Dead space is the empty space in a coffee grinder where previously ground coffee accumulates, some of which isn't ejected as part of the current grind. Good grinders have little dead space and eject most of the ground coffee.
Dead space is created by the gaps around the grinding discs. Long and convoluted ejections that the ground coffee must make on its way to the dispenser after grinding increase dead space. Retention refers to the amount of ground coffee that remains behind . We distinguish between permanent, temporary, and absolute dead space.
Coffee grounds permanently stick together in the permanent dead space. The temporary dead space is emptied and refilled with each grinding. The absolute dead space is the sum of the permanent and temporary dead space.
What are the taste effects of dead space?
The larger the dead space, the greater the impact on the coffee's flavor. Ground coffee is located in the dead space. As long as the coffee remains in the bean hopper as a whole bean, 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 previously formed. The specific surface area increases many times over. While oxygen and light only reacted with the whole bean before grinding, they now easily reach every particle.
As a result, the chemical components contained in coffee react with external factors such as oxygen and light. The coffee's inherent oils oxidize at an accelerated rate. Aromatic components evaporate. Just 15 minutes after grinding, a significant portion of the volatile components have evaporated.
The oxidation of particles is strongly noticeable in the taste. A sensory difference can be perceived in darker coffees as early as 30 minutes after grinding, and this difference increases significantly. If a lot of old coffee sits in the grinder, the coffee tastes unpleasantly sharp, rancid, and pungent. The aftertaste leaves a dryness that makes the tongue ache.
In addition, old coffee becomes bland and loses complexity due to the loss of volatile aroma components. With a large dead space, this can make up half of the new espresso. The consequences for the taste are correspondingly serious.
Dead space impact on extraction
Old coffee in the dead space also affects the extraction and pour-over behavior of the coffee. This is especially significant when preparing espresso . Old coffee creates a different resistance in the portafilter than freshly ground coffee. In high humidity, the older coffee sometimes creates greater resistance as it clumps and sticks together. At the same time, this coffee also no longer flows easily into the portafilter, which can lead to channeling .
A large dead space makes adjusting the grinder more difficult. For example, if an espresso is brewed too quickly, we adjust the grinder to a finer setting. However, when the next espresso is brewed and the dead space is 9 grams, there will still be 9 grams of "old" coffee, including coffee with the previous grind size, 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 size adjustment will be based on the wrong basis. This quickly causes the inexperienced home barista to lose track and become desperate when adjusting the espresso. Therefore, the important rule: with grinders with a large dead space, always grind out the dead space between adjustments, even when adjusting the grind size!
Permanent dead space
In the permanent dead space, coffee grounds permanently stick to the grinder. Once the permanent dead space is filled, they are not replaced with each grinding. Typical places that create permanent dead space are the cracks in screw heads. A permanent layer of coffee grounds also forms around the grinding discs in some grinders . A small amount of coffee grounds also tends to build up in the area in front of or behind the flapper, permanently sticking and rarely or never coming off.
If it comes loose, it causes sudden fluctuations in the amount of coffee powder from one extraction to the next.
Theoretically, the permanent dead space can be considered relatively neutral. It has no direct impact on the ground coffee produced 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 does not normally develop mold, this is relatively unproblematic from a food hygiene perspective. However, the abrasion of the old coffee does have an effect on the taste. The effect is pronounced if the coffee in a larger temporary dead space is in contact with the coffee in the permanent dead space for a long time. If a lot of coffee is dispensed regularly and the permanent dead space is less than 2 grams, the effect is minor.
Temporary dead space
The temporary dead space empties and refills with each grinding. The coffee remains in the grinder as a residue from a grinding. Instead, the old coffee grounds, which are in the dead space from the last grinding, are ejected.
Depending on the grinder, dead space can range from 0 to over 10 grams. As part of our espresso grinder test series, we tested over 24 espresso grinders for their dead space. The dimensions of the dead space are sometimes surprisingly large. The larger the burrs, the more susceptible a grinder is to temporary dead space. This is due to the circumference of the burrs, but also the path to the ejection port. If the burrs are positioned above the ejection port or if a grinder is constructed from top to bottom, the dead space is generally smaller.
Depending on its size, the temporary dead space has a significant impact on the flavor, extraction, and grinder adjustability. The coffee grounds in the dead space become part of the espresso if they aren't ground first. This influences both the flavor and the extraction. If the coffee grounds are only in the dead space for a short time, the impact is less pronounced. If the last coffee was brewed the day before, the pre-ground coffee will make the coffee taste unbalanced and have a rancid tendency. In addition, extraction is less effective because the grounds can't build up any resistance.
Mills with low absolute dead space
In the largest espresso grinder comparison in German-speaking countries, we placed particular emphasis on measuring the absolute dead space. Absolute dead space is the sum of temporary and permanent dead space. It is the area in the grinder where the ground coffee is permanently or temporarily located.
Grinders with the smallest absolute dead space 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, including the Magnifico (2.1 g) and the Specialità (2.7 g), also achieve good values. The Baratza Sette 270 (2.7 g) and the Rocket Faustino (2.8 g) also remain below 3 g of dead space. Not all of the grinders mentioned above are good espresso grinders. The amount of dead space is only one important parameter.
Other mills with low absolute dead space:
- Zuriga G2 1.5 g
- Ceado Life 1.9 g
- La Marzocco Pico 2.2 g
- Macap Leo 55 Essential 2.4 g
Dead space significance for single dosing and time programming
Many of the tested grinders performed significantly better as single-dosing grinders than in their intended mode. In particular, they achieved less dead space.
Single dosing is a principle of espresso preparation that originated in the Barista World Championships . Instead of pouring the entire amount of coffee into the bean hopper, only the desired amount is weighed and ground. This process was initially developed with the Mahlkönig EK43, which does not have a dosing program. For example, 18 grams of coffee are poured into the bean hopper and ground. A good single dosing grinder will then eject all 18 grams. Many grinders achieve this goal more or less well if they grind for 30 seconds. In our test, we tried exactly that. We let the grinders grind for 30 seconds.
For example, a Lelit PL72, which normally has 5 grams of dead space, was surprising. However, when grinding for more than 30 seconds, it ejected the entire amount of coffee, thus leaving no dead space.
This procedure differs significantly from working with a bean hopper. In this case, grinding is paused after a defined time when the target dispensed quantity is reached. However, in this case, a large amount of ground coffee remains in the dead space along the way.
Especially for grinders with a large dead space, it's worth checking whether they perform well as single-dosing grinders. This has the nice side effect of making it easier to adjust the grind size and using less coffee. Switching from one type of coffee to another is also easier when using the single-dosing process.
Conclusions on dead space
If you know your grinder, you can work against your own dead space and improve the quality of your coffee with a simple trick. At the same time, this also makes adjusting the grinder easier and more efficient.
Before each brew, the coffee grounds in the temporary dead space should be ground. This is especially important if no coffee has been brewed for a while. We recommend grinding after an inactive period of one hour. Regular, thorough cleaning of the espresso grinder is also important.
When adjusting the grinder, it's also essential to grind out the old coffee grounds after each grind adjustment. This is the only way to ensure the results of the grind adjustment are visible in the coffee. This is especially crucial for successfully finding the perfect grind when preparing espresso.
If a grinder has a very large dead space but performs well as a single-dosing grinder (see our espresso grinder test), it should be used accordingly. This increases quality, makes adjusting the grinder easier, and quickly becomes routine. This type of use also makes it easy to switch from one type of coffee to the next. It's a joy!
Dead space is an important topic when it comes to the quality of coffee grinders. This blog will tell you everything you need to know about it.
Or have we missed something? Feel free to add your comments, questions, and opinions. Thank you!
