Channelling: The Main Symptom of Unevenness.
Channelling is the term we use for a critical unevenness during extraction, where a large amount of brew water travels through one localised area. When it’s extreme, channelling can be observed with the naked eye. However, it doesn’t have to have to be extreme or even visible to have a profound effect on the quality of your extraction.
When brewing espresso, for instance, there are three aspects of a poorly designed tamper that could potentially have negative effects on evenness: a very loose fit in a 58mm basket, convex in shape, and/or has a heavily bevelled edge or edge radius on the bottom of the tamper base.
This diagram shows a ‘bevelled edge’; a common design used in tamper bases which we find reduces extraction efficiency.
Like the bevelled edge, many tamper designs feature an ‘edge radius’, intended to reduce dents and scratches which we have found reduces extraction efficiency.
An experienced barista may look at these factors and expect the tamper to be less effective. However, an acid test for evaluating equipment design lies in comparing whichever equipment can brew a stronger coffee while keeping brew parameters identical. Evidence for a stronger coffee comes from measuring strength through sensory analysis (which tastes stronger?) and physical analysis (which has more extracted coffee solids in it?)
Uneven Distribution: The Main Cause of Channelling
One of the core technical skills a barista can bring to a cafe environment or competition scenario is the effective distribution of coffee. Distribution of coffee grinds can be achieved through horizontal palm tapping, vertical collapsing, or distribution via fingers or other mechanical tools.