Fermentation chambers
An old fridge on a controller keeps sauerkraut at 20 °C (68 °F) in summer, and a box with a heat mat keeps koji and tempeh at 30 °C (86 °F). How to set them up and where the probe goes.
7 min read Getting good Reviewed October 26, 2026 19 facts, each with its source
What a chamber is for
Temperature decides how long a ferment takes and how it turns out. For good sauerkraut, the USDA recommends 70–75 °F (21–24 °C) while it ferments. Below 60 °F (16 °C) it may not ferment, and above 75 °F (24 °C) it can turn soft12. In a kitchen, in summer or in winter, that isn't always easy.
A fermentation chamber is a closed space that holds whatever temperature you ask of it, with cooling, heating or both. You don't need to buy one. It's almost always built from something you already have, plus a controller.
The temperature each ferment wants
| Ferment | Temperature |
|---|---|
| Sauerkraut, kimchi and pickles | Best at 20–22 °C (68–72 °F); 16–26 °C (60–78 °F) can work3 |
| Kombucha | 20–25 °C (68–77 °F)4 |
| Mead and cider | Steady, between 18 and 27 °C (65–80 °F)5 |
| Koji | About 30 °C (86 °F)6, never above 40 °C (104 °F)7 |
| Tempeh | 20–37 °C (68–99 °F)8; Fervida uses 30–32 °C (86–90 °F), and never above 42 °C (108 °F)9 |
| Yogurt | 43 °C (110 °F) for hours10 |
In practice there are two chambers: one at 18–24 °C (64–75 °F) for nearly everything, and one at about 30 °C (86 °F) for koji and tempeh.
A fridge on a controller
This is the most complete chamber. An old fridge, or a chest freezer, plugged into a temperature controller:
- The controller is a thermostat with a probe and two outlets, one for cooling and one for heating. You set the temperature and the probe decides when each one switches on.
- Cooling comes from the fridge, plugged into the cooling outlet. That way its own thermostat stops being in charge.
- Heating comes from a heat mat or a small heater inside, plugged into the heating outlet. A fridge on its own only cools, and in winter you need heat.
Set a small differential and a delay between starts on the controller, so the compressor doesn't keep switching on and off. Follow your controller's instructions.
A fridge turned into a chamber stops being a food fridge. What you keep cold, finished ferments included, has to sit at 41 °F (5 °C) or below11. It's a house rule: one fridge for fermenting and another for storing.
A box with a heat mat
For koji, tempeh and yogurt, and in a cold house, heating is enough. A cooler, an oven or a heating pad that holds the temperature for hours works as an incubator, just like a yogurt maker10.
- A cooler or an insulated box with a heat mat inside, plugged into a controller.
- A heat mat or a heat belt around the carboy or under the jar, also on a controller.
- An oven that's off with the light on. Every oven runs at a different temperature: measure it for a few hours before putting anything in.
A heat mat without a controller heats nonstop, and in a closed box the temperature climbs more than you'd think. Always use it with a controller, or with a thermometer you check often, and as its maker says. Monitor the temperature throughout incubation12.
Where the probe goes
A loose thermometer measures the air13, and what's fermenting can be warmer than the air around it.
- Vegetables, kombucha and drinks. Tape the probe to the side of the jar or carboy and cover it with a piece of insulation, so it reads the ferment and not the air.
- Tempeh. Its temperature can rise 5 to 7 °C (9 to 13 °F) on its own14. Put the probe between the bags, not in the box's air.
- Koji. It makes heat and, left unchecked, goes past 50 °C (122 °F) and stops growing15. Put the probe in the grain, and mix it at 16 to 18 hours to release heat16.
If the chamber heats and the ferment is already heating itself, a probe in the grain or between the bags makes the controller cut the heat in time.
Humidity and air for koji and tempeh
What a chamber doesn't do
A chamber controls temperature, nothing more. It doesn't replace weighed salt or pH. In a vegetable ferment, what tells you it has worked is pH dropping to 4.6 or lower19, and you have to measure it with a chamber or without one.
Where every number comes from
Every number in the text leads to its figure: the exact sentence from the official guide it comes from, in its own language.
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1
For good sauerkraut, 70–75 °F (21–24 °C) while fermenting
“For obtaining a good quality sauerkraut at home, the USDA recommendation is to store at 70º to 75ºF while fermenting.”
National Center for Home Food Preservation (USDA, University of Georgia) · Sauerkraut · read on October 20, 2026
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2
Below 60 °F (16 °C) it may not ferment; above 75 °F (24 °C), it softens
“At temperatures lower than 60ºF, kraut may not ferment. Above 75ºF, kraut may become soft.”
National Center for Home Food Preservation (USDA, University of Georgia) · Sauerkraut · read on October 20, 2026
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3
Best at 68–72 °F (20–22 °C); 60–78 °F (16–26 °C) can work
“Fermentation is best achieved at 68-72°F, although the range of 60-78°F can work.”
Virginia Cooperative Extension (Virginia Tech) · Vegetable fermentation (FST-328, reviewed 2025) · read on October 26, 2026
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4
Kombucha at 68–77 °F (20–25 °C) for 7 to 10 days
“allow the kombucha to ferment at room temperature (68° - 77°F) for 7 – 10 days.”
National Center for Home Food Preservation (USDA, University of Georgia) · Kombucha Tea · read on October 20, 2026
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5
Must at an even 65–80 °F (18–27 °C)
“try to keep the must maintained at an even temperature between 65 and 80 degrees Fahrenheit.”
NDSU Extension (North Dakota State University) · From the Garden or Orchard to the Table: Getting Started With Home Winemaking (FN1638, reviewed August 2026) · read on October 25, 2026
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6
Koji is incubated at about 30 °C (86 °F)
“covered with a food safe cloth, and incubated at approximately 30oC.”
BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026
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7
Koji never above 40 °C (104 °F)
“Temperatures must never go above 40oC or enzyme development will halt.”
BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026
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8
Tempeh, 1 to 3 days at 20–37 °C (68–99 °F)
“conditions may vary from 1 to 3 days at temperatures between 20oC to 37oC.”
BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.9: Tempeh (November 2023, updated December 2024) · read on October 25, 2026
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9
Tempeh never above 42 °C (108 °F)
“Temperatures should not go above 42oC as Rhizopus mould may die off, permitting growth of spoilage organisms.”
BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.9: Tempeh (November 2023, updated December 2024) · read on October 25, 2026
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10
A yogurt maker, cooler, oven or heating pad holding 110 °F (43 °C)
“A yogurt maker, insulated cooler, thermos, oven or heating pad that will maintain a temperature of 110°F for several hours may be used as an incubator.”
University of Alaska Fairbanks Cooperative Extension Service · Making Yogurt at Home (FNH-00062, reviewed October 2024) · read on October 25, 2026
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11
Storing at 41 °F (5 °C) or below protects safety and quality
“Maintaining a storage temperature of 41 °F or below helps ensure both the safety and quality of the final product”
University of Georgia Cooperative Extension · Homemade Kimchi: A Science-Based Guide to Safe Fermentation (Circular 1362, 2025) · read on October 20, 2026
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12
Monitor the temperature throughout incubation
“Monitor temperature throughout incubation, making adjustments as necessary.”
University of Alaska Fairbanks Cooperative Extension Service · Making Yogurt at Home (FNH-00062, reviewed October 2024) · read on October 25, 2026
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13
A thermometer measures air temperature
“You can use a thermometer to measure air temperature.”
University of Minnesota Extension · Preserving food at home: Fermentation (reviewed 2026) · read on October 25, 2026
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14
Tempeh's temperature can rise 5 to 7 °C (9 to 13 °F)
“As temperature may rise between 5oC to 7oC, fermentation temperatures at or below 37oC or lower is”
BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.9: Tempeh (November 2023, updated December 2024) · read on October 25, 2026
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15
Koji makes heat; left alone, it passes 50 °C (122 °F) and stops growing
“Koji mold consumes carbohydrates in the materials and generates heat; if left untreated, the product temperature exceeds 50 °C and the growth stops.”
Journal of Fungi (MDPI), Yamashita H. · Koji Starter and Koji World in Japan (2021) · read on October 25, 2026
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16
Koji is mixed at 16 to 18 hours to release heat
“In manual methods, koji is mixed by hand at 16 to 18 hr to release heat and to separate rice grains to prevent clumping.”
BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026
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17
Tempeh at 75 to 85% humidity, on trays that drain
“relative humidity should be maintained between 75% to 85%, and trays should drain away excess moisture.”
BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.9: Tempeh (November 2023, updated December 2024) · read on October 25, 2026
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18
Tempeh needs some oxygen, but not too much
“The fermentation conditions and packaging should allow for sufficient oxygen, but not too much oxygen, otherwise sporulation will occur.”
BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.9: Tempeh (November 2023, updated December 2024) · read on October 25, 2026
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19
In vegetables, what rules is pH dropping to 4.6 or lower
“The critical control point in vegetable fermentation is a pH drop to 4.6 or lower.”
BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.1: Fermented Vegetables (August 2024, updated December 2024) · read on October 25, 2026