The method is suitable for the determination of water vapor volatile aroma compounds in beer.
Volatile aroma compounds are driven out of the sample through steam distillation. The ethanolic distillate is saturated with NaCl. Potassium hydrogen sulfite is added to separate carbonyl groups that might interfere with the analysis. The extraction of the aroma compounds is performed by shaking out with dichloromethane and the phases separated by centrifuging.
Determination of dissolved oxygen concentration by electrochemical oxygen sensors with exposed electrodes
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 × CO2, whereupon K = n × F × A × 1/d
I = sensor current
CO2 = 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.
This method is suitable for the determination of steam-volatile aroma compounds in wort.
Volatile aroma compounds are driven out of the sample through steam distillation. The ethanol distillate is adjusted to be alkaline and saturated with NaCl. The extraction of the aroma compounds is performed by shaking out with dichloromethane and the phases separated by centrifuging. The organic phase is further concentrated in a stream of nitrogen gas. An ammonia solution is added to remove the acids, because the acids would co-elute, thus preventing quantification of the target substances.
Determination of the vicinal diketone content (diacetyl + 2,3-pentanedione) as well as the total diketone content in beer
The method is suitable for filtered beers brewed to any original wort or to any alcohol content as well as for fermenting wort.
Diacetyl (2,3-butanedione) and 2,3-pentanedione are detected photometrically in the beer after steam distillation. It is also possible to determine precursors in green beer.
The method is suitable for beer brewed to any original gravity or to any alcohol content.
Volatile compounds in beer are concentrated through distillation and extracted with dichloromethane. The solvent phase is analyzed with a gas chromatograph. The linearity of the detector and the determination of the concentrations of analytes in the sample are achieved by using multiple concentration levels within the relevant range and through evaluation of the relative area under the peaks.
The method is suitable for beers of all original gravities and of any alcohol content.
The gas chromatography headspace method is used to determine the higher alcohols and esters present in beer, i.e., the volatile compounds are transferred from the headspace in the sample vial into the GC system for analysis. The following substances are measured in this analysis:
Acetaldehyde
Propanol-1
Ethyl acetate
2-Methylpropanol
3-Methylbutanol
2-Methylbutanol
2-Methylpropylacetate
Butyric acid ethyl ester
3-Methylbutyl acetate
2-Methylbutyl acetate
Hexanoic acid ethyl ester