Fermentation Calculator

The science of fermentation timing and salt

Two dials govern almost every ferment: temperature sets the speed, salt sets the safety.

Fermentation is one of humanity's oldest food preservation techniques. Whether you are cultivating a sourdough starter, brewing kombucha, incubating yogurt, or packing a crock of sauerkraut, the principle is the same: microorganisms consume sugars and produce acids, alcohols, and carbon dioxide. Controlling and predicting that process is the key to consistent, delicious results.

Vegetable fermentation: salt concentration matters most

For sauerkraut, kimchi, and other lacto-fermented vegetables, salt concentration is as important as temperature. The standard is 2 to 3 percent salt by total weight — vegetables plus any added brine water. This concentration selectively promotes Lactobacillus species while suppressing harmful organisms. Too little salt, below about 1.5 percent, risks soft, spoiled vegetables; too much, above 5 percent, can inhibit even the beneficial bacteria and produce an unpleasantly salty product. To calculate a brine, weigh the water and add 2 to 3 percent of that weight in salt — 25 grams of salt per litre is a reliable 2.5 percent standard. For dry-salted kraut, weigh the shredded cabbage and add 2 to 2.5 percent salt, then massage until it releases enough liquid to submerge itself.

Kimchi typically uses a higher salt concentration during the initial vegetable-wilting step (a 5 to 10 percent brine for 2 to 6 hours), then rinses and settles to a final 2.5 to 3.5 percent. Added garlic, ginger, fish sauce, and chili introduce extra microbial communities and flavor compounds that distinguish kimchi from plain sauerkraut.

Temperature: the master variable

Temperature is the single most important factor influencing fermentation rate. Microbial metabolism roughly doubles for every 18 degrees Fahrenheit (10 Celsius) rise within the viable range, so sourdough at 82 degrees completes bulk fermentation in about half the time it would at 64 degrees. Each culture has an optimal band: sourdough wild yeast thrives at 75 to 82 degrees Fahrenheit, kombucha SCOBYs at 75 to 85, yogurt cultures at a much warmer 108 to 115, and vegetable ferments best at a cooler 65 to 75 for flavor. Exceeding the upper limit does not just speed things up — it can kill beneficial organisms, select for undesirables, and create off-flavors.

Starter percentage and inoculation rate

The amount of active culture you add determines how fast the beneficial microbes take over. In sourdough this is the starter percentage — mature starter weight relative to total flour. A 20 percent inoculation is the baseline for room-temperature bulk fermentation; raising it to 30 percent can shorten the ferment by 25 to 40 percent, while dropping to 10 percent can extend it by 50 to 100 percent. For kombucha, a 10 to 15 percent starter-liquid ratio provides enough acidity to inhibit mold from the start. Higher rates are faster and more reliable but can trade away some flavor complexity.

Hydration and cold retardation

For sourdough, higher hydration doughs (75 percent and up) ferment faster because water aids enzyme activity and gas expansion; a dough at 80 percent ferments 15 to 25 percent faster than the same formula at 65 percent. Refrigeration between 36 and 40 degrees Fahrenheit dramatically slows fermentation without stopping it — bakers use this cold retardation for 12 to 72 hours to build a more complex, slightly more acetic flavor. The same idea moves finished sauerkraut and kimchi to the fridge to preserve a balanced tang.