Why did my fermented hot sauce bottle explode or ooze?

The sauce kept fermenting after you capped it. Live lactic acid bacteria and yeasts found sugar in the bottle, made carbon dioxide, and the gas had nowhere to go, so it pushed sauce out through the cap or, in glass, built enough pressure to break the bottle. It happens most with raw, uncooked sauces, sauces with fruit or sugar added after the ferment, and sauces that sat at room temperature. Cooking the finished sauce before bottling, or keeping a raw sauce cold, prevents it. This page explains the cause, shows the warning signs, sets out how to handle a swollen bottle without getting hurt, and lists the fixes.

What builds the pressure

Three things have to be present at once:

1. Live microbes. A fermented mash is full of lactic acid bacteria and yeasts even after it stops bubbling. Fermentative yeasts turn sugar into alcohol and carbon dioxide [1], and several bacteria in the mix make carbon dioxide too [1].
2. Something to eat. Vegetables naturally contain about 2 to 4 percent simple sugars [2]. A ferment that was bottled before it finished still has some. Fruit, honey, sugar, carrots and fresh garlic blended in after the ferment add more.
3. A sealed container. In a jar with an airlock the gas escapes. In a capped bottle it accumulates. UC Davis food scientists warn that gas produced by fermentation may break an airtight container [3]. Virginia Cooperative Extension says carbon dioxide from fermented products can build to dangerously pressurized levels in sealed canning jars [4]. Ireland's food safety authority notes that glass containers that develop excessive pressure from fermentation "may unexpectedly explode" [5].

A low pH does not prevent this. Yeasts tolerate acid well, and a sauce can test at a safe pH and still be full of active yeast. In a 2019 thread on The Hot Pepper forum, a maker selling pineapple and mango hot sauces reported that the fruit kept fermenting in the bottle and that one or two bottles burst when opened, even though the sauces had tested at pH 4.6 or below [6]. Other makers in the thread pointed to the same cause: the batch had not been heated enough to kill what was living in it [6].

It happens to big brands too

Commercial producers deal with bottle pressure too. On December 30, 2019, the Singapore Food Agency directed two importers to recall Huy Fong Sriracha Hot Chilli Sauce in 17 oz and 28 oz bottles with a best-before date of March 2021. The reason given was a lactic acid build-up that may cause bottles to bloat and continue to ferment, with pressure that could make them explode when opened [7]. A commercial sauce that keeps fermenting is treated as a defect serious enough to pull from shelves.

Signs a bottle is under pressure

• The cap is domed, or a plastic bottle feels hard and swollen.
• Sauce is weeping out around the cap or through the orifice reducer (the small plastic insert that controls the pour).
• You hear a hiss when you loosen the cap, or sauce foams up.
• Bubbles rise through the sauce when it sits still.
• The sauce tastes fizzy, yeasty or boozy when it did not before.
• A sticky ring builds up on the shelf under the bottle.

Any of these means fermentation has restarted. Clemson Cooperative Extension lists a bulging lid or leaking jar as signs of spoilage in home-preserved food, along with spurting liquid, off odor or mold on opening, and says not to taste or use food that shows any of them [8].

Handling a swollen bottle safely

A swollen bottle has two risks: physical, from glass under pressure, and microbial, from whatever is growing inside. You cannot tell from the outside which microbe it is, so treat the sauce as spoiled.

1. Do not open it to check, and do not taste it. Clemson's advice is not to taste or use any home-preserved food with signs of spoilage [8].
2. Do not shake it. Agitation releases dissolved gas and raises the pressure suddenly. Handle it gently and keep it upright.
3. Keep it away from your face. Glass under pressure can break without warning [5]. Hold a swollen glass bottle at arm's length, wrapped in a thick towel, when you move it.
4. Bag it if it is still sealed. For sealed containers of spoiled food, Clemson's instructions are to wear rubber or heavy plastic gloves, place the container in a heavy garbage bag, close it, and put it in the trash [8].
5. Clean up leaks with bleach. If sauce has leaked or the bottle broke, Clemson's cleanup method is 1 part unscented household chlorine bleach (5 to 6 percent sodium hypochlorite) to 5 parts water, left on the surface for 30 minutes, reapplied for another 30 minutes, then rinsed [8]. Wear gloves, keep the bleach off your skin, and bag the paper towels and gloves.
6. Check the rest of the batch. Bottles from the same batch probably have the same problem. Move any that are not yet swollen into the refrigerator and decide whether to discard them or use them quickly.

If anyone has eaten from a swollen or leaking bottle and develops double or blurred vision, drooping eyelids, slurred speech, difficulty swallowing, dry mouth or muscle weakness, get medical care immediately. Those are symptoms of botulism, which Clemson notes usually begin 18 to 36 hours after eating contaminated food but can begin as early as 6 hours or as late as 10 days [8]. Gas in a fermented pepper sauce is far more often yeast than botulism, but nobody can tell which by looking. More on the risk is on botulism and hot sauce.

Preventing it

If you keep a raw sauce for its live flavor, give it a vent. Virginia Extension recommends containers that release gas for refrigerated fermented products [4]. At home, that can mean a loosely fitted cap for the first week or two, or opening the bottle every few days over the sink.

Stopping the ferment before the next batch goes into bottles

The fix for bursting bottles is made at the bench, before you fill: deciding whether the sauce will be cooked, acidified, preserved or kept cold. The four options and what each one does are compared on finishing a ferment for the bottle.

References

1. Breidt F, Pérez-Díaz IM, McFeeters RF, Lee CH, Fermented Vegetables, chapter 33 in Doyle and Buchanan, eds., Food Microbiology: Fundamentals and Frontiers, 4th ed., ASM Press, 2013. https://www.ars.usda.gov/ARSUserFiles/60701000/Pickle%20Pubs/p380.pdf
2. Snyder A, Breidt F, Andress EL, Ingham BH, Manufacture of Traditionally Fermented Vegetable Products: Best Practice for Small Businesses and Retail Food Establishments, Food Protection Trends 40(4):251-263, 2020. https://www.ars.usda.gov/ARSUserFiles/60702500/Fermented%20Veg%20Manuscripts/p437.pdf
3. DiCaprio E, Marco ML, Kim H, Ting KC, Safely Ferment Fruits and Vegetables at Home, UC Davis, version 1.0, March 15, 2022. https://eatlac.ucdavis.edu/sites/g/files/dgvnsk13106/files/media/documents/Safely%20Fermented%20Fruit%20and%20Vegetable%20Factsheet.pdf
4. Virginia Cooperative Extension, Enhancing the Safety of Locally Prepared Foods: What do I need to know to sell Fermented Vegetables at the farmers market? FST-308NP, 2019. https://ps.spes.vt.edu/producers/documents/FST-308.pdf
5. Food Safety Authority of Ireland, Guidance Note 37: Good Manufacturing Practices for the Production of Ready-to-eat Unpasteurised Fermented Plant-based Products, 2021 (marked draft). https://www.fsai.ie/getmedia/f8ba3e7e-d7aa-4771-86c3-8d65aa8a038c/GN-37-Good-manufacturing-practices-for-the-production-of-ready-to-eat-unpasteurised-fermented-plant-based-products(1).pdf?ext=.pdf
6. The Hot Pepper forum, Fruit, hot sauce, and unwanted fermentation (thread), August 2019. https://thehotpepper.com/threads/fruit-hot-sauce-and-unwanted-fermentation.72088/post-1645983
7. Singapore Food Agency, Recall of "Huy Fong Sriracha Hot Chilli Sauce", press release, December 30, 2019. https://sfa.gov.sg/newsroom/press-releases/recall-of-huy-fong-sriracha-hot-chilli-sauce
8. Barefoot S, Revisiting Botulism, Clemson Cooperative Extension Home and Garden Information Center, May 19, 2023. https://hgic.clemson.edu/revisiting-botulism/