L-malic acid occurs as the primary acid in apples. Apple juice typically contains a minimum of 3 g/l L-malic acid.
L-Malic acid is an organic acid found in malt and wort in small concentrations. It is also produced during fermentation.
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 L-malic acid by enzymatic means
This method is suitable for malt, wort, beer and beer-based beverages non-alcoholic beverages.
L-Malic acid (L-malate) is oxidized by nicotinamide adenine dinucleotide (NAD) in the presence of L-malate dehydrogenase (L-MDH) to oxaloacetic acid:
L-Malic acid + NAD+ \(^{\underrightarrow{L-MDH}}\) oxaloacetic acid + NADH + H+
The equilibrium of this reaction disproportionately favors malic acid. However, the oxaloacetic acid can be captured with the help of a downstream reaction involving the enzyme glutamate-oxaloacetate-transaminase (GOT) in the presence of L-glutamic acid, shifting the reaction in favor of oxaloacetic acid and NADH:
Oxaloacetic acid + L-glutamate \(^{\underleftrightarrow{GOT}}\)L-aspartate + 2-oxoglutarate
The amount of NADH produced during the reaction is equivalent to the quantity of L-malic acid and its absorption is determined photometrically at 334, 340 or 365 nm.