Though you probably drink a cup (or a pot) of it every morning without thinking twice, coffee is a very complicated and involved beverage. From its origin to how it is brewed, there is a wealth of information to be discovered and discussed when it comes to our nation’s favorite drink. Our very own Lisa Knisely did a great job of breaking it down for us basic bitches; head over to her post to learn about coffee origin, varietals, and preparation!
But coffee is not only complex from a sociological standpoint; it’s complex on a molecular level. When you make a cup of coffee, you are performing a chemical extraction. You are using water and energy (from heat) to create a solution of flavor compounds, browning pigments (there’s that Maillard browning again), cell-wall carbohydrates that provide body, and of course caffeine (McGee, 2004).
Making coffee may seem like a simple task – almost everyone is capable of making it in one form or another – but opinions as to the “best” way to go about it are many and fueled with passion. I can’t tell you what the best way to make coffee is; the word “best” is subjective and taste varies from person to person, but I can give you experimental parameters to play around with. By understanding the science behind this solvent extraction known as “making coffee,” you can tweak your process to suit your individual preferences and taste.
Few of us buy coffee beans in their green state; usually roasting is out of our hands. But once you have obtained a bag of beans, there are three main factors you can manipulate: ground particle size, temperature, and extraction time.
Ground Particle Size
The smaller the particle size, the greater the surface area that is exposed to water.
Think of it this way: if you were to place a whole, roasted coffee bean in a cup of water, only the bean’s outer surface would be exposed to the extracting liquid. If you were to break that bean is half, now the outer surface of the bean plus the newly exposed inside would be available for extraction. Break those halves into even smaller pieces, and even more area is exposed to water, allowing more deliciousness to be coaxed out of the bean and in to your cup at a faster rate.
Optimum ground size varies for each brewing method; according to my personal Bible On Food and Cooking, you want grounds for machine filtered coffee to be fairly coarse – around 1 millimeter in size.
Too coarse a grind and your beverage may be under-extracted, leading to a watery, acidic brew that isn’t very flavorful. Conversely, too fine of a particle size may result in a harsh and bitter beverage as you may be extracting things too quickly. If you find that your daily cup leans a little too far in either direction, try playing around with your grind time until you find the particle size that yields your preferred flavor profile.
[Note: I say “grind time” and not “size” because millimeters are hard to eyeball. Unless you have a micrometer in your kitchen, the best way to control your grind is to keep track of how long you grind your beans; I like to grind mine for six seconds before transferring to my coffee pot.]
The higher the temperature, the more quickly substances are extracted.
The ideal brewing temperature (for a 1-3 minute extraction of fine grounds and a 4-8 minute extraction for coarse grounds) is just below boiling, around 190-200F/85-93C. Go higher than that, and you risk extracting bitter compounds too quickly (McGee, 2014). If you like really bitter coffee (and I really do), then by all means, jack it up to a rollicking boil! Italian stovetop moka pots operate above boiling, and I love the strong, somewhat harsh cups they produce.
But some days, I want something mellow, maybe even a little fruity, and nothing is mellower than good cold-brewed coffee. Cold brewed coffee is a different beverage, chemically speaking, than its hot-brewed counterpart. When you use hot water, you are not just extracting compounds, you’re cooking them. The heat transforms some of the extracts into other substances and drives off others. This could be the reason that one guy you know “loves the smell of coffee,” but “hates the taste.” The poor, confused soul is a fan of those compounds that are driven off by high temperatures.
Cold water extracts more slowly, extracts fewer compounds, and doesn’t change the chemical make-up of those compounds it extracts. Fans of cold brew love it for its less acidic, smooth flavor.
But when you decrease temperature, you have to increase time to make up for it.
The longer the extraction time, the more that is extracted.
If you want to speed up your extraction process, you would need to increase your temperature, decrease your ground size, or both. If you don’t mind waiting a bit, you can afford lower brewing temps. For cold brew (extracted at room temperature), this usually translates into a 12-hour extraction period, but I’ve let mine go for as long as a weekend.
For coffee that is served with the grounds, such as Turkish coffee, the extraction time bleeds over into consumption. This process involves grinding your beans extremely fine (purists recommend a special Turkish grinder), letting the mixture foam – not boil! – several times, and then pouring into small cups for drinking. The grounds remain in the cup, making it important for the brew to be consumed in fairly short order; wait too long and the hot water will continue to extract compounds from the grounds, some of which may be too bitter for your liking.
Extraction time isn’t easily controlled when a conventional coffee pot is the instrument of choice, but a French press allows one to manipulate solvent (water) exposure as precisely as one pleases. Cold brewing gives one similar control.
Pressure is another parameter that can be manipulated in the coffee extraction process, but only with special equipment. The making of espresso requires around 9 atm (most other coffee is produced at 1 atm, which is pretty much ambient pressure). When you increase the pressure of a coffee extraction system you force water (in the form of steam) through grounds, extracting delicious little oil droplets. This accounts for espresso’s unique mouthfeel; more oil equals a creamier texture.
These factors just scratch the surface of the scientific discussions that surround the subject of coffee, but they are an excellent starting point. By manipulating ground size, temperature, and pressure, you can create a cup of coffee that is unique to your individual desires. Just remember to be a good scientist and only change one variable at a time.
I’m a big fan of almost every preparation of this delicious beverage, but during the summer months, cold brew is my go-to beverage. If I’m being lazy, I like to use Trader Joe’s concentrate [Editor's note: Claire Lower is a basic bitch], but making my own cold brew only requires slightly more effort. Most recipes call for room temperature extraction, but I’ve found that by lengthening the extraction time, and grinding the beans very fine, I can extract in even colder temperatures. Three or four days in the fridge creates a strong, smooth, and almost alcoholic tasting brew.
Claire’s Very Lazy, Very Cold-Brewed Coffee
- ¼ pound of whole-bean coffee
- 8 cups of filtered water
Grind your beans very fine, as fine as you can get with your grinder, and pour into a pitcher. [Editor's note: Invest in a burr grinder!] Top with cold water, give a stir, and cover with plastic wrap. Place the pitcher in the fridge for at least 72 hours, but I’ve gone up to five days with excellent results.
After the extraction period is over, decant through a sieve lined with a large coffee filter. Once all of the liquid is decanted off of the grounds, gently press the grounds with a spoon to get that last bit of caffeinated goodness.
Pour over ice, add a dollop of cream, and enjoy through a straw.
Further reading: Harold McGee. 1984. On Food and Cooking: The Science and Lore of the Kitchen. (New York: Scribner), 440-447.