Please help settle a friendly argument about coffee. Does a latte cool faster than a cappuccino, all else being equal (initial temperature, volume, etc.)? My friend thinks the foam on a cappuccino will insulate it better, but I reckon this effect is offset by the cappuccino having a smaller volume of hot milk.
@MsAMOne77
via Twitter
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Happy to be in the testing team!
Graham Smith
Melbourne, Australia
The heat loss from any surface is proportional to its contact area, multiplied by the temperature difference and also multiplied by a variable called the heat transfer coefficient.
For a cup of coffee, there are two surfaces where heat is being lost to the atmosphere: the cup itself and the surface of the beverage.
When my coffee cup is full of latte, it has 44 square centimetres of latte exposed to the air. There is a large loss of heat from the surface of my latte, because latent heat is being carried away very efficiently by the steam rising from it. In other words, the heat transfer coefficient of this surface is very high.
My cup also loses heat from about 180 square centimetres of warm glass that is exposed to the air. However, heat loss from this larger surface is less than from the exposed latte because the temperature difference between it and the atmosphere is smaller, and the convective heat transfer coefficient of the glass is much less than the evaporative heat transfer coefficient of the latte surface. The net result – at least initially, when the coffee is steaming – is that more than half of the heat is lost from the surface of the latte.
Now, what happens if I have a cappuccino, with a third of its volume being froth? Assuming that the froth has little mass and good insulating properties, the heat loss from the glass will be about the same, but there will be minimal heat loss from the liquid surface. Thus the heat loss will be reduced by more than half compared with the latte, while the heat capacity of the liquid, the amount of thermal energy it must lose for its temperature to change by a unit value, will be reduced by only a third, resulting in a lower cooling rate of the cappuccino.
[Ed: Looks like your friend is right!]
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