Not just wasted coffee. What stays behind inside a grinder sits there aging before it eventually works its way into a later grind, which is the part of this conversation that usually gets left out.
Retention is typically measured the straightforward way: weigh the beans going in, weigh the grounds coming out, and the difference is what stayed behind inside the grinder. It sounds precise, but the real number shifts a bit from grind to grind depending on humidity, static buildup, and how long it's been since the grinder was last cleaned out.
That's worth keeping in mind when you see a specific figure like "0.1 to 0.2 grams" for the Niche Zero. These published numbers are generally averages across repeated testing, not a guaranteed identical result on every single grind.
Retained grounds mostly end up in a handful of predictable places: clinging to the walls of the burr chamber through static charge, sitting in corners or ledges along the exit chute, or lingering in the space directly around the burrs themselves where airflow and gravity aren't enough to clear them out. Different grinders are designed with more or less attention to eliminating these specific spots, which is a big part of why retention varies so much between models even within the same burr type.
The obvious cost of retention is straightforward: you paid for beans that never made it into your cup. The less obvious cost is what happens to the coffee that stays behind. Ground coffee left sitting inside a grinder is exposed to air and starts going stale, losing aroma and flavor, well before it eventually gets dislodged by a later grind.
That means retained grounds aren't just missing from today's cup, they're quietly degrading and then showing up in a future one, adding stale, muted notes to what would otherwise be a fresh grind. This matters most when you switch between different beans, since it's not just staleness at that point, it's a different coffee entirely mixing into the one you meant to drink.
A few practical techniques show up repeatedly in how people deal with this. RDT, adding a small amount of water to whole beans before grinding, cuts down on the static charge that causes grounds to cling to the inside of the grinder. Purge grinding, running a small, discarded dose through after switching beans, clears out old residue before the real dose runs through. Some grinders also include or support bellows or brushes specifically for physically clearing out what static and gravity leave behind.
Typically by weighing the beans going in and weighing the grounds coming out, with the difference being what stayed behind. Retention can vary somewhat from grind to grind depending on humidity, static, and how long it's been since the grinder was last cleaned, so published figures like 0.1 grams are generally averages rather than an exact guarantee every time.
Both. It's wasted coffee you paid for and didn't get to drink, but it also sits inside the grinder oxidizing and going stale before it eventually gets dislodged into a future grind, contributing stale, muted flavor to a batch of otherwise fresh coffee.
Running a small, discarded dose of beans through the grinder after switching to a new bean, specifically to catch and clear out old retained grounds before running your actual dose. It's a common practical technique alongside RDT for people who care about minimizing cross-contamination between beans.
Much less. If you're not switching beans, the small amount that stays behind is largely the same coffee getting reincorporated rather than a different one contaminating your cup. Retention matters most specifically when you're rotating between different beans.