Acid and salt keep fermented foods safe, and a few shortcuts undo them

Key takeaways

• Home sauerkraut and pickles stay safe because helpful bacteria make acid; extension scientists aim for a pH below 4.6, and the ideal for finished sauerkraut is 3.4 to 3.6³.
• For sauerkraut, tested recipes call for 3 tablespoons of canning salt per 5 pounds of cabbage, fermented at 70°F to 75°F for about 3 to 4 weeks²⁷.
• A flat white film that isn’t fuzzy is usually kahm yeast, which is harmless; slime, rot or a foul smell means throw it out⁷⁸.
• A couple who drank kombucha brewed in an old glazed ceramic pot every day for six months developed lead poisoning¹.
• Pregnant people, adults 65 and older and people with weakened immunity should choose pasteurized soft cheese or heat it to 165°F; a 2026 outbreak showed pasteurized cheese can still be contaminated¹²¹⁵.

For half a year, a husband and wife started each day with homemade kombucha as a tonic. Their brewing vessel was a quarter-century-old ceramic pot from Spain, glazed inside and never meant to hold food¹.

Then the wife, 58, spent six weeks with worsening constipation and cramping pain. Tests showed anemia and a blood lead level more than 10 times the top of the normal range. Her husband, 63, turned out to have high lead, too¹.

Doctors traced the lead to the tea. Their best explanation was that acids in the brew had drawn lead out of the glaze; the tea tested high for lead, and the culture tested higher still¹. After intravenous treatment to remove lead, both recovered, and their doctors described the case in the Medical Journal of Australia in 1998¹.

The lesson isn’t that kombucha is dangerous. It’s that fermentation is a set of rules, and the rare serious problems come from breaking one of them.

Our article on fermented foods and the Stanford trial covers the benefits. This guide covers safety: how fermentation protects food, where it can go wrong, and how to buy, make or skip each kind.

Acid and salt are the safety system

Fermenting vegetables is simple chemistry. Lactic acid bacteria eat the natural sugars in cabbage or cucumbers and turn them into acid². That happens with little or no oxygen, which is why the vegetables must stay under the brine².

As acid builds, it holds back harmful bacteria. Eventually it stops even the fermenting bacteria, and the batch settles².

Virginia Tech food scientists say acidity is the main safety barrier. They set a target of pH below 4.6 for fermented vegetables and yogurt³. On the pH scale, lower numbers mean more acid. They put the ideal for finished sauerkraut at pH 3.4 to 3.6, well past that line³.

Salt does the setup work. It pulls water and sugar out of the vegetables, favors the fermenting bacteria over spoilage microbes and keeps the texture firm². Sauerkraut needs about 2.25% to 2.5% salt by weight²³.

Timing matters too. University of Georgia scientists note that homemade kimchi takes about two weeks to reach a pH below 4.6, and early in fermentation it may not yet be acidic enough to stop some germs⁴. That’s why tested recipes specify temperatures and times, not just ingredients.

The germ behind botulism, Clostridium botulinum, thrives without oxygen. It can make its deadly toxin unless high acid, enough salt or refrigeration holds it back³.

Botulism follows ferments that lose their old safeguards

Alaska has had some of the highest botulism rates in the nation. From 1990 through 2000, CDC researchers counted 58 botulism events in Alaska affecting 103 people, about 39% of all U.S. foodborne cases in those years⁶.

In all 49 events where investigators pinned down the food, it was a homemade Alaska Native food⁶. Fermented fish, such as salmon heads, caused 14 events. Seal oil caused 14 more, fermented tissue from aquatic mammals caused 11, and fermented fish eggs caused 7⁶.

The CDC’s prevention advice starts with tradition. It describes the old method: fermenting in a grass-lined hole in the ground, in the cold, where fresh air could circulate⁵.

The trouble began with modern shortcuts. The CDC researchers linked part of the problem to fermenting in plastic or glass containers⁶. The CDC now warns that these containers block airflow, and that botulism bacteria can grow in them even without a lid⁵.

The agency’s advice reads like a recipe for honoring tradition⁵:

• Ferment cold, ideally keeping the food below 37°F the whole time.
• Skip plastic and glass containers for these foods.
• Salt fish when drying it.
• Boil fermented foods for 10 minutes before eating, adding a minute for every 1,000 feet above 1,000 feet of elevation. Boiling can destroy the toxin.

One botulism survivor’s advice appears on the CDC’s page: “Follow the traditional ways of preparing food. Don’t try to use any shortcuts to get them ready faster”⁵.

Botulism can kill. In the Alaska data, about 3% of people with it died⁶. Our food storage article covers home-canning rules, another key botulism safeguard.

Home sauerkraut and pickles: follow the tested numbers

Virginia Tech’s extension guide gives the basic sauerkraut formula, and Oregon State’s matches it on salt and temperature²⁷:

• Salt: 3 tablespoons of canning or pickling salt for every 5 pounds of shredded cabbage.
• Temperature: 70°F to 75°F for about 3 to 4 weeks, or 60°F to 65°F for 5 to 6 weeks. Below 60°F, kraut may not ferment. Above 75°F, it can turn soft.
• Submerge it: Keep the cabbage 1 to 2 inches under the brine.

Pickles take more salt. Virginia Tech lists 3% to 5% salt for low-salt pickles and 5% to 16% for high-salt ones². Cutting the salt invites mold and other contaminants².

The salt itself matters. Use canning or pickling salt, not iodized or reduced-sodium salt, and skip table salt with anticaking agents²⁸. University of Illinois Extension notes that iodized salt can slow fermentation and turn kraut brown⁸.

Pick the right container. Glass jars, food-grade plastic and ceramic crocks all work²⁸. Be wary of old crocks. Virginia Tech notes that lead glazes were common in crocks made before the 1970s².

Skip metal other than high-grade stainless steel, trash bags, and rocks as weights, since rocks may contain heavy metals. Wood is hard to keep clean². Plan on about a 1-gallon container for every 5 pounds of vegetables⁸.

Refrigerated, fully fermented vegetables keep about 4 to 9 months, with saltier batches lasting longer². To make them shelf-stable, use a tested canning recipe; our food storage article explains why that step has no room for improvising.

Kahm yeast, mold or slime: when to toss it

Most surprises in the crock are harmless. A few aren’t. This is how extension services sort them.

Verdict	What you see or smell	Why
Keep	A flat white film, not fuzzy, not in round spots	Kahm yeast, common and safe; skim it and push the vegetables back under78
Keep	Soft texture, but a clean, sour smell and taste	Poor quality, not a hazard; often too little salt or too much heat7
Keep	Pink color without slime	Yeast or natural cabbage pigment7
Keep the rest	A rotten layer on top, where brine didn’t cover the cabbage	Air reached the surface; remove that layer7
Your call	One small patch of mold on sauerkraut	Guides differ (see below); when unsure, toss47
Toss	Mold over much of the surface	Too much to skim7
Toss	Slimy texture	Spoilage, often from heat or too little salt7
Toss, untasted	A rotten or foul smell	Spoilage28
Toss	A traditional Alaska ferment you have doubts about	You can’t see, smell or taste botulism toxin; “If in doubt, throw it out”5

Guides disagree on mold. Oregon State allows skimming small patches off sauerkraut but says to discard a batch with large amounts⁷. University of Georgia lists any mold on kimchi as a sign of spoilage and says to discard it⁴.

The two guides cover different foods, but the safer rule is clear. If the batch is for a pregnant person, a young child, an older adult or someone with weakened immunity, take the stricter path.

Kombucha: an acid drink that keeps fermenting

Kombucha is sweet tea fermented by a culture of yeasts and bacteria. It turns acidic fast, and it can contain alcohol⁹.

The 1995 Iowa cases. In April 1995, two women in a small Iowa town became gravely ill with lactic acidosis, a dangerous buildup of acid in the blood⁹. A 59-year-old died. A 48-year-old went into cardiac arrest and survived; she had recently tripled her daily amount, from 4 ounces to 12⁹.

Both brewed kombucha from cultures that came from the same parent⁹. Yet at least 115 other residents used cultures from that source without unexplained illness, and the CDC couldn’t prove the tea caused the cases⁹. Iowa health officials still advised people to stop drinking it until its role was clearer⁹.

The report flagged other lessons. Home brews tested contained 0.7% to 1.3% alcohol, mold had been seen on home batches, and contamination was possible⁹. Ceramic and lead crystal were on the CDC’s do-not-use list, too: an acidic drink can draw toxic metals out of both⁹.

Poison Control lists other reports tied to home-brewed kombucha, including several cases of liver inflammation¹⁰. It advises that young children, pregnant women and people with weakened immunity not drink kombucha, homemade or store-bought¹⁰.

The alcohol rule. Federally, kombucha that hits 0.5% alcohol by volume at any point, even after bottling, falls under the Alcohol and Tobacco Tax and Trade Bureau (TTB)¹¹. At that level it must carry the government alcohol warning and meet alcohol labeling and tax rules¹¹. Kombucha that stays below 0.5% falls under FDA food rules instead¹¹.

Live kombucha can keep fermenting in the bottle, which is how alcohol creeps up¹¹. Some commercial products have been recalled for carrying more alcohol than their labels said¹⁰. Keep refrigerated kombucha cold, and treat it as off-limits if you avoid alcohol for pregnancy, recovery or medication reasons.

If you brew at home, use plain glass rather than ceramic or lead crystal, start with a clean culture from a trusted source, and throw out any batch that grows fuzzy mold.

Soft cheese and raw-milk ferments carry a Listeria risk

Cheese is one of the oldest fermented foods. Soft, moist cheeses are more likely than hard cheeses to carry Listeria, a germ that is riskiest for pregnant people, adults 65 and older and people with weakened immunity¹²¹⁵.

The CDC names queso fresco-type cheeses in particular. Raw-milk cheese, yogurt and ice cream can also carry Listeria¹². Our article on processed foods covers raw milk in depth.

Fermentation doesn’t guarantee safety. In early 2024, Hawaii officials found the outbreak strain in a sample of aged cotija cheese¹³. That find helped tie a long-running Listeria outbreak in 11 states, stretching back to 2014, to Rizo-López Foods¹³.

Of 26 people sickened, 23 were hospitalized and two died¹³. Two women became ill during pregnancy, one lost her pregnancy, and two newborns were infected¹³. The company recalled its cheeses, along with yogurt and sour cream sold under 13 brands¹³¹⁴.

Pasteurized isn’t a complete shield. In a 2026 outbreak tied to soft cheeses, including requesón, 14 of 15 sick people were hospitalized and one died¹⁵. The CDC noted that soft cheese made from pasteurized milk can still pick up germs during cheese-making¹⁵.

If you’re pregnant, 65 or older, or have weakened immunity, choose pasteurized cheeses, and heat soft cheeses to 165°F or until steaming hot¹²¹⁵. Queso fresco cooked into enchiladas or a casserole is a safer way to enjoy it¹².

Buying fermented foods: cold, sealed and checked

A few habits make store-bought ferments safer:

• Buy live-culture foods from the refrigerated case and get them home cold. Refrigeration at 41°F or below slows fermentation and suppresses harmful microbes⁴.
• Check recalls. The Rizo-López recall reached products sold under 13 brands, including Whole Foods Market’s 365 line¹⁴. The FDA posts recalls online.
• Watch the salt. University of Georgia Extension puts kimchi at about 498 milligrams of sodium per 100 grams, about 3.5 ounces⁴. Our fermented foods article covers sodium limits and the stomach cancer link to salt-preserved foods.
• Know your histamine tolerance. Our articles on fermented foods and food intolerances explain histamine and tyramine in aged cheese and other ferments, and who reacts to them.

How strong is the evidence?

Practice or claim	Evidence level	What it rests on
Tested salt, temperature and acidity keep home vegetable ferments safe	Strong evidence	Long-standing food science used by extension services237
Fermenting traditional Alaska foods in plastic or glass is as safe as the old way	Evidence says no, or unsafe	CDC data and warnings link these containers to botulism56
Brewing kombucha in glazed ceramic is harmless	Evidence says no, or unsafe	Documented lead poisoning; CDC advises against it19
Home kombucha in typical amounts seriously harms healthy adults	Not enough evidence	Rare case reports; CDC couldn’t prove cause in Iowa910
Pasteurized soft cheese can’t carry Listeria	Evidence says no, or unsafe	2026 outbreak; contamination during cheese-making15
Heating cheese to 165°F kills Listeria	Strong evidence	CDC guidance12

Tradition is the original safety manual

People who love fermented foods often love them because they’re old. That’s a strength here. Across these cases, the safest choice was usually the traditional one done carefully: the right amount of salt, a clean crock, a cool cellar, cold ground instead of a plastic bucket.

The problems came from shortcuts, such as a decorative pot, a sealed container, a warm counter, or a jar left out too long. Respecting the craft means following its rules exactly.

Questions readers ask

The bottom line

Fermentation protects food with acid and salt, and those two tools do most of the safety work. Problems are rare, but they’re serious when they happen: botulism when traditional Alaska ferments lose their cold, airy conditions, lead when acidic kombucha sits in a glazed pot, and Listeria in soft cheese for people at higher risk.

Follow tested recipes exactly, use the right containers, keep live products cold and toss anything slimy, foul-smelling or doubtful. If you’re pregnant, older or immune-weakened, choose pasteurized dairy, heat soft cheese until steaming and skip kombucha.

Keep exploring: how to choose a probiotic.

Related reading

• Six servings of fermented food a day reshaped gut microbes in a Stanford trial, so start with one
• Date labels mostly track quality, while a fridge thermometer tracks safety
• Frozen peas and candy bars are both processed, and the difference matters
• Most food intolerances come down to dose, while gluten sensitivity remains a puzzle
• Many probiotic bottles can’t be traced to a study, so check the strain code

Sources

1. Phan TG, et al. “Lead Poisoning From Drinking Kombucha Tea Brewed in a Ceramic Pot.” Medical Journal of Australia, 1998;169:644–646. mja.com.au
2. Eifert JA, Boyer RR, Williams RC. “Vegetable Fermentation” (FST-328P). Virginia Cooperative Extension, 2020, reviewed April 2025. pubs.ext.vt.edu
3. Cook C, Ponder MA, Strawn LK. “Making Safe Fermented Foods and Beverages” (FST-435NP). Virginia Cooperative Extension, September 21, 2022. pubs.ext.vt.edu
4. Mahida M, Cullinan S, Ingmundson K, et al. “Homemade Kimchi: A Science-Based Guide to Safe Fermentation” (C1362). University of Georgia Extension, August 5, 2025. fieldreport.caes.uga.edu
5. Centers for Disease Control and Prevention. “Alaska Native Foods” (Botulism: Prevention). CDC, updated May 3, 2024. cdc.gov
6. Sobel J, et al. “Foodborne Botulism in the United States, 1990–2000.” Emerging Infectious Diseases, 2004;10(9). wwwnc.cdc.gov
7. Oehler N. “Sauerkraut: Problems and Solutions” (SP 50-745). Oregon State University Extension Service, 2020, revised January 2025. extension.oregonstate.edu
8. University of Illinois Extension. “Fermenting FAQ.” Undated. extension.illinois.edu
9. Centers for Disease Control and Prevention. “Unexplained Severe Illness Possibly Associated With Consumption of Kombucha Tea — Iowa, 1995.” MMWR, 1995;44(48):892–893. cdc.gov
10. National Capital Poison Center (Poison Control). “Kombucha Tea: Health Tonic or Dangerous?” Undated. poison.org
11. Alcohol and Tobacco Tax and Trade Bureau. “Kombucha” (TTB G 2010-3A). TTB, 2015, updated August 2019. ttb.gov
12. Centers for Disease Control and Prevention. “How Listeria Spreads: Soft Cheeses and Raw Milk.” CDC, January 16, 2025. cdc.gov
13. Centers for Disease Control and Prevention. “Investigation Update: Listeria Outbreak, Queso Fresco and Cotija Cheese — February 2024.” CDC, January 31, 2025. cdc.gov
14. U.S. Food and Drug Administration. “2024 Recalls of Food Products Associated With Dairy Products From Rizo Lopez Foods, Inc. Due to the Potential Risk of Listeria monocytogenes.” FDA, 2024. fda.gov
15. Centers for Disease Control and Prevention. “Listeria Outbreak Linked to Soft Cheese.” CDC, June 4, 2026, updated August 26, 2026. cdc.gov