Kjeldahl process of protein content determination involves digestion of the sample in a Kjeldahl digester in the presence of concentrated sulfuric acid and selenium tablet, which acts as a catalyst for the process.
It is a simple, precise, and a straight forward method, especially for learners, to get acquainted with.
You can apply this process on a large variety of foods to determine the nitrogen content, which you then translate as a protein content using a conversion factor.
After titration, used the obtained titre values to calculate % nitrogen liberated from the food sample. This value that will be instrumental in the determination of the protein content using the illustrated formula below.
Nt = {(V1-V2)N*1.4007}/Weight of sample
Where; Nt – Nitrogen content
V1 – Titre value of the blank
V2 – Titre value of the sample
N – Normality of the acid used
After doing the experiment, get the values and use it in your calculations. Let's use the sample values below for illustration purposes:
Blank titre – 9.9 ml
Cheese sample titres (A – 5.7; B – 5.6)
To obtain the protein content of the sample, first determine the nitrogen content of each then calculate their average. From there, use the protein content conversion factor to get the protein content of the sample under analysis.
Sample A: Nt = {(9.9-5.7)*0.1*1.4007}/0.2 = 2.94
Sample B: Nt = {(9.9-5.6)*0.1*1.4007}/0.2 = 3.01
Therefore; the protein content of the cheese sample = 6.38*{(3.01+2.94)/2} = 18.98%
Note: 6.38 is the protein content conversion factor for cheese. Different foods have different conversion factors as shown in the table below.
Food Sample/Class |
Protein Conversion factor |
Milk and milk products |
6.38 |
Nuts: Almonds |
5.18 |
Nuts: Peanuts, Brazil nuts |
5.41 |
Nuts: Other nuts, Seeds |
5.30 |
Cereals: Whole meal wheat |
5.83 |
Cereals: Wheat bran |
6.31 |
Cereals: Other wheat flours, Pasta |
5.70 |
Barley, Oat, Rye |
5.83 |
Rice |
5.95 |
Soya |
5.70 |
Gelatin |
5.55 |
Other foods |
6.25 |
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