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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.

Glucuronolactone is separated by means of a strongly alkaline eluent with an ion exchange column. It is detected and quantified electrochemically using a pulsed amperometric detector (PAD).

Through creating a potential, the ions are oxidized on a gold electrode and induce a measurable charge. To prevent the electrode from being coated over a short time, the potential is then reversed to reduce and release the ions from the electrode.

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.

The volatile constituents in hops or hop products are obtained by means of steam distillation. The hydrocarbon and oxygen fractions are separated using a process involving column chromatography.

Hop extracts and isomerized hop extracts are dissolved in methanol. The iso-α-acids, α-acids and β-acids are separated using hig-pressure liquid chromatography (HPLC) with gradient elution and measured spectrophotometrically at a wavelength of 270 nm (iso-α-acids) and 314 nm (α-acids and β-acids).