The Relationship Between Time and Grind
The word ‘resistance’ describes how contact times and grind settings interact. If you didn’t grind the coffee at all and then you made a pour-over using whole beans, all the water would come through in about 15 seconds. The water could easily fit through the large gaps between beans. Whole coffee beans put up zero resistance to the flow of water. However, if you tried the same experiment (as we do in the Barista One course) with an espresso machine, it would take less than 15 seconds. The flow takes a little longer in a pour-over because the water is not being squeezed out by an espresso machine’s pump. The filter paper also adds a little bit of resistance. The holes in the filter paper are very small, approximately 20 μm in diameter, so you might expect them to put up a lot of resistance. The holes are so numerous, though, that the combined surface area of the millions of holes add up to the surface area of a rather large hole.
If you used a grind as fine as you could make, it is possible that you could choke up the filter paper. The pores in the paper are very prone to being filled in by fines (fine coffee particles). Fines can jam up the holes so completely as to entirely arrest the flow of water. The paper industry refers to this process as muddying. Pour-over coffee is thus very sensitive to use with coffee grinders that produce an excess of fines. Every coffee grinder produces a significant amount of fines, but some, such as rotary spice grinders, are almost unusable for filter coffee.
The proportion of coffee particles small enough to lodge completely inside a 20-μm hole in the filter paper amounts to less than 1% of the total coffee grinds by mass. Research has shown, however, that if we measure coffee particles by number instead of by mass, on a typical grind profile suitable for pour-overs, for every particle above 100 μm there are 100 million below that size.