It is taught universally that 30lb (15 kg) of pressure is the optimum amount to compact coffee in the portafilter, so as to facilitate the correct denseness of the coffee for extraction. As a person who leans toward scientific proof, I did some research on the subject.
Tamping itself, is a method of compacting coffee as closely together as possible to remove air pockets or channels within the plug. Once tamped, the compacted coffee within the portafilter can be inverted to show its adhesion to each other facilitated by the force of pressure. Once the portafilter is inserted in the grouphead, and water is added, either by pre-infusion or by a straight pour, the tamped coffee loses pressure integrity immediately. Even lightly wetting a tight tamp will negate the effect.
Most modern coffee machines force water through at, give or take, 9 bars of pressure. If you work it out, that is 15-16 times greater than a 30lb tamp. Bearing in mind that a 30lb tamp has already lost its integrity and hold when initially wetted, the coffee itself during extraction, is no longer under tamp pressure, and will also be under much less integrity and way below pump pressure.
So what can we summarize from these observations? Firstly, a lot of barista’s out there are making fine coffee and NOT doing everything by the barista’s bible. I guess the obvious answer is that the coffee machine pump normalizes a poor tamp, or indeed, will pressurize the water through the grinds regardless of the tamp. I ran a test over 2 days of approximately 80 extractions, and found that there was a more basic component to good pours than tamp.
Grind density and dose size.
Make sure the basket is at the correct level for coffee grinds. Make sure the grind is fine enough to force the water through the coffee at the best rate. My two best pours were when the coffee was fine enough and I applied the tamp weight alone and swirled for a light polish with no added pressure!
It would appear then, that if a barista can apply the same pressure consistently to a correctly dialed in grind, that the coffee will be consistent for quality. Conversely, if the coffee is fine enough and dosed correctly, the same quality can be achieved with little or no tamp!
In talking with some “old school” masters of European coffee experience, men in their 60’s who still have restaurants and coffee bars in Italy and Spain, I noted that agreement between them regarding tamping produced some of the finest expressos and Cappuccinos I ever tasted. They filled the portafilter, and LIGHTLY tamped about 5-10lb pressure and then extracted. It was all one fluid movement of making a coffee. The tamping seemed to be just to settle the grind in the portafilter.
I also am experimenting to see if pre-infusion actually “settles” the coffee before full extraction. Some people have commented that a straight pour without pre-infusion, with or without tamping, can lead to channeling. I have not seen this issue but will continue the research.