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Titratable acidity represents the sum of the free acids present in a beverage, with the exception of the dissolved carbon dioxide (carbonic acid). In fruit juices and the beverages prepared from them, they usually consist of malic acid, citric acid and tartaric acid.

The titration of the degassed beverage sample (freed from carbonic acid) is carried out potentiometrically using 0.25 mol/l sodium hydroxide solution either to a pH of 7.0 calculated as tartaric acid or to a pH of 8.1 calculated as citric acid.

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.

Ascorbic acid and other reductones are titrated iodometrically.

Determining the density by using the buoyancy of a hollow body of constant volume requires an aræometer (hydrometer) with a suitable measurement range. The buoyancy corresponds to the weight of the water displaced by the submerged hollow body and is directly proportional to the density of the displaced fluid.

The VitaFast® microbiological assay for niacin is carried out on microtiter plates and is suitable for the determination of total amount of niacin in foods and beverages.

The beverage is sterile-filtered and diluted with sterile water for the determination of added niacin.

The diluted sample and the niacin assay medium are added to the wells of the microtiter plate, which are coated with Lactobacillus plantarum. Lactobacillus plantarum requires niacin to grow. After an addition of niacin, the microorganism exhibits growth until the vitamin is completely utilized. The plates are incubated at 37 °C for 44–48 h.

The growth of Lactobacillus plantarum is dependent upon the concentration of niacin and is determined through measuring the turbidity. These results are compared with those obtained from a series of standard concentrations. The measurement is performed with a microtiter plate photometer at 610–630 nm (alternatively at 540–550 nm). 

Vitamin B₁ is separated by ion-pair RP-HPLC, detected using UV absorption and quantified through comparison with external standards. If necessary, the identity of the substances can be confirmed via spectra comparison.