Fermentation jar

Two jobs, several solutions

Fermenting vegetables produces carbon dioxide, sometimes vigorously. A sealed rigid jar builds pressure, and a jar that builds enough pressure eventually leaks, pops its lid or breaks.

At the same time, oxygen at the surface is what allows moulds and aerobic bacteria to grow. Everything above the brine is at risk; everything below it is protected.

So the vessel has to vent gas without admitting air, and it has to let you keep the food submerged. Airlocks, water-seal crocks, one-way valve lids and daily burping are four different answers to the same question.

The options

A plain jar, lid loose

Works. Gas escapes past the loose lid. Needs the food kept under the brine and a check every day or two.

A jar, lid on, burped daily

Works. Open it briefly each day to release pressure. Easy to forget on a hot week.

An airlock lid

Water-filled trap that lets gas bubble out and nothing in. The classic solution and genuinely convenient.

A one-way valve lid

Silicone valve that opens under pressure. No water to top up. Modern and reliable.

A water-seal crock

A moat around the lid. Traditional, large, expensive, and excellent for quantity.

A weight

The other half of the problem, and the one people skip. A glass weight, a small jar, or a sealed bag of brine.

What actually matters

In order of how much difference each makes.

• Everything under the brine. More failures come from a floating cabbage leaf than from any other cause. Weight it.
• Headroom. Fill to about three quarters. A full jar overflows in the first three days.
• Somewhere to stand it. A plate under the jar. Vigorous ferments push brine out.
• Glass or food-grade ceramic. Not metal, which reacts with the acid, and not any plastic that is not food-grade.
• A wide mouth. Packing and removing food from a narrow-necked jar is miserable.
• Cleanliness rather than sterility. Fermentation vessels are washed thoroughly; they are not sterilised like preserving jars, because you want the bacteria.
• Salt levels and keeping times are safety numbers. Follow Food Standards Agency guidance rather than a website or a jar manufacturer.

Setting one up

1. Wash the jar and any weight thoroughly in hot soapy water and rinse well.
2. Pack the food in firmly, pressing out air pockets as you go.
3. Add brine or let the salted vegetables release their own, following a recipe from a food safety authority for the quantities.
4. Press everything below the liquid and put the weight on top.
5. Leave roughly a quarter of the jar empty.
6. Fit the airlock, valve or loose lid, and stand the jar on a plate somewhere cool and out of direct sun.
7. Check it daily for the first week: everything still submerged, gas escaping, and no fuzzy growth on the surface.

Common questions