I observe that coffee servers (I refuse to use the term barista) will apply pressure to the coffee grounds using a variety of methods and pressure. This seems to vary wildly and I'd like to know how important is this? Is there some general rule of thumb when it comes to recommended pressure?
My thoughts were that the fineness mattered the most for espresso machines and that reasonable enough pressure when tamping is enough but what do expert's use and believe.
Tamping is one of three key controls you have over the espresso brewing process; The others are dose (the amount of coffee used) and grind coarseness. Together, they allow the coffee machine operator to produce a puck of the correct density and consistency through which the pressurized water can be pushed through.
While techniques vary, the "rule of thumb" is to apply roughly thirty pounds of force evenly across the puck; however, the key is consistency. Keep your variables the consistent and you'll be able to dial in the best end-product.
I've been able to pull great shots using anything from just the weight of the tamper itself to my full weight pushed onto a puck, keeping all of the other factors where they needed to be to create the correct puck density.
See this video below for more detail: What is the Right Way to Tamp?
Let's start off with what the function of the tamp is.
The tamp is cylindrical in shape and is ideally just snug enough to fit into the portafilter smoothly. What the tamp does, is prime the coffee bed to be met with water. What we know about the water is that in general, in commercial machines, water is applied with 9 bars of pressure. Physics tells us that pressured water flow will take the path of least resistance. By this logic, the tamp functions by making the coffee bed smooth, even, and robust enough to be hit with 9 bars of pressure and extract evenly.
Note: In no way am I a qualified physician. This is not to be considered medical advice.
If the surface one tamps on is on the proper level, the barista should be able to tamp with their elbo at a 90 degree(ish) angle, and their wrists in a semi-straight path to prevent damage. The barista can then use their body weight, and not muscle, to perform the tamp. By leaning into the tamp with the correct path, one can achieve about 30lbs of pressure.
Personally I have experimented with varying pressured tamps and have found that applying a greater pressure than 30 lbs produced little effect, at most, a longer extraction. There are other variables that should be adjusted instead, such as the grind size, to achieve such an effect.
This is a good question. The answer usually given is that it creates an even density to prevent water channeling through a path of least resistance. But does it work? I volunteer in a church community coffee shop open for two mornings and one afternoon a week. We have a two port commercial espresso machine. There is a wide range of about thirty volunteers most of whom are not coffee drinkers and each volunteer does maybe just a few days each month. While we have automatic measured grinder removing variables at that stage the tamping process is very variable: different pressure and puck not being level.
But, the resultant coffee seems to be consistent and good, we have positive comments from customers who are not locals but passing through our village and see our signage. So, in my opinion, tamping is not as significant as many claim.
