The word arrives on a bag as praise and lands on most people as a warning. Roasters print high acidity meaning something closer to lively. Plenty of buyers read it as a coffee that will taste like it went wrong.
Acidity is the coffee trade's word for the sour basic taste when it is in balance and welcome. Sourness is the same basic taste when it is out of balance and unpleasant. The chemistry underneath both words is identical. The difference is a judgment about quantity, not a difference in kind. Jessica Easto makes this point directly in How to Taste Coffee, and it explains the confusion better than any flavour wheel does.
Which acids, and where they come from
Coffee's acidity is not one compound, and the main ones do not answer to heat the same way. Easto's account of the two big ones is the useful one. Citric acid is inherited rather than made: it comes in with the green seed and the roast can only spend it, which is why a medium roast holds roughly half of what the raw bean carried. Acetic acid runs the other way. Most of it is built during the roast as carbohydrates come apart, so a bean that started sweeter finishes more acetic. Push past medium and that gain begins reversing. Chlorogenic acids, the largest group of phenolic compounds in green coffee, do not survive sustained heat at all.
Hence the shape of the roast. Acidity starts high and falls; bitterness starts low and climbs. James Hoffmann plots sweetness as a curve peaking somewhere between the two, which makes a roaster's real decision a question of where on that crossing to stop.
Brewing moves it more than roast does
The most useful recent finding is not about beans at all. Mackenzie Batali and colleagues at UC Davis ran drip-brewed coffee past a trained descriptive panel and published in ACS Food Science & Technology in 2021. Roast level was one of the variables. It lost. How the coffee was brewed moved the panel's sourness scores further than how dark it had been roasted, and the cups that scored most sour were the concentrated ones that had given up little of the bean: high dissolved solids, low extraction yield.
The instrument they trusted is worth borrowing. Sourness scores tracked titratable acidity, the total quantity of acid in the cup. They did not track pH, which registers only the portion already broken apart in the water. Two coffees can meter the same and taste nothing alike.
Water finishes the job. Bicarbonate neutralises exactly the acids you were trying to taste. Maxwell Colonna-Dashwood's demonstration in The Coffee Dictionary is to drop the smallest pinch of baking soda into a finished cup. The brightness goes and does not come back. It is also why a bottled water chosen for drinking can flatten a coffee it had no business flattening.
What the word will not do for you
Knowing the term will not fix a sour cup, and it will not tell you whether you will like the coffee. Acidity on a bag says the roaster found the sour taste balanced at their extraction, in their water, on their palate. Yours may differ on all three. The word also names no specific acid, so high acidity could mean the clean citrus edge of a washed Kenyan or the sharper, more fermented note of acetic acid in a lightly roasted natural, and the bag will not tell you which.
Anette Moldvaer's system in Coffee Obsession handles this honestly by giving acidity its own wheel, with positive terms like crisp and winelike on one side and sour, sharp and flat on the other. Acidity is not the fault. Imbalance is.
If your cup is sour rather than bright, the fix is a brewing change, not a vocabulary change. Sour points one direction and bitter points the other, and that single question is the whole diagnostic.