Recently I learned about an old, classic and fascinating device called vacuum coffee maker.
Recently I learned about an old, classic and fascinating device called vacuum coffee maker.
http://en.wikipedia.org/wiki/Vacuum_coffee_maker
Those appeared in 1830's and were the very first automatic home appliances.

Let's start from the very beginning

To brew coffee, the ground beans are mixed with hot water or steam, possibly under pressure, for a time long enough too extract what we want, but not so long that the stuff we don't want gets extracted, mostly bitter tannin.

The most primitive way to brew coffee is to put it in pot of water and heat it i.e. Turkish method. Turns out it is better to pour already hot water over coffee. Tannins are very soluble even in cold water, unlike "essences". Even better, let hot water drip through coffee. For the same reasons, we want to grind coffee as fine as possible, but then the water can't easily get through.

Which brings us to "the vacuum coffee maker"
Vacuum Coffee Maker - How does it work? Demo

The first portion - water vapor pushing the water up - is optional. You could just pour hot water into the top container yourself, but the vapor-push method conveniently provides the right temperature of 90-95 C.

The key is the condensation of water in the bottom container, creating vacuum, which pulls water through the coffee at ~0.25 atm overpressure. You may recall from many classic can-crashing demos, how this vapor condensation can create serious vacuum. The reason it only lowers pressure by 0.25 atm is because the "can to be crashed" i.e. the lower container, is always full of hot water. If we cooled it with ice, the overpressure would be 1 atm, and we would get iced coffee.

Showing the demo and asking students to calculate pressures and temperatures is an excellent high-school physics problem, especially if you stipulate no air in the bottom container for simplicity and elegance. Upper container is open to the atmosphere.

Vacuum coffee makers completely disappeared from US in 1950-1970, replaced by drip coffee makers, including the first automaticone - Mr.Cofee. Those also use vapor pressure to deliver water to the top for the same reasons as described above.

Next, in the order of increasing temperature and pressure is Moka Pot, which has pressure of 1.5 atm, and temperature of 110 C.

http://en.wikipedia.org/wiki/Moka_pot
http://en.wikipedia.org/wiki/Moka_pot#Moka_coffee_vs._espresso_coffee
Next is espresso, with pushed water though coffee at pressure of 9 atm, and temperature of 90-95 C.
http://en.wikipedia.org/wiki/Coffee#Brewing
Note that at 9 atm, boiling point of water is 175 C. So it is really water, not steam which is pushed thorough coffee, and it can not be simply pushed by a steam in equilibrium, only by a level, pump, or a steam-driven contraption.

As a result of brewing under high pressure the espresso has 10-15 times more concentrated quantity of coffee to water as gravity-brewing methods can produce) and has a more complex physical and chemical constitution

In USSR they were selling "coffee" made from 100% barley (ячмень).

More references:

http://www.coffeekid.com/coffee/vacpots/vacpotfaq
http://baharris.org/coffee/Physics.htmhttp://baharris.org/coffee/History.htm
http://www.youtube.com/watch?v=FxIs2GVsqgY

Labels: Oleg Zabluda

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