How Do I Keep Mold From Forming During Fermentation?

“I’ve recently started fermenting vegetables at home, but I’ve noticed that every time I try, mold forms on top of my fermenting jars. I’ve tried covering them tightly and even using weights, but it keeps showing up! I’m not sure if I’m doing something wrong, or if this is just part of the process. What causes the mold, and how can I stop it from ruining my ferments? Any help would be great, because I love the idea of fermenting my own food, but this mold issue is frustrating!” Thanks, Lauren, Perth, Australia.

Why Does Mold Form During Fermentation?

Oh boy, Lauren, I hear you! Mold can be such a headache when you’re all excited to ferment and suddenly… ugh, there it is, fuzzy white or green stuff on top. Not exactly appetizing, right? Mold forms because fermentation is all about creating an environment for good bacteria to thrive, and when things aren’t quite right, mold can take over instead.

Mold tends to form when air (oxygen) gets into the mix. Most fermentations, like sauerkraut or pickles, rely on anaerobic (oxygen-free) environments. If your ferment is exposed to air, moldy situations are much more likely to happen. There’s also a battle going on between yeast, bacteria, and potential mold spores. So, the goal is to give the good bacteria a leg up and deny mold the conditions it loves most — oxygen and warmth.

Now, let’s get into the ways we can keep that mold away so your ferments stay healthy and tasty.

Common Causes of Mold During Fermentation

Lauren, you mentioned using weights and covering jars tightly, which is a great start, but there are still a few common culprits that might be causing that mold growth. Let’s go over these quickly because once you understand what might be going wrong, it’ll be easier to troubleshoot!

1. Exposure to Air

This is by far the biggest reason for mold forming in fermentation. Like I mentioned before, molds thrive in oxygen-rich environments. If any part of your ferment is sticking out of the liquid, it’s like a welcome mat for mold spores.

Even if the ferment is mostly submerged, if air somehow sneaks under your lid or cover, it increases the chances that mold will form on top. Some fermenters out there (myself included — guilty as charged!) have used loose-fitting lids or left jars uncovered during the initial stage. It happens! But this can let more oxygen in than the good bacteria can handle.

2. Poor Salt-to-Water Ratio

The salt you use in your brine helps to create an inhospitable environment for mold but a perfect one for those happy lacto-bacteria. If you’re eyeballing or under-salting your brine, the mold struggle becomes very real.

A typical ratio that works for most vegetable ferments is between 2 and 3 percent salt by weight. Now, if you’re anything like me, measuring things exactly might not feel like fun, but a little attention to this detail can save you a whole lot of moldy pain later.

3. Temperature

Most ferments like to hang out in a cool yet not too cold environment, around 65-75°F (18-24°C). If things get too warm, you’re not just speeding up the fermentation; you could actually be creating prime real estate for mold to move in. Summer ferments in hotter locations can be mold magnets if the temperature isn’t well-managed, which might explain what’s happening, especially since Perth can get warm!

4. Contaminated Equipment

All it takes is a smidge of something funky on your jar, lid, or weight, and suddenly you’ve got a ready-made mold farm. It’s a good idea to give everything a thorough scrub before you start. You don’t have to go too extreme — just clean is fine — but make sure nothing’s been left to harbor old nasties from a previous ferment.

Tips To Prevent Mold In Your Next Ferment

Now that we know what’s causing the mold, let’s work on stopping it for good. Don’t worry, Lauren, it’s totally doable, and pretty soon, you’re going to be rolling out jar after jar of perfectly preserved goodness!

1. Keep It Submerged

If I had to shout one piece of fermenting advice from the rooftops, it would be: keep everything under the brine! Vegetable pieces that are peeking above your liquid are just waiting to grow mold. A good weight helps with this. Some people use specialized fermenting weights, but if you don’t have one, something as simple as a heavy, clean rock or a smaller jar filled with water can do the trick.

You can also use fresh cabbage leaves as a kind of “lid” inside the jar to keep the smaller pieces from bobbing up too much. It might take a bit of creative thinking at first, but you’ll work out what equipment works best for your jars.

2. Use Proper Fermentation Lids

There are all kinds of lids designed for fermentation that help air stay out, but gases from the fermentation process can still escape. They’re called “airlock” lids or fermentation lids, and they’re worth considering, especially if you ferment frequently. This can significantly cut down on the oxygen exposure, which is a major mold-causing issue.

If those aren’t available to you, no worries — I’ve had success with sealed jars that I burp occasionally to release trapped gas. Just keep in mind, this takes a bit more babysitting.

3. Get the Right Salt Ratio

This is kind of the peacekeeper in the world of fermentation. The right salt concentration creates the ideal conditions for good bacteria to thrive while keeping mold at bay. For example, for sauerkraut, stick to a 2-3% salt-to-water ratio. If you’re new to using a scale, don’t stress, there are lots of online calculators for brining that’ll help you get the exact amount of salt you need for the amount of water you’re working with!

Advanced Tips for Long-Term Success

1. Experiment With Fermentation Vessels

If you’re using mason jars for everything right now, that’s perfectly fine, Lauren, but sometimes a different style vessel can make a big difference in terms of air exposure. Ceramic crocks, for instance, are naturally more anaerobic-friendly, especially the ones with water-sealed tops. It might seem like overkill at first, but for larger batches, crocks tend to produce fewer mold issues, as they maintain a more stable environment.

2. Fermentation Starters: Give Your Batch a Boost

If mold keeps persisting, even after making all these tweaks — consider giving your ferments a little kickstart with a fermentation starter. These are pre-made combinations of bacteria from previous successful ferments. A bit of liquid from a previous batch of sauerkraut or pickles can often inoculate your new batch, helping to crowd out the mold-spore nasties from the get-go.

It’s kind of like inviting a party of good bacteria in before mold can even show up! Just make sure the starter you use is from a healthy, mold-free batch.

Signs of Mold Vs. Normal Fermentation

Not everything growing on top of your ferment is necessarily mold, although it sure can be tricky to tell at first. Usually, mold appears fuzzy, tends to be white, green, or blue, and sits on the surface of the liquid. If it gets especially bad, it can penetrate deeper if left too long.

On the other hand, “kahm” yeast can look like mold at first glance, but it’s actually fine. It’s usually white and has a thin, powdery appearance, not fuzzy. It can be scooped off and shouldn’t affect the overall ferment, whereas mold can give off a distinct “off” smell and taste.

If you think you’ve let the mold get too far down into the ferment, it might be safer to toss it. Sometimes, it’s better to start fresh than risk potential toxins from mold running too deep.

Setting Up Your Fermenting Environment for Success

Where you keep your ferments can play a big role in whether or not mold becomes an issue. Ideally, you want to store your jars or crocks in a cool, dark place. A pantry or a cabinet works well for this.

Watch The Temperature

If the house is running a bit warm (more common in summer months), find a slightly cooler spot — around 65-75°F (18-24°C). Too cold, however, and the ferment will slow to a crawl, which isn’t necessarily bad, but just means longer wait times.

Sanitize Your Space

Keep the jars and tools as clean as possible from the start, and make it part of your normal routine to clean fermenting lids, weights, and crocks with hot, soapy water before the next use. A good cleaning can dramatically decrease the chances of old mold spores hanging around.

Troubleshooting Persistent Mold Issues

If you’re still having continuous mold problems even after taking all these precautions, don’t panic, Lauren! There are a few advanced tricks to troubleshoot those super stubborn cases.

• Avoid Cross-Contamination: Be sure to keep different ferments slightly apart from each other — mold spores can jump! I’ve found that too many ferments crowded into the same spot tend to share their problems.
• Use a pH Test: If the pH of your ferment isn’t dropping enough (below 4.6), mold can thrive more easily. pH strips can easily test your brine to see if the ferment is sufficiently acidic.
• Switch to Filtered Water: If you’re using heavily chlorinated tap water, that can sometimes slow down the growth of friendly bacteria. Switching to filtered water might help maintain the right bacterial balance for better fermentation.

Final Thoughts…

Lauren, thanks again for your question! Fermentation can feel frustrating at times, but once you figure out the right balance of salt, temperature, and keeping things submerged, you’ll be in great shape. Mold happens to most of us at some point, so don’t get discouraged. Keep experimenting, clean those jars, and remember — mold doesn’t stand a chance when the good bacteria have what they need to thrive.