Viili, also called curd milk, is a traditional cultured milk product of Finland, though it originated in Sweden. Similar to yogurt, it is created through the activity of lactic acid bacteria, fungus, and yeast in milk. The dish is characterized by the heteropolysaccharide viilian, a carbohydrate gel, which is produced by the lactic acid bacteria strain Lactococcus lactis subsp. cremoris.
In Finland, viili is usually eaten for breakfast or as an afternoon snack. It is served chilled, and often sweetened with fruit, jam, sugar, or honey. It is thick and gelatinous in texture, and its flavor is mildly sour. If stirred, it becomes rope-like and difficult to eat. It can also be served with a variety of other toppings, such as muesli, cinnamon, nuts, and powdered ginger. A traditional Finnish topping is talkkuna flour, a mixture of semi-cooked, dried, and sometimes roasted grains ground into a course flour.
You can make your own viili at home with a starter kit, or with fresh viili. After making a batch with a starter kit, you will have your own fresh viili to use for future batches. The company G.E.M. Cultures sells Finnish viili culture in the United States. Their culture requires the user to put the culture into a clean bowl, add pasteurized milk, and let sit for about 24 hours at room temperature, until it sets. It forms a thin, velvety layer on top caused by the mold Geotrichum candidum. In order to keep a culture active, you should make a new batch at least every ten days.
It is important to make viili using only pasteurized milk, as the bacteria in raw milk can interact dangerously with the bacteria and fungus in viili culture. However, you can use any kind of pasteurized milk you like. Using non-homogenized milk will cause a layer of cream to form on the top of the product. While whole milk is the most traditional basis for viili, you can also make a low-fat or non-fat version, or even a dairy-free soy version. Soy milk is not ideal for use with a starter kit, however, so if you wish to make soy viili regularly, it is best to keep a dairy starter on hand.
Other Scandinavian cultures have traditional cultured milk products very similar to viili, such as the Swedish langfil and filmjolk, Norwegian tettemelk, and Icelandic skyr. A variation on viili is kermaviili, which is cream curdled with starter bacteria. Kermaviili is smoother, creamier, and more sour. It is similar to low-fat sour cream and can be used in dips, dressings, and sauces.