I have three manual espresso machines and a Nespresso and even after years of making them, it can be difficult to get it just right. It takes pressure, understanding the ground beans, and a touch of finesse, which can be more challenging than it sounds.

As beans age they express gases. The carbon dioxide is high right after roasting and after about five days it has leveled out to where you can get the quality, but de-gassing continues to happen, and after a month they are flat. Nothing can stop that physics. It was 1908 Nobel laureate Dr. Ernest Rutherfordwho said, "All science is either physics or stamp collecting", and chemists agreed so much they gave him a Nobel prize. You can slow it down but it will get you.

Buy LaVazza beans from Amazon and they could have been roasted months ago. They will note their flimsy bag is vacuum sealed but even a pressure canister will let in air after a week. A local roaster will put the roasted date in plain sight, and they can tell you based on experience when to grind and use the beans. Yet the aging that has to take place before freshly roasted beans will be just right continues to happen, so you will likely be adjusting your grinder as they change.

Not an espresso, a latte. Credit: Hank Campbell, Science 2.0

A new study says science can help with that. It won't turn your espresso into art any more than a science of dancing can turn you into Lady Gaga, but it can get you out of any get-together with your dignity intact.

In 1884, Angelo Moriondo of Turin patented the first espresso machine and like pistols and cars from that period, the underlying science hasn't changed all that much.  A whiskey entrepreneur and engineer improved the patent with a portafilter and a pressure release valve. The pressure used to be applied manually but motorized pumps became common in the 1960s.

Yet at its heart it's still the same as 1920; hot water is forced through a puck of finely ground coffee and a filter.

Folk wisdom says you need an ounce of finely ground espresso beans to get a decent pull but if you believed in folk wisdom without hard evidence you wouldn't be here. A 2020 paper sought to determine if extra-fine grinding and ~18 grams of coffee were really needed for espresso.  Coffee freshness and grind will often be the big determinants, I feel like I can pull a great shot with a 1920 manual machine if I can use my grinder and beans roasted two weeks ago. 

I have been at this a while, though. For newer people there is a lot of variation to think about. Too coarse, and you have coffee spraying (at least I do, because I use a bottomless portafilter) and it will be sour while if you are too fine you won't have much and it will be bitter.

The paper found that a coarser grind and less than 18 grams of beans will provide me consistencvy than the 18 in 36 out that is the rule of thumb. The reason is that an overly coarse grind will lead to barely detectable clogging - and that clogging result is hard to predict, much like bad tamping of the puck may lead to channeling and inconsistent flavor.

So if you are unsure of your grinder, or the age of the beans, be conservative and go a little bit coarser with 15 or so fewer beans.