What to know before starting a home ferment

Lactic acid bacteria already live on fresh vegetables, and a home ferment gives them the conditions to multiply. The vegetables are held in a salt brine, or packed with salt, which favors the sour-producing bacteria and slows the microbes that cause spoilage. Before starting one, understand three things: how much salt to use, how to keep the vegetables under the brine, and how healthy activity differs from a batch that has gone bad.

Most first ferments run into the same few problems, including too little salt, vegetables left above the liquid and a jar kept somewhere too warm. Each one is easy to prevent once you know what to watch for.

Salt is the main control

Weigh your vegetables and your salt rather than measuring by spoon, because coarse and fine salts differ greatly by volume. Salt sets the pace of the whole ferment, so the number matters more than the jar.

For vegetables you salt directly, such as cabbage, use a percentage of the vegetable weight. A common range is 2 to 3.5 percent, which means 1,000 grams of vegetables takes roughly 20 to 35 grams of salt. Less salt speeds up fermentation but gives unwanted microbes more room to take hold. More salt slows the process and makes the result very salty.

Vegetables covered with a liquid brine, such as carrots, radishes and green beans, are measured differently. Here the percentage is based on the water: about 20 to 35 grams of salt per liter, or 2 to 3.5 percent of the water's weight. Some guides base the percentage on the combined weight of the water and the vegetables instead, which gives a different salt amount for the same jar. Pick one method, write it down, and use the same method for any top-up.

Use pickling salt, or a salt whose label lists only salt. Many table salts contain anti-caking agents, which can cloud the brine and make it harder to tell normal cloudiness from a problem. Filtered or unchlorinated water is also a good choice, because chlorine can suppress the bacteria you want to encourage.

For a first batch, choose vegetables that hold up well. Cabbage is the most forgiving. Salting and massaging it releases its own liquid, which becomes the brine, so you may need little or no added water. Carrots, radishes and green beans also work well when they are covered with a salt-water brine. Cucumbers and other soft, watery vegetables tend to turn mushy during a long ferment, so they suit a short one better.

Clean tools, the right vessel and submerged vegetables

Wash your hands, jars, weights and tools with hot soapy water and rinse them thoroughly. Glass jars, food-grade ceramic, food-grade plastic and stainless steel all work well. Avoid unlined aluminum, copper and galvanized containers, since salt and acid can react with the metal.

Keep every piece of vegetable below the brine. Pack the vegetables firmly, then press them down with a clean weight, such as a small glass jar or a sealed bag filled with brine. Pieces that poke above the liquid tend to grow yeast and mold. Cover the jar loosely, or use an airlock lid, because fermentation releases carbon dioxide that can push a tightly sealed jar apart. If you use a regular lid, open it once a day to release the gas.

Temperature and timing

A cool room between about 65 and 70 degrees Fahrenheit (18 to 21 degrees Celsius) gives a steady, controllable pace. A warmer kitchen speeds fermentation and can turn a batch soft or overly sour within days. A cooler space slows it, sometimes by weeks. Vegetable ferments typically take several days to several weeks, depending on the vegetable and the temperature. Visible activity usually begins within a day or two of packing the jar, so if the brine stays perfectly still for several days, check the salt weight and the temperature before waiting longer.

Start tasting the ferment after three or four days, using a clean spoon each time. It is ready when it is pleasantly sour and still crisp. Once it reaches that point, move the jar to the refrigerator to slow the process. Taste regularly rather than following a fixed calendar, since the same recipe can behave differently from one kitchen to the next.

Each time you check the jar, look at the vegetables as well as the brine. Push any exposed pieces back under the liquid, skim any film, and make sure the lid is still doing its job. If the liquid level has dropped, top it up with fresh brine made at the same ratio as the batch, about 20 to 35 grams of salt per liter of water, rather than with plain water. Plain water dilutes the salt and weakens the protection it gives.

Telling healthy activity from spoilage

A healthy ferment looks and smells lively. You may see small bubbles, a cloudy brine, a pleasantly sour smell and some sediment at the bottom. A thin white film called kahm yeast can also form on the surface. It looks flat and wrinkled, and it is harmless but tastes off, so skim it away and keep the vegetables submerged.

Spoilage looks different. Fuzzy mold, whether green, blue, black, pink or orange, is a sign to stop. The same goes for vegetables that turn slimy or mushy, or a smell that is rotten, putrid or sewage-like rather than sour. Sourness on its own is not a warning sign. A ferment that smells sharp and tangy is usually doing what it should.

When to discard and how to start over

Discard any batch with fuzzy or colored mold, slimy or mushy vegetables, or a rotten odor. If you are unsure, throw the batch out rather than tasting it to find out. Mold can also spread beneath the surface, so scraping off one spot does not make a fuzzy or colored batch safe.

To start over, wash everything, double-check the salt weight and the water, make sure every piece is under the brine, and make a smaller first batch so you can learn the pace. Keeping notes on salt weight, temperature and the day you started makes each later batch easier to repeat. One small jar teaches more about fermentation than any amount of reading.