Continuing an occasional series of LC questions I enjoy teaching, this is from 2011 (q.7) and focusses on a key intersection between physics and modern life: how to make a latte. The question takes 70 degrees as being the temperature at which the hot drink should be served, which prompted me to do some research (googling).
70 degrees seems to be the upper end of recommended levels. perfectdailygrind.com - amongst others - recommends 55 - 65 as the target range. And reading about this has shown me just how much is going on in that steaming process: apparently milk contains whey and casein proteins. When we heat the milk, the whey protein structures unravel and create spheres around the air. These formations stabilise into bubbles, making the texture we want in our coffee.
Apparently, a little under that temperature range and the foam tends to collapse - the thin membrane of the bubbles will be pierced by the solid fats. And over 70 degrees, there is a risk that the lactose will react with the proteins and destroy the flavour.
They say we should stay well under 70 to avoid that latter risk. But as is so often the case with experts, they might be missing a bigger and simpler point: I want my hot drink to be hot! I'm with the LC people on this one. 70 degrees is perfect - I've checked with my favourite coffee shops and that's what they aim for too.
Another aspect of the question I like is that it makes us think about the temperatures of everyday experiences, and what they tend to be. I've included a few slides about that in the PowerPoint - in a sort of game-playing set-up.
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