The Science of Grinding: Particle Size and Coffee Extraction

Grinding is a key variable in coffee brewing because particle size controls how much surface area is exposed to water, and thus how quickly and how much coffee is extracted.

When whole beans are ground, the total surface area increases dramatically. For example, a 10 g dose of coffee ground to about 400 μm particles can expose more than 1 m² of surface area to water.

Extraction Basics

Brewing coffee is a mass transfer process: hot water acts as a solvent, diffusing into the coffee particles and dissolving compounds in stages:

1. Early extraction: organic acids (like citric and malic) and simple sugars dissolve first.
2. Middle extraction: more complex compounds such as lipids and chlorogenic acids are extracted.
3. Late extraction: bitter compounds, including phenols and tannins, come out toward the end.

If extraction is too low (under-extraction), the coffee tastes sour, sharp, and thin. This usually happens with grind sizes that are too coarse or brew times that are too short. If extraction is too high (over-extraction), the coffee becomes bitter, harsh, and drying, often due to overly fine grinds or excessively long contact time.

Espresso

Espresso requires a very fine grind, typically 200–400 μm. Water at high pressure (about 9 bar) is forced through the compacted coffee bed, with a target shot time of 25–30 seconds. Ideal extraction yields around 19–22% of the coffee’s soluble material.

A common espresso recipe is:

• Dose: 18 g of ground coffee
• Yield: 36 g of espresso (a 1:2 brew ratio)

Dialing in espresso:

• If the shot finishes in under 25 seconds, the flow is too fast → grind finer.
• If the shot takes over 30 seconds, the flow is too slow → grind coarser.

Pour-Over (V60)

Pour-over brewers like the Hario V60 use a medium-fine grind, around 600–800 μm. Typical parameters:

• Coffee: 15 g
• Water: 250 g
• Temperature: 92–93 °C
• Total brew time: about 2:30–3:00 minutes

The paper filter slows the flow and traps many oils and fine particles, resulting in a clean, bright cup with clear flavor separation.

Flow-time guide for V60:

• Brew finishes in under 2:00 → water is draining too fast → grind finer.
• Brew takes over 3:30 → water is draining too slowly → grind coarser.

French Press

French press brewing uses a coarse grind, roughly 800–1000 μm, to keep particles from slipping through the metal mesh filter.

Typical recipe:

• Ratio: 60 g of coffee per liter of water (60 g/L)
• Brew time: about 4 minutes

This full-immersion method allows longer contact between water and coffee, extracting more oils and body for a rich, full-bodied cup.

Adjusting French press:

• If the cup is muddy or silty, too many fines are getting through → grind coarser.
• If the cup is weak or tea-like, extraction is too low → grind finer or extend brew time.

Choosing the Right Grinder

Blade grinders chop beans randomly, creating a very uneven particle size distribution. This leads to some particles over-extracting (bitter) while others under-extract (sour), making it hard to achieve balance.

Burr grinders use two burrs (flat or conical) to crush beans to a more uniform size, which greatly improves control over extraction.

• Conical burrs often produce a bimodal distribution (a mix of fines and larger particles), which can increase body and perceived sweetness.
• Flat burrs tend to produce a more unimodal distribution (tighter particle size range), often enhancing clarity and separation of flavors.

For specialty coffee, it is best to invest in a quality burr grinder and adjust grind size in small increments, tasting each change to find the ideal setting for your brew method and beans.