Kjeldahl Protein Determination

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). 

Protein dispersibility index Dry product is dispersed in water by blending, then centrifuged at 880 x g for I0 min Protein dispersibility index (%) = (% water-dispersible prolein/% total protein) x 100 Advantages Tested in a collaborative study and found to be reproducible between laboratories. Disadvantages Limited to soy products protein is determined by Kjeldahl analysis, which can be a lengthy procedure. AOCS Method B a 10-65 (AOCS, 1999) AACC Method 46-24 (AACC, 2000)... 

The total nitrogen of orange juices was found to increase with the maturity of the fruit and ranged between. 068 to. 120 g per 100 ml (30). The actual protein values obtained by Clements (31) were about 20 percent of the acetone powder. Nearly 30 percent of the alcohol-insoluble solids of juice and about 20 percent of that of vesicular pulp were found to be protein as determined by the Kjeldahl procedure (32). These values are the actual protein that was precipitated by alcohol and are only a fraction of the total protein values usually reported for orange juice (8). The main source of proteins in citrus juice is probably in the form of enzymes and the plastids. At least 47 different enzymes have been reported to occur in citrus fruits (33). Citrus fruits also contain several phenolic amines (34), some of which such as syn-epherine, may have physiological importance (35). 

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. 

Quantitative acid hydrolysis with 72% (w/w) H2SO4 [19] was used to characterize the BSG feedstock. The monosaccharides and acetic acid were determined by HPLC to estimate (after corrections for stoichiometry and sugar decomposition) the contents of glucan (cellulose) and hemicelluloses (xylan, arabinan, and acetyl groups) in the sample. The acid-insoluble residue after hydrolysis was recovered by filtration and considered as Klason lignin after correction for the acid-insoluble ash. Protein was determined by the KJeldahl method [20] using the Ax 6.25 conversion factor. 

Jung, S. D. Rickert N. Deak E. Aldin J. Recknor L.A, Johnson RA. Murphy. Comparison of kjeldahl and dumas methods for protein content determination in soy products. /. Am. Oil Chem. Soc.2W5,80, 1169-1173. 

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. 

Difficulties in the Kjeldahl determination of nitrogen (c), center chiefly on the problem of digestion. Variable factors known to influence the completeness (and speed) of conversion of protein nitrogen into ammonia by sulfuric acid digestion include the following sources of error ... 

High correlations were observed between the protein percentages determined by the Kjeldahl method and those estimated by the InfraAlyzer 400 (Table 20.6). The square multiple correlation coefficients of 0.97 to 0.999 with SEC of 0.18 to 0.34% are acceptable compared with the standard deviation of the reference method of about 0.15% for milk powders. 

Figure 16 Application of multivariate calibration for the direct determination of protein in wheat by NIR. Abscissa protein content determined by the Kjeldahl method. Ordinate protein content determined from NIR measurements at 100 different wavelengths and PLS regression with 10 factors. The plot shows results obtained from a test set containing 39 samples. The calibration model has been calculated from 50 other samples. [Reproduced from Ref. 18, p. 145, with kind permission of Wiley]...

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. 

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

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. 

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. 

The Kjeldahl total nitrogen determination method is not very sensitive, hut it suits well for analyzing insoluble samples without preceding disintegration. Automated Kjeldahl protein estimations are used especially in food analysis. 

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