Coffee extraction

Coffee extraction is the transfer of soluble compounds from ground coffee into brewing water. Its flavor depends on which compounds dissolve and how much of each enters the cup.

Diffusion: The net movement of particles from regions of higher concentration to regions of lower concentration. Dissolved compounds move from coffee particles into surrounding water down concentration gradients.
Espresso: A concentrated coffee beverage made by forcing pressurized hot water through finely ground coffee. High pressure and a compact coffee bed produce a rapid, concentrated extraction.
Total dissolved solids: The combined concentration of dissolved substances in a liquid, commonly measured with a refractometer. Brew-strength measurements estimate how much material from the coffee dissolved into the beverage.
Under-extracted coffee: Brewed coffee in which insufficient soluble material has been removed from the grounds for a balanced flavor. It illustrates the flavor consequences of extracting too little from the coffee dose.
Solubility: A substance’s capacity to dissolve in a particular solvent under specified conditions. Different coffee compounds dissolve in water to different extents.
Pour-over coffee: A brewing method in which water is poured over coffee grounds held in a filter. Pouring pattern and flow rate influence how evenly the grounds are extracted.
Coffee brew ratio: The mass ratio of coffee grounds to brewing water used in a recipe. The ratio sets how much water is available to extract each unit of coffee.
Over-extracted coffee: Brewed coffee in which excessive or poorly balanced soluble material has been removed from the grounds. It shows how continued extraction can produce harsh or unbalanced flavors.
Particle size distribution: The range of particle sizes present in a sample of ground material. Grind size affects exposed surface area and how evenly water passes through the coffee bed.
French press: An immersion brewing method that separates steeped coffee grounds with a plunger-mounted mesh filter. Grounds remain submerged for a set period before the brewed liquid is separated.

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