Coffee Data Science
The Imprecision Filter Basket for Espresso
Pivoting on what makes great espresso
In my previous Moka pot study, I found large holes were effective for espresso brewing without coffee migrating through the basket. So I wanted to modify a basket with an aim at imprecision. Most of the espresso community has pivoted towards precision baskets. The original investigation into precision baskets aimed for a particular shot time not necessarily extraction yield.
My previous work has looked at filter baskets based on how much area of the basket is open. I theorized that a larger area open for the basket was more important than precision.
To further test this theory, I modified an old filter basket (from the Kim Express). I didn’t want to use a newer basket just yet in case this experiment was a disaster.
I pushed the same needle through all of the holes, and I measured the holes before and after.
After widening the holes, I scraped down the top because there was metal pushed up.
I imaged before and after.
Then I did some hole analysis. Spatially, they had a similar randomness.
I compared these to others. The original Kim basket to the Kim D Max or Kim Max. I called it the Kim D for Double because it was the original double basket. Then I did one round of this where the hole openness was only 4%, so I did the process one more time to get the maximum hole size (hence Kim Max).
The Kim Max filter has almost double the hole area as the VST, and the average hole size is not only larger but has a wider distribution.
Then I pulled a shot.
Equipment/Technique
Espresso Machine: Decent Espresso Machine
Coffee: Home Roasted Coffee, medium (First Crack + 1 Minute)
Shot Preparation: Staccato Tamped
Pre-infusion: Long, ~25 seconds
Filter Basket: 20g VST and the Imprecision basket.
Other Equipment: Atago TDS Meter, Acaia Pyxis Scale
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.
Feasibility Data
Let’s start with two pairs of shots. I was pulling these when doing my cooling extraction study, so I have with and without. In terms of taste, the Kim D Max performed a noticeably better. In terms of TDS and EY, there is a mix.
I took a bit more data on 6 Pairs across 6 roasts. The Kim had a taste advantage.
Looking at the individual score markers, the Kim D Max had better performance. In terms of TDS/EY/IR, it is slightly lower.
Coffee Migrating?
One concern is that this basket could cause coffee to migrate out of the basket and into the cup because the assumption is that the holes need to be small for a fine grind. So I pulled a long shot with spent coffee remixed.
The sample of coffee grounds into the cup was not measurable (>0.01g).
This data brings me back to questioning how the basket functions fundamentally, and I think filter basket design should be reevaluated especially for model espresso profiles. I suspect the Wafo basket will outperform others as a result of these tests.
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