Time & Temperature

Temperature is the master variable. It sets how fast the lactic-acid bacteria (LAB) work, which microbes win the early succession, and how the finished ferment tastes. Once you have salt and submersion right, temperature is the dial you turn to trade speed for flavor — and the one most likely to push a ferment off the rails.

The mechanism: Q10 ≈ 2

Microbial metabolism follows the same rule as most chemistry: rate roughly doubles for every +10 °C (a Q10 near 2). A jar that finishes in 7 days at 22 °C will take roughly twice as long near 12 °C and finish in about half the time near 32 °C — but speed is not the goal. Above the ideal band, acidification outruns flavor development and proteolytic spoilage and softening enzymes also speed up; below it, the LAB lose their tempo and the protective drop in pH stalls.

Info

The early-ferment danger window (anaerobic but not yet acidic) is carried by salt and the speed of acidification. A cold, sluggish start widens that window — which is why too-low a temperature is a safety concern, not only a quality one. See Safety First.

Temperature bands by product

Product	Ideal band	Effect of going low	Effect of going high
Lacto vegetables (kraut, kimchi, pickles)	18–22 °C / 64–72 °F	<15 °C stalls; mushy, slow acidification	>24 °C softens, yeasty/off-flavors
Koji (Aspergillus oryzae)	28–32 °C / 82–90 °F	Slow, patchy mycelium	>42 °C kills the mold; bacterial contamination
Kombucha / vinegar	21–27 °C / 70–81 °F	Sluggish, mold-prone	Vinegary fast; thin SCOBY
Cold storage / arrest	2–4 °C / 36–39 °F	— (this is the brake)	Above ~6 °C the ferment resumes

Caution

2–4 °C is the storage and arrest line, never a fermentation temperature. Once a ferment hits its target pH, move it to cold storage to slow LAB activity to a near-stop and lock in texture and acidity. Cold does not make an unsafe jar safe — only a finished pH ≤ 4.6 does.

Time × temperature for a 2 % lacto-vegetable ferment

These are first-taste-ready windows for a typical 2.0 % salt kraut; always confirm the finish by pH (target 3.4–3.6 for sauerkraut), never by the calendar.

Temp	Q10 factor (vs 22 °C)	Approx. ready window
12 °C / 54 °F	~0.4× rate	18–28 days (sluggish)
18 °C / 64 °F	~0.7× rate	10–14 days
22 °C / 72 °F	1× (reference)	7–10 days
28 °C / 82 °F	~1.7× rate	4–6 days (watch softening)

Tip

Cool, slow ferments (15–18 °C) reliably give the cleanest, crispest kraut and kimchi — the classic “cellar” result. Use warmth to rush only when you have to, and taste/test early.

Worked example

A 2.0 % cabbage kraut takes ~8 days to reach pH 3.5 at 22 °C. Move it to a 12 °C cellar (ΔT = −10 °C, so ~0.5× rate) and the same finish takes roughly 16 days. Drop it further to 4 °C storage after it finishes and LAB activity falls to a crawl — the jar will hold for months while drifting only slowly more sour.

See also

• Microbial Succession — which organisms each temperature band selects
• Salt & Osmosis — the other lever on the danger window
• Safety First — the pH 4.6 line and the early-ferment window
• Ferment Predictor calculator — estimate finish time from salt and temperature