The distribution of Extraction Yield (EY) across an espresso puck is rarely uniform. This study by Stéphane Ribes from April 2021 investigates radial uniformity during extraction and evaluates how placing a BPLUS contact screen (also called "puck screen") directly above the ground coffee puck alters both local extractions and the overall shot yield across VST and Pullman filter baskets.
Core research questions
This study aims to answer three key questions:
1. What is the radial distribution of the Extraction Yield (EY) within an espresso puck during extraction?
2. How does using a BPLUS contact screen placed above the puck affect extraction uniformity across different radial distances (center, middle, and outside)?
3. What effect does the BPLUS contact screen have on the overall average Extraction Yield (EY) when using VST and Pullman filter baskets?
Methodology & experimental setup
To isolate the variables influencing extraction uniformity, a precise, highly controlled testing framework was established.
Espresso machine: Decent Espresso Machine DE1PRO v1.1, featuring an S22A Ultem prototype shower and an IMS CI 200 IM shower screen (protruding 9.6 mm into the basket).
Grinder: Levercraft Ultra Grinder equipped with SSP High Uniformity blind burrs (Red Speed coating), run at 100 RPM at a grind setting of 2.0 (100 microns above the burr touch point).
Water specifications: Montcalm water sourced from the Pyrenees, France (4 ppm eq. CaCO3 alkalinity and 10 ppm eq. CaCO3 total hardness).
Extraction profile: Easy Blooming profile set to 90°C, featuring a 2.5 bar bloom exit pressure and a 7.5 bar maximum extraction pressure.
Dose & brew ratio: A fixed 19.0g coffee dose pulled to a 1:2.0 brew ratio across all test iterations.
Filter baskets: 20g VST basket and 20g Pullman basket.
Preparation & puck workflow:
Portafilter and filter basket were pre-heated to 80°C in a kettle prior to testing.
Basket and shower screen were thoroughly dried before every shot.
Single doses of frozen coffee beans were ground directly into the basket.
Distribution was performed using a Levercraft WDT tool (8 x 0.4mm needles), raked with a BPLUS stirrer, and finished with an SworksDesign PorcuPress v1 (109 x 0.8mm needles).
Ground coffee was tamped using a 58.5mm flat base Bravo Tamper at 26lbs of force, applying a light polishing step when no contact screen was used.
Measurement & data logging procedure**
TDS testing: Total Dissolved Solids were measured using an Atago PAL refractometer zeroed with Montcalm water.
Sampling zones: Measurements were logged for the overall liquid shot as well as three distinct radial sections across the puck: Center (0mm–18mm, Middle 18mm–25mm, and Outside (25mm–29mm).
Sample handling: Unfiltered coffee samples were vigorously stirred and brought to room temperature before taking readings. Every reported data point reflects the average of 3 to 5 individual measurements.
Results
The data obtained through the experiments demonstrates a significant improvement in extraction uniformity and overall efficiency when applying a BPLUS contact screen across both VST and Pullman filter baskets. Below are all the Extraction Yields (EY) found:
The data shows the following:
Edge underextraction without contact screen: Without the BPLUS screen, both filter baskets exhibit a sharp decline in extraction yield toward the outer edge of the puck (25% center vs. 16%-17% outside).
Uniformity gains with contact screen: Adding the BPLUS screen dramatically elevates outer edge extraction from 17% to 22% in the VST basket and from 16% to 23% in the Pullman basket, creating a far more even extraction profile across all radial zones.
Increase in total Extraction Yield: By mitigating severe periphery underextraction, the overall shot Extraction Yield increases by +2% in the VST basket (reaching 24%) and +3% in the Pullman basket (reaching 24%).
Conclusion & key takeaways
Based on the findings, the questions can be answered as follows:
1. What is the radial distribution of the Extraction Yield (EY) within an espresso puck during extraction?
Standard espresso extractions exhibit significant radial non-uniformity. Without a contact screen, extraction at the outer perimeter of the puck (representing roughly 30% of the total dose weight) is severely underextracted, reaching only about two-thirds (approx 16%-17%) of the extraction yield measured at the center (approx 25%).
2. How does using a BPLUS contact screen placed above the puck affect extraction uniformity across different radial distances (center, middle, and outside)?
Placing a BPLUS contact screen directly above the coffee puck significantly improves radial uniformity. It lifts the outer extraction yield from 16%-17% up to 22%-23%, balancing extraction across the center, middle, and outer radial sections.
3. What effect does the BPLUS contact screen have on the overall average Extraction Yield (EY) when using VST and Pullman filter baskets?
By eliminating severe underextraction at the outer edges of the puck, the BPLUS contact screen delivers a substantial increase in overall shot extraction yield for both basket types: increasing from 22% to 24% in the VST basket, and from 21% to 24% in the Pullman basket.
In conclusion, using a puck screen significantly improves radial uniformity.
#research #Ribes #coffee #extraction #uniformity #documentation
