Measure alcohol in beer

[doug293cz]

I could be wrong but assuming the sugar be used is 100% fermentable and yeast has an approximate attenuation level, you should be able to assume your abv% within a couple points, the only variable would be attenuation and you could just take a fg, knowing your sugar to water ratio would give you your og .. for example, dextrose has 42 gravity points per pound per gallon. So if you have 5 gallons of water and add 5lbs of sugar your og will be 1.042 there's no need to take an og reading with a hydrometer if this is how you're going about it.

If using simple sugars, like dextrose (corn sugar) or sucrose, most yeasts will be at, or very close to, 100% attenuation. The attenuation estimates for yeast assume typical carbohydrate profiles of wort, which has significant amounts of more complex sugars like maltotriose and dextrins. Many yeasts can metabolize maltotriose, but some can't, and very few yeasts can metabolize dextrins, so different yeasts will have different attenuation depending on the molecular weight distributions of the sugars in the wort.

The gravity potential of table sugar (sucrose) is 46.2 pts/lb. If you mix 5 lb of sucrose with 5 gal (8.33 lb/gal => 41.65 lb) the resulting °Plato is 100°P * 5 lb / (5 lb + 41.65 lb) = 10.72°P. If we convert that to SG, it equals 1.043. The gravity potential of corn sugar (dextrose mono hydrate) is less than sucrose because 9.09% of the weight is water. So, 1 lb of corn sugar mixed with 1 gal of water will result a solution at 100°P * 0.9091 lb [dry wt] / (1 lb + 8.33 lb) = 9.74°P, which works out to an SG of 1.039, not 1.042.

The true definition of points/lb is: The SG that would be achieved by mixing 1 lb with enough water to make 1 gal of solution. Since the solute (sugar in this case) contributes to the volume, it takes less than 1 gal of water to make 1 gal of solution - it only takes 0.962 gal (7.715 lb) of water mixed with 1 lb of sucrose to make 1 gal of solution. If we then calculate the °Plato is 100°P * 1 lb / (1 lb + 7.715 lb) = 11.4744°P, which has an SG of 1.0462.

The incorrect definition of SG potential in the quoted post is an extremely common misconception, which results from the rather convoluted actual definition of SG potential.

Brew on

 

[bailey mountain brewer]

Ahh, makes total sense, I didn't account for the volume of the sugar itself. Thanks @doug293cz for clarifying that. I seldom use this process so forgive me for incorrect information. To touch on what you mentioned about attenuation and dextrins, I believe yeasts with STA1 are capable of metabolizing those dextrins. I don't have documentation on that but I read it somewhere on HBT so it must be true.

 

[doug293cz]

To touch on what you mentioned about attenuation and dextrins, I believe yeasts with STA1 are capable of metabolizing those dextrins. I don't have documentation on that but I read it somewhere on HBT so it must be true.

Yes.

Brew on

 

[Hoochin'Fool]

I saw the winemaker at a nearby winery using some odd looking stainless steel contraption with a little bunsen burner, that measured evaporation of a small sample, and had charts that you plugged in the local barometric pressure and elevation, and he said that's what the ATF required him to use, to prove his ABV claims on wine, rather than just OG/FG math. No idea what it was called, but it looked pretty neat. (this was quite a few years ago), long before I knew anything about gravity and fermenting or anything, so not certain I'm explaining what I remember very well.

 

[BrewZer]

Ok. But if I don't have OG, and just the beer. How do I then measure the alcohol percentage with a refractometer, or what else may be used?

Drink a 12-oz glass. If you get snockered, it's about 8%.

 

[keystonehb]

I saw the winemaker at a nearby winery using some odd looking stainless steel contraption with a little bunsen burner, that measured evaporation of a small sample, and had charts that you plugged in the local barometric pressure and elevation, and he said that's what the ATF required him to use, to prove his ABV claims on wine, rather than just OG/FG math. No idea what it was called, but it looked pretty neat. (this was quite a few years ago), long before I knew anything about gravity and fermenting or anything, so not certain I'm explaining what I remember very well.

These will only accuratly work if there is little to know residual sugar as the sugar suspension affects the "boil"