15 April 2009

Hot Coffee

This is something that I have pondered off and on for a couple of years now, usually while driving somewhere so that it's forgotten by the time I get anywhere it's safe to take notes:

Why does coffee begin to cool more rapidly after you take that first sip, even in an insulated travel mug?

Does the backwash force more circulation and therefore energy loss
Does the increased ratio of air to coffee inside the mug lead to more rapid loss of energy?
Is the reduced volume of coffee related to that (e.g., less energy-holding coffee to share and contain the energy)?
Is it a psycho-sensory effect brought on by the mouth's heat receptors becoming accustomed to the greater heat of the coffee compared to the ambient temperature of the mouth?

I have a hunch this is mathematically solvable using thermodynamics equations, but I think we all know that doing the experiment is more fun than just working some problems. Not that I won't use math to verify my results. I will also conduct a lierature review to see if anyone else has done any work on this phenomenon.
Here's the experiment:


Repeat three (3) times (Weak, normal, strong brews) making coffee as defined here:

A. Weak Coffee - 1 scoop Less than # of cups in pot
B. Normal Coffee - # of scoops = # cups in pot
C. Strong Coffee - 1 scoop Greater than # cups in pot

  • Drip Coffee maker with pot
  • Water (enough to fill the coffee pot)
  • Ground Coffee (enough to make 3 pots of varying strength)
  • 2 matching ceramic coffee mugs
  • 2 matching insulated travel mugs
  • 5 thermal probes


1. Brew a pot of coffee.

2. Pour coffee into all 4 mugs.

2.1 Return pot with remaining coffee to burner.

3. Seal travel mugs.

4. Insert probes into all 4 cups and the pot of coffee.

5. Record initial probe readings.

6. Wait 5 minutes, then record new readings.

7. Take a sip from one ceramic and one travel mug - the other 2 mugs are controls.

7.1 - exclaim about how hot the coffee is.

8. Repeat steps 6 and 7 until the cup of coffee is empty. (For extra fun make a note of when you no longer feel it necessary to perform 7.1)

Expected Results

The open ceramic mug that is sipped from will lose temperature quickest, followed by the sipped-from travel mug, then the open control, travel control, and the pot. Possible confounding variables include variable air temperature, strong/variable air movement, presence of sweetener (we are conducting this experiment with black coffee), and for step 7.1 the relative sensitivity of the experimenter's mouth.


Feel free to offer constructive criticism, replicate this experiment and send me your results in tabular format to lefthandrob at gmail, or point me to where this very thing has already been done. This is FOR SCIENCE!

p.s. - Save all those used coffee grounds; they're good for your compost.

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