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Methods of Protein Analysis and Variation in Protein Results

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Methods of Protein Analysis and Variation in Protein Results Methods of Protein Analysis and Variation in Protein Results C. E. McDonald Premiums on high-protein hard red spring wheat has created much interest in the protein test. The Kjeldahl method, a chemical procedure for nitrogen, is still the basic method used for protein analysis. The Kjeldahl method, the Udy dye binding method and the new infrared reflectance method for determining protein are described in this paper. In the analysis of wheat protein by the Kjeldahl method, the moximum amount of variance in results expected between different laboratories and within the same laboratory is larger than the increments of pro­ tein on which price premiums have been paid. The causes of protein variance and ways of reducing the variance are discussed. In the past year, the price of hard red spring Table I. Basis of Kjeldahl Protein Procedure. wheat has been substantially influenced by the 1. Protein made of amino acid building blocks. protein content. Price premiums have been paid on increased increments of 0.1 percentage points 2. Amino acids made of carbon, hydrogen, oxygen, of protein. The price expected for wheat on the sulfur, nitrogen. basis of protein as determined by a North Dakota laboratory sometimes is changed because the pro­ 3. Protein calculated from measurement of nitro­ tein found later at Minneapolis or Duluth differs gen. more than the protein increment required for 4. Per cent nitrogen x 5.7 = per cent crude pro­ premium. With barley, an unexpected discount tein of wheat. may be applied against the seller because the pro­ Per cent nitrogen X 6.25 = per cent protein of tein is found to be over the discount protein level other grains. at the marketing point. This paper discusses the protein test, how much variance should be expect­ ed between laboratories, and the causes of protein gen found, times a factor of 5.7 for wheat and 6.25 variance. for other grains, gives the content of crude pro­ tein. Other protein methods like the Udy or the Protein Methods new infrared reflectance method are calibrated 1. Kjeldahl Nitrogen Method. Amino acids are against the basic Kjeldahl method. The steps for the building blocks of protein, and they contain the Kjeldahl protein determination are given in carbon, hydrogen, oxygen, sulfur and nitrogen Table 2. This procedure requires the use of boiling (Table 1). In the basic method for protein, nitro­ concentrated sulfuric acid and strong caustic. gen is determined by a method of chemical anal­ 2. Udy Dye Binding Method. In this procedure, ysis known as the Kjeldahl procedure. The nitro- ground grain is shaken with an orange dye solu­ tion. This acid dye forms an insoluble dye-protein Dr. McDonald is professor, Department of Cereal complex with the basic amino acid building blocks Chemistry and Technology. of the protein. The dye-protein complex reduces May-June, 1977 3 Table 2. Major Steps in Kjeldahl Protein Analysis. I. R. LIGHT 1. Grind and weigh 1 gram (rv 1/28 oz.) grain. 2. Digest in boiling concentrated sulfuric acid to convert protein nitrogen to ammonia (NH3). 3. Add 50 per cent caustic to make ammonia vol­ atile. 4. Boil off ammonia and collect by distillation. 5. Measure amount of ammonia collected by titra­ tion. the amount of dye left in solution, the complex is DETECTOR removed by filtering, and the concentration of dye in solution is determined with a color measur­ ing instrument. The amount of dye left in solution is inversely related to the protein content of the sample. 3. Infrared Reflectance Method. In 1971, in­ struments using infrared reflectance (Neotec and Infraalyzer) were introduced into the grain trade to measure protein, moisture and oil. In this meth­ od, infrared light of different wave lengths (ob­ tained by filters) is directed onto a sample of Figure 1. Illustration of infrarec:l reflectance method fOl ground grain in a cell (Figure 1). The light reflect­ protein analysis. ed from the grain depends upon the chemical composition of the grain, and the reflected light at different wave lengths is measured by a photo­ lower for flour than wheat grain. The step of cell detector. The response of the detector is fed grinding the wheat sample by each laboratory into a small computer which calculates complex should have caused at least a part of the higher equations to obtain the protein, moisture or oil variation for wheat. Another factor is the uneven content of the sample. The instruments should distribution of protein found in ground wheat, gain rapid use in the grain trade, provided the which will be discussed later. accuracy is near that of the Kjeldahl method now used in protein laboratories. Table 3. Variance in Protein Results Between Laboratories on Samples Analyzed Variance Expected In Protein Result Monthly. Discussion of protein variance will be con­ Average fined mostly to the basic Kjeldahl procedure, be­ Number Range in standard cause almost all protein laboratories are still using Sample samples Protein deviationS this method. Protein for samples in the grain trade % % are obtained from a single analysis on each sam­ Wheat flour (1975)1 12 11-12 0.14 ple. With a single analysis there is probability Wheat grain (1975)2 10 12-18 0.16 that occasionally there will be a wide variation '1\1 onthly check sample from the American Association from the actual value. More than one analysis on of Cereal Chemists analyzed by some 50 laboratories. each sample would reduce this number of varia­ 2Monthly check sample of wheat grain from Ingman tions. Laboratories, Inc., analyzed by 13 laboratories. 'Standard Deviation - a measure of the deviation from Data shown in Table 3 indicate how much the average value expressed as percentage protein. variance in protein results should be expected between laboratories. Wheat flour samples were Normal variation expected in protein results analyzed on a monthly basis by some 50 different where there is a standard deviation of 0.16 per laboratories in the United States. Wheat grain cent is given in Table 4. There would be a 67 per samples were also analyzed on a monthly basis by cent probability that a laboratory doing a single 13 different laboratories in the spring wheat area protein analysis would obtain a value deviating of the United States. The standard deviation, as a from the actual value by one ± standard deviation measure of deviation from the average value, was (SD) unit. For example, for a wheat of 14.0 per 4 Farm Research Table 4. Expected Variance in a. 14 Per Cent Pro­ for protein analysis by an infrared instrument is tein Wheat Having a 0.16 Standard now being studied at North Dakota State Universi­ Deviation. ty. Probability Expected protein range % % Causes of Protein Variance 67 13.84-14.16 (± 0.16) 1. Sampling. A number of factors that can 95 13.68-14.32 (± 0.32) cause protein variance are listed in Table 6. Under 99.74 13.52-14.48 (± 0.48) sampling there are several areas where errors can occur. For example, the 4-5 lb. grading sample obtained by probe from a box car by a sampler cent protein, the value could be between 13.84 and may not be of the same protein content as the 14.16 per cent protein (14.0 ± 0.16) 67 per cent of whole large bulk of wheat. Uneven areas of high the time. There would also be, for a single analy­ and low protein make it difficult to obtain a truly sis, a 95 per cent probability that the value would representative protein sample. be within ± 2 S.D.; wheat of 14.0 per cent protein could show a range from 13.68 to 14.32 per cent protein (14.0 ± 0.32). What about the 5 per cent of Table 6. Factors That Can Affect Protein Results. the time when the variation is greater than ± 2 S.D.? There is a 0.26 per cent probability that the 1. Sampling. variation would be greater than ± 3 S.D. or for a. Of original grain (4-5 Ib). wheat, a variation greater than 0.48 percentage b. Subdividing grain sample (2 Ib). protein from the actual value. This wide variation c. Sampling grading or submitted sample for can be expected in about one out of every 400 grinding (1 oz). analyses. These variances in protein between pro­ d. Sampling ground sample for protein test (1 tein laboratories is the reason why there can be gram or A..l 1/28 oz). differences in the total wheat price when premi­ 2. Cleanliness of grain. ums are paid on 0.1 per cent increments of protein. 3. Moisture content of grain. The protein variance that can be expected in a. Original moisture. the same laboratory is indicated by the data pre­ b. Moisture loss on grinding. sented in Table 5. The variance is lower than be­ 4. Protein differences in particles of ground grain. tween different laboratories (Table 3), but it would 5. Laboratory techniques. be enough to cause some problems in price when premiums are paid on increments of 0.1 percent­ age protein. Bin samples brought into the protein labora­ tory by individuals should also be obtained by proper sampling procedures to get a truly repre­ Table 5. Variance in Protein Results in a Protein sentative sample. The assumption that the protein Laboratory at North Dakota State Uni­ is exactly the same throughout the bin would be versity. expected to be true only if there is a single variety Number Protein content Standard involved, and the soil, fertilizer availability and Sample analysis' low high Avg.
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