What is "souring"

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 of vice from souring into severity.
--Addison.

Souring \Sour"ing\, n. (Bot.) Any sour apple.

n. (context dated English) Any sour apple. vb. (present participle of sour English)

n. the process of becoming sour

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 itself by a microbe such as Lactobacillus.

Souring is similar to pickling or fermentation, but souring typically occurs in minutes or hours, while pickling and fermentation can take a much longer time.