AS ISO 8968.4:2021 Milk and milk products – Determination of nitrogen content Part 4: Determination of protein and non-protein nitrogen content and true protein content calculation (Reference method).
Filter the contents of the conical flask through a filter paper (6.5) placed in a filter funnel (6.4). Collect the entire filtrate in a clean, dry conical flask (6.2). The filtrate shall be clear and free of particulate matter. If it is not, repeat the process of precipitation and filtration with a new test portion.
if duplicated tests of the same test sample are to he carried out, two separate precipitation and filtration procedures have to be carried out for each test sample.
10.2.3 Preparation of the filtrate
Swirl the filtrate in the conical flask (6.2) to ensure that it is mixed. Pipette 20 ml of the filtrate into a 50 ml beaker (6.7) and weigh. Pour the filtrate from the beaker into a Kjeldahl flask or digestion tube containing boiling aids (optional), potassium sulfate and copper sulfate (TI) solution specified in ISO 8968-1IIDF 20-1. Immediately reweigh, to the nearest 0,1 mg, the empty beaker.
10.2.4 Digestion and distillation
Add the appropriate amount of sulfuric acid to the Kjeldahl flask or digestion tube and continue with the digestion and distillation procedure specified in ISO 8968-1IIDF 20-1, either 9.1 or 9.2 depending on the type of Kjeldahl apparatus being used.
10.2.5 Titration
Titrate the liberated ammonia by using the procedure specified in ISO 8968-1IIDF 20-1, but replacing the 0,1 mol/l hydrochloric acid standard volumetric solution by the 0,01 mol/l hydrochloric acid standard volumetric solution (5.3) as specified in Clause 5.
10.3 Blank test
Digest, distill and titrate blanks comprising of 0,1 g of sucrose and 16 ml ± 0,5 ml of trichloroacetic acid solution (5.2). Carry out a blank test following the procedure described in ISO 8968-1IIDF 20-1. Always titrate blanks with the same reagent and apparatus as used for the test portions.
Keep a record of blank values. If blank values change, identify the cause.
10.4 Calculation and expression of results
10.4.1 Calculation of non-protein nitrogen content
Calculate the non-protein nitrogen content, Wnpn, by using Formula (3).AS ISO 8968.4 pdf download.