Minor quibbles on terminology and completeness aside, I agree with Mark's definition below. However, as I wrote about in Pressure profiles, preinfusion and the forgiveness factor
, some espresso machines nicely demonstrate that preinfusion is not
a prerequisite to exceptional espresso. Jim suggested in the Elektra Semiautomatica review
that uniform water diffusion plays an unappreciated and yet significant role. I think he's right.
In terms of buying criteria, it would be a mistake to vet your choices based on the presence of some preinfusion mechanism. For example, the Elektra A3 has no preinfusion, the Vibiemme Domobar Super has the classic E61 preinfusion chamber
, and the Ponte Vecchio Lusso is a spring lever where you control the preinfusion (I prefer about 3 seconds for the Lusso). Three completely different espresso machines in terms of preinfusion and all very easy to use.
An Espresso Glossary wrote:Preinfusion:
the act of pre-wetting the bed of ground coffee inside an espresso machine before actually commencing the brew. Some espresso machines do this by using the pump; water is pumped to the coffee for a second or two, then halted for another second or two. After this pause, the pump activates again, and continues brewing the shot. Super automatics and some automatic machines use this pre-infusion.
Another type of preinfusion is called "natural" or progressive preinfusion, and occurs in machines equipped with an E61 grouphead. When the pump is activated, a secondary chamber must fill prior to full pressure being applied to the bed of coffee. This gives a 3 to 7 second saturation time for the grounds before the pressure builds up. This type of preinfusion is preferable to pump and pause active preinfusion.
There is a school of thought that progressive preinfusion improves overall extraction from the coffee.