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L-lactic acid (L-lactate) is oxidized by nicotinamide adenine dinucleotide (NAD) in the presence of L-lactate dehydrogenase (L-LDH) to pyruvic acid. For oxidation of D-lactic acid, the enzyme D-lactate dehydrogenase (D-LDH) is required.

The equilibrium of these reactions is much closer to lactate. The equilibrium can be shifted towards the pyruvate and NADH side of the equation by removing the pyruvate with the help of the following reaction involving the enzyme glutamate-pyruvate transaminase (GPT) in the presence of L-glutamate.

The amount of NADH formed during the reactions is equivalent to the amount of lactic acid or D-lactic acid; the absorbance is determined photometrically at 334, 340 or 365 nm.

The fruit juice content of beverages and soft drinks is a very important quality characteristic. In Germany, there are guidelines that define the amount of juice contained in fruit-based beverages and soft drinks.

The guidelines specify that beverages made with fruit juice from seeded fruits or grapes must contain a minimum of 30 % juice, while those made from citrus fruits must contain at least 6 %, and beverages made from all other fruits or fruit combinations must contain at least 10 % juice. At least half of the juice content specified for fruit-based beverages must be present in fruit-flavored soft drinks. 

Under special conditions, a high fruit juice content can also be determined. In order to evaluate these conditions, equations for calculating the juice content have been developed by the working group for fruit juices and fruit-flavored beverages within the Society of German Chemists.

The RSK values serve as the basis for the calculation and are used to aid in the evaluation for evaluating the authenticity of fruit juices and products made from them (the values are not legal standards).

The different levels of importance attributed to individual analytes and, in part, the large range of fluctuation typical of certain values, were taken into account by assigning a weight to the value for each analyte in the equation.

Fruit juices

The positive effect of fermented beverages on the human body has been known for centuries. Current beverage trends, like kvass (Russia) and kombucha (Asia), stem from traditions with roots deep in the past. They have always been consumed as healing beverages. Non-alcoholic forms of fermentation employ microorganisms, such as lactic and acetic acid bacteria. They produce organic acids like lactic acid and gluconic acid, which promote digestion and metabolism. Due for the most part to their slightly acidic flavor, these kinds of fermented beverages are popular with consumers as a healthy natural refreshment.

Malt, fruit juice and tea serve as a base for fermented beverages.

As a rule, fermented beverages contain 0.5 – 15 g/l D-gluconic acid.

Malt, wort and beer

L- and D-lactic acid are produced during fermentation and are already present in malt and wort to some degree. The lactic acid content increases through biological acidification of the wort or through the use of acidulated malt.

L-Lactic acid (L-lactate) is oxidized to pyruvic acid by nicotinamide adenine dinucleotide (NAD) in the presence of L-lactic acid dehydrogenase (L-LDH). The enzyme D-lactate dehydrogenase (D-LDH) is required for the oxidation of D-lactic acid:

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

D-lactate + NAD \(^{\underrightarrow{D-LDH}}\) pyruvate + NADH + H⁺

The equilibrium of the reaction disproportionately favors lactic acid. However, the pyruvic acid can be captured with the help of a downstream reaction involving the enzyme glutamate pyruvate transaminase (GPT) in the presence of L-glutamic acid and the reaction can be shifted in favor of pyruvic acid and NADH:

Pyruvate + L-glutamate \(^{\underleftrightarrow{GPT}}\) L-alanine + α-ketoglutarate

The quantity of NADH consumed during the reaction is equivalent to the quantity of L-lactic acid or D-lactic acid and can be determined photometrically due to its absorption at 334, 340 or 365 nm.