T - Science (Experiment 1)

T-Science - Experiment 1

Does adding milk to tea affect how fast it cools?

This is a classic problem, one that causes arguments in kitchens the world over (okay, it happened to me once). Imagine the scenario, you're making yourself a delicious cup of hot, refreshing, tea when you're called away. Because you couldn't find the teapot, you've made the tea in a mug with a teabag. Although you've removed the teabag, you haven't yet added the milk. You've got time to add the chilled milk before you leave the tea, but should you? Will the tea be warmer upon your return if you add the milk now or will it be better to add it later?

Hypotheses

1	You should add the milk first. Although the milk is going to cool the tea, we know from Newton's law of cooling that the rate at which the tea will lose heat is inversely proportional to the temperature difference between the tea and the room (the bigger the difference, the faster the tea cools). By adding the cold milk whilst the tea is still very hot, and losing heat fast, you will reduce the temperature to a point the tea would have reached before long anyway.
2	You should leave the milk out till later. Although it wont make you popular with anyone else who shares the milk with you, leaving the milk out will allow it to warm up towards room temperature. If you add the warm milk to the tea on your return you won't cool the tea as much as if you added it straight from the fridge.
3	You should leave the milk out till later. Milk contains fats. When you add it to tea, which is essentially just hot water, you alter the thermal properties of the tea, affecting its specific heat capacity and causing it to lose heat faster than it otherwise would.
4	You should add the milk. Yes, it will affect the thermal properties of the tea, but it has the effect of slowing cooling, not heating it.
5	It doesn't matter. By the time you've poured the hot water from the kettle and brewed the tea in the cup you're already past the point where the tea's temperature is decaying fast. With the rate of temperature loss levelling out, the addition of milk: hot or cold, will make very little difference to the tea. If that wasn't enough, most people seem to be taking their tea with semi-skimmed and skimmed milk these days. Even full fat milk only contains approximately 5% fat, so you're not adding enough fat to make a difference.

The Experiment

The video shows how I set about testing out which of the hypotheses was most likely.

I made my tea in measuring jugs rather than cups so I could ensure that there was the same amount of tea in each one (700ml after removing tea bags). I checked that all three jugs were the same temperature at the start of the experiment (83ºC). I used three jugs so that I could compare the effects of adding 100ml cold water at 12ºC with 100ml cold milk at 12ºC and 100ml milk that had warmed.

By using cold water as well as milk I hoped to discover whether the lipid content of the milk had an effect.

I noted the time, added the cold milk and water to their respective jugs and then, every minute or so I stirred the jugs and measured the temperature.

The Results

The graph below says it all really. You can download the numbers as a spreadsheet in Microsoft Excel format if you'd like to repeat the experiment and add your own results.

The Conclusions:

Well, as expected, adding cold water and milk lowered the temperature instantly at time zero. However, the differences are all pretty small. I ended up repeating the experiment a couple of times and got slightly differing results but they were hardly significant. I'll go for hypothesis 5 - by the time the tea has brewed it doesn't make any difference. I used semi-skimmed milk in my experiment so I'm thinking of repeating it with full fat to see if it makes a difference, however, I'm not expecting much. So, the final answer is: "Do what you like, it doesn't matter!

Next Step:

That's up to you. Can you see errors in my method? Do you have suggestions or alternative theories that need to be tested? Would you like to add your own results to the tea pages? Feel free to contribute by emailing me.

Comments from other Tea researchers.
The Maryland Virtual College of Science and Mathematics (Coffee Cooling Problem).
More about Newton's Law of Cooling.

See the Video (900kb) in either Windows Media or RealMedia formats. PC users, left click one of the icons (right) to view the streaming version or right click and select "save as" to download to your computer for viewing later. Feel free to email the video to your friends to get them involved.

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