Kjeldahl’s Method

Amino Acids

What is amino acid?

• Amino Acid: aminated carboxylic acid (R- COOH)

Classification of Amino Acid

1. By the location of Amino-group : /β / γ-AA

2. By its acidity : neutral/ acidic/ basic AA ratio of Amino-group to carboxylic group

3. By whether containing phenyl group aromatic / non aromatic AA

4. By its occurrence in protein Protein / non protein AA

5. By polarity of R group : polar / apolar side chain AA

6. By its nutrient value to human: Essential AA and non-essential AA


What is Protein ?

• polymer of 20 α- amino acids, with mol.wt from 5000 to1000,000 daltons.

• N is most distinguished element: among the composing elements of C,H, N, O, S, for some proteins: P, Cu, Fe, I.

• N content in different proteins ranging from 13.4% -19.1%, and averagely 16%.

• Most abundant component in cells: 50% of dry cells by weight

Protein content in food in %

Cereals : ( % )
Brown Rice 7.9
Polished rice 7.1
Wheat flour, whole-grain 13.7
Corn flour, whole-grain 6.9
Corn starch 0.3

legumes :
Soybean, raw 36.5
Beans, kidney, raw 23.6
Tofu, raw, regular 8.1

Fruits & vegetables :
Apple, raw, with skin 0.2
Strawberry, raw 0.6
lettuce , raw 1.0

Dairy products :
Milk, whole, fluid 3.3
Milk, skim, dry 36.2
Cheese, cheddar 24.9
Yogurt 5.3

Meat, poultry, fish:
Beef 18.5
Dry beef 29.1
Chicken, breast meat, raw 23.1
Ham 17.6
Egg, raw, whole 12.5
Finfish, raw, 17.9

Conversion factors for Foods

N to Protein conversion factors
Foods factors
Egg or meat 6.25
Dairy products 6.38
Wheat 5.70
Other cereal grains and oilseeds 6.25
Almonds 5.18
Peanuts 5.46
Other tree nuts and coconut 5.30


1. Digest the organic compounds with strong sulfuric acid in the presence of catalysts while heating.

2. The total organic N is converted to ammonium sulphate.

3. Neutralize the digested sol’n with abundant alkali. Here, the N is converted to ammonium hydroxide, and then being distilled into a boric acid solution and converted to ammonium borate.

4. Titrate ammonium borate with strong acid. (please notice that N: HCl = 1:1)

5. N content in proteins is averagely 16%.

Kjeldahl’s method


• a. Kjeldahl digestion flask – 500 or 800 ml

• b. Kjeldahl distillation apparatus,

• c. Conical flask, 250 ml

• d. Burette 50 ml.

Apparatus used in Kjeldahl

I. Digestion apparatus
II. Distillation & absorption apparatus

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