The moka pot is sometimes misleadingly referred to as a stovetop espresso maker. There are many variations in design for this device, but the basic function is the same. At its most basic, the moka pot consists of 3 parts:
1) a chamber at the bottom for the water. It has a threaded opening for the top (not middle) section, and a pressure relief valve. This section is almost invariably made from metal. Before using, it is filled with water to just below the relief valve; do not fill completely. 2) the middle part holds the ground coffee. It is a metal ring with a funnel attached to its bottom (all one piece); the funnel is separated from the ring by a screen, which usually spreads across the bottom of the ring before it narrows for the funnel. This component is also made of metal. This section simply drops into the lower section, and finely ground coffee is spooned in until full or slightly heaping. The grounds are not tamped. 3) The brewed coffee flows into and is poured from the top section, which has a threaded opening for the bottom section and is topped off by a lid, commonly one that flips open and closed. This component varies the most, both in shape and composition; there are also models where this section is made from clear, heat-resistant plastic, enabling the user to see the brewing coffee flowing in. The bottom of this section has a screen very similar to the one in the middle section. This screen also leads to a tube, one that points up into the upper section. The upper section gets threaded onto the bottom section, thereby slightly packing the coffee in the middle section.
The assembled device is placed on the stove and the water is heated. Since the lower chamber is airtight (the funnel bottom is situated below the waterline), the expanding air pushes down on the the water and forces it up the lower tube, through the coffee and through the top tube. It spurts out of the tube (which ends near the top of the upper chamber) and drops down into the bottom of the top reservoir. The brewed coffee is them poured off. This is similar to how a vacuum pot works, except that (1) rather than mixing with the coffee and steeping, the water is forced under some pressure though the coffee, and (2) the brewed coffee remains in the top portion. A moka pot can make good quality, strong coffee.
Notes: 1. Start with a somewhat coarser grind than used for drip. The trick is to allow a small amount of resistance to the water flow without creating a clog. Similarly, the coffee should not be packed, as the grounds will expand when they become wet and may otherwise clog the device. Too fine a grind may also choke the moka pot and possibly pop open the safety (pressure-relief) valve. On the other hand, if the coffee holder is not filled sufficiently, water (seeking the path of least resistance) may seek relatively empty "channels" in the coffee, thereby creating a weak brew. Make sure that there are no grounds on the lips of either the lower or middle sections, as this may prevent a tight seal. 2 . Stoves can vary; you want the water to heat in about five minutes, so a low to medium heat setting should be fine. Do not use a very high heat. Get to know your stove. 3. When the device makes a gurgling sound, remove it from the heat and it will finish brewing on its own. Do not be concerned if there is some water left in the bottom section; you would have to overheat the unit to get the last bit out. 4. It is crucial to have airspace in the bottom section; the pressure relief valve must be kept unblocked in case of a clog, to prevent the lower chamber from bursting, and the airspace is needed to help create the pressure used to push the water up.
The moka pots most commonly sold in the United States are made out of aluminum. Although there are no definitive links between aluminum and adverse health effects, (see, Scientific American: Is there any proof that Alzheimer's disease is related to exposure to aluminum--for instance, by using aluminum frying pans? many people insist that aluminum imparts a metallic taste, and will thus only use a stainless steel model. Some varieties have built-in wands that dispense the brew and/or can generate steam for steaming and milk.There is at least one electric moka pot.
As already noted, moka pots are sometimes referred to as stovetop espresso machines. However, these devices cannot achieve the pressure required to achieve the emulsion of oils and colloids unique to espresso; moka pots make strong coffee, but not espresso.
(Source: thecoffeefaq.com)