It is easy to calculate percent alcohol in beer and it only requires two important measurements.  A beers alcohol level is measured in Alcohol by Volume (ABV), it is the calculated amount of the total volume of liquid that is alcohol.  The density of a liquid is often measured in unit of specific gravity, where water has an approximate density of 1.00 g/mL or a specific gravity of 1.000 at 60F.  The specific gravity of a liquid is a comparison of its density vs the density of water.

Gravity Readings

During the fermenting process, yeast is added and the yeast eat the sugars in the wort(sugar water) producing alcohol and CO2.  After fermentation, since alcohol is less dense then wort (sugar water) there will be a change in the specific gravity of the liquid.  ABV is calculated by taking a specific gravity reading of the wort, beer before fermentation, and beer after the fermentation.  The two measurements are referred to as the Original Gravity(wort) and Final Gravity(beer post fermentation).

Measuring Specific Gravity

Measuring the specific gravity is often either done by a hydrometer or a refractometer.   All measurements should be done at what the hydrometer or refractometer is calibrated to, usually 60 °F.

graduated cylinder with hydrometer

graduated cylinder with hydrometer


To measure the specific gravity using a hydrometer fill the cylinder until about 2-3 remain of head space, drop in the hydrometer slowly and spin it.  When reading the hydrometer make sure it is not clinging to the side of the cylinder, it should be completely floating.

fill cylinder with 2-3 inch head space

fill cylinder with 2-3 inch head space

read specific gravity on hydrometer

read specific gravity on hydrometer

Using a refractometer is much easier and requires only a drop of liquid.  A refractometer measures the density of a liquid in units of brix, amount of sugars present in a liquid.  There is a coversion equation below to go from brix to specific gravity.

refractometer

refractometer

Equations to Determine ABV

Here is a basic equation that can be used to determine ABV of a beer.  Two contants are used.

1.05 g — represents the amount of CO2 produced for every gram of ethanol produced
0.79 g/mL  — represents the density of ethanol alcohol, drop the g/mL to get the specific gravity.

 (  ( 1.05  x  ( OG – FG )  )  / FG  )  / 0.79  x  100 = % ABV

*For temperature correction use the following  ~50 °F subtract .001  /~ 70 ° F add .001 / ~ 80 ° F add .002

For example, lets say you are brewing up a dry irsh stout and your OG=1.052 with your FG=1.014.

 (  ( 1.05  x  ( 1.052 – 1.014 )  )  / 1.014  )  / 0.79  x  100 = 4.98% ABV

Here is an equation that can be used to convert units of brix to units of specific gravity.  This is an estimate equation, refer to the USDA Brix Measurements doc for the extact conversion numbers.

Specific Gravity = 1 +(0.004 x °Brix)

 

After doing some research on the topic of brewing sugars, I have put together a quick article that sums up all of the things I have learned.  Grains are soaked in approx. 150 degree water to extract these sugars, this is called mashing.   Next the fermentation process uses yeast to convert these sugars in the wort into alcohol and CO2.  The common sugars associated with brewing, there prevalence in wort, and how yeast breaks them down are described below.

Building Blocks

Here are the very basic things to know about the sugars extracted from grains during brewing.

  • Sugar is basically a ringed structure made up of carbon, oxygen, and hydrogen
  • A sugar ring is named by how many carbon atoms are attached to it.
  • Common brewing sugars, such as glucose and fructose are made up of a single hexose (6 carbon atoms, chemical formula C6H12O6)
  • A single hexose is called a monosaccharide.
brewing sugar -- glucose

brewing sugar -- glucose

Common monosaccharides that existing in brewing sugars are glucose, fructose, and galactose.  When two monosaccharides join they form another sugar structure called a disaccharide.  Common disaccharides that exist in brewing sugars are sucrose and maltose.  Sucrose is made up of a glucose molecule and fructose molecule, while maltose is made up of two glucose molecules.  When three monosaccharides join they form another sugar structure called a trisaccharide.  The most common trisaccharide that exists in brewing sugars is maltotriose, which is made up of three glucose molecules.  When monosaccharides join in structures of 4 or more the resulting structures are called dextrin, which is not fermentable by beer yeast.  Pure dextrin is actually added to some beers during the boil to increase body in the final product, our Northern English Brown ale is a good example.

brewing sugar -- glucose + fructose = sucrose

brewing sugar -- glucose + fructose = sucrose

Common Brewing Sugars Breakdown

  • Monosaccharides — glucose, fructose, and galactose
  • Disaccharides —  sucrose and maltose
  • Trisaccharides — maltotriose

Prevalance in Wort

Here are the typical percentages of the 5 common brewing sugars in wort.

  • 45%   Maltose
  • 14%   Maltotriose
  • 8%     Glucose
  • 6%     Sucrose
  • 2%     Fructose
  • 25%   Unfermentable dextrins

As a side note according to sweetness the order follows this – Fructose(sweetest), sucrose, glucose, maltose, maltotriose(least sweet). 

 

How does beer yeast break down these sugars?

Beer yeast break down brewing sugars by first working on the sucrose structure.  The yeast will break down the sucrose into its glucose and fructose components.  It will then consume the glucose, followed by fructose then maltose and finally maltotriose in that order. 

The yeast usually use two different enzymes to break down the different sugars.  To break down sucrose the yeast use an enzyme called invertase, which breaks them down outside the yeast cell.  To break down maltose and maltotriose the yeast use an enzyme called maltase, which breaks them down inside the yeast cell.  The take away here is that the sugars are broken down into monosaccharides before being completely utilized by the yeast.  The science behind what the yeast is actually doing is a huge topic on its own.

yeast eating sugars in fermentation vessel

yeast eating sugars in fermentation vessel

Hopefully this article was useful in understanding more about some of the basic chemistry in brewing, if you want to know more just comment below.