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Old 05-15-2012, 12:05 PM   #11
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I bet he's going to throw out another trick question that you have supposedly "dodged".

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Old 05-15-2012, 12:07 PM   #12
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Originally Posted by BOBTHEukBREWER
thanks, everybody - but you all dodged the issue of effect of amount of sugar on yeast activity, let us consider 4 beers with OG's of 1025, 1035, 1045, 1055 all 3 gal batches all with a sachet of safale04 sprinkled on top at 25 deg C - I think they would all ferment at same rate but as OG goes up take longer to finish.
Is this scenario a personal experience of yours? What are you drivin' at here Bob?

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Old 05-15-2012, 12:13 PM   #13
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I don't think the yeast activity is dependent on OG, but it is VERY dependent on the number of yeast cells in the carboy and temperature.

Since the cell population doubles every 2-3 hours, the activity will increase exponentially over time, and the more sugars you start with the longer fermentation will take, the more yeast you will have (due to this geometric progression), and the greater activity.

The doubling time is very dependent on temperature. Increasing the temp of the wort will result in lower doubling time, so the population increases more rapidly and so does that activity.

I'd guess that the greater osmotic pressure on the cell in a high OG beer would have a negative effect on fermentation rate, but that would be guessing.
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Old 05-15-2012, 02:05 PM   #14
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Originally Posted by BOBTHEukBREWER View Post
A reply to my last post talked of yeast eating sugar and inferred the more sugar there is, the more active the yeast is. I thought yeast cells need nutrients, the right temperature range, the right degree of oxygenation - and then they grow and divide. As they divide, enzymes are formed or released, and these enzymes break down fermentable sugars. Is this somewhere near the truth. A reply from a practicing professional brewer would be good also.
To add/clarify a couple of points. Yeast are continually making enzymes. Cell division does not have big impact on the formation and release of enzymes involved in fermentation (in a simplistic sense). More yeast does mean more enzymes though. Yeast need oxygen to divide and grow, but not to ferment. The lack of additional nutrients, beyond sugars (that would be vitamins and minerals) will have a large impact on the health of the yeast. Unless you are brewing with distilled water, this is generally not an issue. Like the earlier plant analogy, there is an optimum though and you can buy supplements to insure your have optimally happy yeast.

Brewing yeast are not terribly good at breaking down polysaccharides (as opposed to most other fungi). That is why we have to mash the grains first. They do take up small sugars and metabolize them inside the cell. They also make proteases that they excrete to break down proteins/peptides into small pieces that they can take up to use as nitrogen sources. I read an article a while back warning that too much extracellular protease (too much yeast?) can lead to the loss of foam positive proteins. I'll have to dig that up and reread it.
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Old 05-15-2012, 04:48 PM   #15
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Yeast eat lots of things and are chronic over eaters. They are part of the growing obesity epidemic. Often deprived of foods they have no self control and will gnaw their own face off.

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