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“There’s a law of physics called the no slip boundary condition which means that the velocity of a liquid flowing past a surface is exactly zero at the surface itself. This means that the only way for molecules to escape from the surface is via diffusion, and for large molecules like the bitter tannins this is very slow – a good thing because we don’t want them. The only way for these molecules to escape is to get turbulent flow coming as close as possible to the no slip boundary – and the only way to do this is via the Forcheimer, rather than Darcy flows mentioned in the previous post. Channelling is a sort of Matthew effect – the more flow starts down one path, the more turbulent it gets and the more it flows, creating more turbulence and dragging out more tannins. So anything which reduces channelling will help reduce the bitter extraction.”
In other words, it’s not just over-extraction: tannins and other large molecules need the turbulent flow created by channels to extract into the coffee. Because our tongues are so sensitive to bitterness and dryness, it doesn’t take much of these molecules to ruin an espresso. However, if we’re able to reduce channeling, then we can push extraction higher before these flavours start to become apparent. “‘Over-extraction’ is an overused term,’ Scott explains. ‘These days I never use the term except to describe localized over-extraction due to channeling.’