What's the Difference in Coffee Grind Sizes? How Fine Should Espresso Be? How Coarse Should Pour-Over Be?

The Science of Coffee Grinding: Unlocking Flavor Through Extraction

Extraction is the process of using water as a medium to dissolve flavor compounds out of coffee beans. Before extraction can happen, we must first grind the beans, because grinding lets the flavor compounds dissolve more quickly and easily. That is what we commonly call coffee grinding.

Coffee grinding usually takes priority over other extraction parameters because, as FrontStreet Coffee said at the start, the coarseness of the grind determines how quickly flavor compounds dissolve. Research shows that when coffee beans are ground into powder, their surface area increases roughly 1,000 times compared with their original state. The finer the grind, the greater the increase in surface area, and the shorter the linear distance from the particle surface to its center (affected by burr design). Conversely, coarser grinds produce less increase in surface area and longer distances from surface to center.

The increase in surface area gives the grounds more contact area with water, while the distance to the center determines how long water takes to penetrate into the interior of the particles. So when coffee is ground finer, it has more surface area and a shorter distance from surface to interior, and hot water can dissolve flavor compounds more quickly - and vice versa. That is why, whatever the extraction method, coffee grinding usually has the highest priority. We need to find the right grind size before we can brew a good cup.

Matching Grind Sizes to Different Brewing Methods

So what grind sizes correspond to different extraction methods? This is surely a question many people want answered. Because everyone uses different grinders and beans, though, and because grind sizes cannot be standardized, we can generally only share ranges in broad terms - coarse-sugar grind, fine-sugar grind, flour-like grind, and so on.

For which grind to use with which method, refer to the detailed article "Coffee Grinding Reference." Alternatively, we can calculate the grind coarseness directly from the average extraction time of that method (which, of course, requires a reference point). The longer the required extraction time, the relatively coarser the grind; the shorter the required extraction time, the relatively finer the grind. Take espresso: we need to extract the coffee in about 30 seconds (an example, not an absolute), so the particle size has to be very, very fine - only then can hot water dissolve all the flavor compounds from the grounds in such a short time.

Because espresso extraction is pressurized and involves many factors, though, we can look at it from another angle, such as pour-over coffee and siphon brewing. The average extraction time for pour-over is about 2 minutes, while siphon brewing stays within 1 minute. Siphon therefore requires a finer grind than pour-over. Following this logic, we can work out the grind size needed for different methods. (Turkish coffee is an exception.) As for why coffee grinding can't be standardized, the reason is simple: the particles we grind are not absolutely uniform in size, a point FrontStreet Coffee often makes.

The Importance of Grind Uniformity

Why aren't ground particles absolutely uniform? Whatever the quality of the grinder, the resulting grounds are never absolutely uniform - there are both large and small particles. This relates to the characteristics of the beans and to the burrs and motor of the grinder. It's precisely because they aren't absolutely uniform that the coffee we brew can have more complexity. So we judge the quality of a grind by its uniformity: higher uniformity means better grinding quality, and vice versa. (The image below shows 15g of coffee ground with an Ek43, with particles arranged from large to medium to fine from left to right.)

Whatever grinder or burrs are used, the grinding process is always accompanied by crushing. Crushing promotes the generation of fine particles: the more crushing and the longer the grinding time, the more fines are produced (different burrs also produce different particle shapes). An appropriate amount of fine particles helps increase complexity, but it's a double-edged sword. Too many fines can easily lead to extraction problems such as clogging and uneven extraction. So highly uniform grounds are crucial for brewing - ideally, fine particles should account for 3%-5% of the total grounds.

How Coffee Bean Characteristics Affect Grinding

The characteristics of the beans also affect grinding. Anyone who has used a manual grinder knows this well. Grinding beans with different characteristics feels completely different - some beans are difficult to grind, while others grind quickly with just a few turns. This is mainly influenced by the roast level and density of the beans.

When beans are roasted darker, their density becomes lower, while their brittleness increases, making them easier to grind - like a cookie we can snap in two with a gentle break. When beans are roasted lighter, their density is higher and their brittleness lower, making grinding more difficult. We should note, though, that when beans are more brittle, they more easily produce fine particles. To use the cookie example again: after breaking a cookie, besides the two intended pieces, don't we also get many small crumbs? That's the cookie's brittleness at work. The same principle applies to grinding coffee. At the same grind setting, more brittle beans produce more fine particles, resulting in an overall finer grind than less brittle beans.

Besides roast level, the processing method, growing altitude, and variety of the beans all affect density. When beans have higher density, we need a finer grind, because denser beans are harder to extract. For lower-density beans, determine the grind from the roast level and processing method. Darker roasted beans need a coarser grind to prevent over-extraction from excessive extraction efficiency. If the lower density comes from the processing method, the grind adjustment can be decided by the brewing state and the coffee's taste.

FrontStreet Coffee Recommendation

Because grind size is really a conversation with roast level and density, it helps to have one light roast and one darker roast on hand. FrontStreet Coffee's Ethiopia Yirgacheffe Gudingding, a washed Yirgacheffe, is the light, dense end of the spectrum: citrus, berries, and florals come through with a soft, bright acidity and a green-tea finish, and because denser beans resist extraction, it rewards the finer setting the article describes.

For the other end, take FrontStreet Coffee's PWN Indonesian Golden Mandheling. Roasted to a medium-dark level, it is lower in density and easier to extract, so it wants the coarser setting. It drinks low in acidity and full in body, with herbal and dark chocolate aroma and a faint earthy note, clean and mellow - a useful contrast that makes the point of matching grind to roast very concrete.