DIN 10325:2010-07

This standard specifies an enzymatic method for the determination of the citric acid or citrate content of milk and milk products. The method is applicable to milk, cheese, processed cheese, powdered milk products and butter plasma. In the aqueous extract or in the aqueous extract of the sample from which fats and proteins have been removed by precipitation with trichloroacetic acid or from the microfiltrated butter plasma citric acid is cleaved by the enzyme citrate lyase (CL) to oxalacetic acid and acetic acid. In the presence of the enzymes L-malate dehydrogenase (L-MDH) and L-lactate dehydrogenase (L-LDH) oxalacetic acid and its possible decarboxylation product (pyruvic acid) are reduced by reduced nicotinamide adenine dinucleotide (NADH) to L-malic acid or L-lactic acid. The decrease in concentration of NADH is determined by measurement of the decrease in absorbance in the test solution at 340 nm (Hg 334 nm, Hg 365 nm) and is proportional to the present citric acid or citrates. With respect to DIN 10325:1986-01, the title and scope have been changed from "Processed cheese" to "Milk and milk products". Furthermore, the results of an interlaboratory test of various powdered milk products have been added. 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") at DIN.