# Thread: I Like My Coffee Hot!

1. ## I Like My Coffee Hot!

It's been decades since I took a thermodynamics class, which is probably why I can't figure out the answer to this.
Or, maybe it's because I always think of it before morning coffee.

Imagine I'm a person who likes my coffee hot, but also likes to add milk to it.
Next, imagine that I make a cup of coffee using one of those single cup machines, but before I can add the milk, my dog declares that she needs to go for a short walk.
I can't take the coffee with me (for reasons), so I leave the cup on the counter.

Question: If I want the hottest cup of coffee upon my return, do I add the milk now, or wait until I return?

Case 1: Adding the milk now, will cause an immediate drop in temperature, followed by a gradual decrease while I'm gone.
Case 2: Adding it later will result in a gradual decrease in temperature, followed by an immediate drop upon return (adding milk).

Which to choose?

BTW: Did I mention that differential equations were never my strong suit?

2. To make it even more complicated, if you add the milk later, while you were gone, what did you do with the milk? If you left it on the counter, the milk probably warmed up (room temperature being higher than inside the fridge).

3. I couldn't help but think the thread title needed to be followed by, "... like my women!"

Assuming you leave the milk in the fridge, do it now. Cooling decreases as temperature differential decreases. If you think of it in terms of heat lost, heat will be lost faster while the coffee is hotter. So you'll lose fewer joules if the cooling effect of adding the milk happens sooner.

4. You could always get an espresso machine and steam the milk. Or add it now and microwave later.

But I like my coffee black.

5. Simplifying by ignoring a temp rise for the milk awaiting being added, I expect a higher net temp by adding the milk early. The greatest temp loss is due to the temp differential so hot black coffee will cool at a faster rate than it would with the added milk.

6. Order of Kilopi
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If you're going to be gone long enough for the coffee to cool to near room temperature then adding the milk when you return will put it under room temperature. Adding it early would be better.

Or get a thermos bottle.

Or a robot dog walker.

Or a robot dog, they don't need walking.

7. Does the Mbempba effect come into play? Or is that only if you're trying to make an iced-coffee?

https://en.wikipedia.org/wiki/Mpemba_effect

8. Another way to look at it as simply as possible is to use the old Caloric theory where heat is regarded as a fluid and it flows from hot to cold. So one can look at the net heat flows, from both the coffee cup and ambient heat flowing into the milk, to do a summation. The insulation property of the coffee cup is the key.

9. Adding milk to the coffee will make it whiter, which will cause it to radiate heat at a lower rate than black coffee.
Last edited by clop; 2019-Oct-11 at 08:14 AM.

10. Add the milk as soon as possible and if you are too long gone, microwave the cup for a few seconds, using the experience of last time you did it. Oh and cover the cup with a saucer while you are gone, evaporation is the biggest heat loss. Or make the coffee after dealing with the dog.

11. Whole milk or skim? Milkfat is an insulator.

12. Originally Posted by Noclevername
Whole milk or skim? Milkfat is an insulator.
Is it still insulating as a colloid? However whole milk might cool it less? Powdered milk even less.

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