Courtesy Visy.
Recycling is something most of us support. But would you eat an ice cream made in part from recycled drinks bottles, and with a bit of sometimes toxic bacteria thrown in for good measure? A group of Scottish scientists think you should. By Jeremy Torr.
Edinburgh, June 2021. Two things to consider. First, some 50 million tonnes of polyethylene terephthalate (PET) waste hits refuse tips every year. It is made from carbon-based, non-renewable petrochemical source materials (oil and gas). Second, E.coli (Escherichia coli) is a normally harmless bacterium that lives in the intestinal tract. But some strains can cause bloody diarrhea, vomiting and worse. Including kidney failure.
So what better than to mix the two together to produce the flavouring used in almost all ice cream and cakes – vanillin?
Researchers from the University of Edinburgh have discovered that if they introduce a specific quantity of modified E.coli bacteria to a mashed up mix of PET waste, heat it up and stand back, one of the by-products is vanillin.
“This is only the beginning.” - Dr. Joanna Sadler.
“This is the first example of using a biological system to upcycle plastic waste into a valuable industrial chemical,” says Joanna Sadler, a Discovery Fellow at the School of Biological Sciences, University of Edinburgh. “This (research) has very exciting implications for the circular economy. The results … have major implications for the field of plastic sustainability and demonstrate the power of synthetic biology to address real-world challenges.”
Add to all this that the world’s supply of vanilla and vanillin is soaring, with raw vanilla increasing in price by 120% last year, and the process looks very attractive despite the perceived taste. Indeed, any new source of vanillin (which is also used in cosmetics, herbicides, antifoaming agents and cleaning products) can only help assuage global thirst for the additive which stood at just under 40,000 tonnes in recent years – worth $448.5 million by 2025. If it is a recycled product, even better.
Proposed enzymatic pathway for converting PET to vanillin. Courtesy Stephen Wallace/University of Edinburgh
Sadler’s team discovered that one of the key elements of degraded PET waste, terephthalic acid, when given a spoonful or two of E.coli, can result in a near-80% conversion into a vanillin precursor. The Edinburgh researchers are confident this is just the first step into what could be a seriously viable industrial process that produces human-safe vanillin – one that could end up in an ice-cream or sponge sandwich cake near you.
“This is a really interesting use of microbial science at the molecular level to improve sustainability and work towards a circular economy,” says Dr Ellis Crawford from the Royal Society of Chemistry. “Using microbes to turn waste plastics, which are harmful to the environment, into an important commodity and platform molecule with broad applications in cosmetics and food is a beautiful demonstration of green chemistry.”
The new process takes cleaned recycled bottles, grinds the PET into “flakes” which are then washed and cleaned in a heated process before being broken down into the constituent parts, modified ethylene glycol and purified terephthalic acid.
The team took the terephthalic acid and using oxidation, methylation and reduction with a genetically engineered strain of E. coli, produced enzymes that directly converted terephthalic acid to vanillin in a biocatalytic transformation.
“A really interesting use of microbial science.” - Dr Ellis Crawford.
Stephen Wallace , another of the Edinburgh Biological Sciences team, added that the research results directly challenge the perception of plastic being “a problematic waste” and instead demonstrates its use as a new carbon resource.
“Instead of simply recycling plastic waste into more plastic, what our system demonstrates for the first time is that you can use plastic as a feedstock for microbial cells and transform it into something with higher value and more industrial utility,” asserts Wallace, who notes that the basic process could work equally well on other plastics.
‘To me, this is only the beginning,’ agrees Sadler. ‘I think we are in a really exciting place now that we are realising we can do all sorts of things with waste plastic.”