SOP: Determination of Nitrogen by Micro-Kjeldahl Method

GIFSQ — Global Institute for Food Safety & Quality

Principle

Nitrogen in the juice is determined by the micro-Kjeldahl method: the organic nitrogen is digested with concentrated sulphuric acid to ammonium sulphate, the ammonia is distilled into standard acid, and the excess is back-titrated. The quantity of concentrated sulphuric acid for digestion is adjusted to the sugar content of the juice.

Apparatus required

• 30-ml Kjeldahl flask
• Kjeldahl digestion and distillation assembly

Reagents

a. Concentrated sulphuric acid (H₂SO₄), AR grade — the quantity for digestion is given as: ml of acid = 6 + 4 (Brix over weight of sample, in a stacked layout). The relation between Brix and the weight of sample is illegible in the available copy of the book — check the printed book before use.

b. Digestion mixture (as used for the micro-Kjeldahl method).

Procedure

Accurately weigh the juice sample into a 30-ml Kjeldahl flask

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Add the calculated quantity of concentrated sulphuric acid and the digestion mixture

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Digest until the digest is clear

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Cool and make the volume up to 50 ml

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Take 20 ml of this solution for distillation and proceed as in the micro-Kjeldahl distillation and titration (see the published GIFSQ micro-Kjeldahl SOPs)

Calculation

Nitrogen is calculated from the micro-Kjeldahl titration in the standard way (1 ml of 0.1 N acid consumed in distillation = 1.4 mg of nitrogen), adjusted for the 20-ml aliquot of the 50-ml digest.

Precautions

• The acid-quantity expression is partly illegible in the scan (the relation between Brix and the weight of sample cannot be read); confirm the printed formula from the book before routine use.
• Amino acids in the juice may also be determined separately by formol titration (see Chapter 32), expressed as the formol number or as millilitres of normal sodium hydroxide per litre of juice (potentiometric titration to pH 8.15).

References

Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.