Retention is the term used to describe the empty space in a coffee grinder where ground coffee accumulates that is not expelled as part of the current grinding. Good grinders have low retention and expel most of the ground coffee.
Retention develops from free spaces around the grinding burrs. Long and winding discharge paths that the ground coffee must travel after grinding to reach the output increase retention. In English, this is referred to as retention, meaning the remaining ground coffee. We distinguish between permanent, temporary, and absolute retention.
In permanent retention, coffee powder becomes permanently stuck. Temporary retention is emptied and refilled with each grind. Absolute retention is the sum of permanent and temporary retention.
What are the taste effects of retention?
The larger the retention, the greater the impact on coffee flavor. In retention, there is ground coffee. As long as coffee is in whole bean form in the bean hopper, the rate of aging is slow. During grinding, the coffee is broken into many small particles. This causes it to lose its own protective layer, which it had formed before. The specific surface area increases many times over. Before grinding, oxygen and light only reacted with the whole bean, but now they easily reach every particle.
As a result, the chemical components contained in the coffee react with external factors such as oxygen and light. The oils contained in the coffee oxidize rapidly. Aroma components evaporate. Just 15 minutes after grinding, a significant portion of volatile compounds have disappeared.
The oxidation of particles is strongly noticeable in taste. Already 30 minutes after grinding, a sensory difference can be perceived in darker coffees, which increases significantly. If a lot of old coffee sits in the grinder, the coffee tastes unpleasantly sharp, rancid, and pungent. In the aftertaste, a drying sensation remains that causes the tongue to pucker.
Additionally, old coffee becomes flat and loses complexity due to the loss of volatile aroma components. With a large retention, this sometimes makes up half of the new espresso. The taste consequences are correspondingly severe.
Retention Impact on Extraction
Old coffee in retention also affects extraction and the flow behavior of coffee. This is particularly significant in espresso preparation. Old coffee creates a different resistance in the portafilter than freshly ground coffee. At high humidity, older coffee sometimes creates greater resistance because it clumps and sticks. At the same time, this coffee no longer flows well into the portafilter, which can lead to channeling.
Large retention makes it difficult to adjust the grinder. For example, if an espresso runs too fast, we make the grinder finer. When the next espresso is pulled and retention is 9 grams, there are still 9 grams of "old" coffee and thus coffee with the previous grind setting in the grinder. These must first be ground out to see the effects of the grinder adjustment. If the 9 grams are not ground out first, the subsequent grind adjustment is made on a false basis. This causes the inexperienced home barista to quickly lose track and despair when adjusting the espresso. That's why the important rule: with grinders with large retention, always grind out the retention between grind adjustments!
Permanent Retention
In permanent retention, coffee powder becomes permanently stuck to the grinder. Once the permanent retention is filled, it is not replaced with each grind. Typical spots that create permanent retention are the grooves in screw heads. Coffee powder also builds up around the grinding burrs in some grinders, creating a permanent layer. In the area before or behind the flapper, a small amount of coffee powder also tends to accumulate, which sticks permanently and doesn't release often or at all.
When it does release, it causes fluctuations in the amount of coffee powder from one extraction to the next.
Theoretically, permanent retention is relatively neutral. It has no direct impact on the discharged ground coffee and does not become part of the next extraction. However, this coffee slowly ages and creates a rancid atmosphere in the grinder. Since coffee itself normally doesn't mold, this is relatively unproblematic from a food hygiene perspective. However, sensorially, the wear of the old coffee has an impact on taste. The effect is pronounced when the coffee in a larger temporary retention has been in contact with the coffee in permanent retention for a long time. If coffee is pulled frequently and regularly, and permanent retention is less than 2 grams, the effect is minimal.
Temporary Retention
Temporary retention empties and fills with each grind. The coffee remains as residue from a grinding in the grinder. Instead, the old coffee powder, which is in the grinder's retention from the last grinding, is expelled.
Retention typically ranges from 0 to over 10 grams depending on the grinder. As part of our espresso grinder test series, we tested over 24 espresso grinders for their retention. The extent of retention is sometimes surprisingly large. The larger the grinding burrs, the more susceptible a grinder is to temporary retention. This is due to the circumference of the grinding burrs, but also the path to discharge. If the grinding burrs are positioned above the discharge or if a grinder is built from top to bottom, retention is usually lower.
Depending on its size, temporary retention has a significant impact on taste, extraction, and grinder adjustability. The coffee powder in retention becomes part of the espresso if it is not ground out first. So it affects the taste, but also the extraction. If the coffee powder is only briefly in retention, the effect is smaller. If the last pull was the day before, the coffee tastes unbalanced due to the pre-ground coffee powder and has a rancid tendency. Additionally, extraction runs worse because the powder cannot build up resistance.
Grinders with Low Absolute Retention
In the largest espresso grinder comparison in German-speaking regions, we placed special emphasis on measuring absolute retention. Absolute retention is the sum of temporary and permanent retention. It refers to the area in the grinder where ground coffee is held permanently or temporarily.
Grinders with the lowest absolute retention are the G-Iota (1.5 g), the Niche Zero (1.1 g), the Rommelsbacher EKM300 (1.5 g), and the Delonghi KG521 (1.4 g). Grinders from the Eureka Mignon series achieve good values with the Magnifico (2.1 g) and the Specialità (2.7 g). The Baratza Sette 270 (2.7 g) and the Rocket Faustino (2.8 g) also stay under 3 g retention. Not all of the above-mentioned grinders are also good espresso grinders. The amount of retention is only one important parameter.
Additional grinders with low absolute retention:
- Zuriga G2 1.5 g
- Ceado Life 1.9 g
- La Marzocco Pico 2.2 g
- Macap Leo 55 Essential 2.4 g
Retention Significance for Single Dosing and Timed Programs
Many of the tested grinders performed significantly better as "single dosing" grinders than in their intended mode. In particular, they achieved lower retention.
Single dosing is a principle of espresso preparation that emerged from the World Barista Championship. Instead of putting the entire coffee amount in the bean hopper, only the desired amount is weighed and ground. The process initially developed with the Mahlkönig EK43, which has no dosing program. For example, 18 grams of coffee are put in the bean hopper and ground. A good single dosing grinder then expels the full 18 grams. Many grinders achieve this goal to varying degrees when grinding for 30 seconds. In our test, we tried exactly that. We had the grinders grind for 30 seconds.
Here, for example, a Lelit PL72 surprised us, which normally has 5 grams of retention. However, when grinding for 30 seconds, it expelled the entire amount of coffee input and thus had no retention.
This approach differs significantly from working with a bean hopper. With that method, grinding is interrupted after a defined time once the target amount dispensed is reached. However, a large amount of coffee powder remains in retention along the path.
Especially for grinders with large retention, it is worthwhile to check whether they perform well as single dosing grinders. This has the nice side effect that grind adjustment becomes simpler and less coffee is wasted. Switching from one coffee to another is also easier when using the single dosing process.
Conclusions on Retention
If you know your grinder, you can work against your own retention and improve coffee quality with a simple trick. At the same time, the grinder can be adjusted more easily and better.
Before each pull, the coffee powder in temporary retention should be ground out. This applies especially if no coffee has been pulled for a while. We recommend grinding out after an idle time of one hour. Regular thorough cleaning of the espresso grinder is also important.
When adjusting the grinder, the old coffee powder should definitely be ground out after each grind adjustment. Only this way is the result of the grind adjustment also visible in the pull. Especially in espresso preparation, this is crucial for successfully finding the perfect grind setting.
If a grinder has a very large absolute retention but performs well as a single dosing grinder (see also our espresso grinder test), the grinder should be used accordingly. This increases quality, makes grinder adjustment easier, and quickly becomes routine. This method of use also allows easy switching from one coffee type to the next. That's enjoyable!
Retention is an important topic when it comes to coffee grinder quality. With this blog, you now know everything that matters about it.
Or did we forget something? Then feel free to add with your comments, questions, and assessments. Thank you!
