Uneven Extraction
Once upon a time, coffee nerds would say a coffee was "under-extracted" or "over-extracted". You'll actually see that in a lot of coffee talk still, and it's a shorthand that most folks recognize. In both cases, the coffee feels like it's missing something. For me it's always a feeling of loss - I haven't helped the coffee show it's full potential. What is the coffee missing? Well typically under-extracted means the coffee is missing sweetness, and over-extracted means it's missing acidity. But everyone experiences it a little different.
The beautiful thing about taste is that it's so subjective, and we're all grasping to communicate these highly subjective experiences to each other. Coffee has over a thousand different compounds that make up that sensory experience. Those compounds give us all the aromas and flavours we sense. The compounds available in any particular coffee bean is impacted by the processing. All of the steps along the way have a chance to change what will end up what's in your cup. What kind of bean it is, how the plants are grown, how it is separated from the cherry and dried, how it's roasted and shipped to you can all change what's available in the bean. Modern coffee science tells us that, for the most part when you brew a coffee - however you brew it - you extract the compounds that are in the bean into your final cup. Your experience of the coffee is affected by the balance of those compounds.
In a super simple example think of it this way. If you're making lemonade from lemons, your final glass of lemonade will always have lemon juice, water and sugar. It's the final balance of those ingredients that make your glass of lemonade too sour, too sweet, or just right. In the same way, when you brew your coffee you're getting all the compounds available to you but the interesting part is the balance of compounds. Is the coffee super sour? You have likely extracted more acids from the grinds. It's not that the sugars haven't been extracted, it's just that you haven't extracted enough of them to balance with the acids. Conversely if your cup tastes sweet but kind of dull, you've tipped the balance in the other direction. Here's the amazing and terrible truth - there are over a thousand compounds in coffee (that we know of this far). What does the right balance for a particular bean look like for you? We all have different tastes and so the right extraction could be incredibly different for each bean and for each person. Personally I don't mind acidity in my espressos, I would prefer to have a cup that leans towards staying bright and lively. I know there are plenty of people that do not enjoy anything remotely close to "sour" and they would much rather have a sweet cup that is tipped the other way. There are many different ways to change the balance of your coffee -grind size, amount of ground coffee, amount of final extracted coffee, water quality, water temperature, extraction time, pressure - and the list continues to grow.
It feels to me like coffee science is starting to build some momentum, you'll find plenty of sources to describe different methods of changing what you taste in your coffee. That's a little beyond what I wanted to cover today, but let's come back to that if you're interested. Have a great day and remember life is too short to drink mediocre coffee!