A retention is the empty space in a Kaffeemacher where already ground coffee accumulates that is not part of the current grinding and is not ejected. Good grinders have low retention and eject the majority of the ground coffee.
Retention is created by free spaces around the grinding discs. Long and winding discharge channels that the ground coffee must travel through after grinding on its way to the outlet increase retention. In English, the term "retention" is used, referring to the remaining ground coffee. We distinguish between permanent, temporary, and absolute retention.
In permanent retention, coffee powder sticks permanently. Temporary retention is emptied and refilled with each grind. Absolute retention is the sum of permanent and temporary retention.
What taste effects does retention have?
The larger the retention, the greater the taste impact on coffee flavor. The retention contains ground coffee. As long as coffee is in whole bean form in the bean hopper, the rate of aging is low. During grinding, the coffee is broken down into many small particles. This causes it to lose the protective layer it had formed before. The specific surface area increases many times over. Before grinding, oxygen and light reacted only 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 inherent in coffee oxidize rapidly. Aroma components evaporate. Already 15 minutes after grinding, a significant portion of the volatile compounds have disappeared.
Taste-wise, the oxidation of particles is very noticeable. Already 30 minutes after grinding, in darker coffees a sensory difference can be perceived that increases significantly. If there is a lot of old coffee in the grinder, the coffee tastes unpleasantly sharp, rancid, and pungent. In the aftertaste, a drying sensation remains that makes the tongue feel tight.
Additionally, old coffee becomes bland and loses complexity through the loss of volatile aroma components. With large retention, this can make up half of the new espresso. The taste consequences are correspondingly serious.
Retention impact on extraction
Old coffee in the retention also affects the extraction and flow behavior of the coffee. This is particularly significant for espresso preparation. Old coffee creates a different resistance in the portafilter than freshly ground coffee. In high humidity, older coffee can sometimes build up higher resistance as it clumps and sticks together. At the same time, this coffee also no longer falls well into the portafilter, which can lead to channeling.
Large retention makes grinder adjustment difficult. For example, if an espresso runs too fast, we set the grinder finer. When the next espresso is pulled and the retention is 9 grams, there are still 9 grams of "old" coffee and thus also 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 will be made on an incorrect basis. This causes inexperienced home baristas to quickly lose track and become frustrated with espresso adjustment. That's why the important rule: with grinders with large retention, always empty the retention between grind adjustments!
Permanent retention
In permanent retention, coffee powder sticks permanently to the grinder. Once the permanent retention is filled, it is not replaced with each grind. Typical areas that create permanent retention are the grooves in screw heads. Also around the grinding discs, some grinders develop a permanent layer of coffee powder. In the area before or behind the flapper, a small amount of coffee powder also likes to build up, which sticks permanently and rarely releases.
When it does release, it causes sudden variations in the amount of coffee powder from one extraction to the next.
Theoretically, permanent retention is relatively neutral to consider. It has no direct impact on the ground product output 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 does not mold, this is relatively unproblematic from a food hygiene perspective. Sensorially, however, the abrasion of old coffee has an impact on taste. The effect is pronounced when the coffee in a larger temporary retention is in contact with the coffee in the permanent retention for a long time. If coffee is consumed frequently and regularly and the permanent retention is less than 2 grams, the impact is minimal.
Temporary retention
Temporary retention empties and fills with each grind. The coffee remains as residue from one grind in the grinder. Instead, the old coffee powder that was left in the grinder from the last grind in the retention is ejected.
Depending on the grinder, retention is between 0 and over 10 grams. In our espresso grinder test series, we checked over 24 espresso grinders for their retention. The extent of retention is sometimes surprisingly large. The larger the grinding discs, the more susceptible a grinder is to temporary retention. This is due to both the circumference of the grinding discs and the path to the discharge. If the grinding discs are above the discharge or if a grinder is built from top to bottom, retention is typically lower.
Depending on size, temporary retention has considerable influence on taste, extraction, and grinder adjustability. The coffee powder in the retention becomes part of the espresso if it is not ground out first. So it affects both taste and extraction. If the coffee powder is only briefly in the retention, the impact is less. If the last shot was pulled 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 the German-speaking world, we placed special emphasis on measuring absolute retention. Absolute retention is the sum of temporary and permanent retention. It is the area in the grinder where ground coffee permanently or temporarily resides.
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 range 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 grinders mentioned above are good espresso grinders. The amount of retention is just one important parameter.
Other 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 timer programming
Many of the tested grinders performed significantly better as "single dosing" grinders than in their intended mode. They came in particularly with less retention.
Single dosing is a principle of espresso preparation that developed from the Barista World Championships. 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 programming. For example, 18 grams of coffee are put in the bean hopper and ground out. A good single dosing grinder now ejects the 18 grams completely. Many grinders achieve this goal more or less well when grinding for 30 seconds. In our test, we tried exactly that. We had the grinders grind for 30 seconds.
For example, a Lelit PL72 surprised us here, which normally has 5 grams of retention. However, when grinding for 30 seconds, it ejected the complete amount of coffee input and thus had no retention.
This approach is very different from working with a bean hopper. In that case, grinding is stopped at a defined time once the target output amount is reached. But then a large amount of coffee powder remains in the retention along the way.
Particularly for grinders with large retention, it is worthwhile to check whether they perform well as single dosing grinders. This has the nice side effect of making grind adjustment easier and using less coffee. Switching from one coffee to another is also easier when using the single dosing process.
Retention becomes particularly relevant for grind-by-weight grinders: Here, retention can also be avoided through single portioning. The differences between models range from 1.5 g to over 7 g – with noticeable impact on cup quality.
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, this makes the grinder easier and better to adjust.
Before each shot, the coffee powder in the temporary retention should be ground out. This is especially important if no coffee has been pulled for a while. We recommend grinding out after an inactive time of just one hour. Regular thorough cleaning of the espresso grinder is also important.
Also, when adjusting the grinder, old coffee powder must definitely be ground out after each grind adjustment. Only this way is the result of the grind adjustment visible in the shot. This is especially crucial for espresso preparation when searching for the perfect grind setting.
If a grinder has very large absolute retention but performs well as a single dosing grinder (see also our espresso grinder test), then 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 variety 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's important about it.
Or have we forgotten something? Then feel free to add with your comments, questions, and opinions. Thanks!
