Regular readers will by now be aware of the complexity of making a good cup of coffee but when Ewan Reid, our Technical & Quality Director, started on about pretreatment or pre- infusion it became clear that we hadn't even scratched the surface of the technical bit.

The first thing you've got to understand is that pre-infusion and pretreatment aren't exactly the same thing. You get pre- infusion on semi-automatic machines and it takes the form of the injection of a small amount of the brewing water into the porta filter. There's then a short rest - typically 0.5-1.5 seconds before the remaining brew water is forced through by the pump.

So far so good. Pretreatment is a bit more complex and is only found on fully automatic machines. Here the parameter settings are either preset and stored or made programmable to varying degrees depending on the manufacturer's design.

This software enables many more of the factors involved in brewing espresso to be affected and customised, including the length of time the coffee is in contact with the pre-infusion water and the volume of water injected. But what's the point and what difference does it make? Keeping technical (and after all this is The Technical Bit!) pre-infusion and pretreatment both cause the wad of coffee grounds to expand in the porta filter (on a semi-automatic machine) or brew piston (on a fully-automatic)

Due to this increase in the size of the coffee grounds, known as the "extraction bed" the whole nature of the brewed coffee changes. There are two different forms of extraction: diffusion and flushing. Both have different effects on the coffee being extracted. It's essential to strike a balance between these two forms of extraction or, as Ewan Reid put it, "The ratio of diffusive extraction to flush-through kinetics can greatly alter the character of the cup." Well put. For those of us who didn't attain Ewan's degree in Food Science, it can be simplified thus: Adding some of the brewing water to the wad of coffee grounds makes them swell up, and this change in particle size and extraction results in your brew having a different taste and mouth feel.

From a sensory point of view, then, a careful use of pre-infusion can optimise your cup of espresso if necessary. It ought to raise the perceived body of the drink whilst also reducing acidity. But one has to be careful, as it is all too easy to ruin the character of the blend. As someone once said . . . don't try this at home.

This article originally appeared in Fresh 3