The Fundamentals of Coffee Extraction
Coffee extraction occurs when hot water is poured over coffee grounds, causing desirable compounds such as caffeine, carbohydrates, lipids, melanoidins and acids to be extracted from the grounds. The degree to which extraction occurs depends on a number of factors, such as water temperature, brewing time, grind fineness, and quantity of grounds. This fundamental process lies at the heart of every brewing method, from espresso to cold brew.
Only 30% of a coffee bean is water soluble, and only about 20% of these solubles taste good in a brew—the rest can add bitter or papery off-flavors to the coffee liquor. Luckily for us, the desirable solubles extract first and the unpleasant ones put up a little more of a fight before dissolving into water. This timing differential is crucial to understanding why brewing parameters matter so much.
Coffee extraction process involves (1) water absorption by the coffee grinds, (2) mass transfer of soluble compounds from the ground coffee into the hot water, and (3) separation of the resulting extract from coffee solids. Each stage requires precise control to achieve optimal results, making coffee brewing both art and science.
The Sequential Nature of Compound Extraction
Water will always extract the different flavor compounds in this order: fats and acids, then sugars, and finally the plant fibers. Fruit acids and caffeine dissolve the easiest, washing into the brew first. When isolated, this portion of the brew can taste bright and thin. Understanding this sequence is essential for controlling flavor profiles in the final cup.
Next come the lipids and fats, which release from the coffee cell as an emulsion and enter the brew liquid that way. This is not really news—we already knew that most of the good stuff (fruit acids and caffeine) came out very early on in extraction, followed by lipids and fats, which add more mouthfeel and may offer the beginnings of brown sugar notes that melanoidins continue to bring until becoming bitter. Plant fibers and carbohydrates are the last to emerge from the bean and into the brew, and too many of these compounds could ruin the cup.
Coffee extraction for an individual grain is controlled by two processes: a rapid dissolution of coffee from the grain surfaces in conjunction with a much slower diffusion of coffee through the tortuous intragranular pore network to the grain surfaces. This dual-process nature explains why grind size and contact time have such dramatic effects on extraction efficiency and flavor balance.
Strength, Yield, and Total Dissolved Solids
Strength, also known as solubles concentration, refers to the percentage of dissolved solids per unit of liquid in the final beverage. A higher concentration of solubles is associated with a stronger beverage, and lower concentration with a weaker, more “watery”, beverage. Strength varies between coffee beverage types; for most it ranges from 1.15% and 1.35%.
In scientific terms, it can be measured by the percentage of dissolved coffee to water, a measurement we call the total dissolved solubles, or TDS. For example, a well-made espresso has a TDS of between 9 and 20 percent coffee to water. By comparison, the more mellow pour over coffee has a much lower TDS of between 1 and 2 percent coffee to 98 to 99 percent water.
In a properly extracted cup of coffee, only between 18 and 22 percent of the coffee’s solubles have dissolved. The optimal guidelines set by the SCAA for extraction are 18 - 22%. According to industry standards, the optimum extraction rate for a coffee bean is between 18% and 22%. This narrow window represents the sweet spot where desirable compounds are fully extracted while avoiding over-extraction of bitter elements.
Temperature and Pressure Dynamics
A commonly recommended brewing temperature for traditional coffee beverages is 91–94 °C (196–201 °F), which facilitates full extraction of desired compounds. When brewing coffee, the ideal water temperature is between 195-205°F (90-96°C). At these optimal temperatures, water effectively extracts the desirable compounds from coffee grounds - including caffeine, oils, and various flavor molecules that give coffee its distinctive taste and aroma.
Higher temperatures accelerate the chemical reactions that dissolve coffee compounds. For example: • Acidic compounds dissolve quickly at lower temperatures, enhancing brightness and fruity notes. • Sweet and complex sugars dissolve more readily at slightly higher temperatures, adding balance. Temperature control becomes a tool for flavor manipulation, allowing brewers to emphasize different characteristics.
An optimal pressure of about 9 bars (which is roughly equivalent to 9 times the atmospheric pressure at sea level) is required to push hot water through the finely ground coffee. This high pressure is necessary for achieving the rich body and distinctive crema that are trademarks of a great espresso. The pressure forces the water to interact with the coffee quickly, extracting flavours and oils efficiently and creating a concentrated, flavorful shot.
Espresso extraction is a few degrees lower than drip coffee. Pressure helps extract more TDS at a lower temperature, while preserving volatile components.
Extraction Variables and Troubleshooting
Yield is inversely proportional to grind size; a smaller grain size produces more surface area and faster extraction. A longer brewing time results in a higher yield. Extraction rates vary between brewing methods. These relationships form the foundation for recipe development and troubleshooting.
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.
Salt and acid compounds are more soluble therefore under-extracted coffees are salty and sour, the sugars haven’t had enough time and/or water to dissolve into the brewed beverage. However, here, in order to push extraction yields higher, you may bring EY up to a higher ratio, while finding that the intensity and flavour profile remains similarly muted, yet at the same time the coffee begins to taste bitter and dry.
This may result in the occurrence of two different situations, which we have come to know as: Over-Under Extraction, and Under-Over Extraction. Over-Under Extraction is when coffee grounds are coarser, and as a result brewed coffee is under extracted. However, here, in order to push extraction yields higher, you may bring EY up to a higher ratio, while finding that the intensity and flavour profile remains similarly muted, yet at the same time the coffee begins to taste bitter and dry. Under-Over Extraction happens when the coffee is ground too finely, and the coffee becomes over extracted. Understanding these nuanced extraction problems helps brewers identify when grind adjustment is necessary versus other brewing parameter changes.
Mathematical Models and Future Research
Coffee extraction is a complex mass transfer process that takes place between hot water and ground coffee beans when the water passes through a bed of coffee grounds. In this study, a general set of macroscopic governing equations for coffee extraction was derived using the volume averaging theory. Moreover, lumped parameter analytical solutions for the extraction of drip coffee, espresso coffee, and immersion-brewed coffee (e.g., siphon coffee) were obtained by integrating the macroscopic governing equations.
The complexity of the coffee extraction process coupled with a lack of applied knowledge makes the brewing process for high-quality coffee more an art than a science. It is difficult to produce high-quality coffee beverages in a reproducible manner. Till this date, coffee extraction is a topic that is not understood fully by scientists and coffee professionals alike. It would only make sense for a Coffee Brewing Control Chart created in 1975 to have much room for improvement with the knowledge that exists today.
Despite decades of research, coffee extraction remains an active area of scientific inquiry. Modern approaches combine traditional brewing wisdom with advanced analytical techniques, including refractometry for TDS measurement and mathematical modeling of extraction kinetics. As our understanding deepens, the gap between art and science in coffee brewing continues to narrow, promising more consistent and deliberately crafted coffee experiences for both professionals and home enthusiasts.