Back to the Grind
A discussion of the extraction of water soluble compounds during the brewing process must include a review of surface area vs. volume ratio (SA:V). When trying to dissolve something, think of a sugar cube in water. A single large cube will take far longer to dissolve than ten smaller cubes, even if they have the same total amount of sugar. This is because the amount of sugar exposed to the water in smaller pieces is higher compared to the total volume of the sugar. In chemistry, the ratio of SA:V is essential in determining the potential rate of a chemical reaction.
In the case of brewing coffee, the chemical reaction occurring is the dissolution of soluble organic compounds from the ground coffee beans into the brewing water. It follows that the grind size (SA:V ratio) plays an integral role in the rate, and consequently the time needed, to achieve optimal dissolution of these compounds. The temperature of water also influences the rate of reaction…more about that later. Below is a table showing the influence of common grind sizes on the SA:V ratio of coffee. Note that espresso grinds have 11,115 times more surface area to volume than a whole coffee bean!
Source: Lingle, T. Brewing Handbook. SCAA
I Love It When You Talk Particle Size Distribution to Me
Theoretically, if all of the coffee was ground to exactly the same size, the extra fine dust and the extra coarse particles were removed, then the dissolution reaction rate of the soluble compounds would be identical for every ground coffee particle. The perfect dissolution reaction would then be possible, and the exact duration of the reaction (brew time or steeping) could be defined and applied. Unfortunately, this is not a practical approach to grinding. Good thing it is not the end of the world if your grind isn’t perfectly homogeneous. The tighter your particle size distribution, the less you will struggle between wasted coffee and over extracted flavors, but modest size variations aren’t likely to ruin your coffee experience.
So what does a grind look like? When graphing particle size distribution (PSD), the vertical axis is % of sample by mass, and the horizontal axis is particle size (as defined by a particle being able to pass through a square hole in a sieve or mesh).
The PSD graph below is for illustration purposes only as it is not based on any official size analysis of the grind types in question. This graph shows a target ground particle size of 0.8mm. Each grinder type has a different PDS profile. The better performing grinders have smaller size variance, meaning that more of the ground coffee is at or near the targeted size. A flatter PSD curve means there are lots of different sizes of particles and the dissolution process during brewing is not ideal. As mentioned earlier, while some variation in size is acceptable, a tighter PSD maximizes the amount of brewed coffee that has undergone optimal soluble compound dissolution.
Notice that the PSD of the burr type grinders are centered on the target grind size, this is because they allow the user to set a target grind size. This is unlike blade type grinders where the only control the user has on grind size is grind duration, which inherently skews the PSD towards larger particles.
Apart from grinder type, there are three other factors affecting particle size distribution.
- Moisture content in the roasted beans - Beans cooled after roasting in water are softer than air cooled beans. The softer beans are harder to grind evenly.
- Brittleness of bean - Beans grown at higher altitudes are denser both before and after the roasting process. Beans which were fermented and processed wet, prior to roasting, are also likely to be more brittle.
- The Roast - This is closely related to the moisture content in the bean following the roast, but beans which are dark enough to have undergone second crack during roasting produce more fines in the grinding process.
Still haven’t had enough? Check out Brewing Science Level: Genuis