Experimental investigation of steam pressure coffee extraction in a stove-top coffee maker

The most common household coffee-brewing method in Italy makes use of a stove-top coffee maker known as moka. This device uses the steam pressure, produced by the water contained in an autoclave-type aluminum kettle heated by an external source, to force upwards water itself through a roasted and ground coffee bed contained in a funnel-shaped filter. Despite its well-established usage, the moka has never been the subject of detailed analysis, which led to a series of unclear descriptions or misinterpretations concerning its functioning, such as the consolidated misbelieve that standard atmosphere boiling point temperature is needed to drive the water out. The detailed measurement of the thermodynamics of the moka, described here, sheds light on its actual behaviour. It is shown that extraction commences at pretty low temperatures and depends on the initial amount of dry air in the kettle. Remarks on the time decreasing value of the coffee bed permeability are also drawn. A correct understanding of the extraction phenomenon, together with considerations on the coffee chemistry, serves the purpose of assessing possible ways to improve the quality of moka product.