How Grind Size Affects Espresso Extraction

Espresso grind size should be fine, but too fine is not better. Making espresso has always been a mystery to some degree. Even the most skilled baristas occasionally make mistakes. If you use a super-automatic it’s even worse.

One thing is constant though, the grind size for espresso. In order to nail that perfect shot that retains some sweetness, and is not overly bitter, you need to get the perfect grind size. A good coffee grinder  to deliver consistent grind size, and some tweaking, is the key to a perfect espresso. Contrary to a popular belief, a finer grind size doesn’t necessarily produce a better shot. Using the wrong grind size is probably the most common problem among home baristas.

pulling espresso shot with gaggia carezza
Pulling espresso shot with Gaggia Carezza

Espresso Extraction

Roasted coffee beans are about 28% water-soluble. That means that out of the entire roasted coffee bean, you can extract about 28%. The rest is mostly cellulose and plant stuff which makes up 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. The coffee bean’s structure is very dense and complex so water can’t pass through easily. All the flavor is collected by the water on its way through.

To make coffee taste better, you need to increase the surface area of the beans. Doing this will create gaps that allow water to permeate all the flavor. We can increase the surface area of coffee beans by grinding the beans. The more surface area, the quicker it reacts to water.

Soluble Solids Extraction Order

Water always extracts flavor compounds in this order regardless of the method: fats and acids, then sugars, and finally the plant fibers.

The first compounds extracted from coffee are acids and fats. Acids, which give coffee a sour taste, are the simplest compounds. This means that water is easy to dissolve them into the coffee. Many of the light aromatics, for instance the the floral an d the fruity flavors are extracted at this moment. Acids and light flavors are very important in our final cup, it’s what give coffee its flavor.

Almost at the same time, we extract the coffee fats. The oils in coffee add body to your cup. Fats are are hydrophobic and they wash out of the ground coffee pretty easily. They are an important component in an espresso, unlike filter coffee. Without fats, some of the heavier aromatics will lack. Without coffee oils there would be no crema.

Sugars are extracted next. Water needs more time and energy to fully dissolve them. In an espresso, these sugars are what give sweetness to your cup. This is what creates the classic espresso flavor.

Finally the plant fibers that hold the ground coffee together will start to break down. These fibers taste dry and bitter, and we want to avoid them. A little bit of them give our cup some bite, but too much of these and our coffee will be ruined.

Over Extraction

From an efficiency perspective, if we would grind the coffee into a fine powder and add hot water it will dissolve all of its delicious flavors. Unfortunately, this wouldn’t work. It would give us a bitter cup of coffee. Over-extracted coffee is bitter and overpoweringly strong, with no complex flavors.

As we just showed earlier, not all of the coffee’s flavors are good, so we have to control the extraction and stop it just before the bitter compounds start to break down. We do not want all of the 28% of soluble matter to go into our cup. A lot of those compounds are undesirable, and we want to avoid extracting those.

Fortunately, chemistry works with us  on this, because most of the bitter compounds are harder to extract, so if we stop extraction in time, we only get the good stuff.

However, if we don’t stop the extraction in time, we obtain an over-extracted cup of coffee.

Under Extraction

If you don’t extract enough soluble solids from the ground coffee, the result is a cup that is under-extracted. 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.

pulling under extracted espresso shot
Pulling under extracted espresso shot. Note the very fast pouring and lack of crema.

A shot of espresso is defined by the quality of the extraction, the strength of the shot is equally important. I don’t mean caffeine content, but rather the amount of dissolved solids in the drink.

Coffee strength depends on the ratio of ground coffee to brew water. Too little water will make your coffee taste muddy. Too much water will make your coffee feel thin and watery.

Strength is in a direct relationship to extraction. If you want a very strong coffee, you can use less water to increase the strength of the cup. Not the best idea, though possible. The more coffee you extract, the more difficult it is to extract out all of the good flavors. 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. That’s why when we brew a drip coffee to the espresso strength it tastes bad.

Different coffee brewing methods will impact how your coffee tastes because of the different brew variables. (See the espresso vs drip coffee strength reference above.) Since extraction time is the shortest of all brewing methods, it tends to favor extraction of the most soluble compounds, avoiding extraction of the bitter ones, compared to other extraction types. This why the espresso recipe calls for a 20 to 25 seconds extraction time.

One of the drawbacks of the espresso as a coffee beverage, is that the strength of your coffee will mute more delicate flavors. The stronger a drink is, the more difficult it will be to distinguish individual flavors. Delicate floral and fruity flavors are many times overwhelmed by sugars and oils in espresso.

What is interesting is that a team of baristas, roasters and scientists studied espresso extraction, and they found that grinding too fine will not produce the best taste.

The Grind Size and Extraction

An espresso machine relies on a pressure pump to force water through a “puck” of ground coffee. This produces a thick and 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. In turn this should increase extraction yield. Extraction yield measures the amount of soluble solids that dissolve and ends up in the final beverage.

How Grind Size Affects Surface Area

Imagine that you have one coffee particle and you cut it in half. The total amount of coffee remains the same, but the water has access to a lot more surface area inside that particle. As more coffee particles are divided into smaller pieces, more surface area will be exposed. A smaller coffee particle can saturate faster with water, speeding up the extraction time. In conclusion, regardless of the brewing method, finer coffee grounds will extract faster than coarser grounds.

While this industry practice may sound good in theory, the science found that it might not be the best approach for the flavor.

A study from the University of Oregon led by Christopher Hendon, a computational chemist, and a competitive barista showed that most coffee shops aim for an extraction yield between 17 to 23 percent. Lower extraction yields taste sour, while higher yields are too bitter.

The team brewed thousands of espresso shots and developed a mathematical model to pinpoint the variables required to get consistent yield. They discovered that when coffee is ground too fine, the flow is sometimes too restricted and the shot is over-extracted.

If you ever ground your coffee too fine, you know this. Water just doesn’t pass through the coffee grinds, if the grounds are too fine. The puck is too compact and water cannot pass through the tightly packed coffee grounds.

Part of the problem is the coffee particle size. A good analogy is the comparison between sand and rocks. You have the same quantity by weight. If you pour some water on the rocks, water will instantaneously go through. If you pour the same quantity over the sand, it will take a bit of time to pass through the layer of sand.

The other part of the problem is the tamping. When you tamp very finely ground coffee, you can pack it better, so the coffee puck is more compact. This restricts the flow even further, if you tamp too hard.

Finally, the fines are another variable. Any grinder produces some fines, and this is a good thing. The fines are clogging the puck, and create flow restriction. We want flow restriction so that water is in contact with the grounds for at least 20 seconds. But too much fines could clog the puck too much, and the shot will just not flow at all. Clogging is random and hard to predict.

The research team found out that using a slightly coarser grind and reducing the amount of ground coffee per shot is better. This leaves some extra room in the coffee bed, leading to a fuller, more even brewing process.

The extraction improvement will make up for the missing 3-4 grams. The extraction yield is practically the same, and even the caffeine content is the same. The only thing that changes is the flavor profile.

This change though, could potentially lead to an under-extracted coffee. The shout will pour too fast and coffee will be weak and sour. If that’s the case, just grind a bit finer.

The Other Extreme

However, coarser coffee is just as problematic as finer coffee. You only need very slight adjustments in grind size, these changes are unnoticeable to the naked eye.

Let’s take an extreme example: If you use for an espresso shot a medium grind, what is typically used for a drip coffee, your espresso will pour in 3 seconds. This would be way too fast, and it would only extract the acids. Your coffee will be extremely under-extracted.

Other Variables that Influence Extraction

Coffee Roast and Extraction

All things equal, roast degree will have as well an impact on the extraction. The same coffee bean will extract easier if it’s roasted dark, compared to a lighter roast. Darker roasted coffee is more soluble.

Dark roast coffee is less dense & has a lower moisture content since it’s been roasted for longer. That means you need to grind your beans coarser to achieve the same level of extraction.

Light roast coffee is denser and has a higher moisture content than dark roast, so it’s more difficult to extract solubles. A finer grind can help with this issue. (You will need to adjust the dose though, to compensate for the more compact puck.)

Coffee Dose and Extraction

For a double shot, you should use between 14 and 21g of ground coffee; The manufacturer usually marks the recommended quantity on the side of the filter-basket. Try to stay within 1g of that number for best results.

If your manufacturer didn’t mark that, experiment with a dose of 14 to 21 grams for a double basket.

Espresso is brewed at a 1:2 ratio. This means that a dose of 20g of ground coffee should brew 40g of coffee.

Tamping

Tamping will also affect the flow rate, and consequently the the extraction yield. However, it’s one of the variables that affect your shot the least. Tamping harder will restrict the flow a little, and water some extra time in the coffee bed. Unless you are not tamping at all, or you tamp extra hard, you shouldn’t worry too much about it.

Don’t Be Too Strict

Make sure their goal is not to take the creativity out 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. The scientific component that allows us to make decisions about flavor is important, and it allows us to make decisions to improve our coffee, but creativity and personal taste are equally important.

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Dorian Bodnariuc

My name is Dorian and I am a former barista. I consume coffee in any form, as a beverage, in savory recipes and desserts. My favorite caffeinated beverage is the espresso. I love to share my coffee brewing knowledge and my geeky coffee research. This blog is one of the places I write about coffee. More about Dorian... If you want to learn more about this site and how I started it, check our About Me page, where I explain all about it.