What dictates ABV?

[paulgough]

This might seem like a daft question....

If the rule of thumb is for 2.5 l of liquor for every kg of grist and you should sparge until all the sugar is extracted. What dictates the final ABV of the beer?

 

[c_burgess]

ABV is dictated by the amount of sugars converted to alcohol , this is dictated in turn by which yeast you use and what temps you ferment at. I believe you are talking about mash volumes which again depends on the speed of the mash, temperature and I also assume the amount of grain used.

I know little about all grain brewing but I use brewing software when designing my extract brews, maybe someone else could shed some light.

 

[Dave1970]

it is the weight of grains that determines FG. More grain=More Sugar=Higher OG=More Sugar for yeast to work on=Higher ABV

I see your point though, I think where you're going wrong is that you don't sparge until all sugars have been extracted, you sparge until you've collected the volume of wort you need for your boil OR until the gravity of the runnings falls below 0.990 (temp corrected 1.008) to avoid extracting tannins. With a big grain bill the gravity of your first runnings might be the same (assuming 2.5l/kg) but the last runnings will be higher resulting in a higher OG.