Accurately Calculating Sugar Additions for Carbonation

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Accurately calculating the carbonation is a great exercise for working with apparent and actual attenuations as well as working with the extract % or Plato scale. The latter is not essential, but makes things more intuitive.

The final carbonation of bottle conditioned beer depends on the CO2 currently present in the beer and the CO2 that will be generated during bottle conditioning.

The amount of CO2 already in the beer depends on the CO2 pressure and the temperature of the beer. It can be determined by using a Carbonation Table. These tables show the equilibrium of CO2 content that exists for a given CO2 pressure and beer temperature.

The amount of CO>sub>2</sub> created by bottle conditioning is based on the amount of sugar that is fermented. Each gram of fermentable extract is fermented into equal parts (by weight) of alcohol and CO2 (this is not exactly true, but close enough for this calculation).

corn sugar

The easiest way to add fermentable extract to beer is through the addition of pure sugar. This can be dextrose (corn sugar) or succrose (table sugar). Most corn sugar is actually glucose monohydrate. This means that each glucose molecule bound with a water molecule which adds to its weight but not to the potential of CO2 that can be produced [BYO]. Glucose monohydrate contains 9% water by weight, which means that only 91% of its weight can be considered for the CO2 calculation.

The formula for calculating the carbonation when priming with corn sugar is:

Cbeer = Cflat-beer + 0.5 * 0.91 * mcorn-sugar / Vbeer

  • Cbeer - the final carbonation of the beer (g/l)
  • Cflat-beer - the CO2 content of the beer before bottling (g/l)
  • mcorn-sugar - the weight of the corn sugar (glucose monohydrate) (g)
  • Vbeer - beer volume (l)

table sugar

Table sugar, succrose, does not contain any water and yeast will convert half of its weight to CO2

Cbeer = Cflat-beer + 0.5 * mtable-sugar / Vbeer

  • mtable-sugar - the weight of the table sugar (succrose) (g)

dried malt extract

When using malt extract for priming, its fermentability needs to be taken into account. A typical aparent fermentablility (limit of attenuation) of malt extract is 80%. (a 12 Plato wort will finish at 2.4 Plato / 1.048 OG - 1.010 FG). But to determine the true fermentability the true fermentability needs to be calculated. To convert between apparent and true attenuation, the following formula can be used (see Understanding Attenuation)

Atrue = Aapparent * 0.82

  • Atrue - true attenuation
  • Aapparent - apparent attenuation

With that the carbonation that can be achieved with dried malt extract is

Cbeer = Cflat-beer + 0.5 * 0.82 * 0.80 * mDME / Vbeer

  • Cbeer - the final carbonation of the beer (g/l)
  • Cflat-beer - the CO2 content of the beer before bottling (g/l)
  • mDME - the weight of the dried malt extract (DME) (g)
  • Vbeer - beer volume (l)