Malic acid is my favorite sour powder. I have almost a pound of it.
I'll add it to apple juice or kool-aid to make it sour. I have sour lemon lime kool-aid right now for instance. It didn't work well with the mango. The pineapple was okay. The orange was tasty! I'll try black cherry sour soon...
Warning: Sometimes biased
http://i.imgur.com/V0x5fw8.jpg http://i.imgur.com/IOovUge.gif http://i.imgur.com/zw7bqPH.jpg