Proteins Kjeldahl method

Protein content was determined by a semi-automated micro-Kjeldahl method [4], The conversion factor used was 6.25. 

The quantity of protein present is normally measured by the Kjeldahl method. As wheat endosperm protein is around 17.5% nitrogen a factor of 5.7 is normally used to convert Kjeldahl nitrogen measurements into protein. Tkachuk suggested that 5.7 should be used for whole wheat but 5.6 should be used for flour.24... 

The Kjeldahl method is not a rapid means of analysis but it does have the advantage of being absolute. It is a sobering thought that a batch of bread can be made in less time than it takes to check the protein content of the flour by Kjeldahl. 

A titrimetric method that has been used for many years for the determination of nitrogen or protein in a sample is the Kjeldahl method. Examples of samples include grain, protein supplements for animal feed, fertilizers, and food products. It is a method that often makes use of the back titration concept mentioned above. We will now describe this technique in detail. 

Experiment 12 Determination of Protein in Macaroni by the Kjeldahl Method... 

The Kjeldahl method measures the nitrogen content of a compound and may be used to determine the protein content of a sample provided that the proportion of nitrogen in the protein is known. Protein determinations are complicated by the presence of nitrogen from non-protein sources. The simplest way of eliminating this source of error is to precipitate the proteins using a suitable method and to determine the nitrogen content of the precipitate. 

Although the Kjeldahl method is tedious and cumbersome it can be extremely accurate if the proportion of nitrogen in the protein sample is known and the complete recovery of nitrogen can be assured. This can be checked by recovery experiments particularly for the digestion stage of the process. 

Bound amino spacer or immobilized protein may be determined also by the Kjeldahl method (Protocol 1.1.6). 

Protein solubilities of soy flour and extrudates in the following solvent systems were determined by the micro-Kjeldahl method ( 9). A portion (0.1 g) of finely-ground sample (No. 60 sieve) was extracted with 9.9 ml of solvent for 1 hr at room temperature followed by centrifugation and filtration. An aliquot of the supernatant was used for nitrogen determination. 

Since late in the 19th century, the classical Kjeldahl method has been recognized and accepted universally as the authoritative method of analysis for determining the protein content in a wide variety of ingredients and finished products. As a result of technical innovations there are currently available and in use semiautomated or fully automated protein analysis systems that are based on the classical Kjeldahl procedure. 

The performance of the combustion method compares favorably with that of the mercury catalyst Kjeldahl method for determination of crude protein in feeds. Standard deviations by the combustion method covered the full range of possibilities compared to the Kjeldahl method values were either equivalent, better than, or not as good as for the Kjeldahl method. For the 0.5-mm pairs, values ranged from 0.09% to 0.58% protein for the Kjeldahl method and from 0.14% to 0.33% protein for the combustion method, and (SR) values ranged from 0.23% to 0.86% protein (Kjeldahl) and from 0.30% to 0.61% protein (combustion). 

The nitrogen content of the test protein can be determined by Kjeldahl method (see UNIT... 

For the test protein, disperse 6.25 mg protein (1.0 mg nitrogen as determined by Kjeldahl method see unitbi.2) in 10 ml distilled water into a small reaction vessel or vial. Adjust the pH of the suspension with 0.1 N NaOH while stirring at 37°C. 

The amount of protein in the substrate solution can be determined by using the Kjeldahl method (unit Bl.2) to determine the nitrogen concentration (N) and then multiplying this by a factor of 6.25 (fN). 

Additional reagents and equipment for Kjeldahl, Lowry, or other protein determination method (unitbi.i)... 

Protein determination by the Kjeldahl method is slow and very few samples can be run at one time. The Lowry and many other protein tests are much more convenient. A Lowry assay on a test tube rack of 40 samples can be done in <2 hr. 

Protein content in the feedstock was estimated by the Kjeldahl method (26) using the N x 6.25 conversion factor. Moisture content of the samples was determined by oven drying at 105°C to constant weight. 

Protein Determine the percent of nitrogen as directed in the Nitrogen Determination (Kjeldahl Method), Appendix IIIC, or by the Protein Nitrogen Combustion Method, AOAC 992.23 or AOCS Ba 4e-93. The percent protein equals percent N x 6.25 and is calculated to exclude added vitamins, minerals, amino acids, and food additives. 

The determination of total serum protein is an important clinical measurement used in diagnosing liver malfunctions. Although the Kjeldahl method is capable of high precision and accuracy, it is too slow and cumbersome to be used routinely in determining total serum protein. The Kjeldahl procedure, however, has historically been the reference method against which other methods are compared. Methods commonly used include the Biuret method and the Lowry... 

A 0.7121 g sample of a wheat flour was analyzed by the Kjeldahl method. The ammonia formed by addition of concentrated base after digestion with H2SO4 was distilled into 25.00 mL of 0.04977 M HCl. The excess HCl was then back-titrated with 3.97 mL of 0.04012 M NaOH. Calculate the percent protein in the flour. 

One micro-Kjeldahl method (10-20 mg protein) requires a minimum heating time of 4 hours with HgO as catalyst (S57), while another (Jl)— carried out in a sealed tube at 460-480°C for 30 minutes without a catalyst — has been proposed. An ultramicro method (0.02-0.1 gg N) can be completed in 1 hour using Se as catalyst (E6). 

At several occasions the soil in which the grass was cultivated was fertilised with large amounts of nitrates (ca. 180 kg ha ) up to one month before the harvest. As a consequence, the nitrate content of the material is high (approx. 3 g kg ) which is the reason for the difference between results of Kjeldhal-N and total-N determinations. The Kjeldahl method does not reduce nitrates at low temperatures, so the results of the total-N determinations are therefore significantly higher. For this reason, it was decided to certify both the total-N content and the Kjeldahl-N content as the latter is of great importance for the practical use of the material (Kjeldahl-N can in principle be bio-converted in the living body into protein). 

Protein was determined by the method of Lowry (7) after hydrolysis with 0.2N NaOH (100°C, 15 min). Total nitrogen was measured by the micro-Kjeldahl method with sulfuric acid/hydrogen peroxide reagentj the ammonia was detected with Nessler s reagent. Glucose was measured by standard colorimetric assay using dinitrosalicylic acid. Starch was hydrolyzed with concentrated HC1 and then determined as sugar. 

A 0.300-g feed sample is analyzed for its protein content by the modified Kjeldahl method. If 25.0 mL of 0.100 M HCl is required for titration, what is the percent protein in the sample ... 

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