Why can't you drink yesterday's tea?

Why can't you drink yesterday's tea?

If the drink is left for several hours in the open air, then under the influence of oxygen, bacteria and harmful compounds begin to form in it.

On the surface of tea, they look like a dense dark film, from which a coating remains on the mug.

By the way, these substances have the most destructive effect on the organs of the gastrointestinal tract, mainly the liver.