If you are interested in perfecting your espresso making skills and technique, you need to understand the science behind it. The science behind espresso brewing is called extraction.
Coffee extraction is the process of dissolving ground coffee in water. This chemistry concept is important in coffee brewing because it determines how good your coffee is.
Espresso brewing is particularly interesting, because the espresso extraction is more complex than other brewing methods. Pressure during extraction changes the extraction, creating the unique flavor profile of espresso.
In this article we are going to explore how different brewing variables affect extraction, and how will these affect flavor in the cup. If all you need is an espresso brewing tutorial, you can read our article on “How to Make Espresso Using an Espresso Machine”. That article is a great start for anybody that wants to start making espresso at home.
However, if you are a perfectionist, or you are having trouble getting that perfect shot, you might want to dive a little deeper in this guide, and understand how every little change you make to a coffee recipe affects your coffee.
- Espresso Brewing Variables
- What Is Extraction?
- Over-extraction and Under-extraction
- Grind Size and Extraction
- Espresso Extraction and Coffee Brewing Temperature
- Brew Time
- Roast Level and Extraction
- How Pressure Improves Espresso Extraction?
- Cold Brew Coffee Brews Faster In Denver
Espresso Brewing Variables
The most important factors that affect the extraction rate are: brew temperature, exposure time, (brew time), amount of coffee, (dose), and the particle size, (grind size). In espresso we also have pressure, which is unique to to this brewing method.
These brewing variables are all interdependent, and together they are responsible for the perfect extraction. When one variable is changed, everything else is affected.
IMPORTANT: Follow a recipe that produces coffee that you like, master this recipe first, and only the start tweaking it to improve it. If you start a recipe from scratch, you will have too many things to adjust until you get it right.
Brewing Variables that We don’t Touch
Well, if they are variables we should be able to tweak them, right? Yes and no. If you are a mad scientist that wants to create a new way to brew espresso, yes. Tweak all the variables you want. But if you want to get a great espresso with the least effort, then stick to what’s important.
From practical perspective, pressure, tamping force, and dose are fixed when playing with brewing variables. The extraction time is something that can be easily adjusted, but you should not play with it, and you’ll see why in a bit.
Pressure is normalized at 9 bars on the vast majority of espresso machines. There is no way barista can change this on a normal espresso machine.
The dose is dictated by the filter-basket size. While small 0.5 to 1 gram adjustments are possible, it is not recommended because the space between the shower and the coffee puck, (head space), determines the extraction.
Tamping force is something many experienced baristas play with. While is not inherently bad to play with, I found that sticking to the same tamping pressure makes it easier to perfect your shots.
The last factor that we should not play with is extraction time. There are too many bad shots pulled because of the extraction time. The perfect extraction time for an espresso is 20 to 25 seconds. No matter if it’s a lungo, a doppio, a ristretto, or a triple shot.
The barista should tweak all other factors in order to get a shot pulled for 25 seconds.
The problem is that the inexperienced barista will try to correct a badly configured shot by adjusting the extraction time. The most common example is increasing the extraction time to compensate for a too restricted shot.
If a shot is too restricted, we should adjust the grind size, and not the extraction time.
Brewing Variables that We Play with
The two most important variables in espresso brewing are the grind size and the water temperature.
I know that some baristas will disagree on the brew temperature, but stick with me and you’ll see why sometimes you need to change the brew temperature, for amazing shots.
OK, we need to mention roast degree and origin. You will have to take these in consideration when pulling your shots. Roast level is in fact so important that will decide the two other brew variables that you can play with: grind size and water temperature.
Before explaining how temperature and grind size affect extraction, lets review the coffee extraction definition.
What Is Extraction?
Extraction is the process that pulls flavors from coffee grounds into the water. Extraction is affected by many factors, including brew time, water temperature, grind size, roast level. Coffee extraction is based on the chemical property of coffee solubility.
A lot of the compounds in coffee are soluble in water. It’s like sugar or table salt. You put them in water and they will dissolve. However, these compounds dissolve faster or slower in water. Some of these compounds only dissolve at high temperatures, and some are volatile, so they dissipate at higher temperatures.
Coffee beans are about 28% water-soluble. This is how much you can get out of the whole roasted coffee bean into your cup. The rest of the coffee bean’s structure is made of cellulose and plant stuff. This doesn’t mean we want to extract all of the 28%, some of that stuff tastes nasty.
Espresso extraction is a process that is optimized to dissolve in water the maximum amount of desirable compounds, while minimizing the amount of undesirable elements.
Here is a list with most of the brew variables that affect extraction.
- Grind size
- Brew temperature
- Tamping force
- Dose, (amount of coffee grounds used per brewed cup)
- Coffee beans roast level
- Coffee beans origin
- Extraction time, (how long are coffee beans in contact with water)
Over-extraction and Under-extraction
As a reference system, coffee specialists have created a system that evaluates a coffee cup by the amount of desirable and undesirable compounds extracted. This system qualifies a cup of coffee as perfectly extracted, under-extracted, or over-extracted.
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. You can tell if coffee is over-extracted or under-extracted mainly by taste.
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 floral and 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.
Extractions Yield vs. Total Dissolved Solids (TDS)
Extraction yield refers to the soluble solids drawn from the coffee beans, whereas TDS refers to the concentration of these coffee solubles in the final brewed coffee.
We can see the extraction yield as dissolution efficiency, and TDS as the quantity of solvent.
A ristretto has a lower extraction yield, but a higher TDS than a lungo. So lungo brewing recipe is more efficient at extracting the good stuff from the beans than ristretto recipe. However, lungo will also extract more bitter compounds than ristretto.
One or more of these following could cause over-extraction:
- brew time is too long,
- grind size is too fine,
- brew temperature is too high.
However, some of these factors will affect over-extraction more than the others.
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, but interestingly, not for the reasons you would expect…
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.
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 in the cup, and the bad tasting stuff stays in the grounds.
Because the bitter compounds get extracted later in the brewing process, we need to stop pulling the shot within 25 seconds so we do not over-extract it.
Coffee is under-extracted if:
- the grind is too coarse,
- brew time is too short,
- the brew temperature is too low,
- or a combination of the above.
If you don’t extract enough soluble solids from the ground coffee, the result is a cup that is under-extracted. The flavors that bring balance to your shot are left in the coffee grounds. Acids from coffee beans are the compounds that extract the first, so a an under-extracted shot will taste sour, maybe salty and without sweetness.
Espresso has the shortest extraction time of all brewing methods. Because of that it favors the extraction of the most soluble compounds, avoiding extraction of the bitter ones. This why the espresso recipe calls for a 20 to 25 seconds extraction time.
The short brewing time is compensated by the pressure, which introduces entropy during extraction. Entropy, (molecule agitation), speeds up dissolution, hence the short brew time.
However, the 20 seconds extraction time needs to be doubled by an adequate brew temperature. The best way to calibrate for a 20 seconds extraction is by adjusting the grind size – dialing in the espresso.
If the shot pours faster than 20 seconds it will be under-extracted. At this point you will need to grind finer. If the shot pours slower than 25 seconds it will likely be over-extracted, hence you will need to grind coarser.
We define a shot as 1 fluid ounce of espresso for each 7-9 grams of ground coffee. For a doppio the shot is 2 fluid ounces for each dose of 14-19 grams of coffee grounds. The extraction time remains the same for the different doses – 20 to 25 seconds.
Extraction and Coffee Strength
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. It’s not the best idea, though it is possible.
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 often overwhelmed by sugars and oils in espresso.
Coffee Strength/Concentration vs Over-Extraction
Some home baristas confuse coffee strength with over-extraction. If a coffee is too strong, just add a bit of water and that should correct it on the spot. On the other hand, an over-extracted cup will taste bitter even if it is diluted with water.
Over extraction is a bit contextual, and even subjective. This is why I am always cautious when I use the word. What we perceive as properly extracted in an espresso, qualifies as over extracted in drip coffee.
Some people love their coffee stronger than others. But when we brew a stronger cup, we just do that – brew a stronger cup. We don’t over extract that cup, we just extract a bit more stuff from the grounds.
Did we extract too much tannins from the grounds? Then yes we did over extracted it.
Fortunately, espresso is simpler to judge than drip coffee or French press. But when preparing a lungo or a ristretto, we need to make sure we time the shot within the 20 – 25 seconds in order to avoid over, or under extraction. This is why the lungo you get in a coffee shop is either short, or over-extracted. They cannot adjust grind size. The grinder is calibrated for a a normale, or for a ristretto, depending on the shop.
Grind Size and Extraction
Grind size is perhaps the most commonly discussed factor when it comes to coffee extraction. When brewing espresso, the size of the grind and the amount of coffee, or the dose, in the portafilter are the two components which are adjusted on a regular basis in order to achieve a balanced extraction. Adjusting grind in espresso is also known as “dialing in” 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 burr coffee grinder to deliver consistent grind size, and some tweaking, are 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 inexperienced baristas.
Grind size affects the extraction in two ways. Firstly, it adjusts the surface area, with finer grinds having more contact with the water. Secondly, it adjusts the pressure in the coffee puck, with finer grounds creating more pressure in the coffee bed during extraction. There is also the amount of fines, which is higher for finer grounds.
How Grind Size Affects the Pressure
An espresso machine relies on a pressure pump to force water through a “puck” of ground coffee. As we saw earlier, the pressure introduces entropy and this speeds up dissolution.
The espresso machine can push the water with a pressure of up to 9 bars. But if your coffee puck is ground too coarse, water will pass through very fast, and the pressure inside the puck won’t be nowhere near that. If the coffee is too finely ground, the machine won’t be able to push the water through.
If you ever ground your coffee too fine, you know that water just doesn’t pass through the coffee grinds.
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.
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.
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. This is called a ristretto.
While this industry practice may sound good in theory, it might not be the best approach for the flavor. When coffee is ground too fine, the flow is sometimes too restricted and the shot is over-extracted. When the coffee puck is so tightly packed, we risk channeling, and this is even worse than a little over-extraction.
Grind Size and Tamping
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.
Grind Size and Fines
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.
Finer Grind Makes Stronger Coffee
Espresso brewing calls for a finer grind compared to other brewing methods such as filter coffee or French press. Espresso coffee is stronger than other methods. Part of the reason is the finer grind size, but let’s not forget that espresso is brewed as a small beverage. We cannot compare a 6 fl oz, drink with a 2 fl oz drink.
Coffee is ground finer in order to increase the surface area of the bean that is in contact with water during extraction. This increased contact causes greater extraction. Therefore, if coffee is ground finer and all other brewing parameters are not adjusted, finer ground coffee will extract more coffee properties to be dissolved into the water, resulting in a beverage with a greater TDS which is therefore stronger. However, if more water is added to the recipe, a coffee that is brewed using finely ground beans would have a lower level of TDS and would be a weaker tasting product.
What Is the Perfect Grind Size?
Using a slightly coarser grind and reducing the amount of ground coffee per shot leaves some extra room in the coffee bed, called headspace by baristas. In turn, this technique produces 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.
You will have to be careful though, as this change could potentially lead to an under-extracted coffee. The shot will pour too fast and coffee will be weak and sour. If that’s the case, just grind a bit finer.
Espresso Extraction and Coffee Brewing Temperature
Brewing temperature is one of the most important factors in coffee extraction. Incorrect temperature, more than any other variables can lead to over-extraction. Other brewing variables are also important for avoiding over-extraction. However, they are amplified at higher brew temperatures. For instance it much easier to over-extract at 205°F than at 195°F, yet both temperatures are within acceptable parameters.
And the example I love the most: Turkish coffee uses the finest grind size possible, the grounds are virtually powder. But because the brew temperature is not very high, there is almost no over-extraction.
But let’s dig a little deeper into temperature, and see how is affecting espresso extraction.
Although brew temperature can be accurately controlled on more advanced espresso machines, it’s worth understanding how a little brew temp modification might change your flavor profile:
- When you lower the brew temperature you will enhance acidity.
- Raising the brew temperature will decrease your cup’s acidity.
- Lowering the brew temperature can decrease extraction yield. As a direct application, changing the brew temperature instead of the grind size is a better way to control extraction yield.
- You can compensate with a higher brew extraction temperature for lighter roasts.
- You can use lower temperatures to compensate for high solubility of a dark roast.
- A larger dose will benefit from a temperature increase. A 20g dose of dry coffee needs a higher brew temperature than 14g dose.
- Although we don’t recommend changing the the extraction time, you can compensate for a longer extraction time by lowering the brewing temperature.
What Is the Ideal Espresso Brewing Temperature
The ideal brew temperature for espresso is a range between 190°F and 205°F. And if you saw elsewhere a range between 195°F and 205°F, that is not wrong, but playing it safe.
The rate of extraction increases with higher temperature. Why not increase the brew temperature then? Because undesirable bitter compounds are extracted at higher temperatures. We do love some of that bite into our cup, but not too much. The bitter flavors cannot be extracted if we lower the temperature enough, (cold brew). But that is the other extreme.
Brewing methods with a high rate of extraction like espresso, can, and should use a slightly lower brewing temperature. In espresso, the pressure compensates for the slightly lower temperature and the short extraction time.
The espresso machine is built to maintain the correct temperature. Some machines are equipped with a PID, which increases the precision of the thermostat. Machines without a PID, require the barista to temperature surfing, in order to ensure the correct brewing temperature.
Remember, a little kick in your brew is not bad. It’s only when you extract too much of the bitter compounds, that we have a problem. That becomes an over-extracted coffee.
Brew time, (or contact time), is the last main element that is responsible for coffee extraction. Brew time is the amount of time that ground coffee is in contact with the water. If other factors are adjusted so that the brewing time is correct, it is likely that the coffee will be extracted well.
Ideal brewing time for espresso is between 20-30 seconds. This quick brewing time is achieved by pressure that is added during brewing.
I personally aim for 22 to 25 seconds, but I can accept a 27 seconds shot of the flow is slightly restricted.
Roast Level and Extraction
Roast level is an important factor to be considered when pulling shots. Dark roasted beans are more soluble, so they will extract faster. Lightly roasted coffee, on the other hand, extract slower.
In order to compensate for a light roast, the home barista need to grind finer and reduce the dose, or increase the brew temperature.
How Pressure Improves Espresso Extraction?
The pressure is used to push the water through the coffee grounds with a high force. This increases the agitation in the brew and speeds up the dissolution of the soluble solids in coffee.
Along with the soluble solids dissolution, we also emulsify the oils in the coffee, and mix in gases from the beans. The result is the crema on top of every successfully pulled espresso shot, and a coffee with a distinct flavor, like no other coffee extraction method.
Pump driven espresso machines push 9 bar pressure, standard pressure at sea level. Pump driven espresso machines are high pressure coffee makers. High pressure is also the reason that well brewed, fresh espresso has a layer of foam or “crema” on top of the coffee.
A a properly brewed espresso is extracted between 20 and 30 seconds. Shots pulled over 30 seconds are overextracted. The reason is again, we expose grounds to a high temperature for an extended time.
Steam based espresso machines typically generate roughly 3 bars of pressure. From a definition point of view, they are still espresso machines. However, the extraction is different. The low pressure does not emulsify enough oils into the shot, hence we don’t have crema.
To a lesser extent, other brewing methods also make use of pressure such as the Aeropress and the percolator or the Moka pot. These methods generate up to 1.5 bars of pressure.
Cold Brew Coffee Brews Faster In Denver
What does this have to do with espresso? Well, a lot.
In Denver, because of the high altitude, water boils at 202 degrees Fahrenheit, compared to 212 degrees in New York. This means that at the same room temperature, coffee will steep faster in Denver than in New York.
Similarly, an espresso pulled in Denver, will taste smoother, with less bitter flavors.
The chemistry of coffee extraction is the key to making great espresso.
The most important factors affecting the quality of the espresso are: grind size, extraction time and the temperature of the water at the time of extraction.
All espresso brewing variables are interdependent, so you need to make tiny adjustments when tweaking your recipe.
Coffee brewing is a science, and it’s important to get it right. But if you get it right you will be rewarded with great tasting coffee, better than in a coffee shop.