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I notice that some barista's keep whacking the doser on the coffee grinder when they fill the espresso holder. What's the theory behind this?
I would've thought that the fact that the coffee is being ground and falling into the holder that it is being aerated enough already so why do this?
This may be something to do with specific grinder models, it may be just the lever to release coffee, I don't know but would like to hear from someone who knows better than me.
Short reason: Tradition conflicting with modern brewing practices.
Long reason: This is more a result of an industry-wide legacy when purchasing equipment, where coffee shop owners will pick up mechanically dosed grinders instead of doserless, or electrically metered models because "that's how it's been done," or because the technology simply wasn't affordable until simple microcontrollers became cheap.
Traditionally, you would grind enough coffee to fill the doser, dispensing grounds for brewing as needed and refilling the doser when the level dropped too low. Dosers exist to allow for predictably size quantities of grounds to be dispensed into the portafilter, and that's basically it.
This, however, conflicts with the "Third Wave" coffee culture, where flavor and texture are paramount. In this new culture, coffee beans are ground for immediate use, rather than drawing from a bin. The moment beans are ground, the gasses contained within (gasses that are the reason Crema exist) are allowed every opportunity to escape. The longer they sit, the less gasses will be left behind by the time they're used to brew.
So when a Third Wave coffee shop grinds using a doser, the Barista rapid-fires the dosing lever, essentially moving the coffee straight into the portafilter.
A better plan is just to use a direct-dosing grinder where the grounds are metered by time or weight straight into the filter. Advances in the packaging options for load cell sensors and simple controllers have made this somewhat common in both consumer and commercial equipment.
