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Old 12-22-2011, 03:49 PM   #1
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Default Why is lots of yeast even needed

When brewing, how little yeast can you get away with? If I use 1 packet of dried yeast for a bathtub of apple wine, won't it just multiply until either the sugar runs out or the 14% alcohol kills the yeast? If not, what's stopping it?



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Old 12-22-2011, 03:57 PM   #2
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It's better to have a good amount of yeast because if not they can be stressed out and create esters. You can get away with minimum yeast but why take the chance of problems with fermentation and off flavors when you can just give it a healthy amount if yeast.



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Old 12-22-2011, 03:58 PM   #3
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If the yeast has to multiply a lot, it's going to be stressed and give a lot of off-flavors. And it will probably won't be strong enough to finish the job.

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Old 12-22-2011, 03:59 PM   #4
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I brought this up in the Fermentation area....what I found out is that unlike wine fermentation where esters are usually a good thing, in beer making you want to minimize them. One good way is to have a sufficient yeast population right from the start.

So this makes pitching rates much more significant in beer making. In wine making you only need 1g/gallon and everybody uses dry yeast...not so in the beer world.

You also want the yeast to become the dominant critter as fast as possible to reduce the risk of infection.

But I'm thinking you could do your apple wine example no problem with one packet of dry wine yeast Just clean the ring from around the tub first !!

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Old 12-22-2011, 04:00 PM   #5
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Yeast only reproduces during the aerobic phase. Once the O2 is used up the yeast stop reproducing and start consuming the sugars. There's a limited amount of O2 that can be dissolved in liquid so once it's used up that's it, no more reproduction.

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Old 12-22-2011, 04:03 PM   #6
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Quote:
Originally Posted by JonK331 View Post
Yeast only reproduces during the aerobic phase. Once the O2 is used up the yeast stop reproducing and start consuming the sugars. There's a limited amount of O2 that can be dissolved in liquid so once it's used up that's it, no more reproduction.
I'm pretty sure there is a form of anaerobic reproduction in which they use reserves.... but I'll need to find citation.
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Old 12-22-2011, 04:11 PM   #7
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While this doesn't point directly to my discussion, this study does show that anaerobic reproduction is possible, even in a medium not easily utilized.

Quote:
Xylose utilization is of commercial interest for efficient conversion of abundant plant material to ethanol. Perhaps the most important ethanol-producing organism, Saccharomyces cerevisiae, however, is incapable of xylose utilization. While S. cerevisiae strains have been metabolically engineered to utilize xylose, none of the recombinant strains or any other naturally occurring yeast has been able to grow anaerobically on xylose. Starting with the recombinant S. cerevisiae strain TMB3001 that overexpresses the xylose utilization pathway from Pichia stipitis, in this study we developed a selection procedure for the evolution of strains that are capable of anaerobic growth on xylose alone. Selection was successful only when organisms were first selected for efficient aerobic growth on xylose alone and then slowly adapted to microaerobic conditions and finally anaerobic conditions, which indicated that multiple mutations were necessary. After a total of 460 generations or 266 days of selection, the culture reproduced stably under anaerobic conditions on xylose and consisted primarily of two subpopulations with distinct phenotypes. Clones in the larger subpopulation grew anaerobically on xylose and utilized both xylose and glucose simultaneously in batch culture, but they exhibited impaired growth on glucose. Surprisingly, clones in the smaller subpopulation were incapable of anaerobic growth on xylose. However, as a consequence of their improved xylose catabolism, these clones produced up to 19% more ethanol than the parental TMB3001 strain produced under process-like conditions from a mixture of glucose and xylose.
Sonderegger M, Sauer U, Evolutionary engineering of Saccharomyces cerevisiae for anaerobic growth on xylose. Appl Environ Microbiol. 2003 Apr;69(4):1990-8.
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Old 12-22-2011, 04:18 PM   #8
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Oxygen is used during the lag phase, not the exponential growth phase....that's why we don't add O2 once fermentation shows visible signs.

From Wyeast:

Log Phase
The log phase is a time of exponential growth of the yeast culture. The preparation the yeast made during the lag phase allows rapid multiplication of cells and consumption of sugar. Yeast reproduce asexually by budding; the adult cell forms a daughter cell that is an exact genetic copy itself. Nitrogen, amino acids, nutrients, and sugar are consumed while the cells are reproducing.

Many of the significant aromatic and flavor compounds are by-products of cell growth and are produced during the log phase. Many large breweries try to limit the amount of yeast growth by pitching larger quantities of yeast and therefore minimize ester synthesis. Keeping fermentations cold also limits ester production by limiting the rate of growth.

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Old 12-22-2011, 04:19 PM   #9
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They definitely can and do reproduce anaerobically, but it's not optimal for making tasty beer. This is a stressful process on the yeast, during which all sorts of weird tasting compounds are created which can flavor your beer oddly. Oxygen is definitely the superior reducing agent, forcing the yeast to use other ones to complete their metabolism and reproduction is not friendly to them.

By pitching the correct amount (lots) of healthy yeast, we are ensuring that they have to do this as little as possible (ideally not at all) so we get the best tasting beer possible. An underpitch is a sure way to piss off your yeast and they have no qualms about messing up your beer if provoked.

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Old 12-22-2011, 04:23 PM   #10
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The least amount that I've read used commercially is 5 million cells per milliliter. That's right about 5 grams dry yeast or one vile/smack pack per five gallons on a homebrew scale.

Some yeast use more oxygen than others. At least 40% of the oxygen they use is scientifically unaccounted for. Dry yeast do not need any more oxygen because they are dried at peak sterols.



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