The positive effects of fermented beverages on human health have been known for hundreds for years. For this reason, the cult drinks kvass (Russia) and kombucha (Asia) can be traced over a long history and have been consumed for their healing powers through time. Microorganisms such as lactic acid and acetic acid bacteria are used for non-alcoholic fermentation. The products of fermentation include organic acids such as lactic acid and gluconic acid, which facilitate digestion and metabolic processes. Fermented beverages are especially favored by consumers as healthy, natural refreshments due to their slightly sour flavor.
In addition to other criteria, the acid spectrum typical for certain types of fruit serves as the tool for evaluating whether a fruit juice is pure, and its composition has not been altered. Generally, tartaric acid, citric acid and L-malic acid are analyzed. These are the primary acids, with a few exceptions, that are responsible for the total acid in fruits.
Determination of lactic acid/lactic acid by enzymatic means
This analysis is suitable for malt, wort, beer, beer-based beverages and soft drinks
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.