Acid, sourness and why juices are not interchangeable

Lime and lemon juice are both very acidic, at roughly 6% acid by weight in the figures commonly cited from Dave Arnold's work. Lemon's acid is almost all citric; lime's is mostly citric with a share of malic. Orange, grapefruit and pineapple are far less acidic — orange and pineapple nearer 1% — and much sweeter. That gap is why a sour made with orange juice instead of lemon is sweet and flabby rather than sharp.

These are typical figures, not fixed ones. The acidity of fruit changes with variety, ripeness, season and storage, and two limes from the same bag can differ noticeably. That variability is the whole reason bars adjust acid, and also the reason a home calculator would mislead.

Titratable acidity is how much acid there is. pH is how strongly acidic the liquid is at that moment, and juices buffer their acids so pH does not follow acid content simply. Sourness is what you taste, and it depends on both, on which acids are present, and on how much sugar is masking them. Citric acid tastes sharp and quick; malic is rounder and lingers; tartaric, from grapes, is harder.

So 'add citric acid until orange juice is as sour as lemon' is not one number. Matching acid content still leaves a different acid profile, a different aroma and more sugar. The adjusted juice tastes like sharp orange juice, not lemon.

A useful calculator would need the acid content and acid profile of the reader's actual fruit, which varies too much for a table to stand in for it, and the reader would need to weigh powdered acids to a fraction of a gram. Without a measured starting point it would produce confident numbers built on guesses. CocktailHQ's rule is that a figure we cannot govern does not get shown as precise.

What does work at home: taste your juice, and adjust the syrup rather than the acid. If the limes are sharp this week, a few millilitres more syrup brings the drink back. The Recipe Doctor works this way.