What is the difference between titratable acidity and total acidity?

Titratable acidity measures only organic acid protons recoverable with titration. Total acidity measures "expected" protons through quantification of organic acid anions.

It is important to note that while titratable acidity and total acidity are terms that often get used interchangeably, they are NOT the same thing. 

Titratable acidity (or TA) measures the number of organic acid protons (hydrogen ions) in a sample that can be recovered during titration with a strong base. 

This method cannot capture all "undissociated" protons.

Total acidity, in contrast, measures the number of protons in a sample by way of quantifying all organic acid anions, and then assuming a proton equivalence. This is done using spectrometry or chromatography.

As total acidity measures all protons as if they were undissociated (i.e., reads both dissociated AND undissociated protons), this method will always give a slightly higher result than titratable acidity.