Why should one always use the same kind of coffee for an Espresso machine?

There are a lot of good detailed answers on this page already, but I guess I could add my own $0.02 to the conversation and maybe a different angle to the answer..

I have been home roasting and brewing for about 2 years now and I think the #1 thing I've learned about pulling espresso shots is that it is all about consistency.

In order to effect consistency you have to control your variables, and the more variables you get control of, the better chance you have of maintaining consistency.

Over the past two years I have continuously refined my process by modifying the entire chain of coffee production, whether it meant ordering a new piece of hardware or changing how I did something.

For instance, I installed a PID on my Racilio Silvia in order to control brew temperature, I bought a naked portafilter to get better feedback on the quality of my shots, and bought a pressure gauge to make sure I was getting proper brew pressure at the group head, the list goes on and on.

So when I saw your question about "always using the same coffee" I interpreted it as just another one of those variables that you can get control of and that will effect the quality of the shots you pull.

In the beginning I stuck with the same coffee and roast to try to minimize the number of variables I was working with. Once I was more familiar with how each variable effected my shot and how I could manipulate them to get what I wanted, I began to experiment.

So I guess for me keeping the coffee the same boils down to removing some noise from the system to make it easier to manage...

Are there any mechanical issues that stem from using different kinds of espresso in the same machine over the course of many years of usage?

No, the quality of the grind will be much more important to the machine than the type of beans. And even then, not so important at all. Coffee is coffee with regard to this piece of machinery.

Or is it only a matter of taste?

Is a machine somehow "branded" with a kind of coffee after some time of use?

No. I suppose in theory over time one could begin to see some form of flavour buildup on their machine however there shouldn't be porous materials to absorb oils and if you keep it clean that should make little to no difference.

My guess is it's either a matter of taste or laziness on the part of the people who said those things. There is no logical nor apparent reason for any of it to be true.

There is no technical reason for sticking to one kind of coffee only. In contrast, you should really try different brands (the fresher the better) to find out what you like best.

However, there's a small truth behind the statements for the portafilter type of coffee machines. It's not related to the machine itself but to the user, because usually it takes some time to adjust the grinder properly for one sort of coffee. Especially unexperienced users of portafilters need more time to get the perfect cup out of beans, so switching the beans to often is not a good idea at the beginning.

Many of the small roasting facilites offer test packages with 250g of beans. These are definitely too small for a beginner. Try to find the optimal dosage and grind with at least 1000g before you try a different bean/roast.

• Are there any mechanical issues that stem from using different kinds of espresso in the same machine over the course of many years of usage?
• Or is it only a matter of taste?

The answer is no. There are no mechanical issues, ever, that stems from using different kinds of coffee bean on the same machine over many years. The only thing that touches the coffee ground is the portafilter (and the water source above the portafilter, and the cup for the matter). The portafilter should be cleaned regularly. If anything build up there it would be rancid coffee oil which are NOT tasty I suppose.

See above. For beginners of coffee making, take the following advice:

1. the fresher the coffee, the better (that is, short time from harvest to roast, short time from roast to ground, and short time from ground to pulling the espresso.
2. Fill the portafilter well first, then tamp it well. No amount of tamping will save an uneven powder filling of a portafilter.
3. Clean your machine well. Nothing will save you from rancid oil.
4. Use high quality water. Poor quality water will make coffee taste worse. Invest in a filter or use distilled water.

If you are really into it, you can consider using a bottomless filter to troubleshoot coffee making similar to a debugger to a programmer...

Good luck!