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Separation by ion chromatography followed by conductivity detection.

Oxalic acid is primarily derived from malt. By reacting with the calcium ions in the brewing liquor, haze caused by calcium oxalate can form. These crystals also serve as nucleation sites for the spontaneous and rapid release of carbon dioxide (gushing). The precise determination of oxalic acid is therefore of great importance in brewing technology.

Oxalic acid (oxalate) is oxidized to carbon dioxide and hydrogen peroxide by the enzyme oxalate oxidase.

\(\text{ Oxalate} \hspace{0.5em}^{\underrightarrow{oxalatoxidase}}\hspace{0.5em} H_2O_2\hspace{0.3em}{+}\hspace{0.3em}CO_2\)

In the presence of the enzyme peroxidase (POD), hydrogen peroxide reacts with MTBH (3-methyl-2-benzo thiazolinone hydrazone) and DMAB (3-dimethyl amino benzoic acid to form a blue quinone complex.

\(H_2O_2+MTBH+DMAB\hspace{0.8em}^{\underrightarrow{POD}} \hspace{0.8em} \text{quinone complex} \space + \space H_2O\)

The intensity of the color is proportional to the concentration of the oxalate in the sample and is measured at 590 nm.

Acetic acid (acetate) is converted to acetyl-CoA in the presence of the enzyme acetyl-CoA synthetase (ACS) by adenosine-5'-triphosphate (ATP) and coenzyme A (CoA). 

Acetate + ATP + CoA   \(^{\underrightarrow{ACS}}\)    Acetyl-CoA + AMP + pyrophosphate

Acetyl-CoA reacts with oxaloacetate in the presence of citrate synthase (CS) to form citrate.

Acetyl-CoA + oxaloacetate + H₂O   \(^{\underrightarrow{CS}}\)   citrate + CoA

The oxaloacetic acid required for reaction (2) is produced from malic acid and nicotinamide adenine dinucleotide (NAD) in the presence of malate dehydrogenase (MDH). In doing so, NAD is reduced to NADH:

Malate + NAD⁺    \(^{\underleftrightarrow{L-MDH}}\)   oxaloacetate + NADH + H⁺

The formation of NADH forms the basis of this analysis, which is measured as an increase in the absorbance at 340, 334 or 365 nm. Since this concerns a previous indicator reaction, the quantity of NADH is not linearly proportional to the acetic acid concentration.

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.

A cation exchanger based upon a sulfonated, crosslinked styrene/divinylbenzene copolymer is used to determine various organic acids in wort. Due to the high ligand density, the separation mechanism is based upon a combination of ion exclusion, ligand exclusion and steric exclusion; detection is performed using a UV detector.

Potassium dichromate oxidizes many organic and certain inorganic substances to various extents in an acidic solution. Since the level of oxidation depends upon the kinds of substances, the concentration of potassium dichromate, the pH of the solution, and the temperature and reaction time, the procedure described below must be followed precisely. The volume of potassium dichromate required in the analysis is determined potentiometrically. In an acidic solution, the dichromate ions are reduced to chromium(III) ions:

Cr₂O₇^2- + 6 e^- + 14 H₃O⁺ → 2 Cr³⁺ + 21 H₂O

Dichromate ions in excess of those required are determined through titration with an ammonium iron(II) sulfate solution:

Cr₂O₇^2- + 6 Fe²⁺ + 14 H₃O⁺ →2 Cr³⁺ + 6 Fe³⁺ + 21 H₂O