We agitate, or mix, immersion brews in three main ways: manual, mechanical, and via the movement of bubbles in convection currents rising up through the slurry. (We will explore the complex topic of convection in chapters 4 and 5, when we discuss the syphon and ibrik brewers.) The reason we need agitation in coffee making is so that extraction can take place more quickly than just waiting for flavour molecules to diffuse from the surface of coffee grinds. Diffusion can be a very slow process, so agitation, when done correctly can introduce turbulence to get things happening more quickly. When too much agitation occurs, things can get out of control. That’s why we don’t usually recommend whisking or blending a coffee slurry. Instead we prefer to apply agitation through stirring. Making agitation as consistent and repeatable as possible is key to good barista technique.
Manual agitation is always required for the cupping method. When coffee cuppers break the crust that forms on the surface of a cupping bowl, they usually do so by pushing through the crust with the back of a cupping spoon. Some cuppers prefer to immerse the spoon into the coffee bed and stir all the grinds. The former approach does not disturb the grinds that have settled down onto the coffee bed, and therefore it produces much less turbulence.
It’s easy to think that reducing turbulence can make a recipe more repeatable; in fact, however, when a coffee bed is allowed to remain undisturbed for the entire brewing process, it tends to lead to underextraction. On the other hand, if we apply an excess of manual agitation, we encounter drastic problems with turbidity. Cloudy brews can be a particular problem with static immersion brews such as the French press, if they are stirred just before serving. (We will explore the science of turbidity in the next lesson.)