DIN 10344:2015-05

This standard specifies an enzymatic method for determination of lactose and D-galactose content in milk and milk products. The method is also applicable in the presence of other sugars. In the fat and protein-free aqueous extract of the sample, lactose is split by the enzyme β-galactosidase into D-glucose and D-galactose. D-galactose is oxidized to galactonic acid by nicotinamide adenine dinucleotide (NAD) in the presence of the enzyme galactose dehydrogenase. The amount of the reduced form of nicotinamide adenine dinucleotide (NADH) formed in this reaction is determined by measuring the increase in absorbance in the sample solution at 340 nm and is proportional to the D-galactose present. In a parallel approach, the existing free D-galactose without cleavage of lactose is determined in the same way by oxidation to D-galactonic acid. The difference in extinctions between the sample solution treated with β-galactosidase and the sample solution without splitting of the lactose is proportional to the D-galactose released from lactose and thus also to the lactose content of the sample. This document is intended to replace DIN 10344:1982-08. In comparison to this standard, the list of chemicals has been updated and supplemented, the calculation factor for lactose monohydrate has been changed, Clause "Terms and definitions" has been deleted and information on the required precision of the method has been added. The document has been technically and editorially revised and supplemented and thus adapted to the currently applicable design rules. The committee responsible for this standard is NA 057-05-13 AA "Milch und Milchprodukte - Probenahme- und Untersuchungsverfahren" ("Milk and milk products - Methods of sampling and analysis") at DIN.