The question is about adjusting the size of particles of coffee powder to temperature.
It is said that properly grinding the coffee grains is cricual for making good coffee (I have an espresso machine in mind), as it affects the efficiency of the percolation process:
- if the powder particles are too big, the water freely flows between them, and the extraction of coffee is inefficient - the resulting drink is watery
- if, on the other hand, the power is too fine, the water cannot pass (we are on the other side of the percolation transition), so it ends up with punching a hole, producing an equally bad result
The whole thing is complicated by this process apparently being sensitive to the temperature, humidity and other conditions, so that the size of the powder particles has to be adjusted daily.
Question: If the outside temperature suddenly drops (in comparison to the previous day), should the powder be made more or less fine to achieve good result? (I mean a drop of temperature of about $10^\circ$C. I also think that this is an indirect effect via the humidity or air pressure change, since the room temperature remains more or less constant.) Equations and good references are welcome.
Update
Here is an article that gives a rather good overview of the coffee extraction process (particularly its dependence on the size of the powder particles, as discussed above). This article points out that the poweder particles are smaller at lower temperatures and their size fluctuations are smaller. This is however unlikely to explain the changes with the drop of the outside temperature, if the conditions of storage and preparation remain the same (either frigo or room temperature).
