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Coffee machines are emerging on the market which claim to superheat the steam they use in heating and aerating milk via a submerged nozzle for latte/cappuccino type beverages.

So instead of the saturated steam typically used to heat and aerate milk coming from a closed boiler at around 1.2 barg or 124°C, they are heating that same steam to 180°C. It is said to produce drier steam (therefore less dilution of milk with water), and sweeter milk (higher Maillard/caramelisation). It is observed however to take longer to bring milk up to the desired drinking temperature.

I've seen a video indicating that superheated steam has extremely high thermal conductivity, yet I've read elsewhere that superheated steam is a bad conductor of heat, having a thermal transfer coefficient (U) of 341 BTU/m² compared to 3960 BTU/m² for saturated steam.

So I'm confused. There is undoubtedly more energy in superheated steam, but does it take longer to release? What happens at the steam/milk interface?

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  • $\begingroup$ Is your question specifically in regard to processes that go on in a coffee machine, or are you asking questions in general? What is steam losing its energy to? In the steam/liquid interface which is the liquid? $\endgroup$
    – Deep
    Jul 12, 2016 at 4:27
  • $\begingroup$ I'm trying to understand the nature of superheated steam that despite being hotter, leads to a slower heating of milk in the coffee use case. Is heat trapped inside the steam bubbles for longer leading to a slower heating the surrounding milk, for instance. By liquid, I mean milk, I'll edit. $\endgroup$
    – jontyc
    Jul 12, 2016 at 5:19
  • $\begingroup$ isn't it simply that if the same volume of steam is hotter then less of it will be needed to heat up a given mass of liquid and so less condenses. Also super-heated steam presumably contains fewer water droplets so presumably worse thermal conductivity (i'm guessing here) but this may be overcome by it being so much hotter. Also its probably quicker and if you have waited for ages listening to hissing and gurgling while someone makes a cappuccino that can't be bad :) $\endgroup$
    – porphyrin
    Jul 12, 2016 at 8:18

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If you are using only the "superheat" in the steam, and letting it leave the coffee before it condenses, you are using specific heat only. This means that you will not be diluting the coffee, but it will take a relatively large volume of steam to do the job because the specific heat of the steam will be somewhat lower for a given mass of steam than the latent heat of vaporization will be for the same mass of steam.

I am assuming that you are bubbling the steam into the coffee, else you wouldn't be worried about dilution. If this is the case, the steam and coffee will be exchanging heat via the vapor-liquid interface of the steam bubbles. The use of heat transfer coefficients in this case would tend to be problematic, because you will have a difficult time determining the area of contact, and indeed, this area of contact would no doubt vary as steam is bubbled into the coffee.

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  • $\begingroup$ I don't get what you mean by "letting it leave the coffee [i.e. milk] before it condenses" but feel it's important to my understanding. Will not the superheated steam bubbles start condensing as soon as they touch the milk? And will the amount of liquid water produced from condensation be the same as if it were just saturated steam? $\endgroup$
    – jontyc
    Jul 12, 2016 at 5:30
  • $\begingroup$ Whether steam in a steam-bubble will condense depends on temperature and pressure of the steam inside the bubble; higher pressure implies that steam will begin to condense at higher temperature. So the question is by how much does the steam cool down when it comes into contact with the milk?(I don't know the answer to that) For same amount of heat removal more liquid water will condense from saturated steam than from superheated steam, because part of the heat removed will serve only to reduce the temperature of superheated steam until it is brought down to saturated state. $\endgroup$
    – Deep
    Jul 12, 2016 at 10:08
  • $\begingroup$ The superheated steam bubbles will start cooling down when they contact the coffee. The conditions that you cited indicate that there is 80 deg C of superheat. It will take time for that steam to cool to 100 deg C. If the steam bubble is in the coffee for less than that time, it will not condense. $\endgroup$
    – David White
    Jul 12, 2016 at 14:32

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