Extraction is the removal of mass from the grounds. Extracted substances are either soluble or insoluble. In drip coffee and espresso, “solubles” are solids and gases dissolved in the brewing liquid. Soluble solids contribute to taste and brew strength while soluble gases, or volatile aromatics, contribute to aroma.
In drip coffee, “insolubles” are solids and oils held in suspension. Insoluble solids are made up primarily of large protein molecules and fragments of coffee fiber. Insoluble solids and oils combine to form brew colloids. These contribute to aroma, body, and taste and alter flavor by trapping and later releasing soluble solids and gases and by buffering acidity.
In espresso, insolubles are held in either a suspension or an emulsion. The suspended solids are primarily coffee bean cell wall fragments that contribute to body but not flavor. The emulsion is a dispersion of tiny oil droplets surrounded by liquid; these oils contribute to aroma, body, and taste and also act to decrease the perception of bitterness of an espresso by coating the tongue (an espresso tastes more bitter when made into an Americano because the addition of hot water dilutes the oil content, which prevents the oils from completely coating the tongue).