/ WORT, BEER AND BEER-BASED BEVERAGES / Beer / Carbon Dioxide

B-420.26.032 [2020-10] Carbon Dioxide Measurement, Volumetric Expansion Method

The quantity of dissolved carbon dioxide is an important quality attribute of beer. A satisfactory level of carbonation (bottom-fermented beer 0.40–0.60 % w/w, top-fermented beer 0.40–0.80 % w/w) contributes significantly to the perceived freshness and drinkability of beer.

Beers containing little carbon dioxide taste flat. Additionally, the carbon dioxide in beer promotes the formation of foam. The flavor thresholds of the flavor and aroma components in beer are altered by the quantity of carbon dioxide in solution.

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This method is based on the Henry-Dalton law, according to which at a given temperature, the concentration of an ideal gas dissolved in a liquid is proportional to the partial pressure of the gas in the gas phase, given that equilibrium exists. This steady state is reached by forcefully shaking the beer. A manometer is used to measure the total pressure.

Aside from carbon dioxide, there is always a certain amount of air present in the neck of bottled beer as well, causing the partial pressure of the carbon dioxide to shift. Therefore, in order to obtain accurate results, besides the total pressure, the “quantity of air” in the beer must be measured, i.e., the values measured for carbon dioxide, due to the analysis principle, are always too high. For precise measurements of the CO₂ content, the partial pressures of the foreign gases (e.g., O₂, N₂) must be taken into consideration.

The result is determined according to the so-called Haffmans equation:

\(CO_2 \space \text{in % w/w} = (p \space + \space 1.013) \space \times \space e^{\left(-10.73797 \space + \space {2617.25 \over T \space + \space 273.15}\right)}\)

p	= overpressure in bar (CO₂partial pressure)
T	= temperature in °C
1.013	= atmospheric pressure in bar

\(CO_2 \space \text{in % w/w} = P \space \times \space e^{\left(-10.73797 \space + \space {2617.25 \over T \space + \space 273.15}\right)}\)

P	= absolute pressure in bar (CO₂ partial pressure)
T	= temperature in °C

For p = overpressure in bar and P = absolute pressure in bar, insert the partial pressure of CO₂.

The Haffmans CO₂ equation is based on the solubility of CO₂ in beer with an original gravity of 12 % w/w and an apparent attenuation of 80 %.

The solubility of CO₂ in beer changes 1 % for every 3 % (w/w) original gravity.

The Haffmans equation is used independent of the alcohol content of the beverage, given that the constants (-10,73797 und 2617,25) are already present in it.

For carbonated beverages with a sugar content of more that 10 g/l, the reduced solubility of CO₂ due to the dissolved sugar must be taken into account. For this reason, for every 30 g/l of sugar subtract 0.098 bar from the measured pressure.

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Application/Purpose

Determination of the concentration of dissolved carbon dioxide in carbonated beverages by means of volumetric expansion

Scope of Application

This analysis is suitable for determining the concentration of dissolved carbon dioxide in carbonated beverages by means of volumetric expansion at concentrations between 2.5 and 6.2 g/l.

Principle

This device makes use of a method patented and entitled by Anton Paar "Multiple Volume Expansion". Here, the volume in the measuring chamber is increased in two separate steps, and at the end of each step the equilibrium pressure and temperature are recorded for use in the calculation. The true carbon dioxide content is determined independently of any other gases which may be dissolved in the beverage [1, 2, 3].

The figure below depicts the calibration correcting for air in the sample and shows how the effect of any dissolved gases other than CO₂ (air or nitrogen) on the result of the analysis is eliminated using the volumetric expansion employed in this method. The CO₂ content is determined after two separate volumetric expansions in the measuring chamber. If no dissolved other gases are present in the sample, the two results are identical, and no correction is necessary. If gases other than CO₂ are dissolved in the sample, this causes the result to be lower after the larger volumetric expansion (the second measurement) than the result at smaller volumetric expansion (the first measurement). A correction value is calculated based upon the difference between the two results, which completely eliminates the influence of the dissolved foreign gases (air or nitrogen) on the ultimate result of the analysis.