Unlocking the microbial diversity and the chemical changes throughout the fermentation process of " hákarl", Greenland shark

Abstract

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 (16S rRNA) 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.

Keywords: Fermentation; Methylamines; Microbial community; Preservation; Shark; Somniosus microcephalus.

Fig. 1

Diagram of the methodology. Triplicate samples (n = 3) were collected for each sampling group and sampling point (weekly during fermentation and biweekly during drying), where selective microbiological plate counts, 16S rRNA amplicon sequencing, TMAO, DMA, TMA, pH and water content were analyzed. X = RTE hákarl.

Fig. 2

Methylamine levels throughout fermentation and drying. TMAO, DMA, and TMA content in shark flesh during fermentation and drying in A) fresh sample groups and B) frozen sample groups. TMAO is represented by the blue line with the triangles, TMA is represented by the red line without points and DMA by the green line.

Fig. 3

Microbial growth during fermentation and drying of fresh and frozen cuts of Greenland shark. Total viable psychotrophic counts on IA and H₂S producer levels in A) fresh sample groups and B) frozen sample groups. Aerobic heterotrophic bacteria and anaerobic heterotrophic bacteria counts on ½ Marine Broth Agar in C) fresh sample groups and D) frozen sample groups. Pseudomonas spp. counts on CFC in E) fresh sample groups and F) frozen sample groups.

Fig. 4

Non-metric dimensional scaling plot based on Bray-Curtis dissimilarity, illustrating the microbial community dissimilarity between fresh and frozen shark cut samples during the fermentation and drying process.

Fig. 5

Microbial community succession in A) fresh back cut samples, B) frozen back cut samples, C) fresh flap cut samples and, D) frozen flap cut samples.

Fig. 6

Principal component analysis (PCA) scores plot (PC1 vs. PC2) of all shark samples based on chemical variables (TMAO, TMA, DMA) and 16S rRNA microbial communities' variables. A) Scores overview of the PCA and B) zoom in on the left part of the scores plot. Fre = Fresh; Fro = Frozen; B = Back; F = Flap.

Fig. 7

Correlation loadings from the PC1 and PC2 of the PCA of the shark samples. The chemical variables are identified in red and the 16S rRNA microbial communities' variables are identified in blue.