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When I'm making lots of individual drinks I tend to clean the portafilter basket thoroughly and then dry it before filling it again etc.
I've watched baristas and they tend to just knock the old coffee out and immediately refil the portafilter without cleaning. I'd have thought that this would create bitter coffee as the grounds will already have been through one extraction.
Is it accepted that you should clean the portafilter between shots, or is it ok to reuse?
There's a trade-off here. If you're working in a fast-paced coffee shop, you're likely to favour the faster option (knock out the coffee) over cleaning. Generally knocking out the coffee will get rid of >99% of the grounds from the previous pull, so the impact on the flavour of the next one will be so slight as to be unnoticeable. Usually the portafilter won't even have time to dry out, so the coffee won't dry on and clog up the portafilter.
If you're making multiple coffees one after another, reusing the portafilter is probably acceptable.
However if you're leaving the coffee machine alone, it's best to at least rinse out the portafilter before you (or the next person) comes to use it. Once the grounds/coffee start to dry, it'll clog up and be much harder to clean further down the line.
To Expand on @Tom Medley's Answer; the pivotal questions are:
The portafilter should be rinsed with the espresso machines line water after every pull. This has the effect of removing the majority of remnant oils and grounds, pre-heating the portafilter, and removing potentially colder temperature water from the head of the line.
If you are pulling shots of espresso with less than 3 minutes between pulls, than the above process is enough, as there isn't enough time for the minute amount of remaining coffee oil to oxidize and become rancid at a tasteable threshold. If there is longer than 3 minutes between pulls, the portafilter should be thoroughly cleaned.
Likewise, if you are pulling different espressos (for example, a standard blend and a single origin), you should thoroughly clean the portafilter between pulls.
