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Pre-infusion is simply a wetting of the coffee grounds before extraction begins. Others will probably be able to give a better explanation as to how this improves the end result.
Pre-infusion is a process that introduces brew water under low(er) pressure (about 4-5 BAR) for a few seconds, to ensure that the coffee puck is properly saturated and allowed to expand up against the shower-screen before full brew water pressure is applied.
The object of doing this is to try and create a homogeneous cake of coffee that presents an even overall resistance to the water flow when under brew pressure, so as to reduce as much as is practical, the tendency for channelling or fracturing of the coffee cake to occur.
Manual E-61 Groups achieve this via a set of valves controlled by spring compression contained within the Brew Head. Other designs achieve a similar end result by a combination of switching valves via electronic means, in and out of circuit with preset Pressure Bypass Valves. No matter which method is used, the aim is the same.... Prevent channelling and fracturing of the coffee puck.... 8-)
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