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# Æ 4.05 Recap and Glossary

• The permeability of the coffee bed determines how fast water flows through the puck. Baristas can control this by adjusting the grind, but also by controlling the way the particles of coffee are packed together.
• How tightly coffee particles are packed together has a strong effect on flow rate, which is why small variations in density lead to uneven flow and channeling.
• The ‘jamming transition’ is the point at which particles being pressed together (e.g. by a tamper) can no longer move.
• Once the bed of coffee is jammed, then tamping harder will have no further effect
• How tightly the coffee can be packed before it jams depends on the size and shape of the particles
• Longer, narrower particles are more likely to be touching other particles and therefore become jammed more easily. Conversely, more spherical coffee grounds can pack together more tightly before they jam.
• Jamming can be overcome by changing the direction of force. This is what happens during nutation, which is why nutation allows the coffee bed to become more tightly packed.
• Water can also reduce the friction between coffee particles, so adding water might allow coffee to pack more tightly together.
• Manipulating particle size distribution, for example by sieving out boulders, could also allow tighter packing and more uniform, easier tamping

## Glossary

Pore Pores, in fluid dynamics and hydrogeology, refer to the gaps between soil or rock particles (filled with air or water). In coffee, the same principles apply to the gaps between coffee particles.

Jamming transition When increasing the density of a powder (for example by packing it into a container) the jamming transition is the point at which the particles are so tightly packed that no further movement is possible. In coffee, the jamming transition is the point at which tamping harder stops significantly compressing the coffee bed.

Aspect ratio The ratio between the longest and the shortest dimensions of an object,