Science Behind Coffee Extraction
Once you've got a bag of amazing coffee from any of the many great roasters out there, it is important to understand how to extract the best cup of coffee possible. Coffee extraction is the process of transferring soluble compounds in ground coffee to the water passing through it. In the following paragraphs I’ll break down exactly what that means. Attention to detail when it comes to coffee extraction is very important. Not following the standards of coffee extraction could have you end up with under-extracted or over-extracted coffee, producing a lower quality and potentially less caffeinated cup. Over extracted coffee can create a very bitter cup, while an under extracted coffee can taste overly sour. I’ll explain why this is the case in a later paragraph. A main mistake that leads to an under-extracted coffee is air pockets in the coffee grounds. A skilled barista goes through the process of extraction attempting to create a coffee somewhere in the middle so you can taste a balance between the coffee's sour and bitter profiles.
The flavor of your coffee is dependent on (amongst a few secondary variables) three key components:
The temperature of your water/the quality of your water
Grind size (this determines the length of time the extraction takes)
Freshness of coffee used
We use water to brew coffee (and to dissolve so many other things) because water is arranged as a polar molecule (two hydrogen bonds, one oxygen bond), which allows it to be the perfect solvent. This polar molecular arrangement gives water the ability to easily pull apart the bonds of coffee molecules, or in other words dissolve them! The reason we use hot water (ideally between 185 degrees and 205 degrees Fahrenheit) in coffee brewing is because the higher the temperature, the more we're able to trap compounds in the brew. When water is hot, its molecules are moving around at a faster speed allowing for it to pull apart more coffee molecules in the time it gets with the coffee. Too hot of water, though, will evaporate caffeine and organic acids. Temperature affects solubility. That’s why cold brew takes several days to extract. But temperature is not everything. You need to make sure you’re using quality water to begin with. Your brew could be ruined if you’re not using good water. In Chicago, your regular tap water should be fine. Chicago has probably the best tap water in the country. Water is broken down into two types: soft water and hard water. Soft water is water that contains low levels of both calcium ions and bicarbonate. It will produce more acidic coffee. Hard water is the opposite, it contains high levels of both calcium ions and bicarbonate and it will produce a chalk-like cup. Ideally we use water somewhere in between. Magnesium levels in your water are also key to consider. Water rich in magnesium helps the extraction of flavors, and water with high amounts of sodium and bicarbonate could end up distorting/ruining the taste.
The most crucial part of coffee extraction in my opinion is the amount of time it takes. This is determined by grind size. Coffee can be ground coarse, very fine, or somewhere in between. Finer grinds create more surface area in the brewer, but each ground is smaller so the individual ground surface area is small. Fine grinds packed together create a higher concentration making the fine grind great for flow through brewing, most notably espresso. A coarse grind equals less surface area in the brewer, and the individual grounds are bigger which creates more space between grounds. This makes coarse grinds good for full immersion brewing, like a French Press. For other pour over methods, like brewing in a Chemex, it is good to find a sweet spot somewhere in the middle. You know you’ll have that sweet spot if your Chemex brew lasts between 4:30-5 minutes. The industry standard espresso brew time is 22-26 seconds, but produce a smaller amount of coffee so we want to grind very finely in order to maximize contact in a short brew time (if it’s taking longer than 32 seconds, perhaps give it a coarser grind the next time). French Press standard is 5-10 minutes, depending on desired strength.
The reason coffee extraction is so time sensitive is because flavor depends on the amount of time the water and coffee grounds interact. Water dissolves coffee compounds in a specific order. During extraction, caffeine gets extracted first. Then oils/sugars which determine flavor and aroma. Water extracts compounds in order from simple to complex: First to get dissolved are fats and acids, then sugars, then plant fibers. So first to get extracted are oily and sour characteristics, then the sweet and syrupy ones, then bitter and earthy features of the coffee. The amount of time it takes water to pass through the coffee also determines the strength of the coffee. Filter coffee is about 2% dissolved coffee compounds and 98% water. Espresso shots are 7-12% dissolved coffee compounds and the rest water. Incorrect brewing practices will alter these percentages, taking you away from the desired brew.
Freshness of coffee will change all of the above. Freshly roasted coffee has a high number of CO2 and other compounds, which dissipate over time. When roasting coffee, the roaster is breaking down the bean’s proteins and sugars, and about 800 compounds are created during what’s called the “thermal degradation reaction”. Over the roast time these break down to about 20 major compounds that create the coffee’s final taste and aroma. However all the 800 compounds breaking down have contributed to the final product. The less fresh your coffee, the more time has passed, therefore these compounds are floating away never to return to the coffee. Less compounds equals less flavor, Simple! Most of the flavor adventure we go on when drinking coffee comes from the smell. Our mouths can only pick up simple tastes like sour, bitter, salty, and sweet. Your nose does the rest of the work. So you want to make sure you’re using fresh coffee in order to get the best aromatic experience. Your freshest and best tasting coffee will be from beans roasted 3-15 days ago. During this period you might also experience a different taste on different days as the flavor notes settle. This is because initial off-gassing is still occurring and flavors are changing and settling. Coffee remains fresh for 3-5 months, depending on its origin. The amount of time passed since the roast is going to largely determine your coffee’s freshness, but exposure to moisture and heat can also be detrimental for your coffee beans. In order to contain freshness, keep your beans in an air-tight container, preferably an opaque one. While the other components mentioned are extremely important, coffee freshness can make or break your brew.
So that was coffee extraction in a nutshell. The keys to remember are that you want to be using quality water at an adequate temperature, pouring it through properly ground coffee, depending on your brew method. Coarse for full immersion, fine for Espresso, and a good medium grind for pour over/drip. Keep all of the above in mind to ensure you’re drinking the best possible coffee!
Thank you for reading, and as always if there is a topic you wish for me to cover in these Coffee Talks, let me know via Instagram or utilizing the chat feature on this website.
Works Cited
Magazine, Smithsonian. “The Chemistry and Physics behind the Perfect Cup of Coffee.” Smithsonian.com, Smithsonian Institution, 28 Sept. 2017, https://www.smithsonianmag.com/science-nature/science-behind-brewing-great-cup-coffee-180965049/.
Mehaya, Fathy M., and Ayman A. Mohammad. “Thermostability of Bioactive Compounds during Roasting Process of Coffee Beans.” Heliyon, Elsevier, 16 Nov. 2020, https://www.sciencedirect.com/science/article/pii/S2405844020323513.
Raper, Adam. “How Coffee Extraction Works.” Clive Coffee, Clive Coffee, 30 Mar. 2017, https://clivecoffee.com/blogs/learn/how-coffee-extraction-works.
Ross, Karolyn F, et al. Effect of Storage Conditions on the Sensory Quality of Ground Arabica Coffee - Wiley Online Library. https://onlinelibrary.wiley.com/doi/epdf/10.1111/j.1745-4557.2006.00093.x.
“The Science behind That Perfect Morning Cup of Coffee.” Coffee Science, 23 Mar. 2018, https://www.coffeescience.org/science-behind-perfect-morning-cup-coffee/.
“The Science of Coffee: How Physics Can Perfect Your Caffeine Hit.” Nest, 8 May 2019, https://www.latrobe.edu.au/nest/science-coffee-physics-can-perfect-caffeine-hit/.