Grinding and sifting coffee - what does particle distribution do to your extraction?
Grinding coffee is an important step to unlock the amazing flavors of precious specialty coffee, meticulously cared for from the producers to your local roasters. Grinding whole beans into much smaller particles allows an effective and efficient extraction by creating more surface area, open pores and a much smaller distance for the soluble to travel to the finished brew. Sounds simple enough. But how come some grinders only cost you $20 while commercial grinders can cost thousands and thousands? What’s the difference between a good grinder and the bad one? How does the resulting ground coffee brew differently? What does particle size distribution mean?
Let’s take a closer look at ground coffee. You may have heard of some broad terms, such as coarse grind, medium-fine, to describe the particle sizes of ground coffee. Grinders transform roasted coffee beans into small particles by cutting, pressing or shearing. Ideally, being a more controlled method, cutting is theoretically supposed to produce more circular shape particles that’s uniform in size. However, the practical operation of grinders is usually a combination of different methods. Also, roasted coffee are naturally of irregular shapes. Furthermore, since roasted coffee exhibits a brittle property, regardless of how it’s tackled, when a coffee bean crack, very fine particles flake off. Fines in ground coffee are unavoidable. Physically identical ground particles are very difficult to achieve.
The range of different ground coffee sizes is called particle size distribution. A narrower PSD will result in a more even extraction since particles closer in sizes will experience more similar diffusion.
What happens when you coffee ground consist of large boulders and extreme fines? First, let’s look at pourover coffee and grind size. When using coarse ground, slurry provides an easier passage for water to pass through, resulting in a fast brew that’s often light and tea like, lacking complexity and tasting blank; on the other hand, when using significantly finer ground, brew usually experience a prolonged brew time due to more resistance from the coffee mass and the potential occurrence when fines clog the filter, resulting in a cup that’s bitter, intense with medicinal notes. Needless to say, when your ground exhibit polarizing PSD, brew will more likely experience uneven extraction. A good grinder should produce ground that contains mostly particles within target range with little presence of fines and boulders.
The Ideal coffee grinder should have the following quality:
safe and energy-efficient to operate,
consistent intake of coffee feed,
narrow particle distribution with minimum fines,
minimum coffee retention resulting in the accumulation or fouling of fines,
precise and repeatable grinding parameters,
relatively consistent temperature,
easy access for cleaning, maintenance and parts exchange.
To understand and control the PSD of your brew, we can use sieves to sort out different particle sizes. A readily available and comparatively affordable tool is Kruve sifter by Kruve. I’ve conducted a side-by-side tasting of the same coffee brewed with similar parameters, but one without any particle distribution control, the other only brewed with ground size between 400μm and 1000μm. For the sake of experiment, I used the same grind setting on the grinder.
Here’s what I find in the particle distribution controlled brew:
more pronounced dry aroma. (This may have more to do with the motion of sifting, since it encourage the release VOCs that’s usually retained inside the particle.)
significantly lighter body.
more defined acidity.
In this experiment, the cup that’s brewed with particle distribution controlled within target range shows a well defined acidity. I also got a lot more floral dry aroma. But since sifting doesn’t actively change the chemical compounds in roasted coffee, I’m logically assuming it’s due to the sifting that help “shake off” some volatiles that’s usually “trapped” inside the ground. However, I also noticed that the controlled brew lacked some sweetness and texture. Overall, I preferred the non-controlled brew from this experiment.
This conclusion is purely based on my own sensory assessment and preference. I have a decent grinder to start with, which supports the argument that little presence of fines can help produce a more balanced brew. Also, I believe that tweeting the grind size on the controlled method can yield a better brew.
Will I recommend sifting coffee ground before brewing? Yes, as demonstrated by this experiment, sifting does help brewer get a better assessment of the quality of the acidity in the coffee. Especially for people who prefer a lighter body and cleaner finish, removing fines and boulders can result in a more even brew. However, I’d recommend using a finer setting when using sieves where you see less presence of boulders. This will help reduce waste. The correct grind setting when using coffee sifter is when your ground consist less large particles, meaning keep adjusting your grind size finer on the grinder till you start to see a noticeable decline in the amount of large particles. Removing fines can dramatically change the brew dynamics. You will be able to use a much finer grind setting since there won't be fines to migrating and clogging your filter, therefore increasing extraction.
Is sifting coffee necessary? No. Also based on this experiment, sifting is not the only way to achieve a good brew. Even if the range of the PSD from your grinder is more spread out instead of peaking in the middle, you can still opt for immersion brew method, or adapt a different technique to improve the brew. For example, large mass initial pour on pourover method allows brewers to use finer grind settings, therefore tightening the PSD. (However, if your grinder is producing unfavorable amount of fines, it may be time to investigate if burr replacement is necessary.)
A little caveat here is that the above experiment and personal recommendation is based on the circumstance that you have access to a good grinder and you can grind coffee immediately before brewing. If you have to grind your coffee ahead of time, I’d recommend sifting coffee to reduce the amount of fines. In the scenario where ground coffee are stored over an extended period of time before being brewed. These fines undergo flavor deterioration much faster and can end up making no contribution to the brew but potentially hindering the brew by clogging the filter and choking the percolation.
Martin von Blittersdorff, Christian Klatt, The Grind-Particles and Particularities