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Viili: How to Make Finland’s Stretchy Fermented Milk at Room Temperature

By Updated 2 min read

Viili is Finland’s ancient mesophilic fermented milk — fermented entirely at room temperature, requiring no heating equipment, no thermometer, and no special skills. Its defining feature is a ropy, thread-forming texture from exopolysaccharide-producing Lactococcus cremoris strains found only in Nordic dairy cultures. This complete guide covers the traditional Finnish method, the microbiology of ropiness, starter sourcing, and how to serve viili authentically.

At a glance

Viili is Finland's ropy fermented milk, made at room temperature: stir 1 tablespoon of viili starter into each cup (250 ml) of pasteurized whole milk and leave it undisturbed at 20–22°C for 18–24 hours until set and stretchy. Mesophilic Lactococcus lactis strains ferment it, and particular slime-forming strains give the stretch – so you need a real viili culture. Save some from each batch to start the next.

  • Finnish fermented milk with a stretchy, ropy texture.
  • Made at room temperature (20–22°C) in 18–24 hours – no heating.
  • Fermented by Lactococcus lactis, with Leuconostoc and aroma strains.
  • Ropiness comes from particular slime-forming strains; you need a viili starter.
  • Often topped by a velvety Geotrichum candidum surface layer.

Ferment profile

Origin
Finland
Fermentation
Lactic acid
Main microbes
Lactococcus lactis (subsp. cremoris and lactis, biovar diacetylactis), Leuconostoc mesenteroides; Geotrichum candidum on the surface
Live cultures
Yes
Alcohol
None / trace

Viili is Finland’s traditional fermented milk, famous for its texture: a spoonful stretches into long, silky threads instead of breaking off cleanly. It’s mild and gently sour, often eaten with sugar, cinnamon, berries or jam, and – unlike yogurt – it’s made at ordinary room temperature, with no heating or incubator. Traditional viili often has a velvety white skin on top from Geotrichum candidum, a mold-like fungus that grows on the surface. Sweden’s filmjölk is a close, less stretchy relative.

What Makes It Stretch

Viili is fermented by mesophilic (room-temperature) lactic acid bacteria. Industrial viili starters of traditional origin contain Lactococcus lactis – subspecies cremoris, lactis or both – plus aroma-producing L. lactis biovar diacetylactis and Leuconostoc mesenteroides (Kahala et al., 2008). The ropiness comes from particular slime-forming lactococcal strains that coat themselves in exopolysaccharide (Kitazawa et al., 1993). Not every strain does this: in a study of 60 bacteria isolated from natural viili starters – mostly L. lactis plus some enterococci – none produced exopolysaccharide on its own (Rampanti et al., 2024). That’s why you need a real viili culture: plain yogurt or buttermilk won’t turn stretchy.

Viili’s polysaccharides have interested researchers – in mice, small doses shifted gut bacteria and kept fecal butyrate from falling (Yamane et al., 2021) – but there are no human studies, so treat that as a curiosity, not a benefit.

Keeping a Culture Going

  • Get a starter: a few spoonfuls of fresh viili from a friend, a Finnish grocer, or a dried viili culture from a fermentation supplier.
  • Re-culture weekly: save a few spoonfuls from the middle of each batch (not the surface) to start the next within about 7 days.
  • Use pasteurized milk: raw milk can carry harmful germs that fermenting doesn’t reliably remove (CDC). Ordinary pasteurized whole milk sets best; ultra-pasteurized milk can set softer.
  • Don’t disturb it while it sets – moving the jar breaks the gel.

Discard a batch that turns pink, orange or fuzzy, smells putrid, or doesn’t set within about 36 hours, and restart from a fresh culture. For other room-temperature milks see amasi; for warm-incubated ones, yogurt and Bulgarian kiselo mlyako.

Recipe: Room-Temperature Viili

Makes
4 cups (1 litre)
Hands-on
10 min
Fermentation
18–24 hours at 20–22°C
Temperature
20–22°C (68–72°F)
Difficulty
Easy

Equipment

  • 4 cups or small bowls (or one 1-litre jar)
  • Spoon
  • Cloth or loose lids

Ingredients

  • 1 litre pasteurized whole milk (not ultra-pasteurized if possible)
  • 4 tbsp viili starter (1 tbsp per cup, from a previous batch or a dried culture)

Method

  1. Let the milk come to room temperature (or warm it gently to about 20–25°C).
  2. Spread 1 tablespoon of viili starter over the bottom of each clean cup.
  3. Pour 250 ml milk into each cup and stir gently once.
  4. Cover loosely and leave undisturbed at 20–22°C for 18–24 hours, until set and stretchy when lifted with a spoon.
  5. Refrigerate. Eat within about a week, saving a few spoonfuls from the middle to start the next batch.
  6. Discard a batch that turns pink, orange or fuzzy, smells putrid, or hasn't set after about 36 hours.

Frequently asked questions

What is viili?

A Finnish fermented milk made at room temperature, known for its stretchy, ropy texture and mild sour taste.

Why is viili stretchy?

Particular strains of Lactococcus lactis in viili cultures produce long exopolysaccharide chains that make the milk ropy.

Can I make viili without a viili starter?

No – yogurt or buttermilk won't produce the stretch. You need a viili culture, from a previous batch or a supplier.

What is the white layer on top of viili?

On traditional viili it's Geotrichum candidum, a mold-like fungus that forms a smooth, velvety skin. It's normal; fuzzy or colored growth is not.

How is viili different from yogurt?

Viili is made at room temperature with mesophilic Lactococcus cultures and is milder and stretchy; yogurt is made at about 43°C with thermophilic cultures.

How long does a viili culture last?

Re-culture at least weekly, using spoonfuls from the middle of a fresh batch.

Sources

  1. Kahala M et al. (2008). Characterization of starter lactic acid bacteria from the Finnish fermented milk product viili. J Appl Microbiol 105(6):1929–1938. Link
  2. Rampanti G et al. (2024). Technological and enzymatic characterization of autochthonous lactic acid bacteria isolated from viili natural starters. Foods 13(7):1115. Link
  3. Kitazawa H et al. (1993). B-cell mitogen produced by slime-forming, encapsulated Lactococcus lactis ssp. cremoris isolated from ropy sour milk, viili. J Dairy Sci 76(6):1514–1519. Link
  4. Yamane T et al. (2021). Exopolysaccharides from a Scandinavian fermented milk viili increase butyric acid and Muribaculum members in the mouse gut. Food Chem (Oxf) 3:100042. Link
  5. CDC. Raw milk. Link
#Nordic countries
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