The Nano World of Espresso: Oil Droplets and Rigid Polymer Structures Shape the Shot

Coffee is one of the most popular beverages worldwide, and the preparation of the perfect espresso has become an entire culinary discipline that combines science and craftsmanship. Espresso involves the extraction of finely ground coffee under high pressure, yielding a concentrated coffee rich in emulsified lipids, polysaccharides, and solids that contribute to its unique aroma and mouthfeel. While the aroma and flavor compounds in coffee have been meticulously studied for decades, relatively few studies have addressed these colloidal structures. We here combine imaging, scattering, and rheological techniques to unravel the size, structure, and interactions of espresso colloids. We find extracted oil droplets, mostly in the submicron range, stabilized by protein-polysaccharide complexes at their interface and relatively long fiber-like polymer structures. The rheological characterization of espresso revealed that only the first milliliters of espresso exhibit a considerable shear viscosity and viscoelasticity. Combined, our results show that the size and structure of extracted lipids and polymers strongly depend on the extraction time (or volume). Mostly the first 12-ml fraction of espresso contributes to the ‘body’ of espresso due to its highest concentration of dispersed colloids and the early extraction of unfolded polymers.