Words by Maya Hey, Art by Reena Mistry
“You’re not going to eat that, are you?” my friend asked, pointing to the inch-thick gelatinous film floating on top of a jar that I had vaguely labeled ‘experimental.’
“Oh, no. That’s just vinegar underneath. Well, at least it should be by now,” I responded, glancing at the jar I had forgotten about for a few weeks. A finger-dip later, I knew my batch of homebrewed beer was now homemade malt vinegar thanks to the help of a booch starter I’d thrown in one month prior. Out of curiosity—as well as to allay any further friend concerns—I then measured the pH. At 5.6, my experimental jar was now just a smidge milder than apple cider vinegar.
SCOBYs Don’t Just Float in Booch
Booch or western kombucha is a sweetened, effervescent, fermented tea often referred to simply as kombucha. Many celebrate it as an elixir, while others find it off-putting because of its tartness or the jelly-like film that forms partway through the brewing process. Although casual kombucha makers often call this film a SCOBY—short for Symbiotic Community of Bacteria and Yeast—this film is produced by only one species of bacteria (Komagataeibacter xylinus). The more the booch ferments, the more this bacterial species makes the film. What starts as a liquid surface becomes a gradually thickening solid mass that might elicit concerns from observant friends.
Booch represents one of many SCOBY examples where bacteria and yeasts work in tandem to ferment foods that we know and like. Other SCOBYs that help jumpstart ferments include sourdough for bread-baking and starters for saké brewing. In each of these examples—booch, bread, and saké—bacteria and yeast take cues from each other to produce a ferment. And, these biodiverse communities are not just important for the mechanics of fermentation, they also produce an array of complex flavors—some desirable, and some best avoided.
In booch fermentation, the film that forms on top serves as a liferaft for all the other species in the SCOBY. One species of bacteria produces the film and a dozen or so others band together on the film’s underside. But even this does not represent the entire community (or ‘C’ in SCOBY) of microorganisms. That’s why booch recipes often need the liquid starter from a previous batch as well, ensuring that the bacteria and yeast in the starter are diverse and plentiful as possible.
Having both bacteria and yeast in this microbial community is crucial because booch fermentation relies on a two-step process. First, the yeasts break down sugar into smaller pieces, then the bacteria take those pieces and make acids out of them. The film is a living testament to the collaborations between bacteria and yeast.
Setting the Stage: Acid-tolerant Bacteria are Enablers
Unlike commercially available yeast packets that get purified down to single strains, sourdough starter—the SCOBY responsible for naturally-leavened bread—contains bacteria (mostly lactobacilli) that can tolerate acidic environments. These bacteria produce lactic acid and help ‘set the stage’ for yeasts.
This happens in several ways: 1) since bacteria multiply quicker than their fungal counterparts, the fast-growing bacteria populations crowd out other opportunistic microbes, 2) with more lactobacilli, the environment becomes too acidic for other bacteria to take over while yeast and molds are tolerant enough to continue working, and 3) when lactobacilli digest wheat flour in particular, they produce an antifungal compound that deters mold.
This is why feeding a sourdough starter is so important. Feeding the starter helps give the bacteria a head start so that the acidic environment they create allows yeasts to thrive, all the while preventing molds from hijacking the fermentation process.
Although an acidic environment might be helpful at first, too much acid built up over too much time can compromise bread’s texture. Texture comes from gluten. We knead dough to develop what’s called a gluten matrix, and this matrix helps bread dough rise. Think of a well-developed gluten matrix as cubby shelving at a daycare center where the air bubbles produced by yeast fit neatly inside each hole like backpacks.
More air inside the holes means the dough inflates and ‘rises.’ But here’s the catch: gluten has two proteins, glutenin and gliadin—think of them as the wood and screws of our cubby shelf— and lactobacilli can digest the gliadin protein. When the lactobacilli in sourdough overpopulate, they look for other sources of food and start to break down the gliadin protein in gluten. It’s as if the bacteria eat all the metal screws in the cubby shelf, but none of the wood, and the shelving falls apart.
When part of the scaffolding collapses, the air bubbles escape the gluten matrix and we’re left with flat, unleavened bread. This is why sourdough-leavened bread tends to be denser, as is the style in Germany and Denmark, or deliberately unleavened as a flatbread like in the case of Ethiopian injera.
So is a fluffy sourdough still possible? Of course, because every ferment has a bell curve with a Goldilocks point for harvesting it. For bread, the challenge of using sourdough starters is finding the moment when the yeasts have produced enough air to leaven the dough, but before the lactobacilli have completely dismantled the gluten matrix.
For the Sake of Flavor: Acid-tolerant Bacteria are Tastemakers
What makes sourdough special is the complex flavors it features in contrast to plain white bread. These flavors are all thanks to the lactobacilli in sourdough starters which produce secondary flavor compounds like ethyl acetate (brandy-like aromas), methional (smells like potato and tomato), and fruit-forward furanones that bring subtle but distinct layers of taste.
While rapid-rising yeasts have their merits, they leaven bread too quickly for microbes to develop these secondary metabolites and flavors. Flavor development falls victim to expedience. The same is true in saké brewing.
The traditional ‘kimoto method’ for brewing saké follows a similar logic to sourdough starters: acid-tolerant bacteria—like Leuconostoc mesenteroides and Lactobacillus sakei—help create acidic conditions for wild yeasts to congregate. But this process fell out of favor in the Meiji era (1868-1912) with the advent of the ‘sokujo method’, where sokujo (速醸) translates to ‘fast-brewing.’ Instead of letting bacteria ferment rice into lactic acid, about 90 percent of saké brewing now follows the fast-brewing method where purified lactic acid is added from a tube rather than allowing it to develop naturally.
In the kimoto method, the combination of wild yeasts and bacteria form a starter culture called shubo (酒母), or ‘mother of saké.’ Brewers store their shubo in cold rooms for about a month to slowly break the rice starches down into lactic and acetic acids before they’re ready to move on to the next step. It’s a lengthy process, but once the bacteria are well-established the yeasts convert rice starches into umami-packed amino acids like glutamate (that’d be the ‘G’ in the notorious MSG). These yeasts go on to produce succinic acid (which tastes milder than wine’s tartaric acid) and esters like diethyl succinate (responsible for tropical fruit aromas) and sotolones (caramel, maple syrup aromas). These flavors are not absent in sakés made by the fast-brewing style thanks to the artificial addition of lactic acid, but they are less varied and less distinct.
But more flavor is not always a positive. The longer incubation time of the kimoto-style brings more diversity in the kinds of microbes that make flavor compounds, but this method fell out of favor because the combination of wild yeasts and bacteria proved too inconsistent to produce a marketable saké. And, since kimoto starters take double the time compared to fast-brewing starters, breweries had to dedicate twice the amount of human resources towards caring for the shubo. This never used to be a problem because rice farmers brewed saké during their off-season, but with skilled brewers/farmers aging and scant interest from younger generations, the saké industry largely chose to cut costs and labor by moving towards automation, purified strains, and reproducibility.
Booch homebrewers often complain that their SCOBYs make increasingly sour batches. Booch fermentation relies on yeasts acting first: yeasts breakdown sugar into alcohol so that the bacteria can take that alcohol and make acids with it afterward. Whatever tastes sweet becomes boozy (temporarily), and then turns acidic. This is why all booch contains a small amount of alcohol and why all neglected booch tastes like vinegar.
High acidity and high sweetness cannot happen at the same time because of the successive conversions of sugar-to-alcohol-to-acid. The ‘just right’ moment to harvest booch is when just enough sugar is used to make some acid and tame the sweetness but before all of it gets used up and made into acetic acid. But many home-brewers miss this point, let the booch ferment too long, and simply accept their puckery beverages as a given. Here’s the thing: attempting to use a previous puckery booch as a starter for the next batch means that its SCOBY will contain more bacteria than yeast. More bacteria means more acid, and any subsequent batches made from this starter can only yield acid-heavy booches that may not always be palatable.
Fortunately, there is a way to curb this acerbic reality. Try adding freshly picked herbs or fruit at the point of secondary fermentation, just before bottling the booch for bubble production. The naturally-occurring yeasts on these (unwashed!) plants will transfer to the booch liquid and become integrated as part of a future SCOBY. The added yeasts become part of a community and help it become more balanced in composition and taste. Just be sure to forage away from dog-walking areas!
The symbiosis of SCOBYs lies in starting something—what happens after depends on how we choose to join the fermentation process as co-conspirators. Too little acid and ferments risk contamination. Too much acid and the ferment becomes structurally compromised or unpalatable.
What matters is balance: between bacteria and yeast, between time and taste, between ambient microbe and human intervention.
A version of this article originally appeared in Acid League Magazine Volume 1.
Maya Hey is a foodmaker, researcher, and educator at Concordia University. She combines her backgrounds in nutrition, gastronomy, and communications to study fermentation.