To keep your coffee hot for as long as possible, should you add cream right away or wait until just before you drink it? Will the addition of cream make coffee cool faster or slower?
The Modernist Cuisine team sought to answer these important questions. It turns out that coffee with cream added actually cools about 20% more slowly than plain black coffee, so it is best to add the cream immediately. But why is this?
A sidebar in Modernist Cuisine reveals the three major principles of physics that determine why coffee with cream cools more slowly. Check out the high-speed videos below and see if you can guess these three factors!
Tags: high-speed video
Mark
3:07 pm
1. The lower-density fat slows evaporation
2. Increased viscosity slows convection
3. Decreased density slows conduction???
Great videos.
Erik Halvorsen
2:19 pm
The fat in the cream is better at retaining heat and floats rather well, locking the heat in, distributing heat more evenly around in the liquid giving it a much slower evaporation, so in other words 1 and 2 yes, but not 3.
To determine that the effect of pouring the cream rather than pouring hot coffe in a cup with a small measure of cream has no effect, I’d suggest that as a third experiment.
Mark
3:11 pm
Another thought: an opaque body should lose less thermal energy though radiation.
green design
1:32 am
I’m suck at physics, after seeing your video, I still don’t get it. But let me try to guess ... is it because the cream makes the coffee thicker and hold the heat longer?
Scott
5:36 pm
Wow, that is counterintuitive. Here’s my guess, though.
1. The cream has a lower thermal diffusivity than the coffee, so when they are combined together, the rate of heat transfer out of the cup of coffee is slowed.
2. The cream adds mass. However, since the cream is likely colder than the ambient temperature, I can’t wrap my head around why this would help. Perhaps the equilibrium temperature of the cream and coffee times the new mass is higher than that of the coffee alone? Puzzled by this one.
3. Perhaps the increase in viscocity from the added cream also slows the rate of heat transfer.
Just guesses — my thermodynamics knowledge is a little rusty. But, those slow motion videos are absolutely sexy!
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Teddy Devico
1:38 pm
Maybe the fat of the cream rises to the top and keeps in the heat.
http://teenchefteddy.blogspot.com/
Gael
10:06 pm
I thought Herve This covered this already. According to Newton’s law of cooling the rate of cooling is directly proportional to the difference in temperature of the object and the medium it’s in contact with. So the reason why it cools slower is simply because it’s cooler.
Now if you are saying that the cream is at the same temperature as the coffee before you pour it in then that is different. In which case I would guess it’s because fat molecules are much larger then the water molecules and coffee aromatics and tend to hold on to energy a bit longer then smaller particles.
dragon
4:12 pm
- Mixing causes any stratification of heat layers to be removed. I would expect that the hottest coffee was at the top of the cup (through hot water having lower density than cold). Having a hot layer of coffee at the surface is not desirable, as it would have a high rate of heat loss through evaporation (steam) and radiation of heat.
- The lighter colour radiates heat more slowly.
- Cream (particularly high-fat cream) would be less dense than coffee and may rise to the top, adding a layer of insulation. (Insulation: due to fatty material conducting heat less well than watery material; due to the thicker, more viscous cream impeding heat transfer by convection.)
- As Gael said above, adding the cold cream without waiting eliminates the fastest-heat-shedding at the high temperature.
Rorschach
1:08 pm
I can easily accept that cream in a coffee solution retains heat better than coffee alone. But I’m unconvinced this is the primary reason that, if one wants to drink one’s coffee hot, one should therefore add the cream ASAP, for this simple reason: The rate at which temperatures equilibrate is proportional to their temperature differential, right? So very hot coffee will lose its temperature more rapidly than coffee w/ cream, independent of its chemical solution. Which effect is the more important, if my goal is to drink my coffee hotter, for longer? Indeed, is it possible that I should let my cream warm up on the counter, if it starts sufficiently cold?
Simply unsure, but I was never good at chemistry, so any guidance appreciated!
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Tyler Herriage
8:16 am
Very simple.... It’s Newton’s cooling law which says the greater the temperature difference of a liquid and it’s surroundings, the faster it will cool off. By adding creamer, it not only lowers the temperature difference but also white objects/materials are good reflectors.