"souring" in the noun sense
1. souring
the process of becoming sour
"souring" in the verb sense
1. sour, turn, ferment, work
go sour or spoil
"The milk has soured"
"The wine worked"
"The cream has turned
2. sour, acidify, acidulate, acetify
make sour or more sour
Source: WordNet® (An amazing lexical database of English)
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