Turning to vinegar

• Turning bad
• Becoming disillusioned
• Going off (like milk)
• Losing enthusiasm

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Sour \Sour\, v. i. [imp. & p. p. Soured ; p. pr. & vb. n. Souring .] To become sour; to turn from sweet to sour; as, milk soon sours in hot weather; a kind temper sometimes sours in adversity. They keep out melancholy from the virtuous, and hinder the hatred ...

Wikipedia Word definitions in Wikipedia
Souring is a cooking technique that uses exposure to an acid to effect a physical and chemical change in food. This acid can be added explicitly (for example, in the form of vinegar , lemon juice , lime juice , etc.), or can be produced within the food ...

Wiktionary Word definitions in Wiktionary
n. (context dated English) Any sour apple. vb. (present participle of sour English)

WordNet Word definitions in WordNet
n. the process of becoming sour