Filmjölk (also known as fil) is a Sweish mesophilic fermented milk product that is made by fermenting cow's milk with a variety of bacteria from the species Lactococcus lactis and Leuconostoc mesenteroides. The bacteria metabolize lactose, the sugar naturally found in milk, into lactic acid. The acid gives filmjölk a sour taste and causes proteins in the milk, mainly casein, to coagulate, thus thickening the final product. The bacteria also produce a limited amount of diacetyl, which gives filmjölk its characteristic taste. Filmjölk is similar to cultured buttermilk, kefir, or yoghurt in consistency, but fermented by different bacteria and thus has a slightly different taste. Compared with yoghurt, filmjölk tastes less sour. In Sweden, it is normally sold in 1-liter packages with live bacteria. The bacteria help maintain the balance between beneficial and harmful bacteria in the intestines.