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Retention is ground coffee left inside the grinder after a dose, such as in the chute or burr chamber. It can mix with the next dose, affecting freshness, weight and the response to grind adjustments.
Every grinder retains some ground coffee between uses — stuck to burrs and surfaces through static electricity, sitting in a chute waiting to fall, or held in a doser chamber. When you grind for your next shot, this old coffee is pushed out first, mixed into your fresh dose. If you ground yesterday, that retained coffee is a day old. If you changed your grind setting, the retained coffee is at the old setting. In both cases, your dose contains coffee that is not representative of what you just ground.
Retained grounds can delay the visible effect of a grind adjustment because the next dose may contain coffee from both settings. How much carries over depends on the grinder and its use. Keep your dosing routine consistent and judge an adjustment after accounting for the retained grounds.
Grinders differ in how much coffee remains inside and how much of it is exchanged into the next dose. Doser chambers can hold additional grounds, while single-dose designs aim to reduce carry-over. Total retention and exchanged retention are different measurements. A near-zero claim does not mean there are no grounds left anywhere inside.
Some grinders benefit from purging a small amount of coffee after an adjustment or a long pause. Follow the grinder’s guidance rather than assuming one purge weight works for every model. Weigh the grounds used for your recipe so retention does not silently change the dose.
Single-dose grinders are designed to grind a weighed portion of beans. RDT means adding a very small amount of moisture to beans to reduce static, but it is not suitable for every grinder. Use it only if the manufacturer permits it and follow their quantity limit. Do not spray water into the grinder or assume moisture cannot affect grinding or extraction.
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