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How Grind Size Affects Espresso Extraction

Jan 7

Espresso should not be too fine. Espresso has always been a mysterious process. Even the most skilled baristas sometimes make mistakes. If you use a super-automatic it's even worse.

One thing that is consistent though, is the espresso grind size. You need the right grind size to ensure a perfect shot.

Espresso Extraction

About 28% of roasted coffee beans is water-soluble. This means that about 28% can be extracted from the entire roasted coffee bean. The rest is cellulose, plant matter and other materials that form the coffee bean’s structure.

Water needs help to dissolve soluble chemicals. If you throw coffee beans in hot water, they only dissolve the outside layer. Because the structure of coffee beans is so dense and complex, water can't get through it easily. The water that passes through the coffee bean collects all the flavor.

You need to increase the coffee's surface area in order to make it taste better. This will leave gaps in the beans that allow water to penetrate all the flavors. The surface area of coffee beans can be increased by grinding them. The more surface area, the quicker it reacts to water.

Water always extracts flavor components in this order regardless how the method is used: fats and acids first, then sugars and finally the plant fibers.

Acids, and fats, are the first compounds taken from coffee. Acids give coffee a bitter taste. It is easy to dissolve these compounds into coffee. This is when many of the light aromas, such as the floral and fruity flavors, are extracted. It's the final cup that gives coffee its flavor.

It is possible for coffee to have different flavors. Therefore, we have to control extraction and stop it as soon as the bitter compounds begin to break down. We do not want all of the soluble matter to be in our cup. Many of these compounds are not desirable, so we try to avoid extracting them.

Chemistry is helpful because most bitter compounds can be hard to extract so we need to stop extracting too soon.

But if we don’t stop coffee extraction in time, the result is a coffee that has been overextracted.

Under Extraction

Under-extraction is when the coffee doesn't contain enough soluble substances. A lot of the flavors that bring balance to your shot are left unextracted from the grounds. And because acids are the compounds that extract the fastest, an under-extracted shot can taste sour, weirdly salty and without sweetness.

Extraction is directly related to strength. To get a strong cup of coffee, you can reduce the amount of water you use. While this may be possible, it's not the best. It is more difficult to extract all the flavors from coffee the more you extract it. The brew saturates. What is more important is that compounds in coffee have different saturation points , so we can extract more of them during brewing. It is because we don't want to brew coffee at the right strength that it tastes bad.

Espresso extraction is directly influenced by the grind size. This is the most important variable in the espresso brewing.

It's fascinating to note that scientists, baristas and roasters studied coffee extraction and found that too fine a grinding won't produce the best tasting cup.

The Grind Size (and Extraction)

A pressure pump is used to force water through the "puck" of ground espresso. This produces thick, concentrated coffee.

A very popular recipe for espresso is extra-fine grind settings around 20 grams to brew a single shot of espresso. The reason is to increase the coffee's surface area to water. This should lead to a greater extraction yield. The amount of soluble liquids that dissolve in the final beverage is called extraction yield.

Surface Area: How Grinder Size Influences

An experiment by Christopher Hendon, a computational chemist and a competitor barista, revealed that coffee shops strive for extraction yields between 17 and 23 percent. Lower extraction yields taste sour, while higher yields are too bitter.

The team brewed thousands and thousands of espresso shots before developing a mathematical model that could pinpoint the variables necessary to ensure consistent yield. They found that coffee that is too finely ground can result in too much extraction.

Coffee that is too finely ground will not be brewed. Coffee grounds too fine will prevent water from passing through them. The puck is too tight and water cannot pass through the coffee grounds.

The size of the coffee particles is part of the problem. You can compare sand to rocks. The same amount of sand and rocks is equal in weight. The rocks will absorb water if you pour it on. If you pour the same quantity over the sand, it will take a bit of time to pass through the layer of sand.

Tampering is also a problem. Tamping finely ground coffee will allow you to pack it better so that the coffee puck is compact. This reduces the flow of coffee if you tamp your beans too hard.

Researchers discovered that a coarser coffee grind and a lower amount of ground coffee per cup is better. This results in a more full and even brewing process.

The Other Extreme

But, finer coffee can be just as troublesome as coarse coffee. The only thing you need to adjust is the grind size. These changes are not noticeable to the naked eye.

Let's consider an extreme example. An espresso shot made with a medium-ground coffee will yield a 3 second pour. This would not extract the acids, as it would pour too quickly. Your coffee will be extremely under-extracted.

Espresso Variables (and Extraction)

The roast degree can have an effect on extraction, but it is not a determining factor. It will extract the same coffee bean more efficiently if it is roasted darkly than if it is roasted lighter.

Double shots of coffee should weigh between 14 to 21 grams. The best results are achieved when the quantity is within one gram.

Tamping can alter the flow rate and therefore the amount of coffee that is extracted.

Fines from a grinder are good as they clog your puck and increase flow. They allow water to contact the coffee grounds for 20 seconds. Too much finesse can clog the puck and cause the shot to not flow.

Don't be too strict

Don't let your creativity get in the way of coffee brewing.

One of the beautiful things about the industry, and why people like coffee so much, is because there's a human component you can't remove. While it is important that we can make decisions about flavor, the scientific component of coffee allows us also to make decisions for improving our coffee. Creativity and personal taste are equally important.