Ever had a Kveik Yeast in your fermenter? If so, you will probably appreciate how kveik is distinctly different from conventional brewing yeasts. The question, though, is why?
Kveik (Norwegian for yeast) represents an old and domesticated group of yeast strains with distinct fermentation properties. Specifically, Kveik yeast strains are thermotolerant, thrive in higher ethanol concentrations and produce a distinctive flavour profile through compounds that are yet unknown.
Scientific interest in Kveik yeast
The good news is that as kveik yeast has become ever more popular in craft brewing, this domesticated Norwegian super yeast now also attracts scientific scrutiny. There is a significant interest in understanding kveik fermentation characteristics and profiles. Here we will describe recent work investigating the performance of kveik yeast strains at a range of different temperatures to assess fermentation kinetics. In addition, the authors studied sugar metabolisms and flavour compound production in a series of characterized kveik strains.
By contrasting results with commonly used ale yeasts, the authors reveal what distinguishes kveik yeasts from conventional counterparts. Furthermore, the work identified a possible way by which kveik achieves temperature tolerance, a strategy that may lead to the development of more ethanol tolerant yeast strains. Let’s dive in!
An investigation into Kveik yeast diversity and thermotolerance
First, the authors tested kveik attenuation levels and how temperature affects performance. Small fermentations at a range of temperatures revealed that most (of the six) kveik strains feature accelerated fermentation at higher temperatures (30-40 oC). Not only did the strains reach maximum attenuation, but they also did so in a significantly shorter amount of time (three days for most strains). Interestingly and in contrast to the kveik yeasts, control ale strains performed poorly at higher fermentation temperatures, indicating significant cellular heat stress.
Another exciting observation stands out from these experiments. Kveik yeasts (at least the ones tested here) ferment perfectly at lower temperatures (15-20 oC), with attenuation levels similar to the control ale strains. This means that brewers can use kveik yeast at a broader temperature range.
Sugar consumption rates in Kveik strains are temperature dependent
The question then, of course, becomes, how do temperatures impact attenuation speed? The authors investigated and compared sugar consumption rates between strains and temperatures to tackle this question. Analyses of changes in wort sugar composition over time, prompted by the growth of each yeast strain in a series of different temperatures, revealed that (i) all kveik yeast strains alter their sugar consumption rates in a temperature-dependent manner. Furthermore, they found that each kveik strain exhibits a unique response to temperature. Not surprisingly, these strain-specific responses also translated in both glycerol and ethanol production rates.
The basis of Kveik beer flavour and aroma
The big question now is how the kveik secondary metabolite profile, responsible for beer flavour and aroma, is dependent on strain and temperature. To look into the chemical signatures of kveik across a range of temperatures, the authors used HS-SPME-GC-MS analyses on four major metabolite groups: fatty acids, ethyl esters, alcohols, and acetate esters. By collecting spectral data from each sample and combining this data, the authors constructed a PCA-biplot that could (i) visualize clusters based on metabolite profile (dis)similarity and profile shifts attributable to temperature change. These analyses revealed the following Kveik features.
- As expected, all the kveik yeasts formed clusters distinct from those created by classical ale yeast strains.
- The temperature profoundly impacted kveik metabolite profiles, agreeing with sugar metabolism observations.
Kveik yeast like it hot
The increase in metabolic rates and profiles at higher temperatures prompted the authors to investigate the possible mechanisms responsible for thermotolerance. One such mechanism is the production of trehalose that acts as a cellular protectant against various stressors. By shifting kveik yeast strains from a low (30 oC) to a high-temperature regimen, the authors showed that kveik rapidly produced substantial amounts of trehalose compared to the classical strains. These results invoke a mechanism by which rapid trehalose accumulation leads to enhanced resistance to heat stress.
Trehalose biosynthesis is executed by a complex that features two enzymes (TPS1 and TPS2) regulated by two regulatory proteins (TSL1 and TPS3). The authors were interested to see how these proteins have evolved (or are different) in kveik yeast strains. Sequencing of all isolates and comparisons between sequences revealed the presence of small changes (single nucleotide polymorphisms or SNPs) that could explain differences in trehalose synthesis rates.
Trehalose synthesis, however, is balanced by its hydrolysis (breakdown) through degradative enzymes called ATH1 (acidic trehalase 1), NTH1 (Neutral Trehalase 1) and NTH2. Expression of the genes encoding these proteins occurs in the stationary phase, which coincides with trehalose breakdown. Interestingly, sequence analyses revealed mutations in NTH1 and NTH2, possibly inactivating these enzymes, which would result in the accumulation of trehalose. Thermal adaptation via trehalose synthesis appears to work in two ways: increased trehalose synthesis rates and reduced neutral trehalase activity.
At least to some degree, these two mechanisms seem responsible for the thermotolerance found in Kveik yeast.
Why this work is interesting and important
We have only started to understand the biology and characteristics of kveik yeast. Research aimed at understanding those features that set kveik apart from other domesticated yeast will undoubtedly better understand the fermentation process. Similarly, discoveries made in kveik may accelerate the investigation of different wild and domesticated yeasts of interest.