In-laws are convinced my kraut jars are a botulism risk — what should I actually be watching for?

Short version: acid is the entire safety mechanism, and a lacto-ferment that is working makes plenty of it. Clostridium botulinum does not grow in a properly acidic environment — the line usually drawn is pH 4.6 — and a finished kraut or hot sauce sits comfortably below that. The cases people actually get hurt by are the opposite of what you are doing: low-acid vegetables improperly home canned and sealed, or garlic sitting in oil in the fridge, neither of which is a fermentation. What I would say to your mother-in-law is that the sourness she is suspicious of is the thing keeping it safe. If you want that in a form she will accept, your national food safety or extension service publishes tested fermentation guidance, and it is worth reading once properly rather than trusting a thread.

Practical red flag list from about a decade of this. Fuzzy growth of any colour is mould: I remove the whole affected layer and reassess. Black or pink fuzz and the jar goes out entirely. A flat white wrinkly film is usually kahm yeast and is a nuisance rather than a danger. Anything that smells putrid rather than sour is not a judgement call. The thing that has caused every problem I have ever had is vegetable poking above the brine, every single time.

I bought a cheap pH meter because I got tired of exactly this argument. It cost about the same as two takeaways, needs calibrating with sachets every few months, and the numbers ended the debate instantly. My krauts read in the mid 3s once they are done and my fermented hot sauces land a bit lower again. Strips work and cost less, but the colours are hard to read against a cloudy brine, which is why I gave up on them. If you are going to make a lot of hot sauce, the meter earns its place.

Slightly different emphasis to the top answer. Everything it says is correct, but it can leave people believing all fermentation is automatically safe, which is not quite the message. The projects that need real care are the low-acid, low-salt ones people wander into after a year of confidence: garlic in honey, fruit under oil, anything sealed tight and left warm. Kraut at 2 percent under brine is about as forgiving as home preservation gets. A sealed bottle of garlic honey is a different project with different rules, and that is the one where I would go and read properly first.

On the family diplomacy half of this: do not argue the science at the table, it never works. I started serving mine as one component of a dish rather than a jar with a spoon in it, and about a year later my mother-in-law asked for the recipe without ever conceding the earlier argument. Some of this is not about pH.

Weigh it, salt at 2 percent, keep it under brine, ferment somewhere cool, taste as you go. Five rules, and four of them are the same rule.