Traditional Norwegian Kveik Are a Genetically Distinct Group of Domesticated Saccharomyces cerevisiae Brewing Yeasts

Abstract

The widespread production of fermented food and beverages has resulted in the domestication of Saccharomyces cerevisiae yeasts specifically adapted to beer production. While there is evidence beer yeast domestication was accelerated by industrialization of beer, there also exists a farmhouse brewing culture in western Norway which has passed down yeasts referred to as kveik for generations. This practice has resulted in ale yeasts which are typically highly flocculant, phenolic off flavor negative (POF-), and exhibit a high rate of fermentation, similar to previously characterized lineages of domesticated yeast. Additionally, kveik yeasts are reportedly high-temperature tolerant, likely due to the traditional practice of pitching yeast into warm (>28°C) wort. Here, we characterize kveik yeasts from 9 different Norwegian sources via PCR fingerprinting, whole genome sequencing of selected strains, phenotypic screens, and lab-scale fermentations. Phylogenetic analysis suggests that kveik yeasts form a distinct group among beer yeasts. Additionally, we identify a novel POF- loss-of-function mutation, as well as SNPs and CNVs potentially relevant to the thermotolerance, high ethanol tolerance, and high fermentation rate phenotypes of kveik strains. We also identify domestication markers related to flocculation in kveik. Taken together, the results suggest that Norwegian kveik yeasts are a genetically distinct group of domesticated beer yeasts with properties highly relevant to the brewing sector.

Keywords: Saccharomyces; ale; brewing; domestication; fermentation; kveik; yeast.

Figure 1

Geographical distribution of kveik yeast samples sourced for this project. Map was generated using Google Maps and Scribble Maps. Parks, including the Jostedalsbreen (Jostedal glacier) National Park are highlighted in green.

Figure 2

Phylogeny of the six sequenced kveik strains compared with two control strains and the 157 S. cerevisiae strains sequenced in Gallone et al. (2016). (A) Maximum likelihood phylogenetic tree based on SNPs at 142120 sites in 166 S. cerevisiae strains (rooted with S. paradoxus as outgroup). Black dots on nodes indicate bootstrap support values <95%. Branches are colored according to lineage, and strain names are colored according to type (kveik, red; control, blue; reference, green). Branch lengths represent the number of substitutions per site. (B) Maximum likelihood phylogenetic tree produced as in (A), but using the phased haplotypes of the kveik yeasts instead of their consensus genotypes.

Figure 3

Population structure of the six sequenced kveik strains and the 157 S. cerevisiae strains sequenced in Gallone et al. (2016). (A) Population structure of 163 S. cerevisiae strains estimated with STRUCTURE based on SNPs at 26583 sites. Each strain along the x-axis is represented by a vertical bar partitioned into colors based on estimated membership fractions to the resolved populations for K = 6, 7, 8, 9, and 10 assumed ancestral populations. K = 9 best explains the data structure according to the “Evanno” method (Evanno et al., 2005). B1O: Beer 1–Other. (B) Principal component analysis of SNPs at 26583 sites in 163 S. cerevisiae strains. Dots are colored by population.

Figure 4

Fermentation kinetics and terminal ethanol concentration of small-scale wort fermentation (12.5°P original density) at 30°C. (A) CO₂ evolution in the fermentations was calculated by weighing the fermentation vessels (50 mL) and normalizing for mass loss in the fermentation airlocks. The data were then multiplied to represent a 100 mL volume. Yeast strains (black) are compared to a control ale strain (WLP001; red). The first 3 days of fermentation are shown. (B) CO₂ evolution at 24 h, calculated as in (A). Control ale strains are marked in red. Error bars represent SD, n = 3. (C) Ethanol concentration was measured via HPLC following 12 days of fermentation. Error bars represent SD, n = 3. Control ale strains are marked in red. (D) Maltotriose utilization as calculated from residual maltotriose values and original maltotriose values of the wort. Control ale strains are marked in red.

Figure 5

Flocculation capacity of kveik yeasts. Flocculation was assessed using the spectrophotometric absorbance methodology of ASBC Method Yeast-11. Values are expressed as %flocculance, with <20% representing non-flocculent yeasts, between 20 and 80% representing moderately flocculant yeast and >80% representing highly flocculant yeast. Strains are sorted in order of flocculance. Error bars represent SD, n = 3.