The Economist, 6 de julho de 2017 – Edição impressa
A better way to make drinks and drugs
Print me a brewery
SINCE the dawn of civilisation, people have used yeast to leaven bread, ferment wine and brew beer. In the modern era, such fermentation has extended its range. Carefully selected moulds churn out antibiotics. Specially engineered bacteria, living in high-tech bioreactors, pump out proteinaceous drugs such as insulin. Some brave souls even talk of taking on the petroleum industry by designing yeast or algae that will synthesise alternatives to aviation fuel and the like.
But fermentation remains a messy process, and one prone to spoilage and waste. Whatever the product, the reaction must generally be shut down after a matter of days to clean out the detritus of biological activity—both cells that have died and the surplus of living ones which growth and reproduction inevitably yield. Alshakim Nelson, a chemist at the University of Washington, in Seattle, and his team, propose to change all that. They have developed a bioreactor that not only keeps bugs alive and active for months at a time, but can also be made in minutes, using low-cost chemicals and a 3D printer.
Dr Nelson’s bioreactors are composed of a substance called a hydrogel, which is about 70% water. The remaining 30% is a special polymer, infused with yeast. Unlike edible jelly, which, as parents of small children will know, breaks into tiny lumps when squeezed, Dr Nelson’s hydrogel has a consistency resembling peanut butter. That permits it to be extruded smoothly through the nozzle of a 3D printer.
Dr Nelson’s team have built a printer specifically designed to do this. Their device lays down thin strips of hydrogel in a cubic lattice structure (see picture) intended to maximise the amount of surface area for a given volume of material. The cube, which has sides 1cm long in the current design, is then cured by a burst of ultraviolet light, to increase its rigidity. Turning one out takes about five minutes.
The fun starts when such a cube is plopped into a solution of glucose. The hydrogel is permeable to this solution, so the yeast is able to get to work on the glucose, converting it into ethanol as if it were the sugar in the wort of a brewery.
This, Dr Nelson had predicted. The surprise was that it keeps on doing so, day after day, week after week, as long as the fermented solution is regularly replaced with fresh. The team’s bioreactors have continued to produce ethanol in this way for over four months now, with no signs of slowing down. The cause of this desirable phenomenon is not yet clear. Dr Nelson believes that immobilising the yeast cells in the hydrogel somehow stops them both ageing and reproducing, without affecting their ability to ferment. Somehow, the cells’ confinement is signalling to them to stop growing without affecting their normal metabolism.
That discovery has enormous potential. If it could be industrialised, it would pave the way for continuous fermentation to replace today’s batch-processing approach, with all the advantages such continuity of production would bring. To this end, Dr Nelson now plans to scale up the size of the cubes. He also proposes to experiment with yeast cells engineered to turn out more complex molecules than ethanol—proteins, for example—that might have purposes other than getting people drunk. This may require tweaking the hydrogel, the current structure of which is likely to be too dense to permit the passage of a large protein molecule.
In the longer term, it is possible to imagine a chain of bioreactors, each specialised for a single step in the synthetic pathway that leads to a desirable product such as a drug. Dr Nelson’s first task, though, will be to increase the concentration of glucose in the bioreactor design that he knows, without question, works, in the hope of brewing up something stronger in his laboratory. “Can we take our yeast,” he wonders, “embed it in hydrogel, print it as a cube, put it in fruit juice and convert it to alcohol?” That thought, of a cheap, domestic hooch plant which works for months on end, will have brewers around the world wanting to pour themselves a stiff drink.