Theoretically, ANYTHING can explode! But let’s look at this logically, and according to fermenting methods.
This is actually a question we get asked sometimes, and mostly about Fermenta Lock. Because of that, I’d like to answer the question for each of the common fermenting methods.
1. Open fermenting. Since the top is open, though it may be covered by a cloth or cloth cap, explosion is not in the list of things that can happen. This does not mean it may not be messy – if you overfill the jar, and it expands more than you anticipated, the jar may overflow. If the food is thick, it may mash up into your cloth or cloth cap… not fun to clean. That is usually as messy as this method gets.
2. Fermenting with a Water Lock. This has only one potential risk for explosion, and that is if the jar is overfilled, and some solid food impacts up into the water lock. Theoretically, this could cause pressure to build up to the point of causing the jar to break, but is more likely to cause the water lock to pop out like a cork – and with similar force. More commonly, brine is forced up into the water lock, which may cause the water lock to overflow. Messy. But not dangerous.
3. Fermenting with a Valve Airlock (Fermenta Lock). Again, we have only a single potential risk for explosion, which requires a fairly remote combination of a series of errors. IF the jar was overfilled, AND the food was fairly thick and sticky, AND the airlock happened to get stuck closed from that food, AND the airlock were glued to the lid by dried sticky food, then, theoretically, pressure could build up until something popped. It would be unlikely to cause jar breakage, and more likely to pop the valve from the lid – and the imperfect home manufactured valve is made in such a way that if it DID do that, it would not fly out like a cork, but would be more likely to just pop loose on one side, probably spewing jar contents as it did so. Again, the worst would be messy, not dangerous, and even that is a fairly remote possibility. The valve is designed to release air pressure before it can build to dangerous levels. Overfilling the jars and then ignoring them for days after they overflowed is the only thing that would cause a potential risk of something going pop.
4. Fermenting with a Canning Lid, or Fido Jar, without an airlock, with the lid on tight. Ok, HERE is where the real danger lies, and even then it is more likely to be MESSY than dangerous. When pressure builds up, the preparer usually opens the lid to vent the jar, and the gasses trapped in the food cause the food to rapidly expand. Enough pressure, rapid enough expansion, and the food bursts from the jar. Very messy. Theoretically if the jar is ignored long enough, it could cause the jar to break. The force and velocities involved here are things I can only guess at. I do not know if it would actually send missiles around the room, or if the jar would simply crack and the food kind of splat out around it. THIS ONE, at least theoretically, could be potentially dangerous. But I’ve only heard stories of MESSES (some quite spectacular), not any actual glass shrapnel terrorist attacks from inside a stealth lacto-fermentation device.
So… as long as your jar has a vent, of SOME kind, explosion really isn’t anything you need to worry about.
And the rule to avoiding messes, other than venting the jar, is to make sure to give your foods adequate headroom.
So what constitutes adequate headroom?
- Brined foods (including kraut) which have a liquid brine – 1-2″ headroom (larger jars, more headroom, smaller jars can have less)
- Thicker liquids such as condiments or kefir – 1/4 to 1/3 of the jar should be left empty.
- Pastes and fairly solid foods – 1/2 of the jar should be left empty, and you may need to work a clean table knife or spatula into the food to manually release bubbles.
The more solid the food, the harder it is for air to escape. The bubbles have a harder time working to the surface, so the food expands instead.
So for all you budding science geeks who are looking for a way to “accidentally” blow something up, this probably isn’t it!