/ WORT, BEER AND BEER-BASED BEVERAGES / Beer / Oxygen and Air

B-420.28.036 [2020-10] Dissolved Oxygen in Beer – amperometric, TÖDT and TESKE

The significance of oxygen over the course of the different stages of beer production varies:

If oxygen enters the process during mashing and wort handling, unsaturated fatty acids can become oxidized and phenolic compounds diminished, while the processes of starch degradation, lautering and extract yield are generally considered to be negatively affected [1]. However, the significance of the influence of oxygen at this stage of the process has been called into question [2, 3].

Oxygen is required for yeast reproduction immediately prior to the beginning of fermentation. However, excess oxygen can foster the formation of elevated concentrations of undesirable fermentation by-products. Downstream from primary fermentation, oxidation exerts an undesirable influence on the substances in beer, where the most harmful effects occur following filtration.

In the packaged beer, aside from the dissolved oxygen taken up during filtration and filling processes, the oxygen remaining in the headspace of bottles and cans is of consequence, as it can enter the beer through gas exchange.

Through the use of a properly designed filter with a well-devised system of bright tanks, wherein the tanks are thoroughly purged with CO₂ prior to filling, as well as suitable bottling and kegging lines, values for oxygen as low as 0.1 mg/l can be reached at the filter inlet and below 0.3 mg/l in the finished beer [4–7].

Literatur

L. Narziß, BWelt 127, 207, 1987
F. Schur, BR 97,117, 1986
H. Hug, P. Anderegg, H. Mändli und H. Pfenninger BR 97, 113, 1986
F. Ullmann BR 92, 143, 1981
H. G. Bellmer, BWelt 122, 1150 und 1166, 1982
L. Narziß, BWelt 122, 2293, 1982
P. Hoeren, MB 30, 36, 1977

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Application/Purpose

Determination of dissolved oxygen concentration by electrochemical oxygen sensors with exposed electrodes

Scope of Application

Suitable for measurements with beer, beer-based beverages, degassed water, carbonated and non-carbonated beverages. This method can be used to measure low oxygen concentrations, e.g., in beer 0.2–1.0 mg/l.

Interference may be caused by compounds, such as sulfur dioxide, hydrogen sulfide, chlorine and formaldehyde. The effects of these compounds vary according to which sensor is employed. A generally applicable rule can be given.

Principle

The measurement process using a Digox Analyzer works according to the principle of the potentiostatic three-electrode measurement system developed by TÖDT and TESKE and does not require a membrane.

The measuring electrode consists of solid silver, while the counter electrode is made of stainless steel. The reference electrode is composed of silver/silver chloride.

After generating a defined “polarization voltage”, an electrochemical reaction occurs at the measuring electrode, inducing a reduction of the oxygen molecules in the sample.

Measuring electrode (silver):

O₂ + 2 H₂O + 4 e⁻ → 4 OH⁻ (cathodic process)

Counter electrode (stainless steel):

4 OH⁻ → O₂ + 2 H₂O + 4 e⁻ (anodic process)

The flow of current as a result of this reaction is directly proportional to the amount of dissolved oxygen in the sample, if the polarization voltage is fixed as close to the level of the diffusion threshold current as possible.

In this case, the relationship can be represented as follows:

I = K × C_O2, whereupon K = n × F × A × 1/d

I = sensor current

C_O2 = oxygen concentration

F = Faraday constant

n = number of electrons per molecule

A = cathode surface

d = thickness of the “undisturbed layer” along the wall

The thickness of the undisturbed layer along the wall is determined by the hydrodynamic relationships at the measurement electrode and the transportation of oxygen molecules across the boundary layer brought about by temperature-dependent diffusion processes. Both of these clearly defined factors are precisely measured and compensated.

In order to adjust the polarization voltage between the two electrodes, a third electrode, the reference electrode, is employed in Digox measurement devices. This reference electrode remains in contact with the surface of the measuring electrode over a diaphragm in order to prevent mass transfer [1, 2, 3].

Active calibration:

In-line calibration is integrated into the device and is initiated by pressing a button. Taking advantage of Faraday’s Law, an exactly defined amount of oxygen is produced through the electrolysis of water.

I × t = m × F

I = current required for electrolysis

t = time

m = mass, g/mol

F = Faraday constant

The oxygen dissolves in the medium as it flows through and is detected at the measuring cell. The hydrogen liberated during the electrolysis reaction is not relevant for the measurement. The microprocessor monitors the calibration values and carries out any necessary corrective measures. The electrolysis enables the calibration of the device to be carried out under the same conditions and in the same medium as the analysis. Measurement operations are not disrupted during the calibration process [3].

The following applies to Digox 6.1 and all later models: In order to precisely determine the necessary potential for the measurement system, the Digox Analyzer possesses a scanner, which records the product-specific behavior of the medium subject to analysis. This can establish, whether the medium – due to the additives or supplements – must be measured at another potential. In this way, all types of beer-based beverages, non-alcoholic beverages and wine can be analyzed. Moreover, oxygen-reducing substances which can cause the calibration to be inaccurate may also be detected using the calibration scanning process. The device automatically implements the necessary compensative measures with the factors it has determined.