Grind Size Matters: The Science Behind a Great Cup of Coffee

Most of us have seen many social media influencers showcasing the coffee-making process while enjoying the view of a mountain or camping outdoors. The first fancy thing that captures the viewer’s attention is the manual grinder. The person measures the beans and transfers them into the hopper, then magically starts adjusting the dial and rotating the handle in slow motion (which, by the way, looks so fancy and zen). The process that follows becomes a blur because we get stuck trying to search for the device they’re impressing us with.

When you search for the picture on Google, you’ll definitely come to know that it’s a manual burr grinder.

When you dig deeper, you’ll discover the role it plays in making that cup of coffee, every coffee enthusiast loves.

Yeah! It’s a grinder, but why is it used?

What is the dial for? And how long should it be done for?

A lot of questions arise. This blog is for people who want to understand the chemistry and physics of coffee grinding and learn how exactly you can end up with that great cup of coffee maybe while making your own video somewhere on a mountain or at a rainy camping site.

Let’s grind in

Wait why do we even grind? Why can’t we just pour water over roasted beans and expect the extraction? Nah! it doesn’t happen like that. 

Let’s understand extraction first.

What Is Coffee Extraction?

Coffee extraction is the process through which water dissolves and carries flavour compounds out of ground coffee.

When hot water comes into contact with coffee grounds, it doesn't simply absorb “coffee flavour.” A series of physical and chemical processes begins.

Think of each coffee particle as a tiny storehouse of compounds. Water enters, interacts with those compounds, dissolves the ones that are soluble, and carries them into your cup. 

Chemistry: What Is Inside Those Particles?

Roasted coffee contains hundreds of chemical compounds responsible for:

• Aroma 
• Acidity 
• Sweetness 
• Bitterness 
• Body 
• Flavour 

Grinding opens up the coffee bean. Along with exposing the flavours we want to extract, it also exposes the coffee to the environment.

The moment coffee is ground, compounds that were protected inside the bean become exposed to:

• Oxygen 
• Moisture 
• Heat 
• Light 

This is one reason freshly ground coffee smells so incredible and why its aroma gradually disappears.

Particle Size and Extraction

This is where physics directly affects taste.

When hot water meets ground coffee, it begins dissolving soluble compounds from the particles. Smaller particles generally extract faster because water has easier access to more surface area. Hot water generally speeds up this process because molecules move faster at higher temperatures.

The compounds move out of the coffee

Once compounds dissolve inside or near the surface of the coffee particle, they move into the surrounding water.

This happens partly because of diffusion.

Simply put, molecules tend to move from an area where they are highly concentrated to an area where they are less concentrated.

So, the dissolved coffee compounds gradually move from the coffee particle into the brewing water.

Fresh water keeps the extraction going

As brewing water flows through or around the coffee, it carries dissolved compounds away.

Fresh water then comes into contact with the particle and can dissolve more compounds.

This is why water movement matters in brewing.

But for all of this to happen efficiently, water needs better access to the coffee, and that is where grinding comes in.

Grind Size

We have understood the interaction between water and coffee particles and how extraction works. Now, let’s find out what different grind sizes can do to your cup of coffee.

Too Coarse

When coffee is ground too coarsely, the particles are larger. Larger particles have less surface area exposed to water, so extraction happens more slowly.

If the brewing time is not increased to compensate, water may not be able to dissolve enough of the desirable compounds from the coffee.

The result can be a cup that tastes:

• Sour or overly acidic 
• Thin or watery 
• Weak 
• Underdeveloped 

This is called under-extraction.

Imagine trying to dissolve a large block of sugar in water compared to finely powdered sugar. The smaller particles dissolve much more easily and quickly. Coffee works on a similar principle.

 Too Fine

When coffee is ground too finely, the particles become much smaller and expose far more surface area to water.

This means extraction happens very quickly.

But there is another problem. In brewing methods where water needs to flow through the coffee bed, fine particles can slow down the flow. This increases the contact time between water and coffee.

As a result, water may continue extracting compounds beyond the desirable range.

The cup can taste:

• Bitter 
• Harsh 
• Dry 
• Astringent 

This is often referred to as over-extraction.

However, an important point is that fine coffee isn't automatically bad. Espresso, for example, requires a very fine grind because water passes through the coffee quickly under pressure.

Balanced Extraction

This is the sweet spot.

The grind size, brewing time, water temperature, and brewing method work together to extract the right amount of soluble compounds from the coffee.

A balanced cup may allow you to experience:

• Bright but pleasant acidity 
• Natural sweetness 
• Aromatic complexity 
• Appropriate bitterness 
• Good body and texture 

The goal isn't to remove everything from the coffee particle. In fact, extracting everything would not necessarily make better coffee.

So, does that mean we need to use a medium-fine grind for balanced extraction?

No.

The concept is simple. We all like our coffee differently. Some like it strong and bold, while others might prefer it sweet and palatable.

Just like different people like different kinds of coffee, different coffees and brewing methods need different grind sizes.

A simple guide to grind size and brewing.

For all the coffee lovers out there—whether you’re a black coffee, espresso shot, iced cold brew, cappuccino, or filter coffee person, remember one thing:

“Grind size matters!”

Cheers! ☕