Temperature and time
Temperature sets the pace of a ferment, and the days in a recipe only estimate it. What tells you it's done is the pH.
6 min read First steps Reviewed October 26, 2026 26 facts, each with its source
Why temperature is in charge
Lactic acid bacteria work fast when it's warm and slowly when it's cool. The temperature where the jar sits decides how long it takes to sour, and also how it turns out: crisp, soft or not fermented at all.
The first days are the touchy ones. Early on, the pH is not yet low enough to stop certain pathogens1. That's why a ferment should start inside its range: in a cold corner, the acid takes longer to arrive.
Dry-salted vegetables
For sauerkraut and other shredded vegetables salted dry, the reference is the NCHFP, the USDA-backed center for home food preservation. It recommends fermenting at 70–75 °F (21–24 °C) for good quality2. At that temperature it's done in about 3 to 4 weeks; at 60–65 °F (16–18 °C) it may take 5 to 63. Below 60 °F (16 °C) it may not ferment, and above 75 °F (24 °C) it may go soft4.
| Temperature | Approximate time |
|---|---|
| Below 60 °F (16 °C) | May not ferment |
| 60–65 °F (16–18 °C) | 5 to 6 weeks |
| 65–70 °F (18–21 °C) | 3 to 6 weeks (house rule) |
| 70–75 °F (21–24 °C) | 3 to 4 weeks |
| Above 75 °F (24 °C) | May go soft |
Between 65 and 70 °F, the NCHFP gives no times. Fervida uses the span of the two bands on either side, 3 to 6 weeks. That's a house rule, and the app labels it as one.
Brine, kimchi and drinks
Each method has its own range and its own clock:
| Ferment | Temperature | Time |
|---|---|---|
| Brine pickles | 68–72 °F (20–22 °C) | 3 to 4 weeks |
| Other vegetables in brine | 68–72 °F (20–22 °C) | Several weeks |
| Kimchi | About 68 °F (20 °C) | 1 to 2 days, then the fridge |
| Kombucha | 68–77 °F (20–25 °C) | 7 to 10 days |
| Milk kefir | 65–85 °F (18–29 °C) | 18 to 24 hours |
Brine pickles ferment at 68–72 °F (20–22 °C) in about 3 to 4 weeks5; at 59–68 °F (15–20 °C) they typically take 5 to 66, and above 77 °F (25 °C) they may spoil7. For other vegetables in brine, such as carrots or radishes, the ideal is also 68–72 °F (20–22 °C)8, but the guides give no day count: it generally takes several weeks9, and taste and pH decide.
Kimchi wants a steady room at about 68 °F (20 °C)10. It ferments out for 1 to 2 days and is tested daily until the pH drops to 4.6 or below11.
Kombucha ferments for 7 to 10 days at 68–77 °F (20–25 °C)12: cooler, it goes slower; warmer, faster13. Milk kefir is the quickest: between 65 and 85 °F (18–29 °C)14, it's ready in 18 to 24 hours15.
Cool or warm?
The guides don't fully agree on the ideal. The NCHFP prefers 70–75 °F (21–24 °C) for sauerkraut2. The University of Minnesota puts it at 68–72 °F (20–22 °C), warning that above 78 °F (26 °C) a ferment over-ferments and spoils, and below 60 °F (16 °C) it stalls8. Kansas State goes slower: it says lactic acid bacteria like it a little cooler than we do, and that a slow ferment at 60–70 °F (16–21 °C) gives better results16.
Where it matters, they agree. The floor is around 60 °F (16 °C) and the ceiling between 75 and 78 °F (24–26 °C). Inside that window, cooler gives a slower ferment, which Kansas State finds better; warmer gives a faster one that is more likely to go soft. Choose by your home and your taste, but stay inside the window.
The days guide you; pH decides
The days in a recipe are an estimate for one particular temperature. Your kitchen isn't the recipe's kitchen, and every vegetable brings its own sugars and bacteria. What tells you a ferment is done is the pH.
The University of Minnesota puts it this way: it's ready when the pH has reached 4.6 or lower and bubbling has stopped17. If the bubbles have stopped but the pH is still above 4.6, let it keep fermenting and test it every 1 to 2 days18. When it's cool, that moment comes later.
4.6 is the safety limit. Fervida recommends waiting for 4.2 before moving vegetables to the fridge: that's a house rule, to leave a margin.
Where to put the jar
- Measure, don't guess. A room thermometer is enough19. What counts is the temperature right where the jar sits, not the thermostat or another room.
- Find a cool, steady spot, away from the oven, radiators and sunlight. In summer, check your ferments more often20.
- If your home drops below 60 °F (16 °C) in winter, find a warmer spot: at that temperature, sauerkraut may not ferment4.
- If it's very hot, kimchi can ferment slowly in the fridge, over 3 to 4 days21.
The fridge slows it, it doesn't stop it
A ferment is a living food. Left out once it's done, it keeps fermenting until it's very sour and past its best, in flavor and in safety22. The fridge slows fermentation, lets the flavor mature gradually and also holds back harmful microbes23. Keep it at 41 °F (5 °C) or below24.
In the fridge, a ferment doesn't stand still: its flavor keeps changing slowly25. Tightly covered, fully fermented sauerkraut keeps for several months26.
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
Early on, the pH is not yet low enough to stop certain pathogens
“During the early stages of fermentation, the pH may not yet be low (acidic) enough to prevent the proliferation of certain pathogens.”
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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2
For good sauerkraut, the USDA recommends fermenting at 70–75 °F
“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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3
At 70–75 °F, 3 to 4 weeks; at 60–65 °F, 5 to 6
“At temperatures between 70º and 75ºF, kraut will be fully fermented in about 3 to 4 weeks; at 60º to 65ºF, fermentation may take 5 to 6 weeks.”
National Center for Home Food Preservation (USDA, University of Georgia) · Sauerkraut · read on October 20, 2026
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4
Below 60 °F it may not ferment; above 75 °F it may soften
“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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5
Pickles: 68–72 °F for 3 to 4 weeks
“The ideal temperature for a cucumber pickle fermentation is 20-22°C for approximately 3 to 4 weeks.”
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
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6
At 59–68 °F, 5 to 6 weeks
“At temperatures of 15-20°C fermentation periods of 5 to 6 weeks are typical.”
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
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7
Above 77 °F it may spoil
“fermentation at temperatures higher than 25°C may result in spoilage.”
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
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8
Ideal 68–72 °F; above 78 °F it over-ferments; below 60 °F it stalls
“A temperature between 68 and 72 degrees Fahrenheit is ideal. Temperatures over 78 degrees Fahrenheit can cause over fermentation and spoilage. Temperatures below 60 degrees Fahrenheit will inhibit fermentation.”
University of Minnesota Extension · Preserving food at home: Fermentation (reviewed 2026) · read on October 25, 2026
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9
It generally takes several weeks
“it generally will take several weeks.”
K-State Research and Extension (Kansas State University) · Taking a New Look at Fermented Foods (MF3491, June 2020) · read on October 20, 2026
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10
Kimchi: ideally a steady room at about 68 °F
“with a relatively constant room temperature, around 68°F is ideal.”
Colorado State University Extension · Understanding and Making Kimchi (2025) · read on October 20, 2026
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11
Kimchi: 1 to 2 days out, testing daily until pH 4.6 or below
“Ferment for 1 to 2 days at room temperature, testing it daily until a pH of 4.6 or below is reached.”
Colorado State University Extension · Understanding and Making Kimchi (2025) · read on October 20, 2026
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12
Kombucha: 7 to 10 days at 68–77 °F
“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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13
Cooler, kombucha goes slower; warmer, faster
“Cooler temperatures may slow fermentation, while warmer temperatures can accelerate it.”
University of Georgia Cooperative Extension (National Center for Home Food Preservation) · Crafting Kombucha at Home: Frequently Asked Questions (2026) · read on October 20, 2026
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14
Milk kefir: between 65 and 85 °F
“Set in a dark place at a temperature between 65° and 85° F (18°-29° C).”
Colorado State University Extension (FoodSmart Colorado) · Understanding and Making Kefir · read on October 20, 2026
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15
Milk kefir: 18 to 24 hours
“Allow to ferment for 18-24 hours.”
Colorado State University Extension (FoodSmart Colorado) · Understanding and Making Kefir · read on October 20, 2026
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16
Lactic acid bacteria like it a little cool: 60–70 °F gives better results
“Lactic acid bacteria prefer slightly cooler temperatures than we do. A slower ferment at 60° to 70° F will yield better results than one fermented at higher temps.”
K-State Research and Extension (Kansas State University) · Taking a New Look at Fermented Foods (MF3491, June 2020) · read on October 20, 2026
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17
It's done when the pH is 4.6 or lower and bubbling has stopped
“When a pH of 4.60 or lower is reached and bubbling has stopped, the ferment is ready.”
University of Minnesota Extension · Preserving food at home: Fermentation (reviewed 2026) · read on October 25, 2026
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18
No bubbles but still above 4.6: keep going and test every 1 to 2 days
“If bubbling has stopped but the pH is still above 4.60, continue to ferment and check the pH every 1 to 2 days.”
University of Minnesota Extension · Preserving food at home: Fermentation (reviewed 2026) · read on October 25, 2026
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19
A room thermometer is enough
“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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20
Find a cool spot and check more often in summer
“Look for a cool place in your home and when fermenting in the summer, plan to check your ferments often for doneness.”
K-State Research and Extension (Kansas State University) · Taking a New Look at Fermented Foods (MF3491, June 2020) · read on October 20, 2026
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21
Kimchi can also ferment in the fridge, over 3 to 4 days
“Kimchi may be placed in refrigerator so it ferments slowly over 3 to 4 days.”
Colorado State University Extension · Understanding and Making Kimchi (2025) · read on October 20, 2026
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22
Left out, it keeps fermenting until it goes too far
“these are living foods so if left out at room temperature they will continue fermenting, becoming very sour and ripening beyond palatability and safety.”
K-State Research and Extension (Kansas State University) · Taking a New Look at Fermented Foods (MF3491, June 2020) · read on October 20, 2026
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23
The fridge slows fermentation and harmful microbes
“Refrigeration slows down the fermentation process, allowing flavors to mature gradually while also suppressing the growth of harmful microorganisms.”
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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24
In the fridge at 41 °F or below
“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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25
In the fridge, the flavor keeps changing slowly
“Expect subtle changes in flavor even during refrigeration (the process slowly continues).”
University of Illinois Extension · Fermentation handout (2020) · read on October 20, 2026
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26
Fermented sauerkraut keeps for months in the fridge
“Fully fermented kraut may be kept tightly covered in the refrigerator for several months”
National Center for Home Food Preservation (USDA, University of Georgia) · Sauerkraut · read on October 20, 2026