Thought long and hard on this one and knew that etoh has it's own bitterness. Struggling to articulate as to how it is bitter- came up with that the harshness of "ethanol" on the tastebuds registers as bitterness. The brain can't process it any other way. It can't register as any of the other four basic tastes.I don't agree. The harshness of ethanol can be felt other ways than through the bitter taste buds, or even any of the taste buds. Your tongue has nerves too, and the skin in your mouth is pretty delicate compared to your hands. When you drink straight alcohol and it burns (although maybe it doesn't burn you euge ) all of the way down your throat, that burning is not from your taste buds, it is from your nerves. The sensation is more complex than the taste buds would allow.
If you want to argue that some vodkas have things in them that taste bitter that's fine, it's probably true. But I don't believe ethanol itself triggers a reaction in the bitter taste buds, because I've never experienced it despite ample opportunity I'll need some real proof of that. I'll start with some everclear if I can get my hands on any, they don't sell the high-test stuff around here.