AS ISO 8968.4 pdf download

AS ISO 8968.4 pdf download.Milk and milk products — Determination of nitrogen content
3  Terms and definitions
For the purposes of this document, the following terms and definitions apply. 3.1 non-protein nitrogen content NPN mass fraction of substances determined by the specified procedure Note 1 to entry: The non-protein nitrogen content is expressed as a percentage by mass. 3.2 protein nitrogen content mass fraction of substances determined by the specified procedure, directly or, alternatively, indirectly Note 1 to entry: The protein nitrogen content is expressed as a percentage by mass.
4  Principle
4.1  Indirect protein nitrogen Precipitation of protein from a test portion by addition of trichloroacetic acid solution such that the final concentration of trichloroacetic acid in the mixture is approximately 12 %. Removal of the precipitated milk protein by filtration, with the remaining filtrate containing the non-protein nitrogen components. Determination of the nitrogen content of the filtrate by the procedure described in ISO 8968-1|IDF 20-1. Where the total nitrogen content of the milk sample has previously been determined, the true protein nitrogen content may be calculated as the difference between the total nitrogen content and the non- protein nitrogen content.4.2  Direct protein nitrogen Precipitation of protein from a test portion by addition of trichloroacetic acid solution such that the final concentration of trichloroacetic acid in the mixture is approximately 12 %. Separation of the protein precipitate by filtration. (The precipitate contains the protein nitrogen of the sample.) Determination of the nitrogen content of the precipitate by the procedure described in ISO 8968-1|IDF 20-1.
5  Reagents
5.1  General Use only reagents of recognized analytical grade, unless otherwise specified, and distilled or demineralized water or water of equivalent purity. The reagents specified for the determination of total nitrogen by the method described in ISO 8968-1 | IDF 20-1, together with the following, are required. 5.2  Trichloroacetic acid (CCl 3 COOH) solution Dissolve 15,0 g of trichloroacetic acid in water in a 100 ml one-mark volumetric flask. Dilute to the mark with water. Do not use concentrations of trichloroacetic acid and volumes of solutions other than those specified. NOTE  The performance of the method with respect to mean value and between-laboratory performance characteristics will be different, if using other than specified concentrations of trichloroacetic acid and volumes of solutions. 5.3  Hydrochloric acid standard volumetric solution For the direct approach, the 0,1 mol/l hydrochloric acid solution is as described in ISO 8968-1| IDF 20-1. For the indirect protein nitrogen approach, the following hydrochloric acid solution is required c(HCl) = (0,01 ± 0,000 1) mol/l in addition to the 0,1 mol/l hydrochloric acid solution required as described in ISO 8968-1|IDF 20-1.
6  Apparatus
Usual laboratory apparatus and that specified for the determination of total nitrogen described in ISO 8968-1|IDF 20-1 and, in particular, the following. 6.1 Water bath, capable of maintaining a temperature of between 38 °C and 40 °C. 6.2 Conical flasks, of capacity 125 ml (indirect approach only). 6.3 Pipettes, of capacities 5 ml, 10 ml and 20 ml. 6.4 Filter funnel, made of glass, of diameter 75 mm. 6.5 Filter paper, nitrogen free, of diameter 15 cm. 2) 6.6 Automatic pipette, piston pump, capable of delivering 10 ml.
9  Procedure — Direct protein nitrogen approach
9.1  Test portion Pipette 5,0 ml ± 0,1 ml of the prepared test sample (see Clause 8) either into a dry and clean Kjeldahl flask or digestion tube, pre-weighed to the nearest 0,1 mg. Weigh the test sample to the nearest 0,1 mg. Immediately add 5,0 ml ± 0,1 ml of water to the flask or tube, rinsing any test sample on its neck into its bottom. NOTE  The use of either a Kjeldahl flask or a digestion tube is dependent on the laboratory’s choice of digestion apparatus. 9.2  Determination 9.2.1  Precipitation and filtration Add 40 ml ± 0,5 ml of trichloroacetic acid solution (5.2) to the Kjeldahl flask or digestion tube containing the test portion (9.1) and swirl to mix the contents. Let the flask or tube stand for approximately 5 min to allow the precipitate to settle. Pour the contents of the flask or tube through a filter paper (6.5) placed in a filter funnel (6.4). Collect the filtrate in a clean conical flask. Some of the precipitate will remain in the Kjeldahl flask or digestion tube and some will be collected on the filter paper. It is not necessary to remove all of the precipitate from the flask or tube. Immediately after pouring the mixture and so as not to allow any precipitate to dry on the neck of the flask or tube, add by means of an automatic pipette (6.6), 10 ml of the trichloroacetic acid solution (5.2). Use the solution to rinse any precipitate from the neck of the flask or tube down into the bottom. Swirl to mix the contents. Pour the thus-obtained contents of the flask or tube through the same filter paper. Add the filtrate to that previously collected in the conical flask. Again, immediately rinse the neck of the flask or tube with a further 10 ml of trichloroacetic acid solution and swirl to mix the contents. Pour the contents of the flask or tube for the third time through the same filter paper, adding the filtrate to that collected previously in the conical flask.