espresso reproducibility

The coffee formula: Could lowering pressure, dosage, and grind size be the keys to reproducibility?

Lower pressure, reduced dosage, and coarser grinds could be the key to espresso reproducibility, faster shot times, and saving thousands of dollars per year.

Too many times, a barista will brew an excellent espresso and repeat the recipe to the exact same specifications only to end up with a different result. This drove Michael Cameron mad.

During an online conversation in 2016 with Prof. Christopher Hendon of the University of Oregon, author of Water for Coffee and various coffee-related works, Michael – at the time a Café Manager at Frisky Goat Espresso in Queensland – expressed his frustration and asked how he could make better espresso more consistently. Christopher said he could start by reducing his line pressure from nine bar to six bar.

“I dropped the pressure and it was amazing coffee. I talked to Chris about it a few days later, and he suggested we write a paper,” Michael says.

“It started with the idea of testing if you can make high extraction coffee on a consistent basis by dropping your line pressure. We thought the finer you grind, the more surface area you expose to water, so the higher your extraction yield would go, and that it would be a higher extraction yield at six bar than nine bar.”

Prof. Christopher Hendon of the University of Oregon has published several coffee-related works.

Michael started going in on weekends to produce espresso in a controlled setting and identified a “tasty point” at 22 per cent extraction yield. Meanwhile, Chris developed a mathematical model which predicted how grind size, coffee mass, and water volume would affect the extraction yield. The model matched Michael’s experiments up to a certain point when extraction yields started to decline.

“We were expecting this to be a quick and easy study, because we assumed that you could just grind finer and get more out the coffee,” Christopher says.

“But we couldn’t pump water through at nine bar through a fine grind setting. It choked the machine. And then after we went to six bar, the extraction yield from the ground coffee bed was actually lower than we expected. That’s when we knew we were onto something unusual and unfortunately, we had a lot more work to do.”

Four years later, their work was published in ‘Systematically Improving Espresso: Insights from Mathematical Modeling and Experiment’ by scientific journal Matter.

Christopher identified clumping as the issue, causing certain areas to be over extracted, others under extracted, and some untouched by water. With Michael’s tasty point on the uneven side of the peak extraction yield, Christopher realised the same yield could be achieved more consistently in less time with a coarser grind size.

He says while the percentage of compounds extracted by the two shots is the same, the flavour may not be.

“They’re going to taste different because they’re different types of extractions. One of them is even throughout the bed – or as even as possible – and the other one is 22 per cent made up of some components extracted to 25 per cent, 24 per cent, all the way down to zero per cent,” Hendon says.

“If you’re happy with the flavour of these unevenly contacted coffee grounds, then you have to accept that you’re going to have shot-to-shot variation because of the mechanics of water passing through a bed made of those particle sizes.”

Chris and Michael encourage baristas to find their own tasty points.

Using this knowledge, they identified two other methods of consistently reaching the same extraction yield. One means identifying the maximum extraction yield and using less water to reach the tasty point.

The other involves reducing dry coffee mass volume and grinding even coarser. In the study’s case, Michael was able to reach the tasty point by reducing coffee mass from 20 to 15 grams and increasing grind size from 1.7 to 2.3 on a Mahlkönig EK 43.

“We’re talking coarser than you would for an espresso grind. We’re not suggesting a grind as coarse as you’d use for a French press,” Michael says.

“What that means for a café is, if they have a recipe that uses 20 grams of coffee, they can hit an approximate recipe or flavour profile using up to five grams less coffee. If you serve 400 coffees per day, using five grams less coffee, you save two and a half kilograms daily. Multiplied over a week, month, or year, that’s a lot of money you’re saving without seeing a reduction in coffee quality.”

Christopher trialled this method over 12 months at a café in Eugene, Oregon in the United States. From September 2018 to September 2019, the café served 27,850 espresso drinks. By reducing dosage from 20 to 15 grams, Christopher says this café reduced shot times to 14 seconds and saved 13 US cents (about 20 cents) per shot. This totalled US$3620 (about $5470) additional revenue over the year.

Though the experiment and café began with a 20 gram dosage and reduced it to 15, Christopher says this is only one possible recipe and their findings can be translated to other brew ratios.

Michael – who now writes for St Ali – says while the idea of cafés using less coffee may be unattractive to some roasters, reduced wastage and higher evenue is better for the sustainability of the industry.

“From a café owner perspective, coffee is a very hard industry to be in,” he says. “There’s a co-dependent relationship between the roaster and the café. Roasters need cafés to be sustainable so they are around for a long time to keep buying coffee. Otherwise they have to find another café to supply coffee too.

“There is a middle ground that needs to be discussed about how roasters can allow their customers to be more sustainable. This is one way to do it, but it’s going to put questions to the roaster about how they’re going to efficiently use their coffee. As a forward-thinking industry, we can come up with a solution that benefits everyone.”

Alongside the obvious financial ramifications of the study, Michael suggests that the coffee industry not forget their original hypothesis, that lowering pump pressure will result in higher extraction yields that are more reproducible.

“That was something statistically significant from the very first data set that I gave to Chris. It’s almost so obvious and certain from the data now that it gets overlooked,” Michael says. “If you want reproducibility and higher extractions, and if you believe that those higher extractions also equal tasty coffee, then lower your pump pressure.”

He encourages baristas who question the study to carry out their own experiments with their own tasty points.

“The ideas of grinding coarser, lowering pressure, and reducing coffee mass are strategies. They don’t have be used in combination. The questions is ‘does it still taste good?’ That’s where individual preference comes in,” Michael says.

“When people dial in, they grind as fine as they can until the machine starts choking up and then dial back a little bit. They do that because it’s the status quo. We need to move forward as an industry and see what we can do to push those boundaries.

“We’re showing them how to reproducibly create that coffee every single time. That makes the barista’s life easier and encourages them to question things. I think it’s a win-win.”

This article appears in the June 2020 edition of BeanScene. Subscribe HERE.

Systematically Improving Espresso: Insights from Mathematical Modeling and Experiment can be found online at www.cell.com