How to keep vegetables submerged

Why it matters this much

The organisms you want are anaerobic or nearly so — they work perfectly well with no oxygen. The organisms you do not want, above all the moulds and the film-forming yeasts, need air.

Brine excludes air. Anything below the surface is in an environment where moulds simply cannot establish. Anything above it is sitting in the open, wet, salty, at room temperature, which is close to ideal conditions for exactly the growth you are trying to avoid.

There is a second layer of protection people underuse: the carbon dioxide the ferment produces is itself a shield. Being heavier than air, it sits in the headspace above the brine and displaces oxygen. A jar that vents gently keeps that blanket; a jar opened repeatedly to check on it loses it every time.

Ways to hold things down

From the purpose-made to the improvised. All of them work.

• Glass fermentation weights. Flat discs made for the job, sized to jar necks. Easy, effective, and the least fuss if you ferment often.
• A smaller jar or ramekin that fits inside the neck, filled with water for ballast. Free, and works well.
• A brine-filled freezer bag. Sits on the surface and conforms to the shape of whatever is beneath it, sealing the whole surface. Fill it with brine rather than water so a leak does not dilute the jar. Probably the most effective improvised method.
• A large outer cabbage leaf, folded and tucked over the top of the shreds. The traditional sauerkraut method: it forms a lid that catches anything trying to float.
• A whole cabbage core, a carrot slice, or a wooden skewer wedged crosswise under the jar shoulder to trap things below.
• A boiled, well-rinsed stone, if you are fermenting in a crock. Traditional and perfectly sound.
• Not a metal weight unless it is stainless steel. Ferments are acidic and salty and will corrode most metals.

Getting it right when you pack the jar

1. Pack the vegetables firmly enough that they interlock and do not drift, but not so hard that you crush them.
2. Leave real headspace — a few centimetres — so brine has somewhere to rise into as gas is produced. A jar packed to the rim will push liquid out and end up with the food exposed.
3. Make sure the brine covers everything before the weight goes in, topping up if it does not.
4. Put the weight in and check it is holding all the solids down, including small pieces at the edges. Loose fragments floating at the surface are the usual starting point for a film.
5. Wipe the inside of the jar above the brine line. Stray shreds stuck to the glass are outside the liquid and will grow things.
6. Cover so gas escapes but air does not enter, and stand the jar on a plate.
7. Check the level after a day or two. If it has dropped, top up with brine of the same strength rather than with plain water.

What grows at an exposed surface, and how quickly

Everything on this list needs air, which is the entire reason submersion is the whole technique.

Kahm yeast

The commonest, and it can appear within a couple of days of a surface being exposed. A flat white film. Harmless, tastes bad, and adds softening enzymes to the jar.

Surface moulds

Days to a week. Fuzzy and coloured. The batch is finished — see mould on a ferment.

Oxidation and off-flavours

Almost immediate. Exposed vegetable browns, dulls and picks up a stale, papery taste even where nothing has grown on it.

Softening

Progressive. Exposed pieces go limp well before submerged ones, both from the air and from enzymes contributed by any film above them.

Nothing at all, below the brine

The point of the comparison. Fully submerged food sits in an environment none of the above can establish in.

Common questions