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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.

Volatile acids are distilled using steam, and the distillate is analyzed through titration. Sulfurous acid present in the distillate is determined iodometrically and subtracted from the total.

Fruit juices:

The acid spectrum typical of certain types of fruit are used, along with other criteria, as a basis for recognizing unadulterated fruit juices. Tartaric acid, citric acid and L-malic acid are recorded here, which, with a few exceptions, determine the total acidity of the fruit.

Citric acid occurs as the primary acid in citrus juices and other juices. Orange juice usually contains 3–17 g/l citric acid (AIJN).

In citrus juices, an addition of citric acid can be detected via the citric acid/D-isocitric acid ratio, as this lies within relatively narrow limits. In orange juice, values below 130 are found.

D-isocitric acid is partly present in fruit products as a lactone. The lactone must first be saponified prior to enzymatic determination in order to detect the total D-isocitric acid content.

Malt, wort and beer:

Citric acid is an organic acid and is present in malt and wort and is also produced during fermentation.

Citric acid (citrate) is converted to oxaloacetic acid and acetic acid catalyzed by the enzyme citrate lyase (CL):

Citrate oxaloacetic ^{\(^{\underrightarrow{CL}}\)} acid + acetate

In the presence of the enzymes malate dehydrogenase (MDH) and lactate dehydrogenase (LDH), oxaloacetic acid and its decarboxylation product pyruvic acid are reduced to L-malic acid and L-lactic acid, respectively, by reduced nicotinamide adenine dinucleotide (NADH):

Oxaloacetate + NADH + H^{+ \(^{\underrightarrow{L-MDH}}\)} L-malate + NAD⁺

Pyruvate + NADH + H^{+   \(^{\underrightarrow{L-LDH}}\)}L-lactate + NAD⁺

The sum of the quantity of NADH consumed during the reaction is equivalent to the quantity of citric acid. The absorbance is determined photometrically at 334, 340 or 365 nm.

Through the addition of a sodium hydroxide solution, the carbon dioxide in beer becomes bound as sodium hydrogen carbonate or sodium carbonate. Sulfuric acid is added to an aliquot of the beer treated with sodium hydroxide. This causes the carbon dioxide to be released again, after which a stream of air conducts the carbon dioxide into a barium hydroxide solution. Through titration of the excess barium hydroxide, the carbon dioxide content of the beer can be determined [1].

The thermal conductivity is measured in a small chamber, which is in turn separated using a semi-permeable membrane from the medium being measured. The diffusion through the membrane alters the thermal conductivity in the measurement chamber. The gas volume in the measurement chamber is completely replaced in 10–20 s cycles. The changes in the thermal conductivity over time are a function of the quantity of CO₂ diffusing across the membrane. Using this value and taking into account the temperature, the concentration in the medium being measured can be calculated. Other dissolved gases, such as nitrogen and oxygen, do not affect the result of the measurement, since either nitrogen or air is used to replace the gas in the measurement chamber [1].