I made milk from a mature brown coconut by blending the flesh with water and straining through mesh. That was a week ago. I haven't used it all, and now I've discovered that it has soured. It's kind of slimy and has a tangy smell.
Does anyone know if it's safe to use? Is this probiotic-filled and healthy or is it just rotten?
I'd go with "certainly not safe" - and probably rotten.
Even for fermenting foods like yogurt or sauerkraut you need to follow a certain protocol to ensure the end product is safe. Yes, a pot of milk that was left out can turn into a delicious soured milk if the right bacteria grows in it. It can also turn into a nasty rotten mess.
If you want to ensure a specific product, you should add the appropriate bacteria, the traditional way would be a spoonful of an established product. Alternatively the bacteria can be obtained in liquid or dry form. This would give you a safe product.
While it is possible to ferment coconut milk with "yogurt bacteria", it's pretty unlikely that the "desired" bacteria mysteriously appeared in your milk. So no, your coconut milk is not safe. And if you write "slimy", I assume it's simply rotten.
Generally, for plant-based foods, you can trust your nose. If it smells nasty, don't eat it. If it smells okay--just tastes different from what you expected, has gotten fizzy, etc., it's likely to be okay. This isn't 100% true, of course--botulism being the most notable exception. But botulinum grows in sealed containers, usually on alkaline substrates, and over a fairly long period of time.
Other considerations, in assessing whether something fermenting is edible or should be dumped:
Check to see whether it's gotten moldy (though not all molds are problematic--blue cheese. for example) or just bubbly. Molds have a higher chance of being problematic than do bacteria, though sometimes they can just be skimmed or cut off (with a good margin, since molds send out "roots" that aren't visible but will still taste bad).
The acidity of a food (fats are acids), as well as it's salinity, are also worth considering: more acidic or salty foods are less likely to support the growth of organisms that are harmful to people.
Did it ferment with exposure to air (at least initially) or in an airless environment (such as a factory sealed pouch or can)? The latter is much greater cause for concern. [NEVER eat food that made it's hermetically sealed packaging swell up (home canned items, factory cans/tetra paks/pouches)--that very well could be botulism.]
Will your accidentally fermented food get thoroughly cooked, such as going in a soup or cake batter? Sufficient cooking will kill most bacteria, though on rare occasion, even dead bacteria can cause harm.
If you're still not sure, and nothing in you wants to gag when you observe it (though even many gag-worthy fermented foods are happily eaten across the world, such as Japanese natto and, again, certain cheeses) carefully taste a tiny, tiny bit. Keep it on your tongue for at least 30 seconds and notice how your body responds.
Fermented coconut of all kinds (yogurt, kefir, water kefir) are popular, probiotic, health foods these days, so don't dismiss it out of hand--just learn how to observe it.
If you want to be prepared for next time, get a copy of one or both of Sandor Katz's books: Wild Fermentation (his first, in which he talks a lot more about his own experimentation) and his larger, more extensive and instructional volume, The Art of Fermentation.
It's fine to eat. I just had some the other day was left in frigerator too long. It sours and becomes a yogurt. May need to add a sweetener. It was tasty though. Fermented coconut milk becomes yogurt. I think its just slower fermentation versus actively fermenting right away with other additives and methods.
Fresh coconut and its water is short lived, it may last overnight in a cold fridge, but only if it goes straight in without delay or in the morning you will smell it, if you lack the sense of smell you will spit it out for the taste, if you lack both you will vomit, if your guts are iron, you will get sick. We took water straight from the shell into a bottle on a 4hr hike in Malaysia, it was delicious but had to be discarded well before we finished, I now understand the great waste in that the water is tipped on the ground. Even the milk is short lived and must be used immediately, I now understand why canned coconut is used in curries where fresh is plentiful. Some advocate the milk can be fermented with kefir grain, others claim they ferment the water with tibicos, I have even seen coconut yogurt in supermarket, I do not think that is possible as in a warm situation it will rot before it can nicely ferment. I have ordered the three, kefir grain tibicos grain and a so called coconut yogurt culture, when I stipulated fresh water, the lady said bottled, concerning milk she could not comment, concerning yogurt she said canned. I love coconut and soon I will be testing it for myself, my girlfriend buys the milk and the grate fresh for curries, I cringe at the waste, liters upon liters a day tipped on the ground. Traditionally it not heard of, research on prolonging its life naturally has been futile, traditionally the water is drunk or discarded and the meat used immediately. Hopefully this is because coconuts are abundant year round and the ancients had no need to preserve it or dry it. Today we hate waste and may find the way.
