Here’s a quick scenario for you. You’ve poured yourself a fresh cup of black coffee, and you want to keep it hot until you’re ready to drink it. Are you making a mistake by adding cream to that coffee? Does coffee with cream cool faster than black coffee left alone? Intuition says yes. The laws of physics lead to a different conclusion.
Last year, the web site Modernist Cuisine gave three reasons why “coffee with cream cools about 20% slower than black coffee” alone. To summarize:
1) Black coffee is darker, and dark colors emit heat faster than light colors. As such, “by lightening the color of your coffee, you slow the rate at which it cools,” if only slightly.
2) The Stefan-Boltzmann Law (apparently) says that hotter surfaces radiate heat faster than cooler ones. So if you add cream to a cup of black coffee, it might lower the temperature of that cup of coffee. However that cup could still cool at a slower rate than a cup of hot black coffee.
3) Finally, and perhaps most importantly, “adding cream thickens the coffee (adds viscosity), so it evaporates slower.” And, in turn, less heat gets carried away by the evaporation.
To top things off, Modernist Cuisine also produced a video showing cream being poured into coffee in super slow motion. Even if you don’t care to consider the physics of coffee & cream, it’s pretty cool to watch an average cup of joe getting turned into a roiling sea.
via Petapixel
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Ill buy that a coffee-cream mixture might cool more slowly than black coffee. But clearly, adding a volume of cool liquid to hot coffee will result in a mixture that’s cooler than the original coffee was. Further, if it’s cool enough to drink after that, you probably don’t want it to cool faster anymore. Cheers!
8 oz cup of coffee at 95 degrees C, 1 tsp of cream at 5 degrees C. 64 tsp = 1 cup, so average temp is 93.6 degrees C after adding cream. Add 2 tsp, temp is 92.3. Add 1 tbsp. (= 3tsp), temp is down to 90 degrees. nHow much cream do you put in your coffee? How much is a ‘splash’? And how hot (cool?) does coffee have to be before you can drink it? So many ill-defined amounts, it’s hard to give a definitive answer!
I drink it black.
8 oz cup of coffee at 95 degrees C, 1 tsp of cream at 5 degrees C. 64 tsp = 1 cup, so average temp is 93.6 degrees C after adding cream. Add 2 tsp, temp is 92.3. Add 1 tbsp. (= 3tsp), temp is down to 90 degrees. nHow much cream do you put in your coffee? How much is a ‘splash’? And how hot (cool?) does coffee have to be before you can drink it? So many ill-defined amounts, it’s hard to give a definitive answer!
I had some dreams, they were clouds in my coffee…
You know that (I believe) myth about how pouring hot water into ice cube trays will produce ice cubes faster than cold water? This kinda feels like the same deal here, notwithstanding the color difference (which I find a bit of a reach) and the other reasons listed. One sort of just “knows” the coffee with cream combo will cool faster because it starts cooler. Your little story has given me doubts, but I’m unconvinced.….
A little university physics helps here. The rate of cooling is proportional to the difference of the hot object (Th) and the ambient temp (Ta). That is, a very hot object (Tvh) cools more quickly than a merely hot object (Th) in the same ambient temp (like a freezer). However, as the very hot object cools, it MUST pass through the hot’s object initial temperature. At that point, it’s as if the very hot object was inserted into the freezer with the same initial conditions as the hot object, and thus must cool at the same rate. But at that point, our hot object has also been in the freezer, and its new temp (Tn) MUST be lower than Th. nApply the same logic as “Zeno’s paradox”, and you can see the cooler ice tray will always freeze faster than the hotter tray. nBut.. if you fill the ice with hot water from your tap, the longer time it takes to cool allows more of the dissolved air to escape, and you end up with CLEARER ice cubes. It just takes longer.
I wish the video was longer, I just sit and watch the cream whenever I have coffee and it swirls for a long time. There’s so much more beauty and chaos I would have loved to see in slow motion.
So, if the coffee is black, but in a completely dark place, is the cooling rate affected? Should we choose black mugs over white mugs? I might stop by the physics classroom at my school and see what the kids think about all this.
STIR IT! STIR IT!!!!!!
The addition of cream increases turbulence, so it would probably increase cooling as hotter internal coffee is swirled to the outside. The website gives no justification for th 20% cooler calculation, so I would like to see an actual experiment done.nAlso, from the reasons given, adding cream would tend to keep the coffee at a drinkable temperature (not too cold) longer.
95C coffee will cool faster (in degrees per minute) than 90C coffee. But if you start two otherwise identical cups of coffee cooling, one initially at 95C and one at 90C, the one that starts at 95C will alway be warmer as they both slowly approach room temperature.
Wow, You take one interesting logical well written article and the morons come out of the woodwork to make their dumb irrelevant comments.
This is rather simple, the colour of the liquid is irrelevant as heat loss to the air from the top is very little, and one could argue that good black coffee has a creme surface which is close to white. After all thermos flasks work by using an air gap, Most heat loss is directly to the cup and then the surface area of the cup is most relevant as its where most of the heat loss will occur. The loss of heat is also not linear, the hotter a liquid is, the faster it will lose heat, but they are the using this fact to confuse the issue, The black coffee being hotter will lose heat faster, but when it reaches the temperature of your white coffee’s starting temp, it will also slow its loss of hear by the same amount, and by that time the white has cooled more, they will eventually meet at room temperature.