R-262.03.112 [2016-03] D/L-Lactic Acid (D/L-Lactate) in Wort from Sour Malt – Enzymatic Method
The wort analysis methods are written, for example, for wort that is produced using the Congress mash method. These methods may also be applied in the analysis of wort which has been produced using a different mash method, particularly the isothermal 65 °C method. However, the reference values listed in the analysis method for Congress wort are not transferable.
Caramel (crystal) malt and roasted malt (earlier referred to as colored malt) are considered specialty malts. Other kinds of specialty malts include smoked malt, acidulated malt, diastatic malt as well as brumalt and chit malt.
Application/Purpose
This method describes how to determine the respective amounts of L-lactic acid and D-lactic acid in acidulated malt. The wort is obtained using a modified Congress mash method (refer to R-260.02.080 Extract Content in Roasted and Caramel Malt).
Scope of Application
Acidulated malt intended for use in beer brewing or elsewhere in the food industry
Principle
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.