What is more sour: a lemon or a lime? Which has higher acidity?

The most common food group that contains naturally sour foods is the fruit, with examples such as the lemon, grape, orange, and sometimes the melon. Wine also usually has a sour tinge to its flavor. If not kept correctly, milk can spoil and contain a sour taste.

https://en.wikipedia.org/wiki/Taste


Being that limes are not mentioned in the above post, lemons are more sour and have greater acidity.

As they are both citrus fruits with high citric acid content, they are both distinctly acidic.

This technical fruit analysis site gives the following values for the index they use to measure fruit acidity:
Lemon 7.6062-12.7485^c, 9.9631^c /100g
Lime 5.2922-8.9132^c, 6.3957^c /100g

This means that their ranges overlap (an individual lime could be more acidic than an individual lemon), but the lemon is more acidic on average. The pH ranges in the table lead to a similar conclusion. Limes tend to taste less sour because they have a higher sugar content, and taste is a subject interpretation of a mixture of qualities.

http://www.brsquared.org/wine/CalcInfo/FruitDat.htm