| I ferment a lot, and know of only a few dangers: some rare fruits or veggies can produce toxins when fermenting, but non that I can easily buy in a European market. Spices and some herbs can be dangerous too, but the quantities needed for toxin levels that pose a danger, are insane. Fungi are more dangerous. If something starts growing hair, it's out. Dairy fermentation is more dangerous ¹, and quite hard, so I avoid that. Alcohol is obviously a toxin too, and rather common in fermentation ;) In nearly all cases, I'll trust one of the best fine tuned measurement devices to protect humans from toxins: my taste and nose. (Q: how do you know the milk is beyond it's expiry date? A: Smell it! Far more reliable than a date printed on some package) ¹https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6356804/ Edit: another thing to be careful with, are kernels, pits, seeds and peels. Peels can often contain lots of sprayed on chemicals that will probably kill your fermenting bacteria. Pits and kernels can contain insane amounts of toxins that we usually won't eat because they are enclosed. But crushing/long fermenting might release them |
Also watch out for methanol. Fruit pectin gets fermented into methanol so using pectase is recommended to break it down, especially if one ferments citrus juice.