In 2020, Stéphane Ribes conducted a series of experiments to answer key questions regarding the radial extraction uniformity of espresso pucks. He published his findings in a March 2020 presentation. The following article is based on said presentation.
What does radial extraction uniformity mean?
Radial extraction uniformity describes how evenly water dissolves coffee solubles from the center of the coffee puck outward to its perimeter during espresso extraction.
In a perfectly uniform radial extraction, water flows at the same rate and extracts the same proportion of coffee grounds regardless of where they sit along the radius of the basket. This means the water flows at the same rate and extracts the same proportion of coffee grounds in the center vs. the outer edges.
What questions did Stéphane try to answer?
Stéphane attempted to investigate and answer the following questions:
1. How uniform is coffee extraction across different radial zones of an espresso puck?
Stéphane measured the Extraction Yield (EY) across three different concentric sections of the coffee puck (outer, middle, center) compared to the overall espresso EY to evaluate extraction distribution.
2. Does placing a paper filter below the coffee puck improve extraction uniformity?
He tested whether placing a V60 paper filter under the puck creates a more uniform extraction across the puck (center vs. outer grinds) and whether it increases the total average extraction yield.
3. What effect does a convex tamper base have on radial extraction uniformity compared to a flat base?
Stéphane evaluated how using a convex tamper base (US curve) alters flow in the center of the puck compared to a flat tamper base and how it affects the average extraction yield.
4. Does the diameter of the perforated surface area of a filter basket affect extraction yield and radial uniformity?
He compared precision filter baskets with smaller versus larger perforated bottom diameters (e.g., comparing a 40 mm Reneka basket to 49 mm baskets like VST, IMS, and Decent) to see if expanding the perforated area could improve extraction uniformity.
What was Stéphane's setup?
Equipment & tools: Decent Espresso Machine DE1PRO v1.1 (with a red Cafelat 8.0 mm silicone gasket and an IMS SI 200 IM screen without spacer), Mahlkönig EK43 S grinder fitted with SSP "High Uniformity" burrs, and a 15g VST ridgeless filter basket.
Water & coffee preparation: Montille water adjusted to 50 ppm CaCO3 alkalinity and 125 ppm CaCO3 total hardness using sodium carbonate and Epsom salts. Single doses of frozen coffee beans (12g per shot) were ground into a double-walled stainless steel cup.
Puck prep & tamping: The basket and shower screen were dried with a clean tissue before every shot. Distribution was performed using a Londinium WDT tool with a Decent funnel (no vertical tapping), followed by gentle raking with a hog tool to smooth the surface. Tamping was completed using The Force Tamper (58.5 mm smooth flat or US curved base) applied twice sequentially.
Brewing & profiling: Extraction used a hybrid "Lever-Blooming" profile at a flow rate of 2.5 mL/s with a 1:2.5 brew ratio.
Measurements: Samples were extracted, divided into radial zones (center, middle, outside), and evaluated along with the final espresso shot. Total Dissolved Solids (TDS) and Extraction Yield (EY) were measured at room temperature using an unfiltered Atago PAL refractometer (zeroed with the test water) after vigorous stirring. Each recorded data point represents the average of 3 to 5 measurements.
What were Stéphane's core findings & Extraction Yields (EY)?
Thanks to his experiments, Stéphane found the following EY:
To sum it all up, Stéphane found the following:
1. Baseline extraction deficit: In a standard espresso shot, the outer edges of the coffee puck (outer 1/4 of total dose weight) achieve only half the extraction yield of the center grinds.
2. Impact of paper filters: Placing a V60 paper filter below the puck significantly improves radial uniformity and increases average Extraction Yield from 21% to 24%.
3. Impact of tamper shape: A convex tamper base (US curve) increases flow toward the center, worsening radial unevenness and dropping average EY to 20% compared to a flat tamper base (21%).
4. Impact of the filter basket perforated surface area: The Reneka Micro-Sieve basket (40 mm perforated diameter) has a surface area 33% smaller than standard 49 mm baskets (Decent, VST, IMS), causing a typical loss of 3 EY percentage points.
Expanding the perforated surface area on precision baskets from 49 mm to 53 mm increases active hole coverage by +17%, which is hypothesized to significantly improve radial extraction uniformity.
What conclusions can be drawn from the experiments?
Based on the findings, the following conclusions can be drawn:
1. With standard baskets, the outer parts of the puck get extracted way less than the center. So there's massive radial inconsistency in standard shots.
2. Placing a V60 paper filter below the coffee puck significantly improves radial uniformity across all zones.
3. The perforated surface area of filter baskets directly affects extraction yield. Consequently, expanding the perforated surface area of precision baskets could further enhance radial extraction uniformity.
#research #Ribes #coffee #EY #extraction #documentation
