My brother and his wife came up to join us in Scotland a few years back for their summer holiday. On this particular outing we had hiked the area around Loch Katrine and returned to the car park to enjoy a welcome cup of tea at the visitor centre. We opted to sit outside so took our teas to an outside table while the ladies went off to do whatever it is they do in the room of dark secrets.
This is where the discussion began. Once we were seated I poured milk into my wife’s tea, much to my brother’s great surprise. He asked why I had done that and I explained it was so that when my wife returned, say in 5 minutes, her tea would be hotter than if I had not poured the milk in. He clearly thought I had lost the plot. However, there is good science to support this. Because I had cooled my wife’s tea down it was closer to ambient air temperature, which was around 15C in June, we are talking about Scotland here, thus losing heat at a slower rate than the tea without milk that was subsequently hotter and therefore losing heat faster. When the ladies returned and milk was added to the other teas they had lost more heat than mine over that period and adding milk resulted in their teas bring colder.
In summary, milked espresso lost heat slower than un-milked tea because it was colder to begin with. The un-milked tea though initially hotter lost heat faster and when eventually the milk was added it became colder than the milked coffee.
I have raised this with a few people, none of whom think I am right. Obviously there are a number of factors to take into account that are all variables. The heat of the tea, the temperature of the milk, the volume of each, the insulating properties of the cups, the period of cooling time and the ambient air temperature. However, by and large, in average circumstances as I have described, I am sure my thinking is correct.
Feel free to add your opinion.