I was told once that a lemon had almost as much sugar as an apple but the high acid content of the lemon overpowered the sugar and accounted for the sour taste of the lemon.
I expect this is not the case, and I did some research. I can not locate the “truth” in a form that I could understand
Can someone explain how the mix of sugar and acid combine to influence how we perceive sweetness? Does a lemon really contain a lot of sugar?
This article lists the brix of lemons and watermelons to be the roughly the same. Both are high in brix at 12.
http://www.nutritionsecurity.org/PDF/Brix.pdf
One tastes sweet and the other sour. Enough citric acid and the sweetness is mask. Also as citrus matures they taste sweeter. That’s as much because the acid level drops as the sugar level increasing. Many fruits are like that, as they ripen the acid drops and sugar increases so they taste sweeter.