Why can black coffee leave brown stains inside a cup?
Show answer & explanation
Answer: Colored coffee compounds build up on the cup surface
The hot drink fires coffee into the ceramic glaze — A hot drink is not hot enough to fire material into a ceramic glaze. Coffee compounds can leave a surface deposit without kiln temperatures. That is why even a cup that has never been baked can acquire brown marks.
Added sugar caramelizes against the cup wall — Added sugar cannot explain stains from unsweetened black coffee. Brown compounds are already present in the brewed drink, including products formed when the beans were roasted. They can remain on the cup after the liquid is gone.
Colored coffee compounds build up on the cup surface ✓ — Colored compounds in coffee can adhere to the cup and accumulate with repeated use. These include brown roasting products called melanoidins; the stain is not simply one substance called tannic acid. How much remains depends on the surface and cleaning.