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How Grind Size Affects Espresso Extraction

Jan 5

Espresso grind size should not be too fine. It's been a mystery how to make espresso for a long time. Even the most skilled baristas have made mistakes. It gets worse if you're using a super automatic.

One thing is certain, however: the espresso grind size. You need the right grind size to ensure a perfect shot.

Espresso Extraction

Water-soluble coffee beans contain around 28%. You can extract around 28% of the whole roasted coffee beans. The rest is mostly cellulose and plant stuff which makes up the coffee bean's structure.

Water needs help to dissolve soluble chemicals. When you boil coffee beans, the outside layer is not dissolved. Coffee beans are very dense and complicated, so water cannot pass through them 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. By grinding coffee beans, we can increase the coffee bean's surface area. The faster it reacts with water, the more surface area.

Water always extracts flavor compounds according to this order, regardless what method it uses: fats and acid, then sugars, then finally the plant fibres.

Acids (and fats) are the first substances to be extracted form coffee. Acids give coffee a bitter taste. It is therefore easy to dissolve these compounds in the coffee with water. At this point, many light aromas such as the fruity and floral flavors can be extracted. Our final cup should contain both acidic and light flavors. This is what gives coffee its flavor.

The coffee may not have all the right flavors so we must control the extraction to stop the bitter compounds from forming. We do not want all soluble matter to go into our cup. We do not want many of those chemicals to go into our cups.

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

However, if the extraction is not stopped in time, then we get an over-extracted cup.

Subtraction

A cup that doesn't have enough soluble coffee solids will result in a cup that is too extracted. Many of the flavors that add balance to your shot are not extracted 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 with strength. To get a strong cup of coffee, you can reduce the amount of water you use. Not the best idea, though possible. The more coffee you extract, it is harder to extract all of its good flavors. The brew may contain 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. Grind size is the most critical variable in espresso brewing.

Interesting is the fact that a group made up of scientists, coffee roasters, and baristas studied coffee extraction. They discovered that finer grinding doesn't give you the most flavorful cups.

The Grind Size (and Extraction)

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

Extra-fine grind settings of around 20 grams are a popular way to make espresso. This allows you to brew one shot. The reason is to increase the coffee's surface area to water. This should lead to a greater extraction yield. Extraction yield measures the amount of soluble solids that dissolve and ends up in the final beverage.

Surface Area and Grind Size

A University of Oregon study by Christopher Hendon was conducted with a barista and computational chemist. It showed that coffee shops tend to aim for extraction rates between 17 percent and 23 percent. A lower extraction yield is more bitter than a higher one.

They brewed thousands upon thousands of espresso shots, and then developed a mathematical model that would pinpoint the variables needed to achieve 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. The coffee grounds that are too fine won't allow water to pass through. Water cannot pass through tightly packed coffee grounds because the puck is too small.

Coffee particle size is a major problem. One good example is the comparison between rocks and sand. You have the same quantity by weight. Water will flow through rocks if you put water on them. It may take a bit longer for water to flow through the layer made of sand when you pour the same amount.

Tampering is also a problem. You can pack finely ground coffee better and the puck will be more compact if you tamp it. This can also reduce the flow if you tamp it too hard.

The research team found out that using a slightly coarser grind and reducing the amount of ground coffee per shot is better. This results in a more full and even brewing process.

The Other Extreme

Finer coffee can also be problematic. You only need very slight adjustments in grind size, these changes are unnoticeable to the naked eye.

Let's use an extreme example. A medium grind is usually used for a drip espresso. Your espresso will pour in about 3 seconds. This would make it impossible to extract the acids. This will cause your coffee to be very under-extracted.

Espresso Variables, and Espresso Extraction

Everything else being equal, the roast degree will also have an impact on the extraction. It'll extract coffee beans more efficiently if they are roasted at a darker roast than if they are roasted at a lighter temperature.

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

Tamping will affect the flow rate of your coffee, which in turn impacts how much of the ground coffee is extracted.

Fines from a grinder can be beneficial as they can clog pucks and improve flow. The coffee grounds and water have a contact time of 20 seconds. However, too much finesse could cause the puck to clog and the shot won't flow.

Don't be too rigid

Coffee brewing is a creative process.

The beauty of coffee and the reason people love it so much is that you can't get rid of the human component. 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.