The (Beautiful) Physics of Adding Cream to Your Coffee

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

Relat­ed Con­tent:

The Physics of Cof­fee Rings Final­ly Explained

Hon­oré de Balzac Writes About “The Plea­sures and Pains of Cof­fee,” and His Epic Cof­fee Addic­tion

“The Vertue of the COFFEE Drink”: London’s First Cafe Cre­ates Ad for Cof­fee in the 1650s

Black Cof­fee: Doc­u­men­tary Cov­ers the His­to­ry, Pol­i­tics & Eco­nom­ics of the “Most Wide­ly Tak­en Legal Drug”

Free Online Physics Cours­es


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Comments (14)
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  • stairbob says:

    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!

    • Kevin Bertsch says:

      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!

    • Kevin Bertsch says:

      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!

  • Jan says:

    I had some dreams, they were clouds in my cof­fee…

  • Johnny Goofball says:

    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.….

    • Kevin Bertsch says:

      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.

  • Artemice Langhorn says:

    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.

  • beacantor says:

    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.

  • kayneeno says:

    STIR IT! STIR IT!!!!!!

  • McDruid says:

    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.

  • RDT says:

    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.

  • Steve says:

    Wow, You take one inter­est­ing log­i­cal well writ­ten arti­cle and the morons come out of the wood­work to make their dumb irrel­e­vant com­ments.

  • Nic Wilson says:

    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.

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