Lactic Fermentation.
Salt does not preserve sauerkraut by itself; at 2 % it is far too little. It holds back the spoilers just long enough for the lactic bacteria to make the acid that does.
Lactic acid bacteria live on plants, in milk and in our guts. Given sugar and no air, they ferment it to lactic acid, and they are unusually tolerant of acid themselves, so they keep going down to about pH 3.5 while most spoilage bacteria and pathogens stop near pH 4.5 (XIII.7). A lactic ferment preserves food by acidifying it from the inside.
The race at the start decides everything. A cut cabbage carries a mixed crowd of microbes, most of them spoilers that would rot it. Salt at about 2–3 % of the weight pulls water and sugars out of the leaves into a brine (III.8), slows the spoilers more than the lactic bacteria, and lets the lactic bacteria pull ahead; once their acid drops the pH below about 4.5, the spoilers are finished. Too little salt and the spoilers get further before the acid stops them; too much and the lactic bacteria slow as well. The workshop runs that race.
Below: the pH of a vegetable ferment and the growth of spoilage bacteria against days at the salt and temperature you choose (microbe bench, teaching organisms and a teaching acid-to-pH curve for a lightly buffered brine), with pH 4.6 marked.
- Explain how lactic acid preserves.
- Explain how salt selects lactic bacteria.
- Relate fermentation speed to temperature.
- Choose a salt level.
Salt starts it; acid finishes it
At 20 °C with 2.25 % salt the ferment passes pH 4.5 in about 2.0 days in this model; at 12 °C, about 6.2. Before the acid stops them, the spoilers gain about 2.7 logs with 2.25 % salt, 3.3 logs with none and 0.6 with 5 %: salt is the brake that lets the lactic bacteria win the race, and the acid is what ends it. Unsalted vegetable ferments are much more likely to go soft, slimy and smelly.
Practical rules. Weigh the vegetables and use 2–3 % salt for sauerkraut and kimchi, more (often a 5–8 % brine) for whole vegetables in 泡菜 jars; keep everything under the brine, because the surface in air grows moulds and yeasts; ferment cool — 15–22 °C — for crisp texture and a balanced sourness; and taste as it goes, moving it to the fridge when it is as sour as you like. Use non-iodised salt without anti-caking agents, and clean, not sterile, jars: the lactic bacteria come in on the vegetables.
Shred 500 g of cabbage, weigh it, and massage in 2 % of its weight in salt until it releases brine. Pack it in a jar under its brine with a weight on top. Taste and test with pH strips every day: within a few days it sours and drops below pH 4.6.
Terms
- Lactic acid
- The acid made by lactic acid bacteria from sugars.
- Brine
- Salty water, here drawn from the vegetables by salt.
- Selection
- Conditions that favour the wanted microbes over others.
- 菹
- An ancient Chinese word for salted, fermented vegetables.
Sources
- USDA Complete Guide to Home Canning (2015), Guide 6, "Preparing and canning fermented foods and pickled vegetables": sauerkraut with 3 tablespoons of canning salt per 5 lb of cabbage (about 2.25 % by weight); ferments in 3–4 weeks at 21–24 °C or 5–6 weeks at 15–21 °C; above about 24 °C it may soften, below about 15 °C it may not ferment.
- Bacterial growth: cardinal-temperature model (L. Rosso et al., J. Theor. Biol. 162, 1993) with cardinal temperatures and optimum rates rounded from ICMSF (1996); teaching values that put each organism in its known range.
- A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.