What happens to the brewed hot tea when it is left in a cup for some time (up to several hours), that spoils the taste of the drink and change its color? What chemical processes lead to such unwanted results and what are the general methods to prevent tea from going bad too quickly?
If there is a significant difference in different tea types' brewing processes, I'd like to know that too.
Assume we make a cup of Earl Grey tea with a spoon of sugar and leave it for a day at room temperature.
I've seen a thin rainbowy layer appear after leaving tea for a few hours (up to a day). I often saw that in teas made from cheap bagged tea. Also the color of the drink becomes distinctively "stale".