Wake Up and Smell the
Assume you take two cups filled with the same amount of hot coffee. Put some milk in the first one, wait 15 minutes and put the same amount of milk in the second one. Will the first cup's coffee be cooler?
First to rise to the challenge was Gnomon with this elegant answer:
Hot things lose heat quicker than cool things. Adding the milk to the first one will cool it by a certain amount, say 10 degrees. So the white one is now cooler than the black one. So after 15 minutes, the white one may have cooled by a further 14 degrees, while the black will have cooled by slightly more, say 15 degrees. Then adding the milk to the second one will cool it by 10 degrees, as it did with the first one. So the second cup is now one degree cooler than the first one.
Of course, it's probably not quite that simple... TRiG pointed out that when milk is added, heat is lost to the cool cup sides (the bit of the cup that wasn't previously in contact with the hot coffee). So, adding milk early means that more heat is lost, because the coffee is hotter at the time, so the heat difference is greater. Although a small effect, this would work in the opposite direction to Gnomon's answer. Further complicating things1, Potholer suggested
that the milk might cause a thin layer of fat to coat the surface
of the coffee, preventing heat loss by evaporation and keeping the coffee warm.
All in all, the consensus seemed to be that if you want your coffee to stay hot for longer, add the milk straight away. All well and good in theory, but would Gnomon's answer stand up to the scrutiny of Skankyrich and his kettle?
I've used 2/3 of a cup of water and 1/3 of milk to
exaggerate the effect. The starting temperatures are:
Water with milk: 55°C
Water alone: 80°C
And Skankyrich's first conclusion is..?
I should probably start warming the cup before I brew my tea.
Hmm... almost certainly correct, but not terribly helpful here. The experiment continues:
The cup I added milk to first is now at 43°C,
while the one I've just added it to is at 41°C
And so experiment matches theory - another success for the SExperts. Of course, a result isn't a result until someone else has reproduced it, so it's over to Gosho:
I boiled a kettle of water and poured 1.5 cups2 into a measuring jug, then divided that equally
between two identical tea cups so that each one got 3/4 of a cup.
I poured 1/4 of a cup of (soya) milk into one and set the timer
for 15 minutes.
After about 14m 30s I put the probe of my cooking thermometer into the one with the milk in and waited for the timer to go off, upon which I took the reading and added 1/4 cup of milk to the second cup and then took the reading from that cup.
Cup 1 (milk poured in immediately): 116°F (47°C)
Cup 2 (milk poured in after 15 mins): 105°F (41°C)
So there you have it. If you're not planning to drink your coffee for a while, it's best to add the milk immediately rather than just before you drink it.