This International Standard specifies a method for the determination of the nitrogen content and crude protein calculation of milk and milk products by the Kjeldahl principle, using traditional and block digestion methods. The methods are applicable to liquid cow's (whole, partially skimmed or skimmed milk), goat's and sheep's whole milk, hard, semi-hard and processed cheese, dried milk and dried milk products (including milk-based infant formulae, milk protein concentrate, whey protein concentrate, casein and caseinate). The methods are not applicable to samples containing ammonium caseinate. Inaccurate crude protein results will be obtained if non-milk sources of nitrogen are present in the products specified in this International Standard. A test portion is digested with a mixture of concentrated sulfuric acid and potassium sulfate. Using copper sulfate (II) as a catalyst to thereby convert any organic nitrogen present to ammonium sulfate. The function of the potassium sulfate is to elevate the boiling point of the sulfuric acid and to provide a stronger oxidizing mixture for digestion. Excess sodium hydroxide is added to the cooled digest to liberate ammonia. The liberated ammonia is steam distilled into the excess boric acid solution and titration with hydrochloric acid standard volumetric solution is carried out. The nitrogen content is calculated from the amount of ammonia produced. This standard replaces DIN EN ISO 8968-1:2002-06 and DIN EN ISO 8968-2:2002-06. With regard to these standards, the title of the standard has been supplemented and the scope has been rendered more precise as well as information on the use of automated distillation systems has been included. Furthermore, the list of apparatus has been supplemented, an equation has been added to calculate the recovery and informative annexes with information on the test portion sizes as well as the results of the collaborative trials have been added. The text has been technically and editorially revised and brought in line with current rules of presentation. This document (EN ISO 8968-1:2013) has been prepared by Technical Committee ISO/TC 34 "Food products" (secretariat: AFNOR, France), Subcommittee SC 5 "Milk and milk products" (secretariat: NEN, Netherlands) in collaboration with Technical Committee CEN/TC 302 "Milk and milk products - Methods of sampling and analysis" the secretariat of which is held by NEN (Netherlands). The committee responsible for this standard is NA 057-05-13 AA "Milch und Milcherzeugnisse - Probenahme- und Untersuchungsverfahren" ("Milk and milk products - Methods of sampling and analysis") at DIN.