Plastic bottles, especially the polyethylene terephthalate (PET) bottles most familiar to North American citizens, are often called out as one of the worst plastic waste offenders. According to a 2017 research article in the journal Science Advances, by 2015 alone, 8.3 billion metric tons of plastic had been produced globally. Only 9 percent of it had been recycled.
Single-use plastic attributes to much of that plastic waste, with over a million plastic bottles being purchased around the world every minute. Plastic waste is everywhere, but one of the most noticeable problems is the impact it has on oceans. The Ellen MacArthur Foundation’s New Plastics Economy report famously estimated that by 2050, there would be more pounds of plastic in the ocean than there would be pounds of fish.
Many initiatives are in place with the goal of tackling plastic pollution, by either removing it from the environment or recycling it into more useful material. Plastic Bank is one organization working to take the waste already present in the ecosystem and find a use for it. The group has recycled more than 22 million kilograms of plastic at 558 locations in Haiti, Brazil, Indonesia, the Philippines and Egypt. Workers collect plastic waste and turn it in so that it can be sorted by base material and color. The waste is recycled as “Social Plastic,” usually consumer goods or packaging. The circular economy envisioned in the Ellen MacArthur Foundation report can be achieved when people continue to find unique ways to deal with the plastic pollution that already exists.
Enzymes Can Break the Plastic Down—But Can We Use the New Material?
Scientists and engineers are tackling the problem of plastic pollution on several fronts. French company Carbios uses a process it calls enzymatic recycling to break down plastic pieces using enzymes. A demonstration production plant is currently under construction in Clermont-Ferrand, France.
Joanna Sadler and Stephen Wallace from the University of Edinburgh recently published research detailing their work with plastic waste. Their article “Microbial synthesis of vanillin from waste poly(ethylene terephthalate)" from the journal Green Chemistry shows the process the pair used to produce vanilla flavoring from PET bottles. The report starts with a discussion of the plastic pollution crisis, noting that the characteristics that make PET so useful to us as a material also make it incredibly difficult to break down.
Sadler and Wallace note that using enzymes to break down PET molecules is a problem that can be solved between the fields of microbiology, synthetic biology, and biocatalysis. The Ideonella sakaiensis bacteria was first found outside of a bottle recycling plant in Japan in 2016 and is currently the favored enzyme used by scientists in the breakdown of PET waste. A super-enzyme discovered in 2018 is expected to eat the plastic even faster, with terephthalate as the by-product. Taking the current problem and removing it is a strong idea, but resources and time are required to remove the plastic waste from the ecosystem. What could happen if the plastic waste was taken and modified into something with value?
The Edinburgh team built a hypothesis that this terephthalate by-product could be upcycled to create vanillin through a five-step process. Aztecs used vanilla as a flavoring way back in the 16th century, but it took until the 1850s to synthesize vanillin as its main flavor component. Vanillin is a food and cosmetics additive that is also used in herbicides, cleaning products, and antifoaming agents. Demand for vanillin is much greater than the current supply of natural vanilla, and chemists are working hard to find synthetic vanillin.
The Testing Methods and Novel Approach to Synthetic Vanillin
The journal article describes the methods used to create vanillin from PET waste, starting with the choice of the base microorganism. Knowing that E. coli MG1655 RARE was previously used to create vanillin from glucose, the team started there. After a successful first test to show production of vanillin using multiple plasmids and different concentrations of terephthalate, the team tried to optimize yield.
In the optimization experiments, several parameters were changed and additives moved into the mix to find the best combination of circumstances that would produce the highest yield.
After the best possible climate for vanillin production was agreed upon, the tests on actual PET waste began. The thermostable enzyme leaf-branch compost cutinase (LCC) was used due to its ability to release terephthalate directly from the plastic without requiring an additional ingredient. A post-consumer PET bottle was treated with the LCC at 72ºC (161.6ºF) and cooled to room temperature, with results tallied up 24 hours later. Without process optimization, the experiment produced vanillin, while a control experiment lacking PET failed to produce vanillin. A control experiment that lacked LCC did create some vanillin, and the hypothesis is that hydrolysis occurred for its creation.
The article concludes with the authors saying that PET waste can be used to create vanillin and that the process can be optimized. Bacterium can be added to waste plastic to synthesize a value-added molecule using a single microorganism. The reactions are mild and occur under ambient conditions, materials required for the reactions are plentiful, and no hazardous waste is produced from the synthesis. A supplementary document that looks deeper into the methods used in the experiment, the chemistry behind the reactions, and the results can be found here. Overall, the journal article estimates that 79 percent of the terephthalate was converted to vanillin, and the group believes that optimization could make that yield much higher.
Final Takeaways
Perhaps the most important part of the conclusion is the sentence: “Fundamentally, this work substantiates the philosophy that post-consumer plastic may be viewed not as a waste product, but rather as a carbon resource and feedstock to produce high value and industrially relevant materials and small molecules.”
Following the idea of a circular economy, it might be possible to take plastic waste and create value from it. This chemical production of vanillin from PET bottles is one example that is in its very early stages. Sadler and Wallace note that process optimization and attempts to duplicate results using other plastic waste will occur in future studies.
We know that the world is using more plastic than it recycles. The net amount of plastic pollution goes up every day, and without broad social, economic or legislative change, that is likely to continue. If our near-future state is plastic pollution existing and more plastic pollution coming, this is a great opportunity for us to engineer our way out of a problem. If more solutions like the chemical synthesis of plastic bottles into vanillin can be found, then maybe we can shift the perception of plastic as waste to being that of a valuable thing, and more people will collect and remove plastic from the environment.