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 acetic acid by enzymatic means
This analysis is suitable for malt, wort, beer, beer-based beverages and soft drinks
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.