Coffee – Why is cream curdled in coffee

Its possible that when you drink a hot liquid, you swish it around your mouth a lot less than when you drink a cold liquid, and thus fewer of the taste buds on your tongue are exposed to it. This wouldn't be surprising: it's hot after all.

Another possibility is that if you're starting with hot coffee, and adding sugar to it, that the sugar isn't fully dissolved when you drink it hot (it takes time, especially if you don't stir it much), but has dissolved by the time the coffee cools. With more dissolved sugar, it is sweeter.

One possible difference is that when you pour coffee (or any other hot liquid) into a ceramic mug, the liquid cools a fair bit (the heat is "lost" to heating the ceramic cup). The double-walled stainless mug is designed to lose as little heat as possible, and the inner wall has much less thermal mass than the ceramic. So, when you pour the creamer into the stainless cup, the coffee is hotter.

The simplest way to test if this is the case, would be to measure with an accurate thermometer. And the solution, then, would be to allow the coffee to cool some before adding the creamer.

Alternatively, temper in the creamer. To do this, put the creamer in the cup first, then add a little coffee, stir, add a little more coffee, stir, then add the rest. The idea is to slowly heat the creamer up to the coffee's temperature. This should prevent curdling, even if your coffee is near-boiling.

It could be some residue on the cup, but I doubt it, because stainless is fairly easy to clean. Also, it'd be hard to imagine enough residue to curdle the cream without also being very evident in the taste of the coffee. Stainless steel itself is pretty non-reactive, so its probably not a reaction with the coffee or cream.