Sour cream is a dairy product rich in fats obtained by fermenting a regular cream by certain kinds of lactic acid bacteria. The bacterial culture, introduced either deliberately or naturally, sours and thickens the cream. Though only mildly sour in taste, the name stems from the production of lactic acid by bacterial fermentation, sometimes referred to as "souring". Sour cream, made out of cream, contains from 12 to 16 percent butterfat (about 14 grams per 4 ounce serving), and gets its characteristic tang from the lactic acid created by the bacteria. Commercially produced sour cream often contains additional thickening agents such as gelatin, rennin, guar and carrageen, as well as acids to artificially sour the product.