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Someone told me that the last drips coming out of a espresso machine after the pull is finished (when there is no water pressure any more) contain bad flavors. He recommended that I remove the coffee cup in the moment the machine stops producing pressure.
Is that true? If yes, why?
It's not really the pressure, that's just timing.
It depends on the beans and roast; your comparison point is the first few (after pre-infusion) drips that came out. The last ones are always going to be weaker and quite possibly more acidic because the puck is both drained and saturated at the same time. It's drained of oils and flavors, and saturated with hot water.
Whether or not you can actually detect that difference should it be added to the rest of the shot is a matter of uncertainty, hinging on your sense of taste and the type of coffee used. If it's a dark-roasted robusta blend then you might be able to detect a slight almost 'metallic' or slightly salty flavor, but you'd need an extremely acute sense of taste. The water you use also has a lot to do with this.
It can sometimes be easy to get caught up in what is probably happening, even if you (or most) can't really taste it. If you get more satisfaction out of your coffee because you've managed to improve the taste even just a tiny bit - then great, and keep doing it, because that's what it's all about. Just be critical and test for yourself, with your setup, and your beans :)
Certainly the brewing process will extract different flavor characteristics from your grounds as it goes through its cycle. But a well-designed machine is designed to produce a full-bodied, well-balanced beverage as the END product.
If your machine is working properly, you shouldn't really have to evaluate the drop by drop extraction process as being "less tasty" or "more tasty" as it goes through that cycle. A good cup of coffee has many subtle complexities, and whatever comes out of that machine at the end is also part of that overall "recipe."
Yes, if you know your coffee to be "over extracted", then perhaps you can compensate a bit by pulling a cup early; but that's a bit like buying a bad pie and just eating out the filling because the crust doesn't taste as good.
You have to adjust your process. You can select your beans, grind them accordingly, and select/use a machine so the end product is to your taste. But if everything is working as it should, pulling that cup early is more likely to create a dull, lifeless brew than just letting the process complete as designed. Second-guessing your machine is a hack, not a solution.
