How Coffee Extraction Works
Coffee extraction is simply the process of dissolving ground coffees with hot water, right? Well, yes. But in order to get that perfect tasting cup of coffee, there is some science that goes to this. The ratio of water to coffee, the precision of your grind, the roast of the coffee, the temperature of the water, and the amount of time that the water is in contact with your coffee are just a few of the things that go into coffee extraction.
If you're having trouble getting that perfect shot or cup of coffee, you might want to dive a little deeper with our guide below on how coffee brewing works and to give you tips to improve your coffee extractions.
Water: The Universal SolventTo make any coffee drink, we take ground coffee and add water. Why water? Mostly because water is an amazing solvent. On a molecular level, it has a polar arrangement – two hydrogen atoms with their positive electrical charge on one side, and one oxygen atom with its negative charge on the other. This makes it super attractive to a wide variety of molecules. It’s so attractive in fact, it will pull apart the bonds of other molecules, causing them to dissolve into the water. If you heat the water, all of its molecules will begin to move around quickly, making it an even more efficient solvent.
When you mix water and coffee, the water will dissolve the different flavor compounds in coffee. But what molecules and compounds are in coffee?
Let’s take one more step back: what is coffee? Coffee beans are just the roasted seed of the coffee fruit, often called a coffee cherry. Because the dream of every seed is to grow into a tree, coffee seeds are full of stored energy: particularly complex sugars, fats, and acids. Because it is part of a fruit-bearing plant, the seed itself is made up of microscopic plant fibers.
To transform the seeds into something we can use to brew, we throw them into a very hot environment, usually a coffee roaster. The heat of the roaster does some important things to all the elements of the coffee seed. First, the heat evaporates any moisture trapped in the coffee. As moisture continues to leave the seed, the plant fibers that make up the structure of the coffee will become dry, hard and brittle. If roasted too much, the plant fibers will continue to break down and most of the oils in the coffee will push to the surface and begin to oxidize.
The heat of the roaster will begin to reduce the complex sugars (long-chain carbohydrates) into simpler sugars (short-chain carbohydrates), making them easier to taste. As the roast develops, some of these sugars will begin to caramelize, creating a nutty, caramel aroma. If a roast is pushed too far, these sugars will carbonize, and the flavor will change from sweet and complex to burnt and smoky.
As the heat increases, many of the acids in the coffee will also break down. If a roast is stopped too early, there will be an abundance of acids in the coffee. If a roast is left to run, the acids will eventually break down completely and the coffee will taste bland and burnt. Most coffee roasters aim to balance the levels of acidity so your brewed coffee will be neither intensely sour or incredibly bland.
The result of quality roasting? Coffee beans that have a careful balance of oils, fruit acids, simple sugars, and caramelized sugars.
Brewing: Understanding Extraction
So now we have good water and good coffee - all you have to do is brew up a cup. In almost every method of preparing a coffee - be it espresso, pour over, cold brew, or french press - the general principle is the same. Take the roasted coffee, grind it into small pieces, and add water. Once the water hits the coffee, it starts to extract out the flavor compounds.
Regardless of the method, water will always extract the different flavor compounds in this order: fats and acids, then sugars, and finally the plant fibers. From a flavor and body perspective, it looks like this:
The first compounds extracted out of coffee are the acids and fats. Acids, which contribute sour flavors, are the simplest compound, molecularly speaking, so the water is able to easily dissolve them into the liquid coffee. The oils or fats in coffee, which add body, are not particularly simple, chemically speaking. Instead, they are hydrophobic and easily wash out of the ground coffee. Additionally, many of the lighter aromatics, floral and fruity, are extracted at this point.
Sugars are extracted next. Even simple sugars are more molecularly complex than acids. As such, water needs more time and/or energy to fully dissolve them.
Eventually, the water will start to break down the plant fibers that hold the ground coffee together. Like all plant matter, including kale and celery, these fibers taste dry and bitter.
When coffee professionals taste espresso or brewed coffee, we’ll often describe it as either over- or under-extracted. Under-extracted coffees taste sour or sharp. This is because the water hasn’t had enough opportunity to break down enough sugars to balance with the acids from the first part of the extraction. Over extracted coffees taste bitter and thin, almost hollow. This is because the water has extracted out all of the available sugars and has started breaking down the plant fibers that make up the coffee.
Brewing: Understanding Strength
A shot of espresso isn’t just defined by the quality of its extraction; the strength of the beverage is equally as important. When we say strength, we don’t mean caffeine content. In coffee, strength relates to the amount of dissolved compounds in the drink.
Filter coffee is roughly 1-2% dissolved coffee compounds; the other 98-99% is water. Espresso is a much more concentrated drink: it’s made of 7-12% dissolved compounds and 88-93% water.
The strength of a coffee is largely the result of the ratio of ground coffee to brew water. If too little water is used, your coffee will feel muddy and overpowering. If too much water is used, the coffee will feel thin and watery.
It’s important to understand that strength has a strong relationship to extraction. If you’re using less water to increase the strength of your coffee, it is more difficult for the water to extract out all of the desired flavors well. For this reason, we recommend finding a ratio that produces the strength of coffee you enjoy before working on improving your extraction.
The strength of your coffee will also determine how easy it is to taste distinct flavors. The more strong a drink is, the more difficult it will be to parse out individual flavors. For reference, think about the difference between a milkshake and an iced tea. A milkshake is intense, sweet and simple where an iced tea can be expressive, aromatic and distinct.
If you’re drinking lots of milk and espresso drinks, it is usually a good idea to have a stronger espresso to cut through the milk. That’s why we’ll recommend a ristretto, or restricted shot, for cappuccinos or lattes. These shots will typically use a 1:1 or 1:1.5 ratio – meaning 1 gram ground coffee for every 1.5 grams of liquid espresso.
If you’re looking to get really clear, distinct shots of espresso to drink on their own or in small americanos, you’ll want a larger ratio. We recommend a 1:2 ratio (1 gram coffee to every 2 grams liquid espresso).
Dialing-in Your Coffee
Once you know you’ve got a target ratio, it’s time to adjust your grind to get the ideal extraction. If your shots taste really sour, you’ll want to extract more out of the coffee. The easiest way to do this is to adjust your grind setting finer and ensure that your tamp is level and not causing channeling. This will have two effects:
- the smaller particles will slow down the flow of water through the coffee, giving it more time to pull out sweetness;
- the smaller particles will have more surface area exposed, making it easier for the water to enter the coffee.
If your shots taste really bland and bitter, you’ll want to extract less from your coffee. Again, the easiest way is to adjust your grind setting – this time you’ll make it coarser. Check out How To Dial In Your Espresso Grinder to learn more!