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Brewing basic concepts: Grinding coffee and grind size

We grind our coffee first every time before we brew, but why we need to grind coffee bean. In fact, you can boil the whole coffee bean in a pot, and you will still get a cup of coffee, but you will find it takes very long and hard to keep the taste consistent every time, moreover, the coffee usually tastes bad in this way. The reason behind is the contact area of whole bean is small, and there is a big contact time difference on inside and outside of the bean. Use the below square to represent a whole bean, water can only reach this bean on red line area. Of course, water will slowly penetrate to inside, but when water just get into the center, the outside has already contact with water for a long time. As we mentioned in last article, the longer coffee bean contact with water, the more solubles will be dissolved. Due to the time difference, the solubles in center were just started to be dissolved while all the solubles in outside were all in in water. That’s why the taste will be bad and inconsistent.

To solve this problem, we try to cut coffee bean into pieces. Still the same square (coffee bean), if we cut it into four pieces, the contact area (length of red line) will increase. We grind coffee beans in order to make the brewing process more efficient and controllable.

OK, now we can talk about grind size. We saw “grind size” when people sharing their recipes: “grind size at Comandante 28 clicks”, “grind medium-coarse”; or when people giving others brewing advise: “if you feel the flow rate too fast, grind finer”. Why people always mention grind size? In my opinion, Adjusting grind size is actually changing the contact time of water and coffee bean. Once again, we go back the square, if other elements remain the same, the time that needed to run through whole red line in the left (coarse grind) will less than the right (fine grind) cause the line is longer. Meaning finer grind has longer contact time with water, while coarser grind has shorter if in the exact same situation. And remember the relationship between solubles and water contact time? That’s why we usually think of grind finer when we want to increase the extraction yield (dissolve more solubles).

That being said, changing grind size is not the only way to adjust water contact time. But compares to methods such as “slower your pour” and “multiple pours”, it has a clear standard and more replicable, that's why people use it more often.

Other articles in this series:

Brewing basic concepts: What are we actually doing in coffee brewing?

Brewing basic concepts: Working principle of brewers