How much does flow rate matter, if brew pressure is same? - Page 2

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#11: Post by gscace »

Noyes-Whitney equation is a reasonable way to describe the relationship between temperature, grind fineness, particle size distribution, pressure and water flow rate. You can read about it here: Dissolving coffee can be assumed to be at equilibrium solubility at the surface of the dissolvable solids that are in contact with hot water. That means that the concentration of material dissolved into the water adjacent to the surface is at the maximum concentration - a property of the system of solute and solvent. If the concentration of solute is lower incrementally away from the surface, then dissolved solute diffuses away from the surface and more solute can dissolve until equilibrium solubility is achieved. The rate at which diffusion occurs depends on "concentration gradient," which is the difference between concentration of solute at the surface and at some distance away from the surface. For engineering folk (and for everyone else, although they might not think of it much), the thickness of the boundary layer near the coffee particle is the portion which we are interested in with respect to concentration gradient. Concentration of solute where the boundary layer and free stream of water meet is equal to the concentration of solute in the free stream. Concentration of solute at the ground coffee surface is the equilibrium solubility. The gradient is the difference in solubility divided by the thickness of the boundary layer. Large differences in coffee concentration across the boundary layer enable higher diffusion rates of dissolved solids across the boundary layer into the free stream, which means that the solute will dissolve more quickly because dissolved material is being more quickly transported (via diffusion) away from the water / ground coffee surface.

Factors that influence concentration gradient include boundary layer thickness, the amount of coffee already in solution. For a given difference in concentration between ground coffee surface and the free stream of water, thinner boundary layers mean higher concentration gradient and more rapid dissolving of coffee solids. Pure water is less viscous than espresso, and for a given flow rate has a thinner boundary layer. So particles near to the top of a bed of coffee dissolve more quickly than those at the bottom. As the water flows toward the bottom of the coffee cake, it has substantial amounts of dissolved material in it, which means that the difference in concentration between the surface and the concentration at the free stream is less. So concentration gradient goes down further. For both the above reasons coffee at the top of a coffee cake extracts much more quickly than at the bottom of the cake. Speeding up water flow transports material away from the boundary layer more quickly, increasing concentration gradient. Since changing flow rate changes boundary layer thickness, and changes the amount of solute in the free stream, you can see how manipulating flow can change what you get out of coffee.


Another Andreas
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#12: Post by Another Andreas »

Changes the ramp up time, higher flow rate, faster ramp up time to come to the desired extraction pressure, given a correct tamp to get the resistance to the water.

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#13: Post by AssafL »

We always discuss flow rate effect on extraction. One thing we ignore is flow rates effect on crema and body/mouthfeel.

While slowing down the flow always extracts more (more time for the solubles to dissolve) - I find that good, heavy crema is highly dependent on the 6ml/s flow rate. Moving to either side (either to a gusher, or to a blocked puck) and crema is reduced until becoming a black hole at the extremes.

I don't know why. My unproven theory is that for crema and mouthfeel - emulsification has to happen. Either micro-emulsification (created by tiny CO2 bubbles escaping form the grinds) or regular emulsification (whisking) that would take optimum shape at some flow rate.
Scraping away (slowly) at the tyranny of biases and dogma.