therefore showcases mi- crobial polyhydroxyalkanoate (PHA) biopolyesters as the prime class of “bioplastics” sensu stricto. In particular, biodegradability of PHA as its central benefit in elevating the current plastic waste scenario is elaborated on the biochemical basis: this covers aspects of the enzymatic machinery involved both in intra- and extracellular PHA degradation, and environmental factors impacting biodegradability. Importantly, PHA degradability is contextualized with potential fields of application of these materials. It is further shown how the particularities of PHA in terms of feedstocks, mode of synthesis, degradability, and compostability differ from other polymeric materials sold as “bioplastics”, highlight- ing the unique selling points of PHA as “green” plastic products in the circular economy. Moreover, current standards, norms, and certificates applicable to PHA are presented as basis for a straight-forward, scientifically grounded classification of “bioplastics”. Keywords: biodegradability, biopolymers, certifications, composting, depolymerases, polyhydroxy- alkanoates Introduction: The plastics pollution – fills and the environment – double the amount that origin and effects existed in 2015.1 In 2016, the Ellen MacArthur Foundation published a report on plastics produc- Being useful, practical and inexpensive, poly- tion with input from numerous industry experts and meric materials or plastics are indeed ubiquitous in organizations. Authors came up with the conclusion our daily lives. During the decades between 1940 that, in 2015, almost 95 million tons of packaging and now, synthetic plastics that are not biodegrad- plastics were consumed, constituting the primary able in nature and of petrochemical origin have source of plastics pollution. They also estimated dominated growth. However, their uncritical dis- that plastics production uses 6 % of the total fossil posal and accumulation in the ecosphere have re- crude oil extracted, and contribute 1 % of the total sulted in an environmental catastrophe. In this con- carbon emission when assuming its post-use incin- text, Geyer et al. published a seminal paper in 2017, eration as common fate of all plastic. Without major outlining the use and fate of all plastics. They con- mitigation efforts, by 2050, plastics would consume cluded that up until 2015, 8.3 · 109 metric tons of 20 % of the world fossil feedstock production and plastics were produced globally, with only 9 % of it represent 15 % of the carbon emissions.2 being recycled, 12 % incinerated, but 79 % (corre- At some point in Earth´s history, massive 9 sponding to 6.3 · 10 metric tons) discarded in land- amounts of carbon-containing plant and animal bio- fills or simply dumped in the environment. Further- 9 mass were buried, which then, under pressure, more, they estimated that by 2050, 12 · 10 metric turned into what we describe today as fossil fuels, tons of plastic waste will be accumulated in land- such as coal, petroleum, and natural gas. After the discovery of these fossil fuels, which was accompa- *Corresponding author: [email protected]; nied by the onset of the industrial revolution, man- Tel.: +43-316-380-5463 kind started to release the carbon that was seques- M. Koller and A. Mukherjee, Polyhydroxyalkanoates… 115–129 116 M. Koller and A. Mukherjee, Polyhydroxyalkanoates…, Chem. Biochem. Eng. Q., 34 (3) 115–129 (2020) tered over a period of hundreds of millions of years. In the meanwhile, mankind has transformed This has caused the increase in greenhouse gases biodegradation into an industrial process by increas- (GHG), particularly CO2, CH4 and NOx, contribut- ing the temperature and/or by adding specific mi- ing to the climate change we are experiencing to- croorganisms; we call it “industrial composting”. In day. Plastics, a natural offshoot of petroleum and industrial composting facilities, compost tempera- natural gas, have played their part, although their tures exceed 50 °C due to the metabolic heat gener- contribution to global warming has been relatively ated by the microbes thriving in it. This high tem- small compared to GHG emissions due to energy perature is also a legal requirement because it generation, transportation, or even agriculture. ensures sufficient pasteurization of the final product However, plastics have had another more serious (compost). The same process of biodegrading natu- and deleterious effect on the planet, including its ral materials can be carried out at lower tempera- oceans and life therein, through the accumulation of tures; this process is known as “home composting”. these fossil-based materials that do not degrade into Here, temperatures as high as in industrial com- CO2, CH4 or water anytime soon. Solutions to plas- posters are typically not reached or only in the core, tics accumulation or pollution is a necessary and because the compost is agitated/turned less often, urgent issue that needs large-scale innovative solu- and has a surface-to-volume ratio beneficial for tions. While numerous methods have been devel- cooling by the ambient. Also, due to a lower aera- oped over decades and are being employed today, tion, microbial growth is less intensive and less heat such as collection and then recycling or incinera- is generated. The timeline is also important in bio- tion, as Geyer et al. notes, these methods do not degradation and/or composting performance, be- address the entire breadth of the problem of plastics cause higher temperatures in industrial composting pollution described above. While these methods and facilities speed up biodegradation, while the process processes can be broadened to cover additional of home composting takes longer; however, in both plastics waste streams, they will not cover numer- processes, biodegradation occurs. ous sources of plastics pollution given their ubiqui- 1 Circularity of natural materials has been at play tous nature. since the origin of life, and the Earth has kept a bal- Recycling is necessary and needs to be im- ance with respect to GHG. Therefore, mimicking proved, but it also comes at substantial costs, in- nature is the best solution when it comes to materi- cluding huge energy consumption and in turn more als production and plastics pollution prevention, in GHG emissions, and it would not address the entire order to ensure early in advance that accumulation plastics waste crisis, including microplastics, gener- of these materials, which have a devastating effect ated from disintegrating larger plastic articles, and on life on Earth, cannot occur beforehand. This in- from fibers and fabrics, paints and coatings, adhe- 1 cludes producing materials using renewable sourc- sives, shoes, and car tires. As a consequence of the es, such as CO and/or CH and water, and, at the plastics pollution, 127 countries have banned sin- 2 4 3–6 end of life span, returning the material to nature, gle-use or other plastics. However, banning plas- i.e., allowing nature to convert these materials again tics use or relying solely on recycling and incinera- into CO2, CH4, water, and biomass. There are nu- tion are insufficient strategies to eliminate the merous materials that can and are being made using negative environmental impacts of plastic waste renewable CO2 or CH4 as the carbon source; how- and GHG emissions, now or in the future. The scale ever, there are only few materials that act like plas- of the problem requires additional innovative solu- tics and also turn into CO2, CH4, water, and bio- tions, especially such solutions resorting to biode- mass. Polyhydroxyalkanoates (PHA), a family of gradable materials. naturally occurring microbial polyesters, are one Biodegradation of polymers such class of materials, which also have many of the beneficial properties of the top seven best-sell- Biodegradability can be considered an end-of- ing types of petro-plastics (PET, HDPE, PVC, life option for consumer plastics produced for sin- LDPE, PP, PS, PA), thereby making them the most gle-use. Known natural polymeric materials typical- suitable material to replace many different plastics ly biodegrade in the environment under various used today, especially but not exclusively those conditions; they turn into CO2, water, and biomass used in single use applications and packaging that 7–9 in the presence of O2, and into CO2, CH4, water and are difficult to collect or recycle. The wide range biomass during anaerobic digestion. Importantly, of chemical, physical, and mechanical properties of the process of biodegradation of natural materials is PHA is due to the fact that these