Botulism bacteria can’t grow or make toxin at pH 4.6 or lower, so a ferment that gets below that line quickly, under brine and
with enough salt, is protected from them. A well-fermented one goes much lower: sauerkraut finishes around 3.5. A jar that sat for
days above 4.6 isn’t made safe by souring later. Most vegetables and milk start well above the line. Fresh cabbage is about pH 5.2–6.8
(K-State Research and Extension), and it’s the lactic acid made in the jar that brings it down.
Why 4.6 Is the Line
US food regulations split foods at pH 4.6. “Acid foods” have a natural pH of 4.6 or below; “low-acid foods” have a pH above 4.6
and a water activity above 0.85 (21 CFR 114.3). Fresh cabbage, at 5.2–6.8, is a low-acid food by that definition.
Pickles made by adding acid to low-acid vegetables (“acidified foods”) are defined as having “a finished equilibrium pH of 4.6 or
below” (same rule).
The reason is Clostridium botulinum. Its spores are common in the environment (CDC, 2013), and they grow and make toxin in airless, low-acid
foods with a pH above 4.6. A brine ferment is exactly that kind of airless place, so acid is what protects it. UC Davis says
botulism “can occur if fermentation is not successful and acid levels are low”, and “the production of sufficient acid will
inhibit” the bacterium (UC Davis). A review for small fermented-vegetable businesses advocates a single critical control point
for the whole process: “a steady and sustained pH decline to < 4.6” (Snyder et al., Food Protection Trends, 2020).
Salt is the other safeguard. UC Davis calls salt “important to prevent growth of these harmful microorganisms”
(UC Davis); the salt and brine calculator gives the weights for dry salting or a brine.
The pH Common Home Ferments Reach
These are end points from studies and food-safety plans. Your batch will vary with temperature, salt, starter and time, but a
finished ferment should land in or near these ranges, and always at or below 4.6, with vegetable ferments well below it. How warm to keep each ferment, and for how long, is in the fermentation temperature chart.
| Ferment | Typical finished pH | How long it took, and at what temperature |
|---|---|---|
| Sauerkraut | About 3.5 (3.4–3.6) | Commercial tanks fell fastest in the first 2 weeks and were just under 3.5 by day 60 (Lu et al., 2003); Virginia Tech gives 3.4–3.6 as ideal for finished kraut (Virginia Tech) |
| Kimchi | 4.0–4.5 at its best; lower as it sours | Reached in 3 days at 15°C and in 47 days at 4°C (Kim et al., 2025) |
| Milk kefir | 4.2–4.6 | Typical after about 24 hours at 20–25°C, as summarised in a study of household kefir; its own kefir measured about 4.5–4.6 after 24 hours at 20°C and 3.9 by 42 hours (Alves et al., 2021) |
| Yogurt, sour cream, crème fraîche | 4.6 or lower | Food-safety plans discard a batch that hasn’t reached it within 2 hours of its usual incubation time (FOODSAFE BC plans for yogurt, sour cream and crème fraîche) |
| Water kefir | 3.45 | Fell from 4.26 during the first 72 hours (Laureys and De Vuyst, 2014) |
| Kombucha | 2.96–3.47 | 3.47 after 7 days at 23°C, 2.96 after 15 days at 28°C (Salinas-Ruiz et al., 2026) |
Kefir and yogurt finish close to the line, so the rest of their safety rests on pasteurized milk, a clean
starter and the fridge (see our milk kefir method). Kombucha is acidified at the start: in the kombucha study the sweet tea
(pH 5.15) was brought to pH 4.14 with liquid from the starter culture before the culture went in, a step the authors took for
microbiological safety (same source). Home kombucha recipes do the same by adding
soured kombucha from a previous batch.
Below 4.6 Doesn’t Mean Sterile
Acid stops botulism, but it doesn’t sterilise a jar. In a Health Canada study, toxin-producing E. coli survived eight
weeks in refrigerated kimchi held at about pH 4.1, falling no more than tenfold (Gill et al., 2024); our guide to
how long kimchi lasts has the outbreak behind that study. A low reading doesn’t excuse dirty jars,
unwashed produce or bare hands in the mix.
Strips or a Meter: What Each Can Tell You
Paper strips are fine for checking that a ferment you expect to be well below 4.0 really is. Near the line, don’t rely on them:
the US rule for commercial acidified foods allows colour (indicator) methods only “if the pH is 4.0 or lower”, and otherwise uses a
meter (21 CFR 114.90). Virginia Tech’s home guidance still suggests strips for checking that fermented
vegetables or yogurt are below 4.6 (Virginia Tech), and that works when the result is clearly lower, at 3.5 or so. A reading
on a strip that sits anywhere near the line needs a meter.
A calibrated pH meter is accurate to about 0.1 pH unit, according to the same rule. That makes a meter
the right tool for anything that finishes close to 4.6: kefir, yogurt, kimchi early on, and fermented hot sauce
before it goes into the fridge.
A reading checks a tested recipe; it doesn’t replace one. It can’t show whether toxin formed while the food sat above 4.6, and it
doesn’t make low-salt ferments, low-acid foods kept in oil or honey, or room-temperature storage safe. Follow a tested recipe for salt,
temperature and time (“Follow a researched and tested recipe”, UC Davis), and keep finished ferments in the fridge unless
the recipe has a tested canning process.
How to Take a Reading You Can Trust
The method in 21 CFR 114.90 is written for factories, but its main steps scale down to a kitchen:
- Calibrate first. Turn the meter on and let it settle, then standardise it in pH 4.0 buffer, repeating until two readings in a
row agree. Check it in pH 7.0 buffer. Give each reading about a minute to settle. - Rinse the probe with water and blot it with soft tissue between buffers and samples. Blot, don’t wipe.
- Test at room temperature. The rule asks for 20–30°C (25°C is best), so let a sample from the fridge warm up first, even if your
meter compensates for temperature. - Test the food, not just the brine. Foods can be “a mixture of liquid and solid components that differ in acidity”, so the rule
blends drained solids into a paste, or blends solids and liquid in the jar’s own ratio. At home, blend a
spoonful of vegetables with brine in roughly the same proportion as in the jar. - Take two readings of the same sample. They should agree; the rule reports to the nearest 0.05.
- Keep oil off the probe. Oil coats the electrode and throws readings off, so skim it before testing an oily sauce.
All the figures in these steps come from the regulation (21 CFR 114.90).
If the pH Won’t Come Down
A vegetable ferment that measures above 4.6, or doesn’t look and smell clearly sour (bubbles, cloudy brine, a sour smell) after a
week at room temperature, hasn’t done its job. Throw it out. Don’t taste it to check, don’t move it to the fridge and hope, and don’t
try to rescue it by warming it up or adding vinegar. Botulinum toxin can’t be seen, smelled or tasted, and the CDC’s advice is “Never
taste food to see if it’s safe” (CDC). Some hot sauce recipes add vinegar to take a sauce that has soured normally further below the
line before it goes into the fridge; it isn’t a fix for a jar that sat warm and never soured.
Then find the cause before the next batch. A cold room is one cause: below 60°F (about 16°C), sauerkraut “may not ferment”
(NCHFP). Our troubleshooting guide covers the others.
What Happens Above the Line: Two Botulism Cases
Both cases below involved foods fermented at home.
- In New York City in 2012, two people became ill (one case confirmed, one probable) after eating tofu they had fermented at
home. One household left cubed tofu at room
temperature for a week before adding chilli and salt and keeping it in the fridge for three weeks; toxin type B was found in it. The investigators pointed to “prolonged storage at
room temperature in a low-acid and low-salt environment” (CDC, 2013). - Four men became ill in a US outbreak described by CDC and Texas public health staff; homemade turshi, a fermented vegetable
dish, was the likely cause. They had eaten it at two places on different days over a month (Harvey et al., 2017).
The CDC report adds that in the US, home-canned foods and traditional fermented Alaska Native foods are major sources of
foodborne botulism (CDC, 2013). A sauerkraut that has soured properly, with enough salt and the cabbage under brine,
is not the risk. A jar that never got sour is.