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.
Whole hops intended for use in beer brewing or elsewhere in the food industry
This method describes how to determine the α-acids and β-acids in hop extract using high-pressure liquid chromatography.
Hop extract intended for use in beer brewing or elsewhere in the food industry
Hop products with isomerized or reduced iso α-acids intended for use in beer brewing or elsewhere in the food industry
Hop products with isomerized or reduced iso α-acids are dissolved with methanol. The bitter acids are separated through reverse phase high-pressure liquid chromatography (RP-HPLC) and isocratic elution. They are then measured at a wavelength of 270 nm.
Acidulated malt intended for use in beer brewing or elsewhere in the food industry
Acidulated malt intended for use in beer brewing or elsewhere in the food industry
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.
|
L-lactate + NAD+ |
L-LDH
|
pyruvate + NADH + H+ |
|
D-lactate + NAD+ |
D-LDH
|
pyruvate + NADH + H+ |
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.
|
pyruvate + L-glutamate |
GPT
|
L-alanine + α-ketoglutarate |
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.