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.
Examples
Dairy products produced by souring include: Smetana, Clabber, Cheese, Crème fraîche, Cultured buttermilk, Curd, Filmjölk, Kefir, Paneer, Soured milk, Sour cream, and Yogurt.
Grain products include: Idli, Sourdough, and Sour mash.
Others foods produced by souring include: Ceviche and Key lime pie.
References
Souring Wikipedia(Text) CC BY-SA