What Yannick Branson is holding in his hands in a laboratory at the University of Greifswald initially looks unspectacular: scraps of foam from an old pillow and a tube with a dark brown liquid. There is an important question behind this: What are we going to do with the plastic waste that is increasing worldwide? One approach: similar to how food is digested, enzymes could break down plastic into its components so that new plastics can be recycled.
"This is actually a very common or hot topic at the moment," says the doctoral student at the Institute of Biochemistry at the University of Greifswald. He is part of a team that found three enzymes that can break down polyurethane into its component parts. Polyurethane is found, for example, in mattresses, insulating materials, but also in sneakers - or in cushions, such as Branson's foam scraps.
Enzymes break down polymers into their units
After a chemical pre-treatment, the material turns into the somewhat unappetizing-looking liquid. According to Branson, part of this can be reused directly, and another part is broken down into its basic building blocks by the enzymes. "All in all, the course has been set for full-fledged recycling." The plastics consist of molecular chains - so-called polymers. If these are broken down into their individual units, new plastics can be made from them.
Christian Sonnendecker from the Institute for Analytical Chemistry at the University of Leipzig praises the people in Greifswald in their search for enzymes. "Impressive results were also achieved." Sonnendecker itself deals with a substance that is familiar from everyday life in the form of plastic bottles, foils or other light packaging: PET (polyethylene terephthalate).
With his team, he discovered an enzyme that can decompose PET particularly quickly. They found what they were looking for in a cemetery. There, the scientists specifically took samples of leaf compost and found in one of them the blueprint of the enzyme PHL7, which decomposed PET in the laboratory at record speed.
Those are enzymes
Uwe Bornscheuer, who heads the Biotechnology and Enzyme Catalysis working group in Greifswald, explains that enzymes are responsible for metabolism in all living beings - whether human, animal, plant, fungus or bacterium. They chopped up our breakfast by breaking down starches, fats and proteins, for example. "And we can use that in the field of biotechnology." The principle can be transferred to plastics.
There are already chemical processes to decompose the plastic polyurethane. However, these require high temperatures and high pressure and therefore a lot of energy. The enzymes that have now been discovered manage the degradation under so-called mild conditions - i.e. under normal pressure and at temperatures of up to around 40 degrees. "There are two important advantages," explains Bornscheuer. "I save energy for the process and at the same time I have good access to the building blocks, so I can achieve recycling of the plastic."
He compares the enzymes found by his Greifswald team with a needle in a haystack. The researchers said they had tested around two million candidates. According to Bornscheuer, however, it will still be a few years before the results can be used industrially.
Millions of candidates tested
The team works with a company. It has also already decomposed polyvinyl alcohols (PVA), which are used as foils for packaging, with enzymes, as a group led by Bornscheuer recently reported in the journal "Angewandte Chemie".
In Leipzig, according to their own statement, they want to use their own research results as part of a start-up soon. In France, for example, work is already being done on PET recycling using enzymes on an industrial scale. One hopes that one's own enzyme will work much faster, says Sonnendecker.
However, the researcher is aware of the limitations of the method. He believes that there will not be a sensible enzymatic recycling option for all common plastics. The Greifswald biochemist Bornscheuer also assumes that enzymes are not a panacea against the plastic flood.
He refers, for example, to the plastic waste in the oceans. "Those are the sins of the past." And currently an estimated 360 million tons of plastic are produced worldwide. And the trend is rising: "Every year there are about 20 million more tons."