First Commission Directive of 25 October 1985 on methods of analysis for edible caseins and caseinates (85/503/EEC)

6. PROCEDURE U.K.

6.1. Preparation of the test sample U.K.

As described in Section 1.2 of the General Provisions.

6.2. Test for presence of ammoniacal nitrogen U.K.

If the presence of ammonium caseinate or other ammonium compounds is suspected, carry out the following test. Add to 1 gram of sample in a small conical flask 10 ml of water and 100 mg of magnesium oxide. Rinse down any magnesium oxide adhering to the walls and close the flask with a cork stopper, inserting a piece of moistened red litmus paper between the stopper and the neck of the flask. Mix the contents of the flask carefully and heat the flask in a water bath at 60 to 65 ^o C. If the litmus paper colours blue within 15 minutes ammonia is present, and the method is not applicable (see Section 1).

6.3. Blank test U.K.

At the same time as the determination of the nitrogen content of the sample perform a blank determination using 0,5 grams of the sucrose (4.4) instead of the test portion, using the same apparatus, the same quantities of all reagents and the same procedure as described in 6.5. If the titration in the blank determination exceeds 0,5 ml of 0,1 mol/1 acid, the reagents shall be checked and the impure reagent or reagents purified or replaced.

6.4. Test portion U.K.

Transfer to the Kjeldahl flask (5.2) 0,3 to 0,4 grams of the test sample (6.1), weighed to the nearest 0,1 mg.

6.5. Determination U.K.

6.5.1. Transfer to the flask a few pieces of porcelain or a few glass beads (5.10.1) and about 10 grams of the anhydrous potassium sulphate (4.2). U.K.

Add 0,2 g of the copper (II) sulphate (4.3) and wash down the neck of the flask with a little water. Add 20 ml of the concentrated sulphuric acid (4.1). Mix the contents of the flask.

Heat gently on the digestion apparatus (5.3) until any frothing has ceased, boil gently until the solution is clear and a pale green-blue colour persists. During heating, swirl the flask occasionally.

Continue the boiling, regulating the heating so as to condense the vapours in the middle of the flask neck. Continue the heating for 90 minutes avoiding local overheating.

Allow to cool to room temperature. Carefully add about 200 ml of water and a few pieces of pumice (5.10.2). Mix and cool again.

6.5.2. Transfer into the conical flask (5.7) 50 ml of the boric acid solution (4.5) and four drops of the indicator (4.8). Mix. Place the conical flask under the condenser (5.4) so that the tip of the outlet tube (5.5) is immersed in the boric acid solution. Using a graduated cylinder (5.8), add to the Kjeldahl flask 80 ml of the sodium hydroxide solution (4.6). During this operation, hold the flask in an inclined position so that the sodium hydroxide solution runs down the side of the flask to form a bottom layer. U.K.

Immediately connect the Kjeldahl flask to the condenser by means of the splash-head (5.6).

Gently rotate the Kjeldahl flask to mix its contents. Boil gently at first, avoiding any frothing. Continue the distillation so that 150 ml of distillate are collected in approximately 30 minutes. The distillate should have a temperature below 25 ^o C. About two minutes before the end of the distillation, lower the conical flask so that the tip of the outlet tube is no longer immersed in the acid solution, and rinse the tip with a little water. Stop heating, remove the outlet tube and rinse its outer and inner walls with a little water, collecting the washings in the conical flask.

6.5.3. Titrate the distillate in the conical flask, using the standard volumetric hydrochloric acid solution (4.7). U.K.