Kermaviili, Finnish curd cream, is a thickish, mildly sour-tasting cream product made by fermenting single cream using the same buttermilk culture bacteria as in making of crème fraîche and buttermilk.

Read more about kermaviili here.

Kermaviili is one of the most versatile ingredients in Finnish cooking and, together with the Finnish curd milk viili, sorely missed by many Finnish expatriates, especially in countries where no low-fat crème fraîche or smetana type products are available. Unlike in (American-type) sour creams, no artificial emulsifiers or other additives are used in making of full-fat kermaviili, but it is curdled and thickened with natural culture bacteria only.

It is possible to make kermaviili virtually anywhere, as long as you have access to buttermilk containing live starter bacteria, ie a product made with fermented pasteurised, homogenised milk that has not been pasteurised again after the fermentation process, as is done in some countries. This second pasteurisation process prolongs the shelf life of buttermilk and enables it to be stored without chilling, but destroys the live bacteria. Finnish buttermilks are never pasteurised.

Great thanks to Heidi A. for contacting the Finnish dairy company Valio Ltd asking for instructions for making kermaviili abroad and kindly sharing the information with the rest of us!

200 ml low-fat milk (about 1,5 % milk fat)
50 ml cream (about 36 - 38 % milk fat)
25 - 50 ml buttermilk with live culture bacteria (about 0 - 2,5 % milk fat), at room temperature

The fat content of regular commercially made kermaviili is around 10 %. While there are single/half/light creams available that have the right fat content for producing kermaviili (between about 10 and 18 %), these products usually contain artificial thickeners, stabilizing agents, emulsifiers or other additives, or just have a "bad" or "cooked" taste as a result of UHT processing, so I would not use them in this recipe.

Instead, I mix together "natural" low-fat milk and regular, "natural" heavy or whipping cream in a proportion to produce an approximate fat content between 10 and 12 %.

Procedure:
Take two to four small and clean, heatproof glass or ceramic bowls just large enough to hold the milk-cream-buttermilk mixture and rinse them with boiling water. Set aside to dry and cool. I find kermaviili to set better if the mixture is poured in several small bowls rather than in one large bowl, in which it tends to remain too watery.

Prepare a cold water bath by filling a large, wide pan or bowl with cold water. See that all the equipment that will be in contact with the cream mixture is clean (pans, bowls, measuring cups, spoons, etc).

Pour the milk and the cream in a small saucepan on a low heat. The whey proteins in the milk are altered by heating, giving the final product a finer, thicker consistency. Gently heat the mixture to a temperature of around 85 °C (between 82 °C and 88 °C), stirring continually to prevent the mixture from burning on the bottom and the cream from rising on top and forming a skin on the surface. (If, nevertheless, a skin forms, strain the mixture through a fine sieve.)

Take the pan off the heat and place it in the cold water bath. Stir the mixture until it has cooled down to a temperature of about 25 °C (warmer than normal room temperature). The starter culture bacteria work more efficiently in a warm environment.

Pour the mixture into the prepared bowls and stir in the buttermilk, dividing it equally between the bowls. Cover the bowls loosely (I use a piece of paper towel) and put them in a draft-free place, like inside a cold oven, with the oven door shut. In about 24 hours, the mixture should curdle into a thickish, spoonable cream with a thin, light-yellowish cream layer on top. Cover the bowls with plastic wrap and store in refrigerator, where they will keep for a few days.

Chilled kermaviili may be eaten with a spoon as a snack, topped with sugar, cinnamon, jam, fresh fruit, berries, etc. In Finland, kermaviili flavoured in a multitude of ways is used as a base for savoury or sweet dipping or dessert sauces, as dressing for salads, fish and meat dishes or mixed in hamburger, meatball or meatloaf mixtures, in fillings of sweet or savoury pies and in bread doughs, pie crusts, pancake batters, etc.

Before using kermaviili in cooking, it should be stirred smooth. As stirred kermaviili is somewhat runnier than thick yogurt, it can be poured in a paper coffee filter to drain and thicken, if required.
Note that because of its low fat content, kermaviili cannot be heated or boiled on its own  —  however, it can be used mixed with other ingredients in cooked dishes like pie fillings, hamburger and meatloaf mixtures, bread doughs, cake or pancake batters, etc, without the fear of curdling.

Homemade crème fraîche:
The method described above may also be used to make crème fraîche at home by mixing buttermilk in cream. Make sure to use pure, natural heavy/whipping cream with no additives.

See recipes for dishes using kermaviili  —  dipping sauces, old-fashioned salad dressing, tartare sauce, creamy beetroot salad, baked potatoes, gingerbread cake, date cake or Herring à la russe, to name just a few.

Recipe source: adapted from instructions provided by Valio Ltd.