We were recently asked the following question:
“Is the final abv or strength of a beer decided before it’s made, or can it change (the final abv, that is) during the brewing process?”
The short answer is that ABV (alcohol by volume) is determined by the amount of malt sugars fermented during the brewing process. Once you reach final attenuation sugar can still be added by priming, to get more alcohol…but that can only be achieved by re-fermentation if the beer is warm enough. Generally, once fermentation is over, that’s it…if you want more alcohol use more malt!
However perhaps it would be helpful to expand on that. During the mashing process, malt is mixed with hot water, and enzymes in the malt convert starches into two main types of sugar – simple, fermentable Maltose, and complex normally unfermentable Dextrose. The higher the percentage of fermentable Maltose in the sugar solution (Wort) which results from the mash, the higher the potential alcohol content. Brewers’ yeast is added to the sugar-rich Wort and most of the fermentable Maltose is turned to alcohol. (Dextrose remains in the beer as sugar, contributing malty flavour and mouthfeel to the beer).
Once the yeast has fermented as much of the Maltose as it can, it stops working, and the alcohol content of the beer is normally fixed at that point. There are two possible ways of increasing the alcohol content at this point – adding more sugar, or using a wild, Brettanomyces-type yeast. If more sugar is added after primary fermentation has finished, the brewers’ yeast still in the beer will ferment it and produce more alcohol. If a wild yeast strain gets into the beer, either deliberately introduced, or by accident, it can start to ferment the Dextrose sugars, which brewers’ yeast can’t, and therefore the alcohol content will increase.
The bi-product of fermentation is carbon dioxide, and priming bottle conditioned beers with a little sugar is a traditional way of ensuring natural carbonation of the resultant beer. The famous Trappist brewery of Orval in Belgium seeds its bottles with a little Brettanomyces yeast as a way of carbonating the beer. In most cases where priming is used to promote bottle conditioning, the primed bottles are stored in heated warehouses for several weeks to give the refermentation time at a suitable temperature to take place.
We were also asked:
“How much does ABV affect flavour of a beer?''
Alcohol is a major flavour component of beer. Anything over 5.5% will have a slightly sweetish/fruity alcohol taste, and body. Also, as a side effect, if the mashing process produces a Wort rich in fermentable Maltose, it will also tend to be rich in unfermented Dextrose. Therefore a beer with a high alcohol level after fermentation will also tend to have a lot of Dextrose in it, which will increase the rich malty flavours and give the beer plenty of body, both of which tend to counter-balance any alcohol “burn”.