A Thermomix® is a wonder weapon for chopping and mincing in the kitchen context. That's why it's no surprise that many households also use the Thermomix® to chop coffee. This is basically possible, but doesn't guarantee good coffee.
The Thermomix® is a chopping expert, but it doesn't grind — it cuts. It's not equipped with grinding discs but rather with impact blades that can achieve almost any grind size of coffee, vegetables, nuts, etc., depending on the cutting time. Unfortunately, for coffee, the precision isn't sufficient for really good coffee. In addition, the surface structure of a chopped coffee bean looks different than the surface of ground coffee.
The grind curve of coffee looks roughly like this on an espresso grinder. This precision of grind size cannot be reliably set with the Thermomix®. The coffee is chopped, but the grind varies from one grinding to the next in terms of fineness or coarseness.
Why does coffee from the Thermomix® sometimes taste sour and sometimes bitter?
The changing taste is partly due to the inconsistent particle distribution (variation in grind size). This varies from grinding to grinding because the impact blades don't always engage the coffee beans in the same way. This is also because coffee has different density depending on its origin. Also, the size of the beans varies depending on the variety, cultivar, and growing region.
Due to the chopping and impact, the spread of particles is relatively large. This is relevant because finer particles tend to over-extract. This leads to bitterness in the coffee. Coarser particles cannot be extracted as well by the brewing water. This leads to what's called under-extraction and thus to a sour-aggressive taste in the cup.
When both fine and coarse particles are present, it leads to a mixture of bitterness and acidity that tastes particularly unpleasant.
Is a Thermomix® not suitable for any coffee?
You can't say that as a blanket statement. There are some very satisfied users who successfully use the Thermomix® for brewing filter coffee. If you know your Thermomix® well and use good coffee, you have a huge advantage over coffee from the supermarket. This is often pre-ground and has already lost quality.
Coffee chopped with the Thermomix® is always better than pre-ground coffee from the supermarket!
However, we recommend using Thermomix® chopped coffee for filter coffee. The precision of particles is not quite as crucial for filter coffee as it is for espresso. With filter coffee, you can control the contact time between coffee and water differently than just through the grind size.
Espresso with the Thermomix®?
With espresso, the precision of the grind size is very important. This is difficult to achieve with the Thermomix®. But there's a trick here too! For once, the so-called "double-wall baskets" make sense. These are sent with cheaper espresso machines. The baskets themselves have only a small opening through which all the coffee is pressed. This way the baskets build up the resistance that cannot be built up by pre-ground coffee powder.
A good espresso has a brewing time of about 20-30 seconds. With the double-wall basket, this is achieved with almost any coarser ground coffee or even older coffee. If we now use the Thermomix and don't grind too finely, we can use the double-wall basket. This gives us a good extraction time. The espresso is not as perfect as with a properly adjusted espresso grinder, but the result is good in accordance with the coffee used.
