You’ve poured yourself a cup of coffee and then there’s a knock on the door – the FedEx guy. You want your coffee to be as hot as possible when you come back in a few minutes. Should you add the milk now or wait until just before you drink it? Most people would say to wait, but that’s wrong – put the milk in now. Here’s why:
- Dark-colored objects radiate more heat than light-colored ones, so the light coffee will radiate less heat than the black.
- The heat loss is proportional to the temperature differential between the cup of coffee and the room air. By adding the milk now, you have slightly cooled the coffee, reducing the temperature differential and the heat loss.
- If you use cream or half-and-half, the fat may lessen the evaporation from the surface of the coffee and the resultant evaporative heat loss.
I agree in theory that your points are right, but how much difference will it make in practice? I don’t have two thermometers and two of the same mug to try, but i imagine it’s not going to make for much of a detectable difference, much like the coffee joulies: http://www.marco.org/2011/08/10/coffee-joulies-review
Wouldn’t the coffee reach the cooler temperature on its own and slow down its rate of temperature change? Kinda like the reverse of the “cold water boils faster than warm water” myth? Or is the coffee different because it’s cooling instead of heating?
In practice, the difference is maybe, MAYBE 2 degrees over 5 minutes. That’s not remotely noticeable. Points 1 and 3 are completely negligible, and 2 is almost unmeasurable. (Calculated using Newton’s Law of Cooling for point 2 in a calculus class once upon a time.)
But how can you calculate and refute without knowing the temperature difference caused by the milk?
why don`t you just bring your mug to the door?
LOL!
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I don’t think this one is quite true. Darker coloured objects in fact absorb more heat than light coloured ones (think wearing a black t-shirt vs a white one on a hot day). The amount of heat radiated is not a function of its color (in fact, for VERY hot objects, like incandescent light bulbs, stovetop elements, and the sun, the color is in fact a function of the heat radiated – scientifically termed blackbody radiators).
In fact, dark objects do radiate more heat: http://www.britannica.com/EBchecked/topic/591461/thermal-radiation
Very interesting topics! I agree with the stated logic of most all of the arguments presented on the website, but on this topic I’m going to have to chime in to express skepticism with points (1) and (2) from a physics standpoint (Ph.D physics, Cornell University)::
(1). Dark objects do radiatively cool faster than light objects at the same temperature. HOWEVER, radiative cooling is a very small effect at the temperature of hot coffee. Convection by air moving over the cup (aided by the evaporative cooling of the coffee) is a much greater effect. If you want to keep the coffee hot, one thing you can quickly do is cover it with a lid. That will greatly reduce convection and the rate of evaporative cooling. Works with my cup of hot tea! Radiative cooling becomes a more important effect as the temperature of the item gets higher. (The rate of radiative cooling increases with temperature to the fourth power.) At the temperature of an incandescent light bulb’s filament, nearly all the cooling is radiative.
(2). The rate of cooling by conduction (and not by convection or radiation) is indeed directly proportional to the temperature difference between the item and its surroundings. BUT, as Nelson here pointed out, prematurely cooling the coffee with cold milk doesn’t help because an untouched cup of coffee would have reached that same temperature anyway and would also have adopted a lower cooling rate as a result. All you’ve done by adding the milk is to sacrifice the time that the cup would naturally take to reach a lower temperature on its own.
As for (3), I’m not sure either way. Evaporative cooling is indeed the biggie here. It sounds plausible that enough fat in the coffee could reduce evaporative cooling, but I would say that a better and more effective way is to quickly find some lid to put on the coffee.
Thanks for the info, I needed it for my research. The exact question was given to me and my class to go home and look up how liquids cool. I was prepared to stumble through dreary science sites but thanks to you, I don’t have to!
I don’t even like coffee T_T
Enjoying the blog, with a nice variety of topics. There seemed to be a bit of debate over this one – this graph should clear things up 🙂
http://www.thenakedscientists.com/HTML/content/kitchenscience/exp/-0c2ee7e805/
Loved your content. I would like to add some information regarding this topic. You can go for simple hacks to keep your coffee warm without electricity.
keep your coffee warn without electricity