ISO 8968-1-2014 pdf free.Milk and milk products – Determination of nitrogen content – Part 1: Kjeldahl principle and crude protein cal culation.
ISO 8968-1 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.
NOTE Inaccurate crude protein results will be obtained if non-milk sources of nitrogen are present in the
products specified in this International Standard.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are indispensable to its application. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 385, Labo ratory glassware — Burettes
ISO 8655-3, Piston-operated volumetric apparatus — Part 3: Piston burettes
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1 nitrogen content
niass fraction of nitrogen determined by the specified procedure
Note 1 to entry: It is expressed as a percentage.
3.2 crude protein content
mass fraction of crude protein calculated as specified
Note I to entry: It is expressed as a percentage.
4 Principle
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.
5 Reagents
Use only reagents of recognized analytical grade, unless otherwise specified, and distilled or demineralized water or water of equivalent purity.
NOTE The solutions specified in this procedure might be different than those required for the operation of automated titrators. An effort Is made to address those, but it Is the responsibility of the operator to follow the directions of the equipment manufacturer.
5.1 PotassIum sulfate (K2S04). nitrogen free.
5.2 Copper (II) sulfate pentahydrate solution, c(CuSO4.5H20) = 5,0 g/100 ml.
Dissolve 5,0 g of copper(ll) sulfate pentahydrate in water in a 100 ml one-mark volumetric flask. Dilute to the mark with water and mix.
5.3 Sulfuric acid (H2S04), with a mass fraction of between 95% and 98%, nitrogen-free (approximately P20= 1,84g/ml).
5.4 Sodium hydroxide (NaOH) solution, nitrogen-free, containing 50 g of sodium hydroxide per
With automated distillation systems, other mass fractions of sodium hydroxide may be used, provided an excess of sodium hydroxide is dispensed to the distillation mixture; for example, a mass fraction of 40% sodium hydroxide solution may be used instead of a mass fraction of 50%, where plugging of the automated flow system is a problem. The total volume of such sodium hydroxide solution should be considered in order to maintain the suitable distillation volumes.ISO 8968-1 pdf free download.