I'm not really asking about flavour, I understand that an inconsistent roast causes inconsistent flavour.
My question is how it affects the extraction itself. Does an uneven roast lead to an uneven extraction? Or does it not really matter?
We can assume other variables are consistent, i.e. a good tamp and a consistent brew temperature within the ideal range for espresso extraction.
That's a tricky one.
It depends on what do you mean by 'extraction'.
"Coffee extraction is the process of dissolving soluble flavors from coffee grounds in water" - from Wikipedia.
"Extraction is everything that the water takes from the coffee" - from BaristaHustle.
"To remove or take out something" - from Cambridge dictionary.
Therefore, the extraction process considers dissolving the flavors. An uneven roast will affect directly the flavours on the final beverage. Thus, even the perfect extraction will not be able to extract all the flavours it could if the beans weren't correctly roasted.
The process of roasting causes both physical and chemical changes in the bean. If you haven't before, watch a video. You'll see the process of roasting produces a lot of smoke. Now think of the idea of inconsistent roasting. This is just another way of saying that some of the stuff that's in that smoke will still be in some of the beans compared to average. Also, the chemical changes caused by the heat will be underdone for some beans and overdone in others, meaning no two inconsistent batches will produce the same extraction.
These factors all affect what compounds water extracts and in what quantities. They will therefore affect the flavor of the coffee beverage by way of affecting the extraction.
