Sophie Jensen, Snorri Páll Ólason, Sigurlaug Skírnisdóttir, Guðmundur Stefánsson, Cecile Dargentolle, Viggó Thór Marteinsson
5 min
The study investigates the chemical and microbial transformations that occur during the traditional production of hákarl, an Icelandic fermented Greenland shark product. Specifically, the authors aimed to understand how different raw material preparations (fresh vs. frozen meat) and processing stages (fermentation and drying) influence the final product’s quality, with the goal of identifying opportunities to optimize or standardize the production process.
The researchers conducted a small-scale industrial study using meat from a single Greenland shark. The meat was divided into two groups: one processed fresh and the other frozen for four days before thawing. Both groups underwent a fermentation period (eight weeks for fresh, seven for frozen) followed by five weeks of drying. Throughout these 13 weeks, the team performed weekly or biweekly sampling to analyze:
The fermentation process acts as both a preservation and detoxification method, characterized by three distinct microbial phases:
This research provides a scientific foundation for the traditional, centuries-old practice of making hákarl. By mapping the microbial succession and chemical degradation, the authors suggest that the fermentation period could potentially be shortened. Furthermore, the findings indicate that the process could be standardized using starter cultures, which would help ensure consistent quality and safety for this unique cultural delicacy.
Hákarl is a unique traditional Icelandic product and is obtained by fermenting and drying Greenland shark (Somniosus microcephalus). However, little is known about the chemical and microbial changes occurring during the process. In this small-scale industrial study, fresh and frozen shark meat was fermented for eight and seven weeks, respectively, and then dried for five weeks. During the fermentation, trimethylamine N-oxide levels decreased to below the limit of detection within five weeks and pH increased from about 6 to 9. Simultaneously, trimethylamine and dimethylamine levels increased significantly. Total viable plate counts, and specific spoilage organisms increased during the first weeks of the fermentation period but decreased during drying. Culture-independent analyses revealed gradual shifts in the bacterial community structure as fermentation progressed, dividing the fermentation process into three distinct phases but stayed rather similar throughout the drying process. During the first three weeks of fermentation, Photobacterium was dominant in the fresh group, compared to Pseudoalteromonas in the frozen group. However, as the fermentation progressed, the groups became more alike with Atopostipes, Pseudomonas and Tissierella being dominant. The PCA analysis done on the chemical variables and 16S rRNA analysis variables confirmed the correlation between high concentrations of TMAO and Pseudoalteromonas, and Photobacterium at the initial fermentation phase. During the final fermentation phase, correlation was detected between high concentrations of TMA/DMA and Atopostipes, Pseudomonas and Tissierella. The results indicate the possibility to shortening the fermentation period and it is suggested that the microbial community can potentially be standardized with starter cultures to gain an optimal fermentation procedure.
Alex: And what did that census show about the chemistry?
Sam: It showed that as the bacteria work, they break down a compound called urea and release ammonia. This is the key step—it pushes the environment from roughly neutral all the way to strongly alkaline. That high-alkaline state is what makes the food safe, because dangerous bacteria simply can't survive there.
Alex: So the bacteria aren't just cleaning the meat—they're essentially cooking it with chemistry instead of heat.
Sam: That's a good way to put it. And then there's a final physical step. The meat is hung outdoors for several weeks to dry in the wind. By the end of the full thirteen-week process, the water content drops dramatically—from roughly three-quarters water down to less than half. Most harmful microbes need moisture to survive, so this drying stage is the last line of defence.
Alex: So the starting lineup of bacteria changes depending on how the meat was handled before fermentation begins. Does that shift who eventually finishes the relay race?
Sam: Surprisingly, the finishers stayed mostly the same. As ammonia levels rose and the environment became more alkaline, a consistent group of specialists took over regardless of how the process started. The researchers used a statistical tool to map all the variables—the bacteria, the chemistry, the moisture—and it showed that while the path starts differently for fresh versus frozen shark, the final dried product tends to land in the same safe, stable zone.
Alex: So the logic of the fermentation is robust enough to handle those early differences. Does that mean producers could actually improve or speed up the process?
Sam: The paper suggests exactly that. Because the researchers identified the specific bacteria that finish the job, they believe these could be used as what you might call starter cultures—a bit like how a baker uses a specific sourdough starter to make sure every loaf turns out the same. By adding these organisms at the beginning, producers might be able to reduce the fermentation time significantly and make the whole process more consistent.
Alex: That sounds like a meaningful development for the industry. But was there anything in the study that makes these findings tentative?
Sam: That's a critical observation. The study was based on a single shark. While the results are clear and detailed, individual sharks can have different biology, and seasonal variation might shift how the bacteria behave. So it's a strong and detailed starting point—a careful map of a very unusual ecosystem—but probably not the final word.
Alex: Right. One shark gives you a clear picture, but not necessarily the full story for every batch. Still, it's a notable piece of work for understanding why this ancient process actually functions.
Sam: It really is. It's a good example of how modern scientific tools can explain—and perhaps improve—techniques that communities have been using for centuries, often without knowing exactly why they work.
Alex: That's it for this look at the science of hákarl. Thanks for listening to ResearchPod.