DIN 10335:2010-09

The standard specifies an enzymatic method for the determination of the L- and D-lactic acid (L- and D-lactate) content of milk products except milk powder. For milk powder, the method described in DIN EN ISO 8069 can be applied. In the aqueous extract of the sample from which fats and proteins have been removed by Carrez precipitation (L and D forms) are oxidized by nicotinamide adenine dinucleotide (NAD) in the presence of the enzymes L-lactate dehydrogenase (L-LDH) and D-lactate dehydrogenase (L-LDH) to pyruvic acid. The increase in concentration of the reduced form of nicotinamide adenine dinucleotide (NADH) is determined by measurement of the increase in absorbance of the test solution at 340 nm (Hg 334 nm, Hg 365 nm) and is proportional to the present L- or D-lactic acid. The equilibrium in these reactions is strongly shifted to lactic acid. It will be shifted by scavenging pyruvic acid with glutamate pyruvate transaminase (GPT) in the presence of glutamic acid and a pH value of 10 to pyruvic acid and HADH. The enzymes L-lactate dehydrogenase and D-lactate dehydrogenase are stereospecific, with the result that the two isomeric lactic acids can be determined successively or in parallel. With respect to DIN 10335:1987-06, the title and scope have been extended to "Milk and milk products except milk powder". Instead of the photometric method, an enzymatic method has been specified and the standard has been technically and editorially revised and supplemented and therewith adapted to the currently valid design rules. The responsible Committee is NA 057-05-13 AA "Milch und Milchprodukte - Probenahme- und Untersuchungsverfahren" ("Milk and milk products - Methods of sampling and analysis").