Kombucha tea is all the rage these days as a handy substitute for alcoholic beverages and for its supposed health benefits. The chemistry behind this popular fermented beverage is also inspiring scientists at MIT and Imperial College London to create new kinds of tough “living materials” that could one day be used as biosensors, helping purify water or detect damage to “smart” packing materials, according to a recent paper published in Nature Materials.

You only need three basic ingredients to make kombucha. Just combine tea and sugar with a kombucha culture known as a SCOBY (symbiotic culture of bacteria and yeast), aka the “mother,” also known as a tea mushroom, tea fungus, or a Manchurian mushroom. (It’s believed that kombucha tea originated in Manchuria, China, or possibly Russia.) It’s basically akin to a sourdough starter. A SCOBY is a firm, gel-like collection of cellulose fiber (biofilm), courtesy of the active bacteria in the culture creating the perfect breeding ground for the yeast and bacteria to flourish. Dissolve the sugar in non-chlorinated boiling water, then steep some tea leaves of your choice in the hot sugar water before discarding them.

Once the tea cools, add the SCOBY and pour the whole thing into a sterilized beaker or jar. Then cover the beaker or jar with a paper towel or cheesecloth to keep out insects, let it sit for two to three weeks, and voila! You’ve got your own home-brewed kombucha. A new “daughter” SCOBY will be floating right at the top of the liquid (technically known in this form as a pellicle). But be forewarned: it’s important to avoid contamination during preparation because drinking tainted kombucha can have serious, even fatal, adverse effects. And despite claims that drinking kombucha tea can treat aging, arthritis, cancer, constipation, diabetes, or even AIDS, to date there is no solid scientific evidence to back those claims.

There are two kinds of fermentation taking place: alcoholic fermentation and acetic acid fermentation, and a really good kombucha will strike the perfect balance between them. The yeast in the SCOBY produces an enzyme (invertase) that breaks apart the sugar into fructose and glucose. The glucose then breaks down into pyruvate, acetaldehyde, and finally ethanol, releasing carbon dioxide as a byproduct to give kombucha that pleasing touch of carbonation.

It’s not a lot of ethanol, since the bacteria in the SCOBY converts much of it into acetic acid. Too much alcohol would actually stop the fermentation process. So most kombucha teas have less alcohol than even a very light beer. (You can get higher alcohol concentrations it you add too much sugar and/or let the stuff ferment too long, but then your kombucha will probably just taste like straight vinegar.)

Beyond its popularity as a beverage, kombucha holds promise as a useful biomaterial. For instance, back in 2016, an Iowa State professor of apparel, merchandising, and design named Young-A Lee gained attention for her proof-of-concept research in using dried SCOBY as a sustainable leather substitute. She thought it might be possible to make biodegradable SCOBY-based clothing, shoes, or handbags.

There are still a few hurdles to surmount to make SCOBY a viable substitute, most notably how the material responds to moisture in the air (it softens) and cold temperatures (it becomes brittle). It also takes three to four weeks to grow SCOBY in the lab, then dry and treat it to get that leathery consistency, which is not conducive to mass production. Nonetheless, Lee thought the material held promise, even making a prototype vest and shoes from the cellulose fiber.

This latest study builds on earlier work by MIT engineer Timothy Lu, who used E. coli to make biofilms embedded with gold nanowires. Those films weren’t really viable for large-scale applications, so he teamed up with colleagues at MIT and Imperial College London to come up with a method for generating tougher materials using microbes. Kombucha was a natural choice.

Lu et al. couldn’t use the wild yeasts typically used in kombucha, because they are difficult to modify genetically. They used lab-grown yeast instead, specifically a strain called Saccharomyces cerevisiae, which they then combined with a bacteria called Komagataeibacter rhaeticus (which can create a lot of cellulose) to create their “mother” SCOBY. (They call it a “Syn-SCOBY.”) They were able to engineer the cells in the yeast to produce glow-in-the-dark enzymes, for instance, or those that could sense pollutants and then break them down after detection. One of their prototype materials senses the pollutant estradiol, for instance, while another could detect luciferase, a bioluminescent protein.

Any number of other strains can be swapped out to achieve different functional properties. The authors contend that their research has yielded a fast, simple method for generating biosensor materials with broad applications. It only took a few days to grow their “living material,” and they found it could grow sufficiently over longer time periods to be the size of a bathtub—at least a thousand times more material than achieved with E.coli, according to co-author Tzu-Chieh Tang, a grad student in Lu’s lab.

In fact, the researchers also demonstrated that they could grow these living materials in tea with sugar, with the possibility that one day people could, say, grow their own water filters at home. “We foresee a future where diverse materials could be grown at home or in local production facilities, using biology rather than resource-intensive centralized manufacturing,” said  Lu.

“Organisms are remarkable, material-producing systems capable of self-assembling complex materials with diverse chemical and physical properties starting only from simple feedstocks,” the authors concluded. “Synthetic material systems capable of recreating all of these behaviors do not exist. … The ability to genetically control the process of material self-assembly seen in natural biological materials could revolution[ize] the manufacture of products for use in numerous areas of human life and society.”

DOI: Nature Materials, 2021. 10.1038/s41563-020-00857-5  (About DOIs).

https://arstechnica.com/?p=1741694