https://en.wikipedia.org/wiki/Denatonium is used to make it unpalatable. Fun fact: the same chemical is also coated onto Nintendo Switch cartridges to discourage children from putting them in their mouths.
We used to use denatonium (Bitrex) in firefighting to test the efficacy of your face mask seal - you'd put on your mask, a filter, and a hood - Bitrex would be aerosolized into the hood and if you could taste it, there was not a good seal.
Nowadays we just measure pressure differentials.