Monthly Archives: August 2014

Physiology of Finnish Baker’s Yeast (Suomen Hiiva)

While ale yeast and baker’s yeast belong to the same species, Saccharomyces cerevisiae, they have over time been adapted to different functions. Brewing with baker’s yeast is not a very common practice, nor generally recommend amongst brewers, mostly because these strains might not exhibit favorable fermentation characteristics, such as production of desirable flavour compounds, adequate attenuation, and flocculation. Of the recognized beer styles, it is more or less only in Northern European Traditional ales (such as Sahti, Gotlandsdricka and Kvass) that it acceptable to ferment with baker’s yeast. As there is little information published on the physiological characteristics of Finnish baker’s yeast in brewing, I thought I’d post some recent results from my related research.


I performed a series of mini-fermentations (30 ml of wort) on a range of yeast strains earlier this spring for screening purposes, and I included the baker’s yeast from Suomen Hiiva, as well as two common homebrewing strains WLP002 (English Ale) and WLP380 (Hefeweizen IV). These mini-fermentations were performed at two different temperatures, 15 and 20 °C. Fermentation progress was monitored by weight loss, and after fermentation the resulting beer was analyzed for ABV% (Anton Paar Alcolyzer ME), Extract (Anton Paar DMA 5000 M), pH (Anton Paar pH ME), aroma compounds (HS-GC/FID), diacetyl (HS-GC/ECD), and phenolics (HPLC/PAD). A 15 °Plato (OG 1.060) wort was used for all fermentations, and the yeast was pitched at a rate of 2.5 g/L (~ 10 million cells/ml).

What first surprised me was how well the baker’s yeast performed during fermentation. Compared to the other two reference strains presented here, it fermented faster at both temperatures and reached a higher final attenuation. It managed to reach a final attenuation of 85% in just 5 days when fermented at 20 °C. Not bad for an all-malt wort at this strength. So the baker’s yeast can definitely give rise to adequate attenuation. Note, a pure culture of the yeast was used (starting from a similar package as in the picture above), so there was no risk of any lactic acid bacteria contamination, which otherwise is probable when using the yeast directly. The pH values of all beers were quite similar irrespective of yeast strain and temperature, and it seems like the baker’s yeast acidified the wort slightly less than the other two reference strains. In the table below you can find a summary of the Extract, ABV% and pH of the beers. In the figure below you can find a plot of the fermentation progress over time (20 °C squares; 15 °C circles).



I have never actually used baker’s yeast for beer fermentation myself, as I’m not that big of a Sahti fan, but all beers (well mainly Sahtis) I’ve tried have had quite a ester-dominated aroma. Especially 3-methylbutylacetate (isoamyl acetate), with its prominent banana aroma, is very pronounced in, and also an integral part of, Sahti. Hence, it comes as no surprise that, compared to the other two reference strains, the baker’s yeast produced more higher alcohols and esters. It was not only 3-methylbutylacetate that was produced in large amounts (even more than the WLP380 Hefeweizen strain), but also the ethyl esters. Ethyl acetate, with its solvent-like aroma, is typically unwanted at higher concentrations in beer, but the other ethyl esters may contribute a fruity aroma to the beer. In the figure below you can see a summary of the flavour impact (i.e. the concentration of the compound in the beer divided by its flavour threshold. An impact above 1 should affect flavour, while an impact between 0.5 and 1 may affect flavour) of the various higher alcohols, esters, and acetaldehyde in the beers fermented with the different yeast strains (20 °C solid; 15 °C striped). As can be seen from the figure, only 3-methylbutanol (the precursor to the banana ester) of the higher alcohols is close to the flavour threshold. As should come to no surprise, less higher alcohols and esters were produced at a lower fermentation temperature. From these aroma compound results, it is clear that the baker’s yeast will produce a fruity, and maybe even slightly solvent-like, beer. Hence, the yeast would probably be suitable for Hefeweizens and Belgian-style beers, and a slightly lower fermentation temperature is probably recommended.sh_aroma


Finally we arrive at two other important aroma compounds in beer, diacetyl (butter-like aroma) and 4-vinylguaiacol (clove-like aroma; 4-VG). Diacetyl is always considered an off-flavour (well in some cases in might be acceptable, but in my opinion it is just a sign of poor fermentation practices), while 4-VG is acceptable (and even required) in some styles (such as Hefeweizens and Belgian-style beers). Diacetyl levels decrease towards the end of fermentation, and are highly dependent on fermentation dynamics, so the concentration that was measured here at the end of fermentation doesn’t say that much. The diacetyl concentration was above the flavour threshold (50 ppb) for all strains at both temperatures, which is not that surprising since the measurements were made 120 hours after pitching the yeast, and the lowest flavour impact was observed in the beer fermented with the baker’s yeast at 20 °C. This result again suggests that baker’s yeast is a good candidate for beer fermentations (and especially more rapid ones). The baker’s yeast is also POF+ (i.e. positive for producing ‘phenolic off-flavours’; the yeast produces a phenylacrylic acid decarboxylase enzyme, that decarboxylates ferulic acid (and other phenolic acids) from the wort into 4-vinylguaiacol (and other phenolic compounds)), since 4-VG was observed in the beer. The baker’s yeast produced slightly less 4-VG than WLP380 (the table below displays the percentage of ferulic acid converted into 4-vinylguaiacol; 77% is the theoretical maximum), but it still produced concentrations above the flavour threshold in an all-barley wort (worts made from wheat malts contain more ferulic acid). This slightly limits the beer styles that the baker’s yeast could potentially be used for, but again it could be used for Hefeweizens and Belgian-style beers.


To conclude, it is evident that the Finnish baker’s yeast is perfectly usable for beer fermentations, and it comes to no surprise that it has successfully been used for traditional beer fermentation in the form of Sahti. Of the two reference strains, the baker’s yeast was closer to WLP380 (Hefeweizen IV), and it can be described as a faster-fermenting and fruitier version of it. As a side-note, it can be mentioned that the baker’s yeast flocculated very poorly, which is also similar to the behavior of WLP380. I would not recommend the use of the baker’s yeast for beer styles calling for a clean yeast aroma and no spicy phenolics, but it would be perfectly suitable for a Hefeweizen, Belgian Blond and why not even Belgian Strong Ale (Sahtis are brewed strong, so the baker’s yeast should be quite tolerant to alcohol levels reaching up to 10% ABV). It fermented quite fast at both 20 °C and 15 °C, and I would recommend a lower temperature if you are after a cleaner finish. As mentioned previously, the yeast packages found in the supermarket are all almost certainly contaminated with lactic acid bacteria, so the use of a pure culture is definitely recommended. Good luck with the brewing!

Feel free to contact me if you have any questions!

Reference for flavour thresholds:

  • Meilgaard, M., Prediction of Flavor Differences between Beers from Their Chemical Composition. Journal of Agricultural and Food Chemistry 30 (1982) 1009-1017.

Article on the differences in diacetyl formation between lager yeast strains and the role of the ILV6 gene

I’ve been researching diacetyl formation and removal by brewing yeasts the past year(s), and we recently did some screening of various lager yeast (Saccharomyces pastorianus) strains for diacetyl production and subsequent transcriptional analysis of key genes in the isoleucine-valine catabolic pathway (diacetyl is formed as an indirect by-product of valine biosynthesis in yeast; more information available here). We did this in order to identify what genes contribute most to the differences in wort total diacetyl concentration we observed between the different strains. We observed that particularly the ILV6 gene, encoding for a regulatory subunit of the acetohydroxy acid synthase enzyme (responsible for converting pyruvate into the diacetyl precursor α-acetolactate), showed greater expression early during fermentation in a strain producing more diacetyl. To confirm the role of this ILV6 gene in controlling α-acetolactate/diacetyl formation, we overexpressed both the S. cerevisiae- and S. eubayanus-form of ILV6 in our standard production lager strain A15, and found that overexpression of either form of ILV6 resulted in an identical two-fold increase in wort total diacetyl concentration relative to a control. These results suggest that both forms of the gene influence diacetyl formation, and different levels of transcription (this could be a result of a gene-dosage effect, since different lager yeast hybrids have inherited different amounts of the parental genomes; for a recent review on the subject click here) may contribute to differences in diacetyl production in various lager yeast strains.

Link to the publication:

E-mail me if you are interested in the article, but don’t have access to the full-text.


A screen of 14 S. pastorianus lager-brewing strains showed as much as a nine-fold difference in wort total diacetyl concentration at equivalent stages of fermentation of 15°Plato brewer’s wort. Two strains (A153 and W34), with relatively low and high diacetyl production, respectively, but which did not otherwise differ in fermentation performance, growth or flavour production, were selected for further investigation. Transcriptional analysis of key genes involved in valine biosynthesis showed differences between the two strains that were consistent with the differences in wort diacetyl concentration. In particular, the ILV6 gene, encoding a regulatory subunit of acetohydroxy acid synthase, showed early transcription (only 6 h after inoculation) and up to five-fold greater expression in W34 compared to A153. This earlier transcription was observed for both orthologues of ILV6 in the S. pastorianus hybrid (S. cerevisiae × S. eubayanus), although the S. cerevisiae form of ILV6 in W34 also showed a consistently higher transcript level throughout fermentation relative to the same gene in A153. Overexpression of either form of ILV6 (by placing it under the control of the PGK1 promoter) resulted in an identical two-fold increase in wort total diacetyl concentration relative to a control. The results confirm the role of the Ilv6 subunit in controlling α-acetolactate/diacetyl concentration and indicate no functional divergence between the two forms of Ilv6. The greater contribution of the S. cerevisiae ILV6 to acetolactate production in natural brewing yeast hybrids appears rather to be due to higher levels of transcription relative to the S. eubayanus form.

Hops Growing Update

I planted some hop rhizomes earlier this year, and they’ve been growing extremely well overall. Shortly after I moved the plants from the indoor pots to the garden in April, we had a spell of some really cold nights, which unfortunately killed both Centennial plants. The other 4 (Cascade, Chinook, Columbus and Nugget) survived and have been growing well (the bines are at least 3 meters long and they’ve reached the roof). Not getting my hopes up for a first-year harvest, but hopefully some of the plants will at least produce something!