What we can extract from coffee, what we should extract, and how this affects the strength of the coffee beverage.
"Somewhat over-extracted and bitter," the barista mutters and adjusts the espresso grinder. Moments later, a few notches coarser, and the coffee runs smoothly—the extraction is just right. The result is a perfect cup that brings out everything the coffee bean holds in terms of flavor potential. A potential that is cultivated by coffee producers in the growing regions and developed by roasting companies.
Brewing coffee is nothing more than unlocking this potential or making it soluble. "Making it soluble" is the key term. Grinding breaks down the cell structure of the coffee beans, enlarges the coffee's surface area, and makes it accessible to water. The solids become gaseous and liquid, and as flavor components they form what we perceive sensorially in the coffee beverage: taste, aroma, body, and a brown-red hue. What we taste are chemical compounds with varying solubility that lead to quite different results in the cup depending on extraction and intensity.
Two perspectives simplify our understanding of the brewing process. What we dissolve from the coffee bean is what we call "extraction." The dissolved particles in the cup we call the "strength" of our beverage, and by this we mean the concentration of dissolved coffee.
Strength and Concentration of Dissolved Coffee Particles
Every coffee beverage is ultimately water in relation to a certain amount of coffee particles—that is, strength. A filter coffee, for example, consists of around 98.5% water and 1.5% dissolved coffee particles. An espresso, which in terms of taste perception is much "stronger" and more intense, consists of around 90–92% water and correspondingly 8–10% dissolved coffee particles. When we speak of strength, we mean the concentration of coffee relative to water. The dissolved coffee particles are called Total Dissolved Solids in English and are abbreviated as TDS. The Konzentration of TDS can be measured with a refractometer.
Coffee Extraction—What We Take from the Bean
About 30% can be extracted from each coffee bean. Everything else is cell material, oil, protein, and not soluble with normal and easily accessible methods. However, not everything we can extract from a coffee bean tastes good. It is a fine line between too much and too little extraction, something we perceive sensorially in every cup of coffee.
Over-extraction—Too Much Extracted
If a coffee tastes more bitter than we know it to be, we have probably over-extracted it. A typical reason for this is a grind that is too fine. A fine grind offers water more resistance and forces it to extract more on its way into the cup. In the case of over-extraction, more than is actually desirable. The result is a coffee that tastes bitter, astringent, and unbalanced. This sensation occurs when we dissolve more than approximately 22% from a coffee bean on average.
Under-extraction—Too Little Extracted
On the other side of the line lurks under-extraction. When we extract less from a coffee bean than what is present, we not only fail to unlock the bean's potential, we also get a sour, grassy, and depending on strength, aggressive cup. We then dissolve only the most soluble particles and have probably extracted less than 18% from the coffee bean.
The Sweet Spot and the Perfect Coffee
The art of brewing a perfect cup of coffee lies in navigating that fine line between over- and under-extraction while also bringing the dissolved particles into the desired strength ratio to water. Successful extraction unlocks the potential in the bean. Strength defines what type of beverage we have brewed.
Strength of Coffee Beverages
The concentration of dissolved coffee particles is set for each type of coffee beverage:
Filter coffee: 1.2–1.5% TDS
Café crème: 1.8–3% TDS
Americano: 3–4.5% TDS
Espresso: 6–10% TDS
These are approximate values that provide a good guideline for the strength of coffee beverages. It is important to note that the rest of the cup's contents is water. With filter coffee, for example, more than 98% water.
