https://en.wikipedia.org/wiki/Magnesium_citrate
In solution with water it tastes somewhere between the worlds worst flattest sprite and a sweet lemon.
Edit: I'd marked Sodium Citrate, which is a similar compound but is used for different reasons. Sodium Citrate is an anti-coagulant. If you've ever donated blood plasma, its that weird sterile taste you get in the back of your neck when they feed the blood solids back in with saline. It is also used in nacho cheese.