I got this question sent to me, which gives me a chance to update my long neglected blog:
Q. Does Saccharomyces generate byproducts as a result of digesting refined table sugar as opposed to invert sugar or candi sugar? In particular, any not so desirable byproducts?
A. The short and annoying answer is, yes and no.
The simple story is that they'll create an enzyme called invertase to digest the sucrose . I doubt we can taste those enzymes (there are 6 or 7 slightly different ones, with some strain variability). That gets you to glucose and fructose in equal proportions, the same as invert sugar.
However, like everything in genetics, it is more complicated than that. In order to know to create invertase, they have to detect the sucrose, so you have the sucrose sensing genes. Once it is sensed, some sort of cascade will be activated to activate the invertase - I don't know if that is a transcriptional cascade or activation cascade or some combination. But the method of activation could lead to tastable byproducts.
Then when a bunch of the sugar gets converted you're going to get repression of the pathway due to the high levels of glucose now present, so there will be a balancing act between inverting the sugar and consuming the glucose. Other genes also affect the levels of invertase secreted as well as invertase activity, and some of these genes have other functions. Then there are transcriptional activators that could up or down regulate genes that are not strictly required in this pathway but can affect overall yeast health. Here is a quick description of one of the genes, you can imagine some of these will have tastable byproducts:
MSN1 - "involved in regulation of invertase and glucoamylase expression, invasive growth and pseudohyphal differentiation, iron uptake, chromium accumulation, and response to osmotic stress; localizes to the nucleus"*
So does it create undesirable byproducts? Probably. But if you keep the level of sugar low enough it shouldn't be an issue. It will taste different though, check out Ted Hausotter's article in Zymurgy where he made a bunch of tripels and added different sugars. I sat in on the tasting panel and the differences were noticeable.
*Source - Saccharomyces Genome Database
Monday, June 8, 2009
Thursday, August 21, 2008
Dry Yeast vs. Liquid Yeast
Keywords: yeast, diacetyl, amino acid, VDK
There are proponents on both sides of the dry vs. liquid yeast debate, both with valid points. Dry yeast is easier to use and can be stored longer, not to mention it’s cheaper. Liquid yeast on the other hand, provides a fresher pitch of healthy cells.
What is a brewer to do? An article appeared last year in the Journal of the American Society of Brewing Chemists that gives me some ideas.
From McGill University in Canada, Cyr et al. reported on their research of two different strains of lager yeast. I think some of the broader lessons from the article can be applied to other lager yeasts, and perhaps to ale yeasts as well although higher temperatures used during the drying process may be better tolerated by the ale yeasts. Further research (or research I haven’t seen) could well prove me wrong on that, we’ll see.
Anyway, here is some of what they found.
This series of events is not seen in the other mechanism studied, isoleucine uptake vs. 2,3-pentanedione (another VDK).
I have some issues with this research though.
The authors rehydrated the dried yeast by pitching it into low gravity wort. For one strain it led to a viability of only 25%, while the other was 71%. The authors corrected for this to make sure the same number of viable cells were pitched into their test fermentations, as they should.
Based on the results listed above, the authors conclude that
Pitching directly into wort can have a number of negative effects. It may simply kill the weaker cells, or it may just act at random in a uniform culture. But worst of all, it may be an artificial selection for cells that can survive being thrown straight into wort.
As soon as you stop selecting your culture based on its ability to produce good beer, you’ve got a problem.
It is strain A, the one with the greater reduction in viability, that takes longest to finish fermenting the beer. It was also strain A that has the highest rate of diacetyl production, roughly four times higher than the other dried strain and the liquid strains tested.
It’s possible that the surviving cells were severely weakened by being tossed directly into the wort, thus their poor fermentation performance. This could be entirely strain dependant, explaining the difference between the two. Maybe strongly fermenting strains could be isolated that easily survive the drying and re-pitching process, I think that would be of great interest to brewers everywhere.
However, although some dry yeast manufacturers recommend it, directly pitching into wort is not the only way to treat your dried culture.
What about rehydrating the yeast in warm water first, as many manufacturers of dried yeasts recommend for their commercial clients? What effect does that have on viability, or the production of amino acids and diacetyl?
That’s not addressed here, and I would really like to have seen them do those experiments and measure the diacetyl levels.
So what does this mean? Well, obviously the dry lager yeasts tested take much longer to reduce the diacetyl levels in the beer than the fresh cultures did. It’s not clear what affect an older liquid culture would have on the diacetyl levels – will it perform more like a dry yeast or a fresh culture? More research is needed to answer that question.
It’s also not clear if this would apply to other lager yeasts or ale yeasts, although it probably has some effect. But ultimately it might not matter too much for brewers if they are in the habit of making a starter.
If your yeast of choice is only available dried, you can still buy it and store it for a long time with little effort, only taking a couple of fairly easy steps prior to making your batch. That is much easier than continually propagating your liquid cultures.
And if you live too far from a homebrew shop to get yeast on a whim, keeping some dried yeast in stock will still let you brew on short notice.
A dried yeast culture could be rehydrated in water and then pitched to your wort. Sure, it takes some of the ease-of-use out of dry yeast, but ultimately you need to be able to make good beer.
References:
Cyr et al. (2007) Vicinal Diketone Production and Amino Acid Uptake by Two Active Dry Lager Yeasts During Beer Fermentation, J. Am. Soc. Brew. Chem. 65, 138-144.
There are proponents on both sides of the dry vs. liquid yeast debate, both with valid points. Dry yeast is easier to use and can be stored longer, not to mention it’s cheaper. Liquid yeast on the other hand, provides a fresher pitch of healthy cells.
What is a brewer to do? An article appeared last year in the Journal of the American Society of Brewing Chemists that gives me some ideas.
From McGill University in Canada, Cyr et al. reported on their research of two different strains of lager yeast. I think some of the broader lessons from the article can be applied to other lager yeasts, and perhaps to ale yeasts as well although higher temperatures used during the drying process may be better tolerated by the ale yeasts. Further research (or research I haven’t seen) could well prove me wrong on that, we’ll see.
Anyway, here is some of what they found.
- "Dry yeast strains took 15–26 hr longer to attenuate the wort to final gravity and 24–48 hr longer to significantly reduce wort amino acid content.”
- “…fermentations made with dried yeasts resulted in 1.5–7 times higher diacetyl indices. With one of the dry strains, diacetyl concentrations remained over the taste threshold value after 7 days of fermentation.”
- “Overall, dry yeasts resulted in a reduced level of performance compared with freshly propagated cultures, suggesting that the direct utilization of ADLY as an inoculum for beer fermentation at the industrial scale is not recommended.”
Diacetyl, which is a vicinal diketone or VDK, is a branch on the path toward valine synthesis. Yeast will create their own valine if there is insufficient available valine in the wort.
Based on the timelines of diacetyl production and valine uptake, the dried yeast first produce their own valine then later use what is available in the wort, while the liquid strains take the valine from the wort first and only later produce it themselves as the valine available in the wort is depleted.
This series of events is not seen in the other mechanism studied, isoleucine uptake vs. 2,3-pentanedione (another VDK).
I have some issues with this research though.
The authors rehydrated the dried yeast by pitching it into low gravity wort. For one strain it led to a viability of only 25%, while the other was 71%. The authors corrected for this to make sure the same number of viable cells were pitched into their test fermentations, as they should.
Based on the results listed above, the authors conclude that
…direct utilization of ADLY [Activated Dry Lager Yeast] as an inoculum in beer fermentation should not be considered an optimal practice, especially when specifications for very low VDK levels are an issue, as in the case of lager beers.And they’re right, “direct utilization” is not something you want to do.
Pitching directly into wort can have a number of negative effects. It may simply kill the weaker cells, or it may just act at random in a uniform culture. But worst of all, it may be an artificial selection for cells that can survive being thrown straight into wort.
As soon as you stop selecting your culture based on its ability to produce good beer, you’ve got a problem.
It is strain A, the one with the greater reduction in viability, that takes longest to finish fermenting the beer. It was also strain A that has the highest rate of diacetyl production, roughly four times higher than the other dried strain and the liquid strains tested.
It’s possible that the surviving cells were severely weakened by being tossed directly into the wort, thus their poor fermentation performance. This could be entirely strain dependant, explaining the difference between the two. Maybe strongly fermenting strains could be isolated that easily survive the drying and re-pitching process, I think that would be of great interest to brewers everywhere.
However, although some dry yeast manufacturers recommend it, directly pitching into wort is not the only way to treat your dried culture.
What about rehydrating the yeast in warm water first, as many manufacturers of dried yeasts recommend for their commercial clients? What effect does that have on viability, or the production of amino acids and diacetyl?
That’s not addressed here, and I would really like to have seen them do those experiments and measure the diacetyl levels.
So what does this mean? Well, obviously the dry lager yeasts tested take much longer to reduce the diacetyl levels in the beer than the fresh cultures did. It’s not clear what affect an older liquid culture would have on the diacetyl levels – will it perform more like a dry yeast or a fresh culture? More research is needed to answer that question.
It’s also not clear if this would apply to other lager yeasts or ale yeasts, although it probably has some effect. But ultimately it might not matter too much for brewers if they are in the habit of making a starter.
If your yeast of choice is only available dried, you can still buy it and store it for a long time with little effort, only taking a couple of fairly easy steps prior to making your batch. That is much easier than continually propagating your liquid cultures.
And if you live too far from a homebrew shop to get yeast on a whim, keeping some dried yeast in stock will still let you brew on short notice.
A dried yeast culture could be rehydrated in water and then pitched to your wort. Sure, it takes some of the ease-of-use out of dry yeast, but ultimately you need to be able to make good beer.
References:
Cyr et al. (2007) Vicinal Diketone Production and Amino Acid Uptake by Two Active Dry Lager Yeasts During Beer Fermentation, J. Am. Soc. Brew. Chem. 65, 138-144.
Tuesday, July 15, 2008
Intro
I plan to use this blog to discuss research that is relevant to beer and the brewing industry. A large part of it will be reviews of various journal articles and perhaps some research I am doing myself. If there are any specific topics or articles you would like me to cover just let me know.
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