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 D-isocitric acid by enzymatic means
This analysis is suitable for non-alcoholic and beer-based beverages.
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.
D-Isocitrate in the sample is isolated as a barium salt through precipitation. The concentration of D-isocitrate is determined enzymatically. The enzyme isocitrate dehydrogenase (ICDH) catalyzes the oxidative decarboxylation of D-isocitrate to α-ketoglutarate with nicotinamide adenine dinucleotide phosphate (NADP):
D-Isocitrate + NADP+ \(^{\underrightarrow{ICDH}}\)α-ketoglutarate + NADPH + CO2 + H+
The amount of NADPH formed is proportional to the amount of D-isocitrate and is measured photometrically through the increase in absorbance.