How many letters does the top answer for “make sour or more sour” have?

The most common answer is acidulate (9 letters).

Crossword clue: make sour or more sour

1 answer found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

acidulate 9 letters wn-def

make sour or more sour

make personal or more personal 50% similar → individualise, individualize, personalise

make regular or more regular 50% similar → regularise, regularize

make sour or bitter 50% similar → acerbate