9. Your coffee was ground too early
Grinding breaks up the structure of the coffee and thus increases the surface area that is exposed to the air and thus to oxidation. This is why the above-mentioned processes take place much faster than with unground coffee. If possible, you should consume ground coffee within 30 minutes.
10. Your coffee has been in the grinder too long
Again, we refer to points 8 and 9. Since your coffee was no longer in the protected and sealed coffee bag, but openly in the grinder, all the oxidation processes take place much more quickly. In addition, many of the coffee beans slide all along the bean container, leaving the coffee oils behind. You've probably also seen grinders that look all sticky and dirty - not a pretty picture for an espresso and crema lover.
11. You have tamped too weakly
This also leads to a too fast throughput time and an under-extraction of the coffee. Orientate yourself to 9kg pressure when tamping. You can find many excellent YouTube videos on this, watch a few of them. You can improve your coffee massively with small changes in technique.
12. You have tamped too much
If you press the coffee into the sieve with too much pressure, the powder is pressed so tightly that the water hardly flows through and your coffee tastes over-extracted, bitter and sharp. The crema is dark brown, uneven and shows air bubbles.
And beware: many baristas knock on the sieve with the tamper after tamping or are a little careless, so that the portafilter still hits somewhere on the machine. This can cause small cracks in the pressed powder. The water seeks the "path of least resistance", flows through these small cracks and can only loosen a little of the coffee powder, so that you get a thin and again under-extracted coffee.
13. The water is too cold
Ideally, your espresso machine has a temperature gauge so you can check the temperature of the water before and during extraction. A water temperature below 88°C will result in under-extraction, sour and thin coffee, and a barely-there crema.
14. The water is too hot
On the other hand, water that is too hot, above 94°C, leads to over-extraction and a thin, dark brown to blackish crema with a white spot or a black hole in the centre.
15. The pressure is too low
See also point 8 - Espresso needs pressure. Too little pressure means that not enough particles are released and your coffee is under-extracted, the crema barely present. The pressure should be around 9 bar. Your espresso machine must be able to do this. At the same time, too coarse a grind, too weak a tamp or too little ground coffee can lead to insufficient pressure building up - so everything is connected on the way to the perfect espresso.
16. The pressure is too high
If you have a reasonably professional espresso machine, it will easily be able to build up more pressure than 9 bar. This will happen (probably no surprise after point 15): if the grind is too fine, if the tamp is too tight or if there is too much ground coffee. So, correct all the above factors to get to the 9 bar pressure.
17. Home roaster only - your coffee is too fresh
If you don't roast your own coffee, you can skip this chapter, because he or she will hardly ever get into the situation where a coffee is too fresh. Most of the coffees on the shelves are weeks, if not months, old, so they almost never show a roast date.
As a home roaster, you may remember the first time you extracted a freshly roasted espresso - the crema was spectacular! This has to do with the fact that there is still enough CO2 in your fresh coffee, which dissipates very quickly after roasting and especially after grinding.
At the same time, this carbonic acid overlays the fine nuances of taste in your coffee. This is why the espresso only becomes really good about 4-5 days after roasting. If you have the patience, we recommend that you leave the coffee to stand for a few days after roasting.
18. You have chosen the wrong coffee
Not all coffee varieties and processing methods produce a perfect crema. That's not too dramatic, because first and foremost you should look for the flavour that suits you best. As a rule of thumb, you can orientate yourself on the fact that natural and honey-pulped beans (see also processing) usually have more sugar and oils than washed coffees and therefore allow more crema.
For home roasters, we recommend these balanced green coffees with nutty-chocolatey or spicy-sweet notes: