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Home Brew Forums > Home Brewing Beer > Brew Science > Aerobic fermentation
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Old 12-07-2010, 05:51 PM   #1
Jungle
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Default Aerobic fermentation

Doing a project in Microbiology, we have two set-ups, aerobic and anaerobic.

We are calculating alcohol using specific gravity, does this still apply for the aerobic set up as not all of the yeast will be metabolizing the sugar into ethanol.

Basically I'm asking would the same equation apply for the aerobic batch as the anaerobic batch?

I'm using this calculation BTW, nothing too complicated (I got the same, using Beersmith)

Here are my current calculations, I've copied and pasted it from my report so sorry if it's a bit long winded...


The specific gravity was measured at the beginning and end of the experiment to enable the calculation of alcohol content/alcohol by volume in the final product.
It also requires two constants (Hough [1985]):
 1.05 g which refers the amount of carbon dioxide produced for every gram of ethanol produced (constant 1)

 0.79 g/ml which represents the density of ethanol alcohol (constant 2)

Aerobic Experiment
 Original Gravity Reading (OR) = 1.030
 Final Gravity Reading (FR) = 1.010

OR - FR = 1.030 - 1.010
= 0.02 g/ml

(0.020 g/ml refers to the amount of carbon dioxide that has bubbled off)

Constant 1 x Amount of CO2 Bubbled off = 1.05 x 0.02
= 0.021


Ʃ (Constant 1 x Amount of CO2 Bubbled off) 0.021 ----------------------------------------------------------------- = ------------ Final Gravity 1.010
= 0.02079


0.02079 0.02079 ------------------- = -------------- Constant 2 0.79
= 0.0263
(%) = 0.0263 x 100
= 2.63 % alcohol by volume (a.b.v)


Thanks for reading

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Old 12-08-2010, 08:54 PM   #2
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Bump

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Old 12-08-2010, 09:33 PM   #3
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I don't know what you're talking about! Oh wait, this is the "brew science" thread, that's why. I'm more of a brewing historian.

In the 1700's, they called it "the art and mystery of brewing." It wasn't categorized with the other crafts and trades of the time, and now I know why they used the word "mystery."

(I'm mostly responding so you know your post isn't being ignored. I just don't know the answer!)

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Old 12-08-2010, 09:37 PM   #4
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I think you are asking if calculating alcohol from OG-FG will work the same for aerobic vs anaerobic fermentation.

If so, yes it will (probably). The yeast shouldn't produce any new compounds (in amounts large enough anyway) for the equation to change. The FG may change, the time it takes to get to the same FG might change (or be unobtainable), and the actual composition of the beer might change (different levels of non-ethanol by-products) but i wouldn't *think* the by-product amounts would be so drastically higher as to need a new formula.

But, the only actual way to tell (w/o literature sources of such info) would be to analyze the make up of the beer, which is probably way beyond what you are trying to do.

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Old 12-08-2010, 10:33 PM   #5
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If it's aerobic enough (really getting a good amount of O2), then the yeast will gobble up the EtOH. Or not make it in the first place; depends on the glucose concentration (Crabtree effect, IIRC).
Got anybody around with a GC or some such to actually measure the EtOH for you?

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Old 12-08-2010, 10:40 PM   #6
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I expect that the differences will be fairly small and don't have much confidence that comparing specific gravities will be accurate enough. SG comparison relies on Balling's assumption that 2.0665 grams of sugar will produce 1 gram of ethanol, 0.9565 grams of CO2 and 0.11 grams of yeast biomass. Based on those assumptions one can find factors, dependent on OG, by which one can multiply the difference between OG and True Extract or OG and Apparent Extract (different factors). As the metabolism of the yeast are going to be different under the 2 conditions I would expect the 0.11 gram number to vary which renders the formulas innacurate. If you are going to use this approach I would at least use TE (True Extract) rather than apparent as the results will be more accurate. The factors are tabulated in Vol II of DeClerck.

It is not so simple as you imply when you state that a OG of 1.030 and a FG of 1.010 implies a loss of 1.030 - 1.010 grams of CO2/cc. The drop in SG is caused in part but that of course but also by loss of yeast biomass and the fact that alcohol being appreciably less dense than water causes the hydrometer to read lower. As noted, there are tables, based on Ballings formula and Tabarie's pricipal that will give you estimates of ABW (easily converted to ABV) based on the differences. You really should consult DeClerck (not that there might not be other sources that handle this well but DeClerck is what's coming to mind).

If you are willing to do some work alcohol can be measured quite accurately by distillation using nothing more sophisticated than a simple pycnometer and laboratory balance (and, of course, the distillation apparatus). A good balance is, of course, expensive but I am assuming you are in some sort of laboratory environment. The pycnometer is a little over $100 US.

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Old 12-09-2010, 05:48 PM   #7
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Aerobic is also known as respiration in which yeast converts the sugar to CO2/water and anaerobic is also known as fermentation in which sugar is converted to CO2/alcohol. I don't think you could tell the difference by checking gravities.

To throw a kink in all of it, some yeast prefer anaerobic fermentation even in the presence of O2.

http://www.departments.oxy.edu/tops/Yeast/YEAST-S.pdf

This also might help you out
http://www.csun.edu/scied/2-longitud...ter/index.html

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Old 12-09-2010, 10:40 PM   #8
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Quote:
Originally Posted by CampFireWine View Post
Aerobic is also known as respiration in which yeast converts the sugar to CO2/water and anaerobic is also known as fermentation in which sugar is converted to CO2/alcohol. I don't think you could tell the difference by checking gravities.

To throw a kink in all of it, some yeast prefer anaerobic fermentation even in the presence of O2.

http://www.departments.oxy.edu/tops/Yeast/YEAST-S.pdf

This also might help you out
http://www.csun.edu/scied/2-longitud...ter/index.html
I looked up some old stuff, it is the 'Crabtree effect' where ethanol production in the presence of oxygen in yeast is glucose concentration dependent, http://mic.sgmjournals.org/cgi/reprint/44/2/149.pdf
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Old 12-09-2010, 11:16 PM   #9
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Quote:
Originally Posted by JohnMc View Post
I looked up some old stuff, it is the 'Crabtree effect' where ethanol production in the presence of oxygen in yeast is glucose concentration dependent, http://mic.sgmjournals.org/cgi/reprint/44/2/149.pdf
That was very interesting. Thanks. It explains technically why you don't need an airlock on a fermenter until the sugar gets low.
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