This question is often asked and the response that is often offered up is ‘the water is not being pulsed through the coffee from a vibratory pump’. This response is quite correctly flagged as being inadequate, as it doesn’t stack up if the pump machine is utilising a rotary pump instead of a vibratory pump.
For me the explanation is lever espresso machines bring the water through into the group and hold it there in the group chamber during the pre-infusion, until the lever is released. Bringing the water into the group chamber means the water temperature at which the water is forced through the coffee is much more even as the extreme water temperature values that tend to occur at the start (too cool in a lever group) and end (too hot in a lever group) of extracting a shot of espresso are evened out by the simple act of bringing all the water into the group chamber and allowing it to mix before forcing it through the coffee.
Contrast this with a heat exchange set-up on a pump machine where the water generally starts off too hot and gradually cools with every second of extraction time. High end ‘rocket-ship’ pump machines try to address this range of water temperature delivery, but we can’t help but think that the low tech approach of the time honoured design of the lever espresso machine is a rather more elegant and effective solution to the problem.
Additionally, the extraction pressure of a lever machine tapers off as the extraction progresses, ensuring that once the most easily extracted solubles are taken from the coffee that the extraction process softens to guard against the bitter elements being extracted towards the end of the shot. It certainly makes you question why so much investment has been thrown at constant pressure profiling in the recent past, which is now being abandoned in favour of trying to mimic the pressure profile of a lever group espresso machine.
Anyway, don’t take our word for it, come and try the espresso from a superb Bosco lever espresso machine and a battery of Mazzer robur electronic grinders this Saturday 24 March from 8am at Exhibit design store.