Protein Content Determination: Kjeldahl Process

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.

Kjeldahl protein content determination procedure (cheese sample)

1. Obtain a duplicate 0.2 g of the cheese sample to be tested and put in kjeldahl flasks
2. Add 10 ml of concentrated sulfuric acid followed by selenium tablet into each flask
3. A control flask should contain no cheese/food sample to be tested
4. Set the flasks into a Kjeldahl digester connected to a fume chamber to extract the fumes produced during the digestion process
5. Heat the samples until the contents of the flasks remain clear. The machine is callibrated to heat the samples to 410°C for 1.5 hours.
6. Allow the samples to cool to room temprature before you start distillation.
7. Distill the samples in the distillation unit for about 5 minutes (the machine is callibrated to dose the required amount of 40% NaOH and steam into the flask containing the digested sample).
8. Collect the liberated gases in conical flask containing 50ml 2% boric acid (make sure the delivery tube is completely immersed into the boric acid solution).
9. Titrate the resultant solution in the conical flask against 0.1N HCl in the presence of mixed indicator (bromocressol green + crystal red).

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

Observations/Results

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)

Calculations:

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.

Summary of protein content conversion factors for various foods (see FAO table for details).