Proteins Kjeldahl titration

Protein Content. The protein content of milk can be determined using a variety of methods including gasometric, Kjeldahl, titration, colorimetric, and optical procedures (see Proteins). Because most of the techniques are too cumbersome for routine use in a dairy plant, payment for milk has seldom been made on the basis of its protein content. Dye-binding tests have been appHed to milk for determination of its protein content these are relatively simple to perform and can be carried out in dairy plant laboratories. More emphasis will be given to assessing the nutritional value of milk, and the dependence on fat content as a basis for payment will most likely change. 

The Kjeldahl titration remains the chemical method for determining the nitrogen and hence protein content of wheat or flour. The method works... 

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. 

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. 

Proteins and Amino Acids Total protein in food and feed samples is commonly determined by Kjeldahl (acid digestion/titration) or Dumas (pyrolysis) or elemental analysis.14 FIPLC can separate major proteins and furnish protein profiles and speciation information. HPLC can be used to further characterize specific proteins via peptide mapping and amino acid sequence analysis. HPLC modes used for protein include IEC, SEC, RPC, and affinity chromatography with typical UV detection at 215 nm or MS analysis. Details on protein separations are discussed in the life sciences section. 

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 ... 

A 2.00-mL serum sample is analyzed for protein by the modified Kjeldahl method. The sample is digested, the ammonia is distilled into boric acid solution, and 15.0 mL of standard HCl is required for the titration of thb ammonium borate. The HCl is standardized by treating 0.330 g pure ( [4)2804 in the same manner. If 33.3 mL acid is required in the standardization titration, what is the concentration of protein in the serum in g% (wt/vol) ... 

O—H, and N—H. These bands enable the quantitative characterization of polymers, chemicals, foods, and agricultural products for analytes such as water, fatty acids, proteins, and the like. In many cases, the use of NIR reflectance spectroscopy has been able to replace time consuming, classical wet chemical analyses, such as the Kjeldahl method for protein nitrogen and the Karl Fischer titration for water content. The NIR region has been used for qualitative studies of hydrogen bonding, complexation in organometallic compounds, and solute-solvent interactions because the NIR absorptions are sensitive to intermolecular forces. 

Proteins in foods can be measured by NIR reflectance spectrometry with no sample preparation. This has replaced the standard Kjeldahl protein nitrogen determination, which required extensive sample preparation to convert protein nitrogen to ammonia, distillation of the released ammonia, and subsequent titration of the ammonia. The replacement of the Kjeldahl method for routine analysis by NIR has permitted online measurement of protein in food and beverage products. The Kjeldahl method is required for assaying the materials used to calibrate the NIR and for method validation. 

In the classical Kjeldahl method, the proteins are digested (wet oxidized) in sulfuric acid with a catalyst (mercury and selenium tablets now succeed by the much safer potassium and copper sulfate tablets). An acid solution of ammonium sulfate is formed which is then diluted in water. The solution is made alkaline with sodium hydroxide and heated to distil off ammonia into excess standard acid sulfuric acid. The excess acid is back-titrated with standard sodium hydroxide to determine the amount of ammonia. It is more usual now to use boric acid in which to collect the ammonia and titrate with standard hydrochloric acid. The Kjeldahl procedure has been partially automated in systems such as the Kjeltec Analyzer. Total protein can be calculated as nitrogen content x 6.38. 

The range of applications of potentiometric titrations for determination of acids and bases is very wide, as illustrated by the following examples. Carbonate, hydrogencarbonate, and hydroxide ions are all bases that can be titrated with a strong acid such as hydrochloric acid. The most popular method for determination of nitrogen, which is found in many important substances such as proteins, fertilizers, drugs, pesticides, natural waters, is the Kjeldahl method, based on the conversion of the bound nitrogen to ammonia, which is then separated by distillation and determined by titration with hydrochloric... 

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