I’ve noticed something weird about re-heating coffee in a microwave; when I go in to work in the morning, I’ll usually nuke the remnants of the pot from the day before, while I’m making a new, fresh pot for that morning. When I put sugar in that microwaved coffee, it forms a thin, almost latte-like foam on the surface of the coffee, and I’m wondering if anyone knows why it would do so? It doesn’t happen with fresh coffee, so why does it happen with nuked coffee? Is it something with the water, the coffee particles, or the sugar particles?
I don’t think it has anything to do with microwaving and everything to do with the fact that the coffee is old. Coffee contains a volatile oil called caffeol (also called coffee oil), which gives fresh coffee its aroma and flavor. Once that oil evaporates away, the coffee can foam up because oil tends to break surface tension–which is why if you stick your finger in a foaming cup of fountain soda, the foam will go down quickly. I’m not sure why sugar makes it foam, but possibly it has to do with creating nucleation sites in the liquid which is near boiling.
Arjuna34