Coffee Data Science

Spraying Water onto the Espresso Puck

Modifying spraying with thermal pre-infusion

4 min readOct 6, 2023

Espresso is challenging, and there have often been tricks to arise to help improve extraction. Some times, it is unclear what tricks to try, but I stay open minded. Some time ago, spraying the top of the puck became fashionable, and I tried it once or twice, but I didn’t see a good gain from it. However, I wanted to try again, but with a twist.

I have been using thermal pre-infusion for the past few months, and I think spraying the puck could help with heat transfer into the coffee. So I tried it again, and I measured how much water I was adding.

I aimed for 2g of water total. I prepare my pucks in two parts, so I added 1g for the bottom half and 1g for the top half. Then I collected some data to see if this technique was helpful.

Equipment/Technique

Espresso Machine: Decent Espresso Machine, Pump & Dump Profile

Coffee Grinder: Zerno

Coffee: Home Roasted Coffee, medium (First Crack + 1 Minute)

Shot Preparation: Staccato Tamped with and without water sprays

5 Minutes of Thermal Pre-infusion

Pre-infusion: Long, ~25 seconds

Filter Basket: 20 Wafo Spirit

Other Equipment: Acaia Pyxis Scale, DiFluid R2 TDS Meter

Metrics of Performance

I used two sets of metrics for evaluating the differences between techniques: Final Score and Coffee Extraction.

Final score is the average of a scorecard of 7 metrics (Sharp, Rich, Syrup, Sweet, Sour, Bitter, and Aftertaste). These scores were subjective, of course, but they were calibrated to my tastes and helped me improve my shots. There is some variation in the scores. My aim was to be consistent for each metric, but some times the granularity was difficult.

Total Dissolved Solids (TDS) is measured using a refractometer, and this number combined with the output weight of the shot and the input weight of the coffee is used to determine the percentage of coffee extracted into the cup, called Extraction Yield (EY).

Intensity Radius (IR) is defined as the radius from the origin on a control chart for TDS vs EY, so IR = sqrt( TDS² + EY²). This metric helps normalize shot performance across output yield or brew ratio.

Data

I took 20 paired shots (but only 17 pairs had taste data). The trend was pretty clear with some large gains in some cases.

This trend was across the board for the taste metrics.

In terms of TDS/EY/IR, the water spray was mostly beneficial.

We can zoom in on just TDS/IR

On a statistical level, these average improvements were statistically significant based on a two-tailed t-test.

I really don’t like adding steps to my routine, but adding a water spray with thermal pre-infusion has improved the experience. I am also curious how much water you can add to the puck and still get benefits.

Initial tests with adding 8 grams of water seem to reduce extraction yield. I have previously tried a technique from over 10 years ago where one would mix water into the coffee grounds for a moka coffee, but the results weren’t interesting enough to continue down that exploration.

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