# Council Decision of 14 November 1992 laying down methods for the analysis and testing of heat-treated milk for direct human consumption (92/608/EEC), Division 6.2: Heat each Kjeldahl flask on the digestion apparatus (5.5.), very gently at first so that all the black froth stays within the bulb. When the initial frothing has ceased and copious white vapour appears, boil vigorously (acid vapour will condense half-way up the neck of the flask) until no black particles remain and until the digest becomes clear pale blue-green in colour. Then boil gently for at least 1,5 hours. Note the following requirements:

Extent: E+W+S+N.I.

(a) The time for the digest to become clear should not be more than one hour, and the total digestion time shall not be less than 2,5 hours. If more than one hour is necessary to achieve clearing, the total digestion time shall be increased accordingly.
(b) The added potassium sulphate promotes the digestion as it raises the boiling temperature of the mixture. If the residual volume of H2SO4 is less than approximately 15 ml at the end of the digestion time, nitrogen may have been lost because of excessive heating. If heating by gas, heat the flask on a plate of heat-insulating material, provided with a circular opening of such a diameter that the free flame only touches the part of the flask that is below the surface of the liquid contents (5.5.).
(c) If black particles enter the neck of the flask and are not all washed down into the bulb by the acid refluxing during the inital stages of the vigorous boiling period (this may be facilitated by rotating the flask) allow the flask to cool sufficiently and carefully wash with the minimum of water. Then continue the digestion as described above.

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Source: https://www.legislation.gov.uk/eudn/1992/608/annex/2/division/IV/division/6/division/6.2 (the official version)
Page: https://openlegislation.uk/eu-decision-1992-608/annex-2/division-IV/division-6/division-6.2
Contains public sector information licensed under the Open Government Licence v3.0. Source: legislation.gov.uk, The National Archives.