Sour/acidic shots can result from a few things.
Espresso extracts like this: Acidity > Sweetness > Bitterness. You can balance out acidity if you run the shot long enough. If your shots are running too quickly, you may have to slow the shot down with a slightly finer (but not too fine) grind.
Also, lighter roasted coffee will exhibit more acidity. Darker roasts mellow the acidity out.
Temperature can also be a factor. Lower temperatures can enhance acidity. If you’re not already doing it, preheat your ROK by letting hot water stand in it while you are doing all your other prep. Also, don’t wait too long to press once you’ve added the water to the top. With only 100ml of water in the top chamber it can lose heat quickly.
