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Fish and seafood from Iceland

Kæst skatakæst skata

Kæst skata is Icelandic skate left to rot and ferment in a container for about three weeks, then boiled and eaten on 23 December. Bacteria break down the urea in the skate's flesh to ammonia, so the fish turns strongly alkaline and smells sharply of ammonia instead of going sour.

Fully documentedFish and seafood9 days – 21 days

Recipe

No fixed amounts recordedThe tradition has no fixed amounts. The study followed a traditional 9-day curing process.
Skate wings (börðin)Only the wings are cured and the rest of the fish is thrown away[reynisson2012][visindavefur-kaesa]
Melted lamb suet, potatoes, turnips and sweet rye breadServed with the boiled skate, not added during curing[westfjords]

Total time across the recorded steps: 9 days – 21 days

Start
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What changes: Starting material: fresh skate

Step 1 of 4
Start with fresh skate
Time
Not documented
Temperature
Not documented
Salt
None given
Acidity
Not documented
Flavour
Not documented
Texture
Not documented
Colour
Not documented

Microbes at work in this step

Not documented for this step.

Method

Measured measured in a studySet fixed by a specification or tested recipeReported one source reports itTypical a general value, not sourced for this fermentUnconfirmed not yet checked against a source

At a glance

Kæst skata is Icelandic skate left to rot and ferment in a container for about three weeks, then boiled and eaten on 23 December. Bacteria break down the urea in the skate's flesh to ammonia, so the fish turns strongly alkaline and smells sharply of ammonia instead of going sour. Made in 4 steps over 9 days – 21 days. The longest step, leave it to rot and ferment in a container (kæsing), takes 9–21 days (Visit Westfjords 2026). Photobacterium and Aliivibrio in the cured flesh, Pseudomonas on the skin and Urea-splitting bacteria: Oceanisphaera, Psychrobacter, Pseudoalteromonas and Pseudomonas do most of the fermenting.

Key numbers
Main step9–21 daysReported[westfjords][reynisson2012][visindavefur-kaesa]Leave it to rot and ferment in a container (kæsing)
AciditypH 6.6 → 9.27Measured[reynisson2012]Leave it to rot and ferment in a container (kæsing)

What makes this one different

Most fish ferments go sour as lactic acid builds up. In skate the urea in the flesh turns to ammonia, so the pH rises from 6.64 to 9.27 instead of falling.

Related ferments

Safety

No salt or acid protects the skate while it cures. Its pH rises instead. Vísindavefurinn says those alkaline conditions leave fewer kinds of bacteria able to cause spoilage, and the one study found fairly slow bacterial growth (Reynisson et al. 2012), but its abstract reports no test for Clostridium botulinum. The FDA puts the top pH for that bacterium's growth at 9, and this cure only ends at 9.27, so the alkali is not a full barrier while it builds. The cured skate is boiled before it is eaten, and boiling is the step that would destroy any toxin (MAFF gives 100 °C for 1–2 minutes). No source tests the curing as a home method, so the steps here are a record, not instructions.

History and sources

Every number is labelled with how it is known. How this atlas is made

The dish comes from the Westfjords, and the custom of eating it on Þorláksmessa spread from there to the rest of Iceland (Visit Westfjords). The historian Árni Björnsson (2007) links it to the Catholic fast before Christmas, when poor fish was the rule on St Thorlak's day, and to skate being caught mainly on the Westfjords banks at that time of year. People who moved from the Westfjords to the south-west in the 20th century took the custom with them; after the middle of the century fishmongers in the Reykjavík area began to stock skate in December, and about a quarter century before 2007 restaurants began to hold skate feasts, which made it fashionable. By that account the Þorláksmessa skate is very old in the Westfjords and only some decades old in the Reykjavík area. Older practice cured it in a pit under stones or turf, or in sacks hung in a cowshed for its temperature, and hung it afterwards to keep it; today it is frozen until it is boiled (Vísindavefurinn, 2011).

Starter
The sources we found do not say how its fermentation begins.
  1. [reynisson2012] Reynisson E., Marteinsson V.Þ., Jónsdóttir R., Magnússon S.H., Hreggviðsson G.Ó. 2012, Bacterial succession during curing process of a skate (Dipturus batis) and isolation of novel strains, Journal of Applied Microbiology 113(2):329-338 (abstract, Europe PMC)paper
  2. [visindavefur-kaesa] EDS, Hvernig kæsir maður skötu? (How is skate cured?), Vísindavefurinn (Science Web, University of Iceland), 23 December 2011web
  3. [visindavefur-bjornsson] Árni Björnsson, Hvers vegna er skata borðuð á Þorláksmessu? (Why is skate eaten on Þorláksmessa?), Vísindavefurinn (Science Web, University of Iceland), 20 December 2007web
  4. [visindavefur-brjosk] Magnús Már Kristjánsson, Hvers vegna er hægt að kæsa brjóskfiska en ekki beinfiska? (Why can cartilaginous fish be cured but not bony fish?), Vísindavefurinn (Science Web, University of Iceland), 21 February 2000web
  5. [fda-app4] US FDA 2011, Fish and Fishery Products Hazards and Controls Guidance, 4th ed., Appendix 4, Bacterial pathogen growth and inactivation, Table A-1gov
  6. [maff-boto] MAFF Japan, hazard profile, Clostridium botulinum (food poisoning cases and control measures)gov
  7. [westfjords] Visit Westfjords, Christmas is approaching (fermented skate on Þorláksmessa), read 2026-10web