How do I keep vegetables under the brine when fermenting?

Lactic fermentation is anaerobic: the bacteria doing the work want no oxygen, and the acid they produce is what makes the ferment safe. Mould and kahm yeast are the opposite — they need air. So the brine line is not a tidiness matter, it is the boundary between the process working and the process being colonised.

Safety

Step by step

1. Pack the jar down hard before anything else.Press each handful in firmly to drive out the air pockets between the pieces. A well-packed jar needs far less weight, because it does not want to float in the first place.
2. Leave real headspace.2 to 5cm between the brine and the rim. Ferments rise in the first few days as gas builds, and a jar filled to the top pushes brine out over the rim and then sits with the contents exposed.
3. Choose a weight that fits the jar.Glass fermentation weights are the tidiest. A small freezer bag part-filled with brine moulds itself to any shape and is free. A scrubbed stone or a small glass jar of water works. A folded outer cabbage leaf tucked round the edges holds loose shreds down and costs nothing.
4. Use brine in the bag, not water.If a water-filled bag splits — and they do — a bag of plain water dilutes the whole jar and ruins the salt percentage. Fill it with brine at the same strength and a split does no harm.
5. Wipe the glass above the line.Stray shreds stuck to the inside of the jar above the brine are the usual starting point for mould. Wipe them down with clean kitchen paper when you set the jar up and whenever you check it.
6. Stand it on a plate and check daily.Active ferments overflow. A plate saves the shelf and tells you at a glance whether liquid has been lost. Look at the level every day for the first week.
7. Top up correctly if the level drops.Make fresh brine at the same percentage — 20g of salt per litre of water for a 2% ferment — cool it, and add enough to cover. Plain water dilutes the salt across the whole jar, which is the one thing keeping the ferment on track.
8. Push floaters back down, or take them out.Use a clean spoon. A single piece that has been above the line for a day, with no visible growth, can be pushed back under. Anything with any growth on it, and the batch it sits in, is a different decision — check it against the spoilage signs.
9. Consider an airlock.An airlock lid lets carbon dioxide out without letting air in, which largely removes both the kahm problem and the burping chore. Cheap and worth it if you ferment more than occasionally.

Tips

• Fermenting in a jar with straight sides and a shoulder-free neck is much easier to weight than one that narrows at the top.
• A cabbage leaf, a weight and an airlock together is close to foolproof and costs a few pounds once.
• The first three days are when the level moves most. After the first week it settles and needs much less attention.
• Do not top up with brine from another jar — you have no idea of its concentration or what is in it.

Common mistakes

• Topping up with plain water — It lowers the salt concentration in the whole jar, not just at the top, and low-salt ferments go slimy and unreliable. Always top up with brine at the ferment's own percentage.
• Filling the jar to the rim to leave no air — It is the right instinct and the wrong execution. The ferment expands, pushes brine out, and then sits below the rim with exposed vegetables and less liquid than it started with. Headspace plus a weight is the answer.

If it doesn't work

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