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How Grind Size Affects Espresso Extraction

Jan 6

Espresso should not be too fine. Making espresso has always been a mystery to some degree. Even the most skilled baristas occasionally make mistakes. It's worse if you use a superautomatic.

One thing remains constant is the grind size of espresso. To get that perfect shot, which retains sweetness but isn't too bitter, you must grind the beans to the perfect size.

Espresso Extraction

Coffee beans are approximately 28% water-soluble. This means that you can extract approximately 28% from a whole 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.

The bean's surface area must be increased to make coffee more flavorful. This will allow water to seep through the beans, allowing for all the flavor. The surface area of coffee beans can be increased by grinding them. 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, which give coffee a bitter taste, are the easiest compounds to extract. It is easy to dissolve these compounds into coffee. Many of the light aromatics, for instance the the floral an d the fruity flavors are extracted at this moment. Coffee's flavor is derived from the acidity and light flavors in its final cup.

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 material to make it into our cups. Many of these compounds are not desirable, so we try to avoid extracting them.

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.

Below Extraction

You will get a cup with too little soluble solids in the ground coffee. You can leave a lot of flavors in the coffee grounds that are essential for balance. Acids are the most efficient extractors of compounds, so a shot that is under-extracted can taste strangely salty, bitter, and devoid of sweetness.

Extraction is directly related to 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, the more difficult it is to extract out all of the good flavors. The brew saturates. It is important to note that different saturation levels of compounds in coffee can be used to extract more. This is why a coffee that has been brewed to espresso strength tastes terrible.

Espresso extraction is affected by the grind size. This variable is crucial in 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

An espresso machine is powered by a pressure pumps to force water through a cup of ground coffee. This produces a thick and concentrated coffee.

Extra-fine grind settings, around 20g, are very popular for making espresso. It makes one shot of espresso. This is because it increases the coffee's surface area relative to water. This will increase the extraction yield. The extraction yield is the percentage of soluble solids that are dissolved and end up in the final beverage.

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. Higher extraction yields can taste bitter, but lower yields will make you feel sour.

The team brewed thousands and thousands of espresso shots before developing a mathematical model that could pinpoint the variables necessary to ensure consistent yield. The team discovered that coffee ground too fine can cause a restricted flow and over-extracted shots.

If you ever ground your coffee too fine, you know this. If the grounds are too fine, water won't pass through. The puck is too tight and water cannot pass through the coffee grounds.

The problem lies in the size of coffee particles. The comparison of rocks and sand is a good example. You have the same weight. The rocks will absorb water if you pour it on. It will take some time for the water to get through the layer of sand if you pour the same amount of water over the rocks.

The other problem is the tamping. The best way to compact coffee is to tamp finely ground coffee. This restricts the flow even further, if you tamp too hard.

Research team discovered that a coarser grind and less coffee per shot are better. This gives you more space in your coffee bed which leads to a better brewing process.

The Other Extreme

However, coarser coffee is just as problematic as finer coffee. These adjustments can only be made to the grind size.

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. It will result in a coffee that is extremely under-extracted.

Espresso Variables (and Extraction)

Roast degree will affect the extraction of coffee beans in all cases. It'll extract coffee beans more efficiently if they are roasted at a darker roast than if they are roasted at a lighter temperature.

A double dose of coffee should not exceed 14 grams. For best results, try to keep the measurement within one gram of the number on the container.

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

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

Don't be too strict

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. It is the scientific component that allows us make decisions about flavor. We can use it to improve our coffee. But creativity and personal taste are equally important.