As a native english speaker, saying "sourer" is strange for me. Either option you've presented would work, though. Personally, I would say "more sour" in this case. Saying "sweeter" is just fine to me and so is saying "more sweet", although saying "sweeter" probably sounds more fluent. Example sentences: This candy is more sour than that candy. This candy is sweeter than that candy. The lemonade is more sour in this country. The lemonade is sweeter in this country.

'sour' is a diphthong. as such it is functionally a double syllable word. so comparing it to 'sweet' misses the point. contrast it, rather, to 'loud' or 'taut'; both of which are monothongs and 'louder' and 'tauter' are correct. 'sourer' may be defensible but it is difficult to pronounce. even in written works it can cause you to pause instead of reading smoothly along.

Hello Vale. You should say 'sourer' and 'sweeter'. Always use this form to make comparatives from adjectives that have only one syllable (unless they are irregular like 'good' and 'better'). (You will find instances however where people don't follow this rule. As a native though my recommendation is to follow it, as the sentence will sound more natural and well-formed).

This is a very old thread, but I'm bringing it up again because many people said that we don't use -er and -est endings with words with over one syllable. This is false. Many adjectives with two syllables can have an -er and -est ending. Many of these adjectives end in -y. Pretty, ready, happy, stupid, common, narrow. Anyway, in the case of "sour", you can say "sourer" and "more sour". Both are fine. Many people might avoid "sourer" because as Alexa noted, the "er-er" sound is awkward, at least in my accent. But certainly everyone should agree that "sourest" is pretty common: "That's the sourest strawberry I've ever eaten."

"More sour" is acceptable in spoken English because it sounds more fluent. However, I would use "sourer" in written English in compliance with the following rules: 1 syllable = add "er" 2 or more syllables = add "more" 2 syllables but ending in y = drop "y" and add "ier"