The Six-Degree Test: Watching Temperature Double the Speed
Every temperature-aware calculator on this site runs on one rule: lactic fermentation goes roughly twice as fast for every 6 °C warmer. It is the single most useful piece of arithmetic in the whole hobby - it is why the temperature-time calculator works and why the winter fermentation guide keeps saying "double the days". This experiment is you checking that claim with one batch of sauerkraut and three jars.
The question
Does the doubling rule hold in a real kitchen - and can you see it with your own eyes, not just in a formula? The prediction is precise. Take a batch that would be ready in 7 days at 20 °C: the rule says the same jar at 14 °C needs about 14 days, and at 26 °C about 3.5 days. Same cabbage, same salt, same recipe - only the thermometer moves.
The setup
- One batch of sauerkraut - 2% salt by weight, cabbage massaged until briny; the vegetable salt guide covers the numbers
- Three identical jars, filled at the same moment from the same bowl, each labeled with its spot: cold (~14 °C), room (~20 °C), warm (~26 °C)
- Three real temperature spots - a cool hallway or unheated room, the kitchen counter, and the top of the fridge or a warm shelf; a cheap thermometer in a glass of water next to each jar logs the actual temperature
- A notebook or phone note - one line per jar per day
The day-by-day log
- Day 0
Fill three jars from the same bowl at the same moment. Label them cold (~14 °C), room (~20 °C) and warm (~26 °C), and park a thermometer glass next to each.
- Daily
Same time every day: taste all three jars with the same question in mind, one line per jar. Cloudy brine and bubbles mean the lactobacilli are working.
- Days 3-5
The warm jar should cross your "properly sour" line first - about half the room jar's time if the rule holds.
- Around day 7
The room jar's predicted line: a batch that would be ready in 7 days at 20 °C.
- Days 12-16
The cold jar, about twice the room jar. Even slower than that? The temperature guide was being optimistic - worth logging.
What to watch
Taste all three jars at the same time every day with the same question in mind. Cloudy brine and bubbles mean the lactobacilli are working. Write down the day each jar crosses your "properly sour" line, then compare the gaps - the predicted schedule for each jar is in the log above.
Expect reality to be a good story rather than a stopwatch. Cabbage lots differ, the hallway jar will drift with the weather, and 6 degrees is the fast end of a 6-8 degree band. A spread of 12-16 days versus 6-8 versus 3-5 would still be the rule working in front of you - and if your cold jar ran even slower, that is the temperature guide being optimistic, and you have discovered something worth logging.
Safety floor, same as always
A properly salted lacto-ferment sours fast enough to protect itself, but the ordinary rules do not pause for experiments: if a jar grows fuzzy, colored mold or smells putrid rather than pleasantly sour, it is compost, not data. The white film guide covers the kahm-yeast lookalike that is harmless.
Questions
Why six degrees?
Classic fermentation tables put lactic-acid fermentation roughly one doubling faster for every 6-8 degrees Celsius; the calculators use the fast end, 6 degrees, which matches the kimchi tables. Sourdough yeast runs hotter at about 5 degrees per doubling and kombucha slower at about 8 - different microbes, different clocks.
What counts as "done" so the three jars compare fairly?
Decide before you start: the same taster, the same descriptor - for example "sour enough that I would stop eating it plain" - checked at the same time each day. Tasting notes drift, so write the day number down the moment a jar crosses the line. A strip of pH paper (target roughly 4 or below for sauerkraut) makes the call less subjective.
Does the same rule work for yogurt?
Same shape, different number. Yogurt cultures speed up with temperature too, but their reference point is the 43 degree optimum and the doubling sits at about 5 degrees below it - the yogurt time calculator handles that math, and the yogurt incubation guide explains why warmer than 43 actually slows the culture down.