What proportion of your extraction comes from fines, or boulders? OK, that’s the last time in this post I’m going to use the arbitrary terms like “fines”, or “boulders”…it was just click bait, sorry about that! 🙂
If we sift coffee grinds using ASTM/ISO sieves we end up with a proportion of the total grind weight at somewhat predictable size intervals. For many grinders, a standard deviation in the grind weight is around 1.4 times/divided by the average grind size. So a 500um average grind will have 2% close to 250um, 16% close to 350um, 50% at 500um, 84% close to 700um, 96% close to 1000um.
We’ll stick with this because the numbers break down nicely in an intuitive way, but a tight distribution could have a standard deviation of little less than a factor of 1.4, a distribution up to a factor 1.5 would still be typical for a well engineered grinder, a Porlex, or similar might have a factor of 1.7.
Kruve sieves tend to make for wider standard deviations at the smaller sieve sizes, so we are not discussing those with respect to this post. I’m not saying that the Kruve sifter has no utility, just that the sieves don’t correlate to ‘microns/um’ in common parlance & that the Kruve standard deviations are less linear in nature, in the smaller particle range. Basically, you can’t mix and match Kruve and ASTM/ISO results.
We are also assuming, as is commonly done, that our particles average out as spherical in shape & extractable volume.
The graphic above illustrates, ‘back of a napkin’ style, what percentage of the total extraction each standard deviation may account for, assuming a similar average penetration & extraction depth across the whole, cumulative grind distribution. For example 75um extraction depth, or 95um extraction depth, or whatever…but a common average depth across the ground sample.
We’ll assume that a 50g dose of coffee was used and it extracted to 20% extraction yield. This then gives us 10g of dissolved coffee, extracted from the 50g dose and makes each 1% proportion of the total extraction, equal to 0.1g of dissolved coffee.
0 to <250um particles – These make up around 2% of the ground weight (this could be more with finer grinds like fine espresso, but sifting is problematic with very fine grinds). Even if these grinds extract to 30%, they only account for 3.4% (0.34g) of the total 10g of extracted coffee. If penetration depth is 100um and maximum extraction for these particles is subsequently 26% extracted, that’s 3% (0.3g) of the total coffee extracted.
>250 to <350um particles – These make up around 14% of the ground weight and likely contribute 17-18% (1.7-1.8g) of the total coffee extracted.
>350 to <500um particles – These make up 34% of the ground weight and likely contribute the second largest proportion of the extraction at 39% (3.9g) of total.
500um particles – It seems likely that the average grind size extracts to pretty much the same extent as the overall extraction yield, around 20% in this scenario.
>500 to <700um – Another 34% of the ground weight & the second largest contribution to the total yield at 28-29% (2.8-2.9g) of the coffee extracted. Therefore, >350 to <700um (+/- 1 standard deviation from the average) provides around 70% (7g) of the total extraction, in this scenario.
>700 to <1000um particles – The larger particles are harder to extract, their contribution thus dropping to around 9% (0.9g) of the total extraction.
>1000um particles – These are likely only half extracted, being about 2% of the ground weight, might only be contributing 1% (0.1g) of our 10g of extracted coffee.
Experiments in extraction (such as Zanoni, Pagliarini & Peri, 1992) suggest that smaller particles are extracted to a lesser depth than the larger particles. Which, if anything, implies that I am over-estimating the contributions of the smaller particles and under-estimating the contribution of the larger particles here?