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

B-420.26.900 [2020-10] Carbon Dioxide, Zahm and Nagel – Manometric 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 in bottles and cans by means of pressure and temperature measurement and compensation of the gases that are note reactive with alkali

Scope of Application

Determination of the concentration of dissolved carbon dioxide in carbonated beverages in bottles and cans

Principle

The total gas pressure in beer is measured after the beer has been forcefully shaken. The carbon dioxide is then bound through the addition of potassium hydroxide. The amount of air in the beer contributes the remaining volume of gas. Once the value for the total pressure has been corrected by subtracting the quantity of air present in the beer, the carbon dioxide can be measured [1].