Hello. Just had a thought and I know for sure I’m not the man to ask. My brain should know better than to ask me stuff like this. Anyway this is what I wondered -
If I wait, say 30 seconds before adding milk to my coffee, will my coffee be hotter 5 minutes later than it would have been if I’d poured the milk in at the same time as the coffee?
I’m thinking if I wait 30 seconds this will give the mug (ceramic) a chance to reach and hold a higher temperature and perhaps give back a little than if I’d lowered the temperature straightaway with the milk at the start.
Given a reasonable degree of approximation, it should not make a difference whether you add the milk right away or a little later. Have you tried it both ways and measured the temperature, by the way?
However, you know how when one brews tea, one pre-heats the teapot using boiling water? One thing that may make a difference is similarly pre-heating your coffee mug so that it is hotter to begin with.
This question should have a definitive answer. One thing that affects heat transfer is the relative difference between the two materials. So putting a 200-degree liquid (straight coffee) into a 75-degree ceramic mug will have a different rate of heat transfer than putting a 150-degree(?) (coffee+milk) liquid into the same ceramic mug. Hopefully someone will come along who remembers more of their differential equations coursework than I do and will be able to give you an exact answer.
Unless the mug gets hotter than the coffee, the mug is going to be sucking the heat out of the coffee until the mug and coffee are at the same temperature. Heat will move to the colder material. In addition, some of the heat that enters the mug will radiate away from the outer surface and no longer be in the mug to help warm up the coffee. One way to combat this heat loss is to get an insulated mug. These mugs are made with special materials which are slow to absorb heat, or they are made in a way where they incorporate a vacuum layer which slows down heat transfer. Another way to combat this is to pre-heat the mug. If you have extra hot water left over when you make coffee (perhaps from pour-over or french press), then pour it in the mug while the coffee is brewing. That will warm up the mug and it won’t suck as much heat from the coffee.
If you start with hot coffee, a room temp ceramic mug, and refrigerator temp milk, the sequence does not matter. Once all 3 are together the coffee will be very rapidly cooling as it tries to heat the cold milk & the cool cup. Then once everything is the same temp, it will all be losing heat & temp together to the cool room.
Now if you do what I do, start with hot coffee, a hot mug, and hot milk, then the result is that once they’re all together they’re nearly uniformly hot. First they’ll exchange a little heat / temp with each other to reach equilibrium, then the slower process of giving up their collective heat / temp to the room begins.
My process: Start the coffee pot. Fill the coffee cup with tap water & microwave for 2 minutes. Discard the hot water & add cold milk / cream / etc. to the now-hot cup. Microwave that for 30 seconds. Caution: boiling milk happens at about 40 seconds, sooner if you skimp on the milk. By now the coffee is brewed. Add the coffee to the hot milk & cup. Now enjoy a hot beverage that stays enjoyably hot for several minutes, not just 30 seconds.
This might be low-brow, but could you simply add some milk to the entire pot of brewed coffee? (Doing so will cool the coffee a little bit, but it will heat back up due to the burner.)
Thanks for the replies so far. All the hints and tips on how to keep coffee are warm are great but also no brainers. I do preheat mugs etc.
Obviously equilibrium in temperature will be achieved over time but what comes out tops in a short time like in the 5 minutes mentioned or even 1 minute etc. The sort of time you want to drink the coffee.
Say I’ve made the coffee in the nespresso machine and the doorbell goes. If I pour the milk 1st then answer the door will the coffee be colder when I get back to it or is it going to be warmer if I wait until I get back to it to pour the same amount of milk?
If you were mixing hotter coffee with identical cooler coffee the answer would be different. But in this case, the milk has a slightly insulating effect and the mixture of milk+coffee will retain heat better than coffee alone.
There is also a slight factor in that a greater heat differential leads to greater heat transfer, so if the coffee is going to cool down by a certain amount when you add the milk, adding the milk immediately decreases the heat transfer from the now less-warm coffee to the colder environment around it.
If your goal is to have hotter coffee 5 minutes later, add the milk immediately.
It will only be slightly hotter though. The difference isn’t all that dramatic.
One somewhat related aspect to this scenario is how the coffee will taste at the later point in time due to how high heat affects coffee. The flavor molecules in coffee are affected by high heat. It could be that cooling the coffee down with milk as soon as possible means that fewer flavor molecules will be broken down as it sits around. It could be that the 30 seconds you wait to add milk is 30 seconds of the flavor molecules being broken down in the hot coffee. By cooling the coffee down quickly, you may be able to mitigate some of the flavor loss.
What about Newton’s law of cooling (heat loss of a body is directly proportional to the difference in the temperatures between the body and its environment).
For those first 30 seconds the heat loss of the coffee will probably be greater than if you added the milk immediately thus reducing the temperature a bit?
Come on experimentalists: let’s get some empirical data here…
I don’t have a decent lab thermometer available at the moment, unfortunately…
On a related note: getting an electric coffee mug warmer as an Xmas present has been a major lifestyle game-changer. No longer do I have to get up a couple times during breakfast to reheat coffee in the microwave, like a medieval peasant.
