Could a Finer Grind Really Make Your Cold Brew Weaker?

Recently, I was chatting with a friend about cold brew. He wanted to brew a richer, more concentrated batch at home, so he did what seemed logical: he adjusted his grinder to a much finer setting. The result? His coffee turned out weaker and more watery than before. He was completely baffled. If a finer grind means more extraction, why did his cold brew taste so bland?

This is a common puzzle, and it’s a great example of how cold brew plays by its own rules.

Why We Can't Apply Hot Coffee Logic to Cold Brew

In nearly every other brewing method, adjusting the grind is a primary way to control extraction. The theory is simple: a finer grind creates more total surface area. When hot water hits these smaller particles, it can penetrate them more easily and dissolve more of the coffee's flavorful compounds. To increase extraction, you grind finer; to decrease it, you grind coarser.

So, why does this trusted principle fail us when it comes to cold brew? The answer is simpler than you might think. To find out for sure, I decided to run a quick experiment here at the Kafava Office.

The Experiment: Coarse vs. Fine Grind

I took one of our favorite beans, a Costa Rica Baja, and prepared two separate batches of cold brew, changing only the grind size.

For the first batch, I used a standard coarse grind suitable for cold brew, which on our EK43 grinder is setting 9. For the second batch, I used a noticeably finer grind at setting 8.

Here are the parameters I used for both batches:

• Coffee Dose: 20g

• Grind Settings: EK43 @ 9 (Standard) & EK43 @ 8 (Fine)

• Coffee-to-Water Ratio: 1:10 (200ml of water)

• Steep Time: 8 hours, refrigerated

The Revealing Results

After eight hours, I filtered both batches and measured their concentration (Total Dissolved Solids, or TDS). The results were clear:

• Standard Grind Cold Brew: 2.3% Concentration

• Finer Grind Cold Brew: 2.1% Concentration

The experiment confirmed it: the finer grind produced a weaker coffee. The reason for this lies in what happens inside the brewing vessel during that long, quiet steep.

The Science: Why Finer Grounds Hinder Extraction

During the immersion process, the coffee grounds are left undisturbed for hours. At first, the grounds float on the surface. As they gradually absorb water, they become heavier and begin to sink. This is where the grind size makes all the difference.

Coarser coffee grounds are larger and more irregular in shape. When they settle at the bottom, they form a loose, porous bed. There are plenty of gaps and channels between the particles, which allows water to continue flowing through and around them, ensuring a steady, even extraction.

Finer coffee grounds, on the other hand, are so small that they pack together tightly at the bottom. They form a dense, compact layer of sludge. This "sludge layer" severely reduces the surface area that is actually exposed to the water. The water simply can't penetrate this muddy barrier, effectively choking the extraction process. This leads to a significant portion of the coffee being under-extracted.

That’s why, counterintuitively, a finer grind can result in a weaker brew.

Better Ways to Boost Your Cold Brew's Strength

While slightly adjusting your grind can be a good tool, making it too fine is clearly not the answer. Since constantly stirring or agitating your cold brew is impractical, here are two far more effective methods for achieving a stronger, more flavorful concentrate:

1. Use a Lower Water-to-Coffee Ratio: This is the most reliable way to increase concentration. Instead of a 1:10 ratio, try a 1:8 or even a 1:7. Using more coffee relative to the amount of water will naturally produce a stronger brew.

2. Extend the Steeping Time: Give the water more time to do its work. If 8 hours isn't enough, try 12, 16, or even 24 hours. The ideal time will depend on your coffee, water temperature, and desired strength.

My final piece of advice: always aim to brew your cold brew concentrate a little stronger than you think you’ll need. You can easily dilute it with ice or a splash of water to hit that perfect balance. After all, you can always fix a coffee that’s too strong, but there’s no saving one that’s too weak!