# Unimodal & bimodal grinds



## MWJB (Feb 28, 2012)

If you grind coffee, then observe the distribution with a laser particle analyser (LPA) the plot may appear to be unimodal (one hump/spike), or bimodal (2 humps generally, but could be multiple little peaks at the left hand side).

What causes this?

Finer grinds (espresso & very fine drip) cause an increase in the incidence of particles in the 40-60um range. This is the secondary, or fines peak & a bimodal grind. To brew espresso with a unimodal grind you would have to find a way to brew with coarser filter grinds, or sift out the smaller particles. Espresso grinders & filter grinders set fine enough to brew typical espresso, do not naturally produce a unimodal grind.

But if you set the same grinder coarser, this 'fines peak' diminishes, until you get to coarse brewed settings (if your grinder will go that far) & it basically disappears, leaving one hump, or peak (unimodal). See the plot here for Baratza Preciso as an example:http://www.baratza.com/wp-content/uploads/2015/07/PRECISO-Graph.pdf

Note that the scale is logarithmic on the horizontal axis, if it were presented in a non-log format, the 'dragons tooth' peak at the right would be a wide hill/hump.

Sieving grinds is not able to identify these different modes because particles in the range we are interested in for bimodal grinds, doesn't sift well & there's usually not useful data, or detail, much below 200um. Sieving is more useful for brewed grind analysis, or for a rough outline of the coarser end of espresso grinds.

Sieving brewed grinds would normally just use 2-4 sieves, with much less detail, a hump style, non-cumulative representation would look more like this:









The sieves could be selected to target the desired range of grind size & provide most useful info.


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## Thecatlinux (Mar 10, 2014)

With that in mind does this mean a typical espresso grind is made up from unimodal and bimodal particles ? And is that why a good distribution is essential ?


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## MWJB (Feb 28, 2012)

Thecatlinux said:


> With that in mind does this mean a typical espresso grind is made up from unimodal and bimodal particles ? And is that why a good distribution is essential ?


A typical espresso distribution will have the fines peak at 40-60um, this makes it bimodal. This peak is generally considered necessary for extraction, without it the liquid would squirt through the puck in a few seconds.

Distribution has a couple of meanings here:

1. The characterisation of the grinds produced, as in narrow, or wide distribution. This is dictated by your grinder & setting.

2. The physical placement of grinds in the basket, in order to ensure a puck of reasonably uniform density, to stop channelling & allow water to pass through the whole puck as evenly as possible. This aspect is always desirable, irrespective of specifics of point 1.


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