The 9 Factors of Extraction
Whenever we talk about espresso – or any coffee really – there are nine factors that we have control over and can use to affect flavor. It’s important to note that most of these factors aren’t readily accessible to us as baristas. They require reprograming the machine, changing water filtration profiles, or otherwise altering the brewing method. There are three that we regularly alter on a daily basis; but first let’s look at the other six.
1. The Coffee
Much of what comes out in the cup depends on the coffee that we choose. All coffee has its own unique character and quality. Where did the coffee come from? How was it processed? How long ago was it roasted? What level of roast was applied to this coffee? There are many external factors that influence the character and quality of a coffee, and our aim as baristas is to highlight these natural characteristics.
2. Water Quality
Coffee is anywhere from 88-99% water, so the quality of water used for brewing is of paramount importance. Brew water should be filtered for taste and odor, and any sediments or chemicals should be removed. There are exacting parameters that SCA puts out as to how many parts per million (PPM) certain elements can fall within, but for the most part they are at or below 150.
3. Water Temperature
What specific flavor compounds are extracted from coffee depends greatly on the temperature of water. Generally, it is best to use water as close to boiling as you can – 205*F seems to be the most advisable given current research. Lower temperatures tend to accentuate acidity but not much else, whereas as you approach boiling (212*F), bitterness and astringency dramatically rise.
4. Pressure
All brewing methods deal in pressure, but you are probably most familiar with the pressure involved with espresso. Using a great deal (10 bars) of pressure, we can force water through a compacted bed of very finely ground coffee, producing a concentrated drink with rich flavor and a syrupy body.
5. Agitation
Good, high levels of extraction rely on saturation of all the coffee grounds with water. Coffee grounds are prone to sheltering air pockets, leaving some coffee (often even at the bottom of the brew bed) dry for long periods of time during brewing. Thus stirring, whether manually with a spoon or other implement, or by a high flow rate of water, can help to saturate the coffee more fully.
6. Time
When we speak about time with regard to extraction, we are referring to the contact time of the brew – that is, the amount of time that water was in contact with the coffee. As baristas, we are often taught that time is a delineator of quality when it comes to extraction, but it is no more a factor than any of the others. Rather, it is more useful to understand that more time equals more extraction, and vice versa.
Now for the three factors that we most often have agency over in our day-to-day work lives as baristas:
7. Dose
Put simply, the dose is how much coffee is put into any brew, be it a pour over or a shot of espresso. Increasing the amount of coffee will brew a stronger cup, but it will also affect brew time and flavor. No factors exist in a vacuum!
8. Beverage Weight
Beverage weight on the other hand, is related to how much water is used for a particular brew. Keep in mind that some amount of water will be retained in the coffee grounds – this is known as the Liquid Retention Ratio. That’s why the beverage weight is different than the total amount of water used in the brew.
9. Grind Size
We grind coffee to create more surface area for the water to interact with during a brew cycle. The finer a coffee is ground, the more surface area is available for water to interact with. In espresso, this is extremely important – most extraction occurs through erosion in espresso, so the more surface area the water can touch, the more extraction will occur. For most brew methods, the grind size will influence the contact time; finer grounds will lead to a longer brew time, while coarser grounds will shorten it. This will have a cascading effect on the overall extraction and flavor of a cup.
Taken together, the dose and beverage weight can be combined to the brew ratio. A brew recipe is most often expressed as a ratio; for example, 28g of coffee to 375g of water (our standard pour-over recipe) or written another way: 13.4:1. It’s helpful to think of time as the answer to the equation of the brew ratio. We’re not actually solving a math problem here, but your grind size and your brew ratio, taken together, will get you a specific brew time as a result. Thus, time can be used to monitor consistency of shots and brews. If the brew time is different, then something has changed.