Grinding
Purging a grinder
Grinders keep some coffee inside. Purging is how you stop yesterday's beans and last week's setting turning up in today's brew.
- Skill level
- Beginner
- How long it takes
- Seconds
- Equipment needed
- Burr grinder
- Steps
- 4
Why it works this way
Retention is the coffee that stays in a grinder between doses, in the chute, around the burr carrier and in the exit path. Every grinder has some. It matters for two reasons: it is stale, and it was ground at whatever setting was in use when it got stuck there. So a grind adjustment does not take full effect until the retained coffee has been pushed through, and a change of bean is not complete until the previous bean has left. Purging is the routine that deals with both, and the amount needed depends entirely on the grinder.
What you need
Required
Helpful, not essential
How to do it
After a grind adjustment, run a small amount of coffee through and discard it.
The purge carries the old-setting coffee out ahead of the new.
When changing coffees, purge more generously.
A few grams of the new coffee pushes out the old. Grinding a small amount and discarding it is cheaper than an unexplained shot.
If single dosing, use a bellows or a brush to clear the chute after each dose.
This is where most retention sits in a single-dosed grinder, and where a puff of air is most effective.
Weigh the output, not the input, if retention is significant.
What you put in and what comes out differ by the retained amount, which varies shot to shot.
How to tell it worked
Consistent doses and shots that respond predictably to adjustments, rather than a shot that behaves as though the setting had not changed.
If it did not work
What you saw, what to change, and why the change works.
Output weight varies noticeably from dose to dose
Change: Purge the chute more thoroughly and weigh what emerges.
Variable retention makes input weight an unreliable guide to dose.
Coffee tastes faintly of the previous bag
Change: Purge more when changing coffees.
Retained grounds mix into the next dose.
Common mistakes
Blowing compressed air into the grinder body
Fines are driven into bearings and the motor housing.
Instead: Use a hand bellows aimed at the chute.
Purging with beans you were about to brew and then using them anyway
The purge is defeated.
Instead: Discard the purge.
Diagnose a specific problem
The science behind it
- Retention (grinder)
Leftover grounds from a previous grind still sitting inside the grinder, mixing into the next one.