One of the easiest things to overlook for a new barista is the importance of the distribution of the coffee. It is essential for great espresso to have the coffee in the portafilter basket fairly evenly distributed. When you think about it the reason for this is pretty obvious. Water under pressure will pursue the easiest path through the puck. If the puck is not evenly distributed then the less dense areas will see more water pass through than the areas where the puck is more dense. The result is the less dense portion of the puck will be overextracted and bitter and the more dense portion will be underextracted and sour. If you can obtain an even and level bed that has relatively even distribution throughout then you minimize these taste flaws.
This is one problem with tamping – it is one of the least important parts of the barista’s ritual as long as they don’t do it badly. Doing it badly means an uneven bed at the end of tamping, in which case again the coffee is not evenly dense or tamping at different weights each time. If you can tamp evenly at the famous 30 pounds of pressure each time that is great, but most believe the same would be true at 20 or 40 pounds and some argue even without a tamp you can achieve great coffee. I haven’t tried it yet, but I will. Note that you can’t fix bad distribution with a good tamp so making sure the coffee falls fairly evenly out of the grinder is essential. If you fail to do that you need to redistribute it so that it is even.
With inexpensive grinders one strategy for this is called the WDT or Weiss Distribution Technique, where you stir the grinds with a thin needle. Commercial grinders do not need the WDT, but most inexpensive grinders are helped by it.
Word of warning though is that if you start doing the WDT you will be officially labeled an espresso obsessive. Then again if you are reading espresso blogs you probably already are. Welcome to the club.
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