Musings on cooling down coffee...

[Haggisman]

So while I'm sat here sipping my slightly-too-hot coffee before work, I started pondering the factors that might affect the rate at which it cools.

Mainly, the timing of the adding of milk.

My thoughts are as such:

You have a cup of coffee which is too hot and you are going to leave it to stand for 10 minutes to cool.

At what point should you add the milk to achieve the optimum cooling performance?

My scientific mind suggests that the most efficient method is to leave the coffee black until the last possible moment. In doing this, the temperature differential between the coffee and surroundings is higher, meaning the energy is lost quicker, and then when you add the milk it obviously drops further.

E.g. it loses 4c through the atmosphere and a further 4 when the milk is added.

I'm also however thinking that it makes no difference, as when adding the milk at the beginning of the waiting period, there is a higher differential, so the coffee loses 5c to the milk, however then goes on to only lose 3c to the environment as the differential is then far lower.

There are also of course the arguments that coffee should be taken black (I agree unless it's instant), or that one should drink tea (these people would have course be wrong).

So, over to you, oh great minds of OcUK, please unite and solve this query for me

 

[Haggisman]

Blow on the coffee. Your breath is filled with cold negative ions which will quickly lower the temperature of coffee. Be careful! If you over deflate the coffee (the scientific term for artificially lowering the temperature of hot drinks) then you have to make a sucking in motion to drag the negative ions back out of the coffee and into your mouth.

This does not work for tea for reasons I was never clear on.

Excellent

That almost sounds scientific enough for someone to fall for

 

[Haggisman]

Get a set of Coffee Joulies, they cool down the coffee and then keep them at that temp for longer. I personally think they are a ridiculous idea but someone buys them (someone on here has a set from the original lot on Kickstarter).

Those look simultaneously awesome and utterly pointless

Without actually interfering with the composition of your coffee (ie something like adding ice), and assuming you are holding it in your hands, the rate at which coffee cools is controlled by how much heat energy it can transfer to the air particles directly above it. So you basically want to blow new air over it continuously, and the colder that air is the better.

So you reckon strapping one of these to my mug would do it?

http://www.overclockers.co.uk/showproduct.php?prodid=BU-024-TL&groupid=701&catid=2330&subcat=

 

[Haggisman]

Add in boiling water, a little cold water and milk to desired drinking temperature.

Boiling water & coffee? Boiling water & coffee? Boiling water & coffee?

Get out. Get out now!

It makes far more sense to add milk when it's too hot to drink than to wait for it to cool then add milk, cooling it further.

To be more sciencey though, the coffee is cooling at the quickest rate at its hottest temperature i.e. the moment its poured. By decreasing the temperature differential between the room temp and the coffee temp as soon as possible the rate of heat loss is slowed. Put the milk in straight away.

But I wanted to know the fastest and most efficient way to cool it. So with your more sciencey bit, it would actually be better to wait until it's cooler before putting the milk in, as it will lose heat to the environment quicker that way.

Did that test in GCSE Physics. Leave the milk out till you want to drink.

Do you have graphs etc. to prove this?!

Well, since this is less musings about coffee, and more musings about thermodynamics and heat transfer, I applaud the OP. I'm sure, from the OP, that this is already common knowledge, but it may help if not

Q = mc*Delta T

Q = heat energy ; m = mass, c = specific heat, delta T = the change in temperature

So you'll need to know the mass of the mug, assume a mass of 1 kilo per litre for the coffee (if it's instant anyway, it's mostly water) and maybe marginally higher for the milk, but we could fudge it a bit.

If you measured the temp before and after adding the milk, we can work out the energy exchanged at different temperatures, and you could compare that with set time periods of heat radiation with and without milk.

Do it. Graph it. Let us know the results.

I will try. Although explaining to my girlfriend why I'm up at 5am making 10 cups of coffee might be interesting...

 

[Haggisman]

Yeah you dont want the taste of that instant coffee to get ruined now do you?

Shush you!

Tell her you're really tired and have a long day ahead of you

Seriously though, you could simply try it with hot and cold (fridge temp) water to get a rough idea. Use a measuring jug (preferably two to make sure the jug temp doesn't influence it) and thermometer and away you go.

Pretty sure the impurities in the coffee would affect the results.