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. 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.
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.