Espresso Masterclass #4 — Grind Science: Particle Distribution, Fines & Heat
By Wing Yuen
Dial in the shot? You're really dialling in a particle distribution — and once you can see it, guesswork disappears.
Welcome to the fourth instalment of the Espresso Masterclass, Tasse Coffee Roastery's ten-part educational series on the science beneath a great shot. In the first three posts we established that espresso is physics, met the numbers that replace guesswork, and dismantled the timing and pressure myths. Now we go upstream to the variable that sets the stage for all of them: the grind.
Grind size and grind quality decide how much surface area the water meets, how fast it can move, and how evenly the puck resists it. Understand the particle distribution your grinder produces — the fines, the boulders, the heat, the static — and you stop chasing symptoms and start controlling the cause. This is the same scientific approach at the heart of our Espresso Alchemy masterclass, where we turn grind from a feeling into a measurement.
The Masterclass at a Glance
| Course | Espresso Alchemy — The Data-Driven Masterclass for Elite Baristas |
| Format | Private, hands-on — maximum 4 participants |
| Duration | One full day, 10:30–16:30 |
| Location | Tasse Coffee Roastery, Takadanobaba, Tokyo |
| Languages | English / 日本語 / 中文 |
| Price | ¥80,000 per person |
| Certification | Certificate of completion + digital resource folder |
Chapter 1
Grind Is the Master Variable
Before pressure, temperature, or ratio, there is grind. Every espresso variable you can name ultimately acts on a bed of coffee particles, and the size and shape of those particles decide how much surface area the water meets, how fast it can flow, and how evenly the puck resists it. Halve the particle size and you roughly double the surface area available for extraction while sharply increasing flow resistance. Grind is not one knob among many — it is the lever that sets the range every other variable operates within, which is exactly why a data-driven approach starts here.
What you'll take away
- Why particle surface area, not weight, governs extraction speed
- How grind setting sets the ceiling and floor for every other variable
- Why two shots with identical dose and pressure can taste worlds apart
Behind the shot — reading the particles
Extraction is a race between solubles dissolving off particle surfaces and water carrying them out of the bed. Grind size is the single input that changes both sides of that race at once.
Chapter 2
The Bimodal Reality: Fines & Boulders
Ground espresso is never a single, uniform size. Under magnification it resolves into a characteristically bimodal distribution: a large primary peak near your target size, and a second, smaller peak of microscopic fines below 100 microns. At the other end sit the boulders — oversized fragments that extract slowly and open fast channels. The goal of grind science is not to eliminate this spread — that is physically impossible — but to understand and control its shape, because the width and skew of that curve is what you actually taste.
What you'll take away
- What a bimodal particle size distribution really looks like
- Why fines slow flow while boulders create channels
- How distribution width — not just the median — drives cup quality
Behind the shot — two peaks in one pile
Researchers separate espresso grounds by size to reveal the twin peaks. The median grind setting only tells you where the main peak sits; the fines fraction hiding beneath it is often what decides the shot.
Chapter 3
Fines: The Double-Edged Sword
Fines are the most misunderstood particles in espresso. A 2024 study in Scientific Reports on the role of fines in extraction dynamics showed they do two things at once: they add enormous surface area that accelerates flavour extraction, and they migrate and pack into the puck, throttling flow and raising resistance. Too few fines and the shot gushes, thin and sour; too many and it stalls into bitterness. The art is a productive fraction of fines — enough to build body and resistance without clogging the bed into uneven, over-extracted pockets.
What you'll take away
- Why fines both boost extraction and restrict flow
- How fines migration compacts the puck during the shot
- Finding the fines balance that gives body without bitterness
Behind the shot — the packed bed
A compacted puck under pressure is where fines earn their reputation. As water pushes through, small particles shift downward and seal gaps — useful in moderation, ruinous in excess.
Chapter 4
Burr Geometry: Flat vs Conical
Your grinder's burr shape writes the first draft of your particle distribution. Flat burrs force every bean through the same precisely controlled gap, yielding a narrower, more unimodal spread with fewer fines — prized for clarity and separation in the cup. Conical burrs crush beans through a varying gap as they fall, producing a wider, more bimodal distribution with more fines that tends to build body and texture. Neither is objectively better; they are different starting points. Knowing which curve your burrs produce tells you what you are dialling against.
What you'll take away
- How flat burrs bias toward clarity and conical toward body
- Why burr alignment and sharpness reshape the whole distribution
- Matching burr choice to the flavour profile you want to build
Behind the shot — the cutting geometry
Flat and conical burr sets side by side make the difference visible: uniform ring versus stepped cone. The geometry that fractures the bean is the geometry that shapes your extraction.
Chapter 5
Grind Heat & the Cost of Friction
Grinding is violent, and violence makes heat. Friction between beans and burrs, the energy of fracturing, and motor resistance all raise ground temperature — and coffee's delicate aromatics do not survive heat gladly. Volatile compounds begin evaporating well before extraction, and studies have measured meaningful losses of aromatic concentration with only modest rises in grinding temperature, alongside faster oxidation of hot grounds. This is why low-speed grinders and cool burrs matter: not for the number on a thermometer, but for the aromatics that reach the cup.
What you'll take away
- How friction, fracture, and motor load heat your grounds
- Why hot grounds lose aromatics and oxidise faster
- The case for low-RPM grinding and cool-down between doses
Behind the shot — keeping it cool
A low-speed grinder trades a few seconds of speed for a cooler grind. The aromatics you protect during those seconds are the ones that survive to the cup.
Chapter 6
Static, Retention & the Water Trick (RDT)
Freshly ground coffee carries a static charge that scatters fines, clumps grounds, and leaves retention lurking in the grinder. The Ross Droplet Technique — a light mist of water on the beans before grinding — is the elegant fix. A 2023 peer-reviewed study in Matter confirmed that roughly 10 microliters of water per gram cuts electrostatic charge by about half, drops retention from over 10% to around 2.5% on dark roasts, and nudges espresso strength measurably upward. RDT doesn't reshape your particle distribution — but it makes sure the distribution you ground is the one that reaches the puck.
What you'll take away
- How static causes clumping, scatter, and grinder retention
- Why a few water droplets neutralise charge so effectively
- The measured gains RDT delivers in retention and cup strength
Behind the shot — ten microliters
A single mist of water before grinding is the cheapest upgrade in coffee. The droplets you can barely see are what stop static from stealing your fines and your consistency.
Chapter 7
Measuring Grind: From Feel to Data
You cannot improve what you cannot see. Dialling by taste alone means guessing at a distribution you never actually observe. Test sieves, laser particle analysis, and disciplined refractometry turn grind from a feeling into a measurement — letting you track your fines fraction, compare burr sets, and prove that a change did what you hoped. This is the heart of the data-driven method: pair every grind adjustment with a number, log it, and build a personal map of how your specific grinder's particle curve translates into extraction yield in the cup.
What you'll take away
- How sieving and particle analysis quantify your grind
- Pairing grind changes with TDS and extraction-yield data
- Building a repeatable log that turns dialling-in into a system
Behind the shot — grading the grind
Stacked test sieves sort grounds by size so you can weigh each fraction. It is the difference between believing your grind is consistent and knowing it.
Who is this series for?
For the barista, roaster, or serious home enthusiast who is tired of dialling in blind. If you've ever changed your grind by "a notch" and hoped for the best, this series — and the Espresso Alchemy masterclass it leads into — is built to replace that hope with a method. No prior science background is needed; only curiosity and a willingness to measure.
What's next in the series
- #5 — Puck Prep: WDT, Distribution & Low-Pressure Tamping
- #6 — Staccato & Layered Pucks: Engineering the Bed
- #7 — Pre-infusion & TCF: Timing the Start of the Shot
- #8 — Pressure Profiling & the SOUP Method
- #9 — Reading the Shot: Channelling, Tiger Stripes & Spent-Puck Analysis
- #10 — Building a Data-Driven Espresso Practice at Home
Espresso Masterclass is a 10-part educational series from Tasse Coffee Roastery, the free lead-in to our data-driven Espresso Alchemy masterclass in Tokyo. Post 4 of 10.