Aerobic vs Anaerobic Fermentation

Among fermenters, there’s a vast diversity of opinion when it comes to a somewhat arcane topic – whether ferments should be aerobic, i.e., open to the outer environment, or anaerobic, i.e., produced in a hermetic environment either through the type of vessel it’s in or beneath an airlock.

First, it’s probably important to understand that “fermentation” involves a wide range of activities and organisms, some of which need to specifically be aerobic and some of which, for best results, you’ll really want to slap an airlock on to keep oxygen (and microbial wind surfers) out.

Here are four basic categories of fermented things, with some commentary about each:

Fermented Vegetables: Sauerkraut, cucumber pickles, and similar products that are produced beneath a brine don’t require any kind of special jars or airlock systems, but they need to be properly weighted to stay submerged. This is especially true early in the fermentation process, when the first wave of microbes come online and churn out a lot of CO2 as they do their work. The escaping gasses will get caught in the plant matter and lift it up, often spilling brine as it goes. Where solids are above the brine and exposed to oxygen, mold can form. I’ve bootlegged dozens of approaches to weighting kraut over the years, but my favorite, by far, is Pickle Pebbles (aff). These are thick, durable glass weights sized for both wide and standard mouth Mason jars, and they do a really nice job of holding everything down. I still get into my vessels for the first few days to scoop out small bits of solids that have escaped with the gasses, but since buying Pickle Pebbles, my kraut making has gotten a lot simpler. I use the normal lid for the jar, screwed on and then backed off a bit so gasses can escape. For me, no airlocks or other hermetic or anaerobic environments are required for fermented vegetables.

Alcohol: My first homebrews were wild ferments of honey, fruit, and air, based on the T’ej recipe in Katz’s Wild Fermentation (aff). Essentially, you stir honey and water together, add fruit or berries, then stir throughout the day, for several days, to oxygenate the mixture, submerge any mold spores, and help airborne yeasts find their way into your brew. In between stirrings, this is covered with a cloth or other breathable lid. After a week or so, the surface turns bubbly and the whole thing goes into an airlocked container for the captured yeasts to do their work.

A few, well, opportunities or problems here, depending on your perspective. One, wild yeasts aren’t usually very alcohol-tolerant, so your ABV will generally be pretty low. Two, yeasts aren’t the only organisms who can join the party, so you can end up with a low ABV drink with slight vinegar or other funky notes. Three, this is not a repeatable process, so no two batches are likely to come out the same way, even if you follow the exact procedure and recipe. For a lot of people, these variables are part of the magic and entirely welcome, but I eventually went a different route.

These days, if I’m making beer or mead, I make them in five- or six-gallon batches and use commercial yeasts. Everything goes under airlock right away – I use the s-style bubblers (aff), but lots of people prefer the three-piece design (aff) – so these pure yeast strains can do their work. This way, I can avoid what most homebrewers would call “contamination,” but which we wild fermenters understand as natural variation. Still, if you drop $50 or $100 on ingredients for some magical brew, that variation can be heartbreaking. So for me, alcohol fermentations are anaerobic all the way.

Vinegar, Kombucha, Jun: These acidic liquids are produced through microbial oxidation processes, and they require oxygen. That said, a lot of jun makers prefer to brew under airlock, which apparently reduces the yeast component in their culture. I don’t really have anything to add to this, except that my jun cultures have done fine in sealed jars for long term dormancy, and my vinegar cultures have done outstanding work covered with towels to give them plenty of air. So for me, these are, generally speaking, aerobic fermentations that should have plenty of access to the environment. I’ll continue researching anaerobic jun, mostly because I’m puzzled by it. That’s fermentation for you.

Sourdough, Tempeh, Natto: These grain-based ferments are themselves very different from each other, but all require oxygen. When I make sourdough starter, I cover it with a towel. The flour itself comes with a complement of yeasts and bacteria, but more arrives through exposure to air with a breathable lid. In tempeh making, cooked and cooled beans are inoculated with Rhizopus mold spores, then incubated at 85 or 90 degrees F for 24 hours or so, letting the mold grow in. Oxygen is required for proper growth. With natto, a Japanese soybean fermentation, cooked beans are inoculated with Bacillus subtilis spores, then incubated around 105 degrees F for 24 hours or so, allowing the bacteria to develop. Again, oxygen is a must-have for this to work, at least as far as I’ve seen (I’ve never attempted it without some oxygen exchange). So for me, grain ferments along these lines are aerobic.

You’ll see articles on the web that argue for expensive, airlocked vessels as the only “safe” way to make sauerkraut, and those assertions are simply false. I’ve made countless batches of kraut over many years using basic methodology, mostly in Mason jars. As long as everything is submerged and there’s little head room between the brine and the lid of the vessel, it comes out great.

I am the proud owner of a five-gallon ceramic crock, and when I made a huge batch of pickled cabbage quarters last fall, this was how I set it up: First into the crock went spices, then cabbage layered with salt, then a lot of brine. That picture was taken before I added still more brine, because keeping the weight submerged is a big deal in the fight against mold. I covered all of this with a towel I taped on, which worked fine, but I’ve seen a lot of set-ups where people cover large crocks with plastic film to protect the surface and deter mold.

Assuming your veggies are beneath the brine, you have nothing to worry about in terms of food safety. Don’t feel pressured to drop huge amounts of money on fancy jars or equipment – but feel free to experiment widely. Plenty of people prefer airlocked vessels for veggies, and plenty of people wild ferment great alcohol that’s open to the air for a while. Some homes seem uniquely disposed to throwing mold spores on brine or growing a yeast layer at the surface, so airlocks or hermetic jars may be the best solution there. All techniques are valid, and no one has a monopoly on fermentation knowledge.

What’s your take on the aerobic vs anaerobic question?

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