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Old 10-07-2011, 07:35 PM   #1
brazedowl
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I know that ABV is calculated by comparing the Specific gravity before and after fermentation. My question is...

The gravity drops because the yeast convert some of the sugar into carbon dioxide gas, which thus decreases the density of the liquid. But at the beginning when the yeast is first added there is Oxygen in the must. So the first bubbling is aerobic respiration and produces no alcohol, but the gravity will still drop because carbon dioxide is produced.

Is this change in the specific gravity taken into account in all the different ways of calculating ABV? Is the change during this portion negligible? If not, aren't all our ABV calculations rounded up?


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Old 10-07-2011, 07:56 PM   #2
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The gravity of your beer drops because the sugar that is in the wort is being converted to alcohol, that is the key. Alcohol has a density below that of water (about 80%) and the sugars in your wort make it more dense then water. That is what we are measuring and why we know (approximately) how much alcohol is in the beer.

The CO2 and O2 levels have very little effect, since CO2 is just O2 with a carbon atom on it, which is pretty light, i.e. not much difference.
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Old 10-08-2011, 12:08 AM   #3
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Quote:
Originally Posted by brazedowl View Post
I know that ABV is calculated by comparing the Specific gravity before and after fermentation. My question is...

The gravity drops because the yeast convert some of the sugar into carbon dioxide gas, which thus decreases the density of the liquid. But at the beginning when the yeast is first added there is Oxygen in the must. So the first bubbling is aerobic respiration and produces no alcohol, but the gravity will still drop because carbon dioxide is produced.

Is this change in the specific gravity taken into account in all the different ways of calculating ABV? Is the change during this portion negligible? If not, aren't all our ABV calculations rounded up?
I see what you’re asking.
But it would seem that any implication is pretty much academic ...

But to your question ...
Specific gravity equals density. So when you ask if specific gravity is taken into account in all ways of calculating ABV you are asking if all methods take density into account, ie. measure by density.

Specific gravity is material to a hydrometer in “density verses water” - for our purposes displayed as buoyancy.
In a refractometer, it is light refraction though a liquid that provides a measurement of brix, a measure of soluble solids (or in our case sugar), again a measure of density.
In a vinometer the measure is of capillary action is a matter of “tensions” (aka. “cohesive forces”) not density ... so NO, a vinometer does not have anything directly to do with specific gravity.

A few other ways ...

Alcohol Determination by Distillation and Hydrometry ... density is used (the hydrometry part)

Distillation plus Chemical Analysis ... utilizes potassium dichromate to react and convert the alcohol present in the distillate to acetic acid, which is then titrated to an end point and the alcohol concentration calculated. (But do we really need to know all this?) ... no density.

Boiling Method - measure the density ... boil out the alcohol ... measure the density again. (this is way more complicated than it sounds.)

There are some other chemical ways but they are not used in this industry.

BUT as I said before ... regarding your question about whether the lag time in the change in specific gravity is taken into any material account ... it is immaterial ... pointless, in regards to the practical application of how we measure ABV.

For those that have not yet turned off the light and fallen asleep ... or finished their business and set this novella back onto the toilet tank next to the kleenex box ...

About that lag asked about ...
The yeast first goes after the free oxygen in the must ... then it goes after the sugar’s oxygen ... the word "then" would seem to assume a delay. But yeast are ravenous and reproduce quicker than rabbits ... so for our purposes, the yeast’s consumption and the SG changing are simultaneous. On a molecular level the yeasties exhaust the free oxygen near them and start on the sugar ... no walking down the block to the corner store to find more O2 ... there’s some right there next to the lay-z-boy in that bowl of sugar ... i.e. in terms of proximity - the sugar’s O2 is right at hand ... and so, in vinting, any “lag” is immaterial ... in chemistry ... maybe ... in vinting ... no. And so for our purposes ... whether or not mathematically “rounded” for that initial period is moot also.

A comment on how to measure ABV ...
Vinometers measure by the measurement of capillary action ... the more ETOH, the less capillary action. The rub is that due to the sugars and solids in wines they are tough to measure with a vinometer ... this is why vinometers are popular with distillers but not with vintners.
Consequently, many vintners use a hydrometer and a refractometer ... and for what it's worth, of the two ... most use a hydrometer and careful technique for their most accurate ABV measurements.
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