Here’s a picture comparing a rotten coconut to a fresh one. Unfortunately, many places will give you rotten coconuts and mix them in with your drinks etc. If you are going to drink coconut juice its better to be able to at least see the meat (e.g., at whole foods some of the coconuts are opened partially so you can see the meat and poke a hole in the top to drink the water). Both of these coconuts were from whole foods. If it hasn’t been partially opened, we find we have about a 40% chance of getting a rotten one.
