Coffee Data Science
Bellowing During Coffee Grinding
Cause and effect
I haven’t used a bellows on my other grinders, and I have been hesitant to get one. However, I bought a new grinder (SPK-38G), and it came with a bellows. After using it for a week, I started to wonder what happens if you use the bellows during the grind?
What would happen if you continuously bellow throughout the grind? In theory, finer particles that could make it between the burrs would be pushed out sooner, lessening the chances of regrinding.
I did a test with two shots, and I looked at particle distribution, extraction yield, and taste.
For particle distribution, bellowing reduces the number of fines quite a bit. For particles 200um or less, it is a 8% difference.
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
Infusion: Pressure Pulsing
Filter Basket: 7g/20g VST
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).
Two Shots
Both shots had a similar development visually.
In terms of flow and water temperature, they were a bit different.
In terms of taste and extraction, bellowing continuously was better.
One difference was that the bellows shot had a faster time to cover the filter (TCF), but the infusion time was longer. I would expect a coarser grind to make the shot run faster overall, but the difference between these two time metrics is intriguing to me.
Bellowing throughout the shot is a bit impractical every shot, but one could make a grinder to push air through the grinder continuously to get the same effect automatically. I don’t know how this effects how unimodal a grind distribution looks, but it certainly points to a technological improvement towards reducing fines.
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