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Old 04-28-2010, 08:53 PM   #1
Gremlyn
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Default Yeast Metabolism: Starters to condition yeast to environment of wort

I keep reading that starters should be made from wort in order to get the yeast used to eating maltose and/or acclimate them to their new environment (your wort). I've not seen this backed up anywhere, just said and propagated around like it's the cold, hard truth. I'm not saying it isn't true and that it didn't stem from somewhere, but I like to know this things for sure.

A little background on sugars:
Maltose is a disaccharide, comprised of two glucose molecules.
Sucrose is a disaccharide, comprised of one glucose molecule and one fructose molecule.

So after doing some digging I cam across a paper talking about glucose repression of maltose metabolism. This paper gives references of the well documented that when yeast S. cerevisiae are in an environment of a lot of glucose, their ability to process maltose is suppressed. Since maltose is made up of two glucoses, the yeast simply prefers to eat plain glucose over breaking down a maltose and eating that, makes sense right?

What about sucrose? Since sucrose is only 1/2 glucose, how do the yeats handle fructose metabolism? Well this paper studying metabolism of an S. cerevisiae strain use commercially for making apple wine compared yeast growth and fermentation using glucose, fructose, and sucrose. The results were that yeast growth was fast faster with pure glucose and slowest with pure fructose, and in the middle with sucrose. There was a noted lag time time in ethanol production with sucrose, attributed to the need of the yeast to first break down the sucrose to it's two constituents.

On the topic of sugar type and getting yeast used to eating a certain type, I would say that besides a slightly reduced growth time in pure sucrose, your yeast will likely have little to no problem adjusting to eating the maltose in wort. The other option would be to use corn sugar, which is basically pure glucose (and/or dextrose - which is virtually the same thing, but that's a chemistry topic for you to research yourself ), which would likely give you the fastest yeast growth, though it's anyone's guess if the result would be measurable over a 24 hour period. Due to the glucose supression of maltose metabolism mentioned before, you may find that there is a lag time as the yeast switch gears and begin to process the maltose again. Again, testing is required to determine exactly how much lag time there would be, if any.

Now there are other factors that may make using wort (or close to it) better for your yeast, and those are a little harder to figure out. There are compounds other than sugar in wort, and some of these compounds may be beneficial, such as nitrogen containing compounds, and I don't have any info on those for now. The pH of the starter with just sugar and water will be different from that of wort, which is another important consideration.

Conclusion
My literature review has convinced me that using a wort starter, either extract based or real wort, is definitely better than using table sugar simply for growing healthy yeast faster. If you're in a pinch you can likely get away with using corn sugar, but I would use yeast nutrient in that starter as well. As I said, this needs to be tested to verify that it'll be effective and not stress out the yeast and yield unsatisfactory results in the final beer.

I'd like to do some real world testing on this, and once I'm a little more settled in my new house I'll look in to doing so.

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Old 04-28-2010, 09:13 PM   #2
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Gremlyn according to that yeast who eat pure glucose for a time are slow to start eating maltose, what does that have to do with sucrose its not glucose ? maltose and sucrose are both disaccharides unlike the pure glucose diet in the document. According to the document the yeast don’t want to take the time to break the maltose in to two monosaccharides after the pure glucose diet, but the same function must also be done to sucrose as it too is a disaccharide ,, is the metabolic pathway and function that breaks maltose into two monosaccharide not the same function that breaks the sucrose in to two monosaccharides?
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Old 04-28-2010, 09:18 PM   #3
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also i think you missunder stood me. i am not advocating a pure sugar diet as my post in the other thread points out, all i am saying is that sucrose is not the problem, wort and must contain alot of sucrose.. if you have a wort with pleanty of Oxygen FAN and vitamins , changing the carbohydrate from sucrose to maltose will not add lag.

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Old 04-28-2010, 09:20 PM   #4
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Gremlyn according to that yeast who eat pure glucose for a time are slow to start eating maltose, what does that have to do with sucrose its not glucose ? maltose and sucrose are both disaccharides unlike the pure glucose diet in the document. According to the document the yeast don’t want to take the time to break the maltose in to two monosaccharides after the pure glucose diet, but the same function must also be done to sucrose as it too is a disaccharide ,, is the metabolic pathway and function that breaks maltose into two monosaccharide not the same function that breaks the sucrose in to two monosaccharides?
I'm unclear if they are actually slow to start (I didn't get to read the article in it's entirety, so maybe I missed it) or if they just don't break down maltose in the presence of excess glucose and immediately switch back. Did you see it in there? I was planning on going through in more detail this evening after work.

As for the maltose vs sucrose, the second article makes the point that fructose isn't as metabolically useful to yeast, and since sucrose is comprised of glucose and fructose it won't be as efficient a food source.
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Old 04-28-2010, 09:26 PM   #5
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also i think you missunder stood me. i am not advocating a pure sugar diet as my post in the other thread points out, all i am saying is that sucrose is not the problem, wort and must contain alot of sucrose.. if you have a wort with pleanty of Oxygen FAN and vitamins , changing the carbohydrate from sucrose to maltose will not add lag.
The articles I found didn't do a direct comparison of maltose vs sucrose, but I would expect maltose to fall between pure glucose and sucrose for growth efficiency. As I said above, the issue is the fructose.
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Old 04-28-2010, 09:32 PM   #6
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The articles I found didn't do a direct comparison of maltose vs sucrose, but I would expect maltose to fall between pure glucose and sucrose for growth efficiency. As I said above, the issue is the fructose.
dont tell that to the wine cider and mead guys thats there bread and butter
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Old 04-28-2010, 09:38 PM   #7
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dont tell that to the wine cider and mead guys thats there bread and butter
Fructose being left over and not metabolised would be good for residual sweetness, so wines and meads would benefit from it I expect.
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Old 04-28-2010, 09:51 PM   #8
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I think the truth to the matter is that yeast don’t need to make Sucrase or Maltase to consume glucose or fructose, they just pull it across the cell wall and get after it,
so yeast eatting those first makes good sense, from an natural selection stand point, if you got after the low hanging fruit first you will out breed your competitors that don’t.

But if yeast spends a long time on just monosaccharides it stops production of the enzymes Sucrase and Maltase. So when these foods do come available a lag is perceived in the time it takes to synthesis the enzymes and then hydrolyze the disaccharide in to a mono...
I can find no proof anywhere that yeast loose the ability permanently , only the observation of the "Lag" and only after very long mono only diets , as brewers we would never have that scenario , we don’t feed yeast glucose only for months on end.

the rumor IMO came not from first hand observation in the brewing wold but from a scientific journal that observed this behavior in yeast that are farmed for enzyme production in continuous biological fermenters
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Old 04-28-2010, 10:34 PM   #9
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The first paper I linked to explained the pathway that suppresses maltose metabolism in the presence of good quantities of glucose, and it seems to be reversible, but as you said, the yeast will have to switch gears and make the requisite enzymes.

Going back to a sucrose vs maltose diet: If the yeast have two different enzymes to process two different types of sugar, then growing yeast in a maltose richer environment such as wort, as compared to a solution made with sucrose, would be better for them. Since their focus will have been on producing sucrase instead of maltase and going into wort from sucrose solution, they would have to switch to making the correct enzyme.

It's the lag time that is the variable in all of this, and since it's once easily eliminated I feel we might as well. With a lag time the potential for infection increases. We also don't know what, if any, negative results from synthesising a lot of sucrase and then having to synthesise a lot of maltase. Could be by-products created from sucrase breakdown, could be an energy drain on the yeast, which may be stressed if pH's aren't similar between the two media, etc.

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