What we can extract from coffee, what we should extract, and how that influences the strength of the coffee beverage.
"Slightly over-extracted and bitter," murmurs the barista, adjusting the espresso grinder. Moments later, and a few grind settings coarser, the coffee flows smoothly and the extraction is just right. The result is a perfect cup that brings out all the flavor potential dormant in the coffee bean. A potential that is created by coffee producers in the growing region and processing roasters.
Brewing coffee is nothing more than unlocking this potential or making it soluble. "Making it soluble" is the crucial keyword. Grinding breaks the cell structure of the coffee beans, increasing the surface area of the coffee and thus making 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 whose solubility differs and which, depending on the extraction and intensity, lead to very different results in the cup.
Two perspectives simplify the understanding of the brewing process. What we dissolve from the coffee bean we call "extraction." As dissolved particles in the cup, we call this the "strength" of our beverage, meaning the concentration of dissolved coffee.
Strength and concentration of dissolved coffee particles
Ultimately, every coffee beverage is water in relation to a certain amount of coffee particles, i.e., strength. For example, filter coffee consists of approximately 98.5% water and 1.5% dissolved coffee particles. Espresso, which is also perceived as much "stronger" and more intense in taste, consists of about 90 – 92% water and accordingly 8 – 10% dissolved coffee particles. When we speak of strength, we mean the concentration of coffee in relation to water. The dissolved coffee particles are called Total Dissolved Solids in English and are abbreviated as TDS. The TDS concentration can be measured with a refractometer.
Coffee extraction – what we get from the bean
About 30% can be extracted from any coffee bean. Everything else is cell material, oil, protein, and not soluble with normal and easily accessible means. However, not everything we can extract from a coffee bean tastes good. There is a fine line between too much and too little extraction, which we perceive sensorially in every cup of coffee.
Over-extraction – too much extracted
If a coffee tastes more bitter than we're used to, we've probably extracted too much. A common reason for this is too fine a grind. A fine grind offers more resistance to the water, forcing it to extract more on its way to the cup. In the case of over-extraction, more than is actually good. The result is a coffee that tastes bitter, astringent, and unbalanced. This sensation occurs when we extract, on average, more than 22% from a coffee bean.
Under-extraction – too little extracted
On the other side of the spectrum, under-extraction looms. If we extract less from a coffee bean than is inherent in it, we not only fail to unlock the bean's potential, but we also get an acidic, grassy, and, depending on its intensity, aggressive cup. In this case, we only dissolve 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 coffee lies in navigating this narrow line between over and under-extraction while also bringing the dissolved particles to the desired strength ratio to water. A successful extraction unlocks the potential in the bean. The strength defines the type of beverage we have brewed.
Strength of coffee beverages
The concentration of dissolved coffee particles is defined for each 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 remaining content of the cup is water. For filter coffee, for example, this means more than 98% water.
