Grinding
Coffee Fundamentals
Course 1 · Chapter 5 · Grinding: Size, Uniformity and Machines
Why Grind at All?
Whole roasted beans barely give anything to water. Grinding breaks them into particles so the water can reach the flavour inside. Wikipedia's Coffee preparation article says brewing methods that keep grounds in contact with hot water for longer need a coarser grind than faster methods. Wikipedia's Burr mill article adds that a finer grind gives more surface contact with water and more complete extraction of caffeine and flavour.
Too Fine, Too Coarse
| Problem | What the article says |
|---|---|
| Too fine | A bitter, harsh, "over-extracted" taste; also slows filtration or screening |
| Too coarse | Weak coffee unless more coffee is used |
| Uneven | A uniform grind is "highly desirable" |
The same article notes that with an adjustable brew time, a finer grind lets you use a shorter brew for similar flavour with less coffee. This is a trade-off, not a free gain (check).
Matching Grind to Method
The Burr mill article groups grinds into qualitative categories only. It gives no millimetre or micrometre figures, so the numbers in the table below are the article's words, not measurements.
| Method | Grind |
|---|---|
| Turkish coffee, espresso | Finest |
| Pour-over, drip | Medium-fine to medium |
| French press, cold brew | Coarse to extra coarse |
Coffee preparation notes that Arabic and Greek (Turkish) coffee need near-powder fineness, finer than most home burr grinders can reach; traditional Greek hand grinders can, and a mortar and pestle can do it by pounding.
Four Ways to Break a Bean
| Type | How it works | Strengths and weaknesses (from the articles) |
|---|---|---|
| Burr | Two revolving abrasive elements (wheels or cones) crush the beans; the gap sets the size | Fairly uniform particles, less frictional heat, avoids excess fines; two types, conical and flat wheel |
| Blade (chopper) | Blades spin at 20,000 to 30,000 rpm | Cheaper, but non-uniform and particles keep shrinking while it runs; makes "coffee dust"; best for drip, not recommended for pump espresso machines |
| Pounding | Mortar and pestle | Reaches near-powder fineness |
| Roller | Corrugated rollers | Even sizes and little heat, but only commercial and industrial |
Electric and Manual
Cheap electric burr grinders use a small motor with reduction gears, which the article says are noisier and usually shorter-lived; heavy-duty commercial machines use direct drive with a large motor and belt. Settings are either numbered steps (repeatable) or stepless. Commercial machines may have a doser, or be doserless and drop grounds straight into a container such as a portafilter. Manual grinders are turned by hand, have largely been replaced by electric ones, and remain popular for cost, size and looks; fast, precise manual grinders gained popularity in the 2020s for single servings. Very fine Turkish-style powder cannot be reached in inexpensive electric or simple manual grinders without straining them.
The Burr mill article gives some history: in 1799 an English blacksmith from Birmingham named Richard Dearmann made the first coffee grinder, and in 1818 Increase Wilson patented the first wall coffee grinder in the US. I have not checked these against other sources (check).
Freshness
Ground coffee deteriorates faster than whole roasted beans because more surface is exposed. That is why many drinkers grind just before brewing, and why chapter 4 gave ground coffee a much shorter optimal life (about 15 minutes) than roasted beans (about two weeks).
Code: Why Finer Means More Surface
For a sphere, surface area divided by volume equals 6 divided by the diameter, so halving the diameter doubles the surface per unit of coffee. Real grounds are irregular, so this is only an illustration, and the diameters are made up for the demonstration, not measured grind sizes. The script also repeats the water-to-coffee ratio range of 15 to 18 to 1 by mass given in the Coffee preparation introduction. I ran it to get the output shown.
Not Yet Verified, and Sources
- Sources. Wikipedia articles Burr mill (read in full as returned) and Coffee preparation. The Coffee preparation article is about 110,000 characters; I read the first 100,000, which included the whole grinding section.
- Not in the pages read. Any measured particle size for any grind; the difference between flat and conical burrs for coffee; the effect of burr material (all marked check or left out).
- Weak support. The Burr mill article cites some community references that the page does not verify, and the history dates are single-source.
- Illustration only. The surface-to-volume figures assume spherical particles and invented diameters.
Exercises
Explain in your own words why a blade grinder gives uneven results and a burr grinder does not. Then say which type you would avoid for a pump espresso machine and why.
Extend the script to print the surface-to-volume figure for a 0.125 mm particle, and the water needed for a 25 g dose at both ends of the 15 to 18 to 1 range. Run it and say how many times more surface the 0.125 mm particle has than the 1.0 mm one.
A brew tastes bitter and harsh, and the filter is slow to drain. Using the chapter, say what is probably wrong with the grind, what you would change, and what you would expect to happen to the taste. Note which part of your answer is the article's claim and which is your inference.
Quick Reference
- Longer contact, coarser grind; shorter contact, finer grind
- Too fine: bitter, harsh, slow to filter; too coarse: weak
- Burr: uniform, less heat; blade: 20,000 to 30,000 rpm, uneven, dust
- Finest: Turkish, espresso; medium: pour-over, drip; coarse: French press, cold brew
- Ratio range given: 15 to 18 parts water to 1 part coffee, by mass