Calculating alcohol content!

[chrisbooth0070]

Not sure if this is in the right section, but I suppose it has some relivance to the recipe.

Anyway, I have been using this link http://www.cascadehomebrew.com.au/brewk ... lators.asp to calculate the alcohol percentage of my recent brews.

When I plug into this the OG and FG readings from other peoples brews I get different values to what they come up with. I then searched google for formulas and found many different ones.

Is there not a strict formula to use with regards to SG values, ie. different ones for different brews/ingredients etc, or are they all wrong apart from one.

Which formulas do you all use? I think I'll go with the most popular one that is suggested

Thanks

Chris

[fizzypop boy]

the tried and trusted formula used for calculating abv is;


OG-FG
____________

7.45

[PieOPah]

eg

og = 1.046
fg = 1.012

46-12 = 34
34 / 7.45 = 4.56

ABV = 4.6%

[Jim]

There's a page on my site here about this. I used a slightly different figure for the divisor (based on a bit of web research) and I still don't know of any definitive answer as to what exact figure to use - unless anyone knows different!

(The difference between my figures and FPBs is probably within the limits of measurement errors of the sg anyway)

[Jonny 2 Gallons]

Take your original gravity minus your final gravity and this gives you the litre degrees of fermentable sugar required.
Each litre degree of fermentable sugar converts to 0.129% alcohol by volume.
Multiply the litre degrees by 0.129 = ABV.

Where most people slip up is by forgeting to calculate another 4/5 of the fermetable sugar required, as abviosly dextrins are not fermentable.

[Jonny 2 Gallons]

Take your original gravity minus your final gravity and this gives you the litre degrees of fermentable sugar required.
Each litre degree of fermentable sugar converts to 0.129% alcohol by volume.
Multiply the litre degrees by 0.129 = ABV.

Where most people slip up is by forgeting to calculate another 4/5 of the fermetable sugar required, as abviosly dextrins are not fermentable.

[BarryNL]

There's a page on my site here about this. I used a slightly different figure for the divisor (based on a bit of web research) and I still don't know of any definitive answer as to what exact figure to use - unless anyone knows different!

(The difference between my figures and FPBs is probably within the limits of measurement errors of the sg anyway)

Plus, you can't calculate it exactly from the difference in gravity. Yeast can respire both aerobically and anaerobically. Both of these methods use sugar but only anaerobic respiration produces alcohol. Theoretically, if you provided the yeast with enough oxygen you could drop the gravity all the way from 1.100 to 1.000 without producing a single drop of alcohol. Bakers love to do this sort of thing...

Also, of course, the gravity will be affected if any evaporation takes place during fermentation.

There may be other variables I haven't thought of too. Basically if you want to know the ABV accurately you need expensive equipment.

[oblivious]

There's a page on my site here about this. I used a slightly different figure for the divisor (based on a bit of web research) and I still don't know of any definitive answer as to what exact figure to use - unless anyone knows different!

(The difference between my figures and FPBs is probably within the limits of measurement errors of the sg anyway)

Plus, you can't calculate it exactly from the difference in gravity. Yeast can respire both aerobically and anaerobically. Both of these methods use sugar but only anaerobic respiration produces alcohol. Theoretically, if you provided the yeast with enough oxygen you could drop the gravity all the way from 1.100 to 1.000 without producing a single drop of alcohol. Bakers love to do this sort of thing...

Also, of course, the gravity will be affected if any evaporation takes place during fermentation.

There may be other variables I haven't thought of too. Basically if you want to know the ABV accurately you need expensive equipment.

Oxygen is used for sterol production

[steve_flack]

Actually it's not just for sterol production. BarryNL is right if you kept a wort saturated with oxygen you'd get no alcohol. That's because aerobic pathway doesn't produce any - it converts sugars to carbon dioxide and water - a process that requires oxygen. This is the yeasts preferred pathway as you get more energy out of each molecule of sugar by doing so. The anaerobic pathway produces ethanol as a by-product. This doesn't produce as much energy for the yeast but is better than cell death....

[oblivious]

Sorry read BarryNL posts wrong

But would you not need to continually add O2 in to the system to get this fermentation, as the surface area of the wort would hardly all the replacement of the O2 been metabolized

[steve_flack]

But would you not need to continually add O2 in to the system to get this fermentation, as the surface area of the wort would hardly all the replacement of the O2 been metabolized

What do you think 'if you kept a wort saturated with oxygen you'd get no alcohol' implies? You'd have to add oxygen to the wort to replace that used up. This is pretty much how the yeast companies make yeast. They take a sugar solution and keep adding oxygen to it. You don't get any alcohol but you do get a lot of yeast.