Mythology Versus Science
The coffee industry has always had a tendency to veer towards dogma, and, in some cases, something not far short of superstition. It’s lessened a little in recent years, as the tyranny of tradition (predominantly from the Italian model of espresso) has been replaced gradually by a more open, scientific approach, but it’s still there in one form or another.
What do I mean by this? In general I’m referring to the primacy of anecdotal evidence and the lack of rigour when investigating a problem or area of development. Nowhere is this more obvious than in the effect of heat on espresso flow rate.
When a coffee shop gets busy and the grinders get used heavily the end result is an increase in burr temperature. This is pretty much unavoidable; although attempts have been made by manufacturers to cool the burrs, usually with air, at high usage levels friction always triumphs. The side effect is that the flow rate of the espresso increases and the barista is forced to set the grind finer to compensate.
Traditionally, this was viewed as being caused by heat affecting the burrs and their carriers and moving them further apart. This seemed like a plausible theory, and was accepted by the coffee industry for decades. The only problem is that it’s completely untrue.
Maxwell Colonna-Dashwood has written about the real reasons for flow rate changes here – https://colonnaandsmalls.wordpress.com/2015/02/03/the-heat-is-on-more-grinding-puzzles/ – and I am indebted to him for bringing it to my attention. To summarise, heat changes the way beans break down when ground, with the result being a lower proportion of fines (very small particles) at higher temperatures. This is what changes flow rate, not burr expansion, which is pretty much non-existent.
So, why does this matter? First, it means that we have a more even grind at higher temperatures (so we can grind finer and extract more without so much over-extraction from the fines). More importantly, though, it provides us with a lesson about mythology versus science in the context of coffee; that is to say, don’t trust received wisdom until it has been empirically demonstrated to be correct.
Speciality coffee has a slightly uncomfortable relationship with the artisan movement, and it’s in this context that much of the tension between fact and folklore is evident. Many people dislike the scientific method impinging on coffee because they view the act of making good coffee as an art, not a craft. They like the notion of the barista as some sort of creative figure who produces stunning espressos just through feel and hard-won experience. Measuring extraction yields with refractometers, or investigating particle size distribution is anathema.
The problem with this viewpoint is that it stifles the development of better tasting coffee. We’ve now reached the limits of where anecdotal evidence can take us; the only way to progress is to be more rigorous in our investigations of coffee production, extraction, roasting and grinding.
Of course, this should always be balanced by sensory evaluation; a perfect extraction on paper will not always be a perfect extraction when tasted. This is where the art and craft of coffee combine and where the skilled specialists can demonstrate their value.