Espresso makers work by bringing water to a certain temperature (typically 198-202°F / 92-95°C), and then extract it at a high pressure (~9 bar). Given that the FAA requires that cabins are pressurized to ~7,500 ft, this results in a few problems I can see.

  1. The pressure differential between the boiler and the cabin would be very large. I fear that this would result in the espresso machine being compromised.
  2. Water would boil at a much lower temperature (198°F / 92°C) resulting in the machinery not working correctly.
  3. A sudden depressurization of the vessel could certainly result in a dangerous pressure differential resulting in explosive decompression.

Although, a Boeing 787 has a higher pressure cabin, in a typical airplane, could you safely brew espresso? Are there any FAA, or other safety regulations preventing you from doing so?

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    $\begingroup$ Expresso makers for aircraft galleys. One at B/E Aerospace. $\endgroup$
    – mins
    Oct 2, 2015 at 9:08
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    $\begingroup$ The machine works with 9 bar (no, decent espresso works with 14). Sea level pressure is, by definition 1 bar. Pressure in the cabin is about 0.7 bar. 0.3 bar is completely insignificant compared to 9 bar. $\endgroup$
    – Jan Hudec
    Oct 2, 2015 at 10:29
  • $\begingroup$ @mins, is it espresso, or just filter coffee machine? I believe most airlines use the later, which does not use pressure anywhere. $\endgroup$
    – Jan Hudec
    Oct 2, 2015 at 12:36
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    $\begingroup$ An airplane cabin is pressurized to 8000' or less during cruise. There are people that live at higher elevations than that and they make espresso with no issues. $\endgroup$
    – casey
    Oct 2, 2015 at 13:38
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    $\begingroup$ @DavidGrinberg -- ISS is pressurized to approximately sea level atmosphere. $\endgroup$ Oct 2, 2015 at 21:06

4 Answers 4

  • The pressure differential between the boiler and the cabin would be very large. I fear that this would result in the espresso machine being compromised.

The pressure in the espresso machine is 9 bars, 14 in a good one. The difference in outside pressure is 0.3 bars. That is comparably tiny value that does not make any practical difference.

  • Water would boil at a much lower temperature (198°F / 92°C) resulting in the machinery not working correctly.

Coffee should not be made with boiling water anyway. 198°F / 92°C is sufficient for making coffee.

Besides, if the boiler was pressurized, the water would not boil until way higher temperature (it isn't, but it does not heat the water above boiling point at that pressure).

  • A sudden depressurization of the vessel could certainly result in a dangerous pressure differential resulting in explosive decompression.

The pressurized content is water, not vapour, so it would not significantly expand if the machine cracked. After all, its pressure is relieved as part of normal operation (when the coffee drips to the pot) and it does not expand.

And its amount is tiny compared to the total volume of the aircraft anyway. Very tiny, because the pressure is only in the pipe from the pump to the ground coffee.

But that all assumes the coffee machine is espresso, which most of them are not. Usually they will be just filter machines where the water flows by gravity at ambient pressure or perhaps at some pressure, but much lower than in a real espresso machine. I believe some airlines do advertise they serve true espresso though.

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    $\begingroup$ The pressure in the espresso machine is 9 bars, 14 in a good one. The difference in outside pressure is 0.3 bars. wut? $\endgroup$
    – Federico
    Oct 2, 2015 at 12:42
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    $\begingroup$ @Federico mean sea level pressure is 1013 mbar, an 8000' cabin is roughly 750 mbar. That is 263 mbar or 0.26 bar difference between sea level and an aircraft cabin at cruise. 0.3 is on the high side, but demonstrates at worst case it doesn't matter for the espresso machine. $\endgroup$
    – casey
    Oct 2, 2015 at 13:36
  • $\begingroup$ @casey, but that's not what the question is about. The OP is not worried about the difference in outside pressure (1013 vs 750) but in water vs cabin (14 atm vs 0.75 atm) $\endgroup$
    – Federico
    Oct 2, 2015 at 13:40
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    $\begingroup$ @Federico yes, and that figure demonstrates that delta p at sea level is 13 bar and delta p in the cabin is 13.25 bar, a 2% difference and one the espresso machine won't even notice. $\endgroup$
    – casey
    Oct 2, 2015 at 13:43
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    $\begingroup$ @Federico: there is no constant 9 bar pressure in an espresso machine, it's not a pressurized container of any sort, so there is no safety risk. The pressure is only present while the machine is actually pushing the water through the ground coffee. A sudden escape of pressure would result in hot water squirting from the machine that can scald somebody standing very close but will not pose any risk to aircraft safety. $\endgroup$
    – Gábor
    Oct 2, 2015 at 22:42

The high pressure in the modern espresso machine is not created by the highly pressurized boiler. It is created by the pump pushing hot water under pressure. Many models do heave the boiler to foam the milk, for instance, but this is not required for making the coffee itself. This pump can even be hand operated. Some machines even need no electricity, just a hot water.

If exactly boiling water is required, it can be kept only slightly pressurized. However water under cabin pressure is also quite hot; maybe it is enough just to add more coffee or increase the brewing duration.

I am not competent to issue "airworthiness certificates" for espresso machines, but as a passenger I can say that the idea may be worth exploring; coffee as it is served now is never the best part of the flight experience.

  • $\begingroup$ Right - but, it is kept warm, and then usually driven by a pump, or a piston (potentially driven by hand). There is still the fear / problem that near the portafilter, there is a ridiculous amount of pressure. I've heard of retail espresso machines being fatigued, and resulting in explosions. $\endgroup$ Oct 2, 2015 at 7:38
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    $\begingroup$ @SargunDhillon, 14 bar is great deal of pressure. But 14.3 bar is only 2% more. $\endgroup$
    – Jan Hudec
    Oct 2, 2015 at 10:37
  • $\begingroup$ @SargunDhillon Well, the difference is whether you have litres of high-pressure liquid, or 50 millilitres of high pressure liquid (enough to make an espresso). I think that a well-made machine will survive plane's explosive decompression without exploding itself. It's more the time (and price maybe) constraint that makes airlines use filter: it takes about the same time to make 3 espressos as to make a kettle of filter coffee. $\endgroup$
    – yo'
    Oct 2, 2015 at 15:08

As the boiling point of water at 0.7bar absolute is 90C, there would be one small difference.

The specific enthalpy of water at these conditions is 376.783kJ/kg and the specific enthalpy of vaporization is 2282.81kJ/kg.

Assuming the machine heats the water to 95C, it will reach a specific enthalpy of 398.017kJ/kg, an excess of 398.017-376.783 = 21.234kJ/kg. There will therefore be flash boiling of 21.234/2282.81 x 100 = 0.93% of the coffee by mass. Although that doesn't sound much, water expands 2461 times when boiled under these conditions. So the volume of steam produced will be 2461 x 0.0093 = 22.89 times the volume of the coffee.

As I understand it, the water in an espresso machine is boiled under pressure at 9 bar and then forced through the coffee and released at ambient pressure. If this is done without adjusting the thermostat on the boiler, significant boiling will occur due to the pressure drop, possibly causing over-agitation of the coffee and / or vapour lock (inability of water to enter due to the volume of steam.) I can't drink coffee for medical reasons so I'm no expert on coffee, but neither of these sounds like a good thing. The thermostat on the water boiler should be adjusted below the boiling point of water.

I understand good quality espresso machines have an independent boiler for generating steam. That should be fine and can be left untouched.

As others have said, the difference between 1 bar and 0.7 bar ambient is of no consequence for the boiler if it normally operates at 9 bar. There is some possibility of erosion of water valves if superheated water flashes to steam within them due to the drop in pressure.

  • 1
    $\begingroup$ The pressure isn't in the boiler, it is between the boiler output and the coffee grounds / portafilter gaskets. If you operate the machine without the portafilter it water just flows out of the boiler at ambient pressure. The pressure is built up by the tamping of the coffee grounds and the portafilter gasket. The pump will only push hard enough to push water through that. If the water was boiled at 9 bar pressure it would have a temperature of around 175 C and be a dangerous machine. $\endgroup$
    – casey
    Oct 5, 2015 at 17:52

Unfortunately, the accepted answer is off the mark. :-)

Espresso is extracted at 9 bars, even in so called "good" or "decent" espresso machines. The source of this misunderstanding is that most manufacturers state the rated pressure the pump inside the machine is capable of delivering (and consumers are often led to believe that bigger is better). The pressure is regulated to 9 bars when the espresso is actually brewed (with slight differences possible depending on the individual service setting of the machine in question). There is a pressure relief valve that simply sends the excess back to the water tank. Espresso coffee extraction is an equilibrium of many factors, including quantity, temperature and time. It's actually not even desirable to have a higher pressure than the traditional, tried and tested 9 bars, together with all the other standard parameters used.

What's more important in the context of the question is that an espresso machine is not a pressurized container of hot water. The 9 bars are created by a pump that only operates while the machine is actually pushing the water through the ground coffee. The amount of water is rather small (a serving of espresso is 30 ml, it's usual to have two-cup sprouts, meaning 60 ml). In any event of breach of pressure (eg. the portafilter accidentally knocked off), the pressure will fall immediately because it is only built up against the ground and packed coffee, nothing else inside the machine. With no resistance, the pump simply continues to deliver hot water, like a tap. So, the result could be a small amount of scalding hot water for anybody standing close, which is a nasty thing but certainly no risk to general aircraft safety.

Actually, it could be added that using a boiler to keep any amount of hot water is more characteristic of the traditional espresso machines used in catering and by connoisseurs at home. The kind of machine more likely to be used on board of an aircraft, the so-called superautomatic type (this is the combined machine that grinds the beans and brews the espresso at the touch of a button) tend to use heaters that simply heat the water flowing through them during the brewing process. This reduces the startup time of the machine and requires less electricity, although it produces espresso of lesser quality.

Because of this setup, the pump would deliver the same pressure at any flying altitude. It's the pressure relief valve that actually decides the pressure, not the pump. If it stays set at 9 bars, that's what will be used during the brewing process. What the altitude would influence is the temperature we can heat the water to before it starts to boil.

Espresso brewing needs to avoid boiling water at the usual ground level primarily because of the temperature limit of 100°C. It's not the boiling itself that would ruin the espresso (apart from the fact that steam, being compressible, would not work in a pump set up to send high pressure water through the coffee puck) but the high temperature: the aromatic ingredients in the coffee bean can only be extracted, exactly as the OP has stated, in a rather narrow temperature range and going near or above 100°C simply ruins the taste. In this respect, if ambient pressure differences would decrease the usable temperature range at higher altitudes (say, 92°C being the highest temperature of water that can be achieved before boiling), this would only require a different selection of coffee on board, one that tastes good when extracted at that temperature (probably meaning that both the composition of the blend and the roasting process should be adjusted accordingly but this task would be simply left to the supplier).


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