Grind Retention: What, why, and is it a big deal?

If you have ever read a home espresso forum or hung out long enough with coffee geeks, inevitably the term "grind retention" will arise. It is a fascinating subject - that combines a bit of engineering, physics, and economics. It affects all coffee grinders and when it comes to espresso it can be the difference between a perfect shot of espresso and... well, a mess. As we have not touched on it before, we produced a video to run through:

1. What is grind retention? 
2. Does it matter?
3. What do you do about it?
4. Which grinders are best at each price point?

Regardless of your budget, brew method, or whether you intend to single dose or not, we recommend giving it a watch so that you can pick an espresso grinder that suits your ideal home espresso setup.

Video Text Transcript Below:

Grind retention is a phrase that is often used by coffee enthusiasts when discussing the merits of different grinders. If you’re not familiar, the concept is quite simple: as beans are ground, there are spaces within the burr chamber and chute where particles can rest without being expelled into your portafilter. In many grinders, this is an intentional design element which helps reduce static and maintain flow for consistent dosing.

While there are benefits, there are also drawbacks to grind retention. Grounds retained in the burr chamber will lose freshness, which is an added variable for home baristas who aren’t using their grinder as frequently as a cafe. Secondly, this retention also means that an adjustment in grind size does not have an immediate effect, with some particles from the previous setting making their way into your dose. Both of these issues can be solved by a quick purge before your first shot of the day.

For those that wish to single-dose their beans, some grinders work exceptionally well, however, grind retention can be challenging in certain models.

So, what does this mean for you? Whether you fill your hopper and use timed dosing or single dose your beans incorporating a purge into your routine is invaluable. If you make just a few drinks in the morning then the next day it’s a good idea to do a purge of a few grams, generally just a second or so of grinding with a filled hopper, to flush out some of the stale grounds from yesterday. If you don’t those stale grounds will cause your first shot to be what is scientifically referred to as a “gusher.” The flow through the grounds will be far too quick and your shot will be watery. Generally, if the gap between shots is around 12 hours or less this effect is negligible. The greater a grinder’s retention, the greater the impact so we recommend purging a bit more coffee based on how much retention your grinder has. A grinder with minimal retention will help you marginally reduce waste by reducing the need for a purge.

If you aim to single dose then a grinder’s retention is likely among the greatest determining factors in whether or not it will suit your needs. There is an added factor to consider beyond purging: the more retention a grinder has the more variation in output you’ll see. Generally, these variations peak at one gram and the best remedy is to stick to a consistent routine in order to ensure consistent output.

Grinder Comparison

Let’s compare the retention of a few different grinders. Many tests we’ve seen start with a completely clean grinder and measure the amount of coffee retained after a single dose. This doesn’t represent the amount of grounds that build up in normal use and paints an unrealistically favorable picture. Our goal with measuring retention is to quantify the amount of loose grounds that rest in the grinder in normal use. Whether you’re using timed dosing or single dosing, these grounds are what will impact your shots and therefore influence your routine. We ground 80 grams of coffee through each grinder until no more coffee was dispensed. Then we opened up the burr chamber and swept out all loose grounds and cleared the chute and screen, collecting all those grounds and weighing them. This measurement is what we counted as the grinder’s total retention.

Small burrs make for small retention. As you can see, the Specialita’s small and neatly designed burr chamber leaves little room for grounds to collect which makes it a very low retention grinder, only ever holding onto about 1.5g of grounds. The Sette 270 in particular, with its conical burrs, has almost zero space to collect grounds thanks to its innovative shape. No burr chamber and an incredibly short vertical chute work wonders, with retention of less than a gram. The Eureka Mignon Specialita is a great example of a low retention flat burr grinder. Its 55mm burrs are on the smaller end of the spectrum, and its impressively compact burr chamber leaves little room for buildup, with only about 1.3g of retention. It works wonders when single dosing - a rare honor for a flat burr grinder.

This price range contains the bulk of prosumer flat burr grinders. Most of these grinders have relatively large burrs and motors, and are built to produce incredibly consistent grounds, and quickly. These larger burr chambers inevitably make for greater retention, but some designs mitigate that better than others. The Eureka Atom, for instance, has mid-sized 60mm burrs with very little space between the burrs and the wall of the burr chamber. This combined with Eureka’s proprietary forked screen, which causes very little retention, means that despite being quite a bit larger than the first few grinders we looked at it only retains around 2g. Though the Atom does have low retention we don’t recommend it for single dosing. It’s designed with the weight of a full hopper of beans in mind, and some pieces, like the augur, make it far faster with a full hopper but are counterproductive when single dosing. The Eureka Zenith has 65mm burrs and the gap surrounding its burrs is considerably larger. This key difference is what generates its 6.5-gram retention. In practice, this difference is nominal and entirely manageable with the same purge.

This price range contains grinders that are built to handle commercial use and are often built with top-notch materials and attention to detail. Having blazing speed and consistency means that these grinders are not designed with single dosing in mind, and therefore have a good bit of retention. Let’s start with the Eureka Olympus. It has huge 75mm flat burrs, but its burr chamber has the smallest gap between the burrs and the inner walls. Additionally, Eureka’s screen design and the immense torque of the Olympus’ motor made for a surprisingly low retention of 3.5g. As you can see the burr chamber of the V-Titan is also designed to hug fairly tightly around the large 64mm burrs and has 3 paddles, where most grinders have two. This design leaves relatively little space for grinds, so the V-Titan only has about 4.5g of grind retention. The Eureka KRE is unique in that it is one of a select few large conical burr grinders in our lineup. While it's one of our favorite grinders with the hopper full, it retains 12g of coffee making it less than ideal for single dosing. Large burrs such as these require the aforementioned purge, but they also benefit greatly from consistent routine. By ensuring that you grind for approximately the same time and use the same technique with each dose, one can ensure that they get an output within 1 gram of their input.

Conclusion

All in all, the extent to which grind retention matters to you depends on your preferred grinding routine, and how often you’re making drinks. For those that wish to single dose choosing a grinder often involves balancing the benefits of larger burrs and motors with their requisite increase in retention. For those that use timed dosing it’s simply a matter of understanding the amount of retention and knowing how much to purge to ensure consistently delicious results. We’ve used every grinder we sell extensively, so if you ever have grind retention questions regarding a specific grinder leave a comment below or give us a call! We’re glad to provide insight.