Why Proper Tamping is so Important

Espresso is a very different drink than plain American coffee. Its proper preparation requires that the water and coffee grounds be in fairly precise proportions, and that the espresso be extracted under certain pressure at a particular temperature. The resulting flavor is very sensitive to variations in how this is done.

How Espresso is Different from Coffee

Espresso is very different from coffee. Coffee is prepared exactly like tea: the grounds are immersed in hot (usually boiling) water, allowed to steep, and then removed. What makes espresso unique is that instead of steeping the grounds, hot (but not boiling) water is forced through the grounds very quickly using high pressure.

Except for some lever espresso machines, commercial espresso machines (and also home espresso machines) use a pump to generate that pressure. The pressure will be at least 9 bars, and as much as 15 bars for any specific espresso machine. (1 bar = 1.02 atmospheres = 14.5 pounds per square inch.) The water temperature should be hot, but not boiling.

An Espresso Extraction

This espresso extraction should be done in about 30 seconds; not less and not more. This means that the pressurized hot water (and specifically, any particular drop of that water) is in contact with the coffee grounds only very briefly - a small fraction of the total 30-second extraction. It is called an "extraction" because this high-pressure quick flow of water through the coffee will extract the intense flavor of the coffee without being in contact long enough to become bitter.

And the fact that an espresso extraction is done with hot water (195-200 degrees) and not with boiling water (212 degrees), and that an espresso extraction is done quickly means that less of the caffeine is absorbed by the water as it passes through. Caffeine is water soluble, and heat is a catalyst. Steeping the coffee grounds in boiling water (as for regular coffee) will extract the most caffeine.

How Espresso Should Look and Taste

Contrary to what most people tend to assume, espresso has less caffeine than regular coffee. Also, contrary to what most people assume (since they have become accustomed to bad espresso prepared by untrained people in restaurants using poorly-maintained espresso machines), the strong flavor of espresso should not have a bitter-edge to it.

Espresso should taste smooth and intensely like coffee. It should have a rich, full, flavor with no bitter undertones to it. It should be an intense coffee flavor that you can almost chew. The color should be dark black.

A properly extracted espresso should have an even golden-brown layer of crema floating on the surface. The crema should not be dark, and there should be no bubbles in it.

Why Proper Tamping is So Important

To do a perfect (and by definition, consistent) extraction requires consistency in all aspects of the extraction process. The extraction should take 30 seconds at constant pressure with a constant water temperature for a constant volume. Specifically, 1 fluid ounce (30 milliliters) of water at 198 degrees should be forced through ¼ ounce (7 grams) of coffee grounds at 9 bars of pressure in 30 seconds to prepare the perfect espresso shot.

Of course a key factor required to make this happen consistently is the density (and the regularity of that density) of the coffee grounds. If the coffee grounds are not ground finely enough, or if they are not packed down (tamped) into the portafilter, or if they are not packed down uniformly, two things will happen.

First, although the pump itself is putting out 9 bars of pressure at the source end, the espresso machine will not be able to hold the pressure at 9 bars at the point of contact with the coffee grounds. This is because there will be insufficient back pressure generated by those coffee grounds. And second, because of this, the hot water will flow through the grounds too quickly and will not make sufficient contact for a full extraction. The extraction will be faster than 30 seconds, and the espresso will be weak and flavorless.

But if the coffee is ground too finely (as for Turkish coffee - which is why many coffee grinders are not made to grind for Turkish coffee), or if the coffee grounds are packed ("tamped") too densely, there will be too much back pressure generated by the coffee grounds, and the extraction will take longer than 30 seconds. The result will be a bitter-tasting espresso.

The Coffee Puck, and the Importance of the Grind

For uniformity and for the perfect espresso extraction, it is important to have uniformly ground coffee grounds, and it is important that the coffee grounds be packed to a uniform density within the portafilter. Tamping is the act of compacting the coffee grounds to achieve that uniform density. The packed coffee grounds within the portafilter is called a "puck" of coffee.

Espresso extraction is the act of pushing hot water through the coffee puck under pressure to extract its basic coffee essence out of the coffee grounds and into the water. After the extraction is complete, the puck is discarded, usually by knocking the group handle against a knock box to dislodge the puck.

Tamping is the act of compressing the coffee grounds into a puck prior to extraction. You should learn to tamp to produce uniform results in a reproducible way. We recommend that tamping be done by hand using a coffee tamper, and not by using the tamper attached to your commercial coffee grinder.