Lab 6: Coffee as a Colloidal Fluid and the Effect of Filtration

Author

Gabriella Carlos

Published

March 4, 2026

Duration: 2 hours

Equipment and Materials

Equipment

Coffee grinder
Brush
Timer
Digital balance
Aeropress
Aeropress filter paper
Aeropress metal filter
French press
Microscope
Graduated cylinder
TDS meter or digital refractometer
Beaker
Electric kettle or Hot plate + Thermocouple

Materials

Roasted coffee beans (≥ 180 g per group)
Water (≥ 2 L per group)

Background

We have learned in previous labs that the extraction time is a critical parameter that affects the final taste of the coffee. In most methods of brewing coffee, the extraction time is controlled by how quickly the hot water moves past the coffee grounds. To look deeper into the fluid velocity, we will need fluid mechanics (yay!).

For this lab, we will focus on how liquids move through porous media, in this case coffee grounds. Recall Darcy’s Law, which states that the velocity of the liquid (in cm/s) is proportional to the pressure difference across the porous medium:

\[ v \approx \frac{\kappa}{\mu}{\nabla}P = \frac{\kappa}{\mu}\frac{(P_{entrance}- P_{exit})}{L} \] where \(\mu\) is viscosity, \(\kappa\) is permeability, and \(\nabla P\) is thepressure gradient. When you use the Mr. Coffee or a pour-over, the pressure difference is provided by the gravity acting on the weight of the water. Other devices provide more control over the pressure gradient, like in an espresso machine. A key goal today is to gaina qualitative appreciation for Darcy’s Law and how quickly liquid flows through the porous grounds and affects the taste of the resulting cup. The lower the applied pressure, the longer the effective extraction time will be, with a corresponding impact on the mass transfer and sensory qualities of the brew.

Previously we also looked at the TDS of a brew. Some of the mass extracted from the grounds are dissolved organic molecules, but there are also dissolved gasses, primarily carbon dioxide and VOCs. The concentration of \(CO_2\) is especially high is especially high in freshly roasted beans, since it hasn’t had as sufficient time to off-gas. When you pour hot water over grounds and you see bubbles rising to the top of the slurry that is a good sign that you’re using freshly roasted coffee!

Additionally, some amount of non-dissolved solids makes it past the filter and into the brew. Ideally during grinding you would make coffee particles all of the same size, but this never happens. Particles smaller than the average pore size in the filter are carried into the brew and get consumed. These small particles are referred to as colloidal particles. Colloids are typically in the range of 1 to 10,000 nanometers and undergo Brownian motion. This is the same sort of random jiggling motion that individual molecules perform, but some colloids are large enough that you can see the Brownian motion with a standard optical microscope. In coffee, the colloids strongly affect the body and mouthfeel by altering the viscosity. The more colloids, the higher the “mouthfeel”.

Finally, there are emulsified oils in the brew. The term “emulsion” simply means a colloidal suspension made of liquid droplets rather than solid, like a vinaigrette. Roasting coffee releases a variety of oils, which you can often see on the surface of dark roasted beans. As with the solid colloids, the emulsified oil droplets also affect the mouthfeel by altering the viscosity. The oil tends to be more potent though since many of the bitter tasting molecules tend to be more oil soluble than water soluble. Typically a little bit of oil is to be desired.

Both the colloids and the emulsified oil are strongly affected by the type of filtration. The filter pore size controls the size of the colloids that make it inot the brew. Less obvious is the composition of the filter matters tremendously. Paper filters are made of cellulosic fibers, which is hydrophobic and oleophilic. In contrast, metal filters are more oleophobic.

Today, you will perform four brews with the Aeropress to quantify the effects of varying grind size/permeability, pressure gradient, and filtration.