46 Nitrogen Methods​

46-09.01 Crude Protein—Automated Colorimetric Method

This method determines crude protein in feeds and feedstuffs by digesting the samples in calibrated tubes and reading the absorbance of NH3-salicylate complex in a flow cell at 660 nm.

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46-10.01 Crude Protein—Improved Kjeldahl Method (ARCHIVED)

This modified Kjeldahl method determines total nitrogen in nitrate-containing materials and animal feeds. The sample is digested in sulfuric acid; ammonia is distilled; and excess acid is titrated. A conversion factor of 6.25 is used for feedstuffs.

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46-11.02 Crude Protein—Improved Kjeldahl Method, Copper Catalyst Modification

This is a modified Kjeldahl method in which CuSO4 is used as catalyst. The method determines total nitrogen in nitrate-free material, wheat flour and grain, cereal adjuncts, yeast foods, and animal feeds. The sample is digested in H2SO4, and N is converted to NH3, which is distilled and titrated.

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46-12.01 Crude Protein—Kjeldahl Method, Boric Acid Modification

This method is a modification of the Kjeldahl method, using boric acid in the indicator solution. The method determines total nitrogen by digestion of sample in H2SO4. It is applicable to wheat and flour mill products only.

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46-13.01 Crude Protein—Micro-Kjeldahl Method

This method determines total nitrogen in very small samples of flour or ground material. The sample is digested, and N is converted to NH3, which is distilled and titrated. The method is not applicable to compounds with N-N, NO, or NO2 linkage.

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46-14.03 Crude Protein—Udy Dye Method

This method measures protein content of wheat flour or ground wheat by colorimetric determination of unbound Acid Orange 12 dye. Ground sample is shaken with dye solution to form insoluble dye complex. After reaction with protein, dye concentrations are inversely correlated with percent protein in sample. This method is applicable to cereal grains, oilseeds, legumes, forages, animal products, dairy products, and formulated feeds or foods. It is not applicable to random mixtures. See Note 1.

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46-15.01 Crude Protein—5-Minute Biuret Method for Wheat and Other Grains

Compounds with peptide linkages produce a blue-violet color with copper ions in alkaline solution. This is named biuret reaction after the chemical compound biuret, first known to produce the color. The color is due to a complex bond formed between cupric ion and four peptide nitrogen atoms, two from each of two adjacent peptide chains. This method is applicable to all cereal grains, such as wheat, corn, barley, rye, and oats.

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46-16.01 Crude Protein—Improved Kjeldahl Method, Copper-Titanium Dioxide Catalyst Modification

This is a modified Kjeldahl method in which CuSO4/TiO2 is used as catalyst. The method determines total nitrogen in nitrate-free material, wheat flour and grain, cereal adjuncts, yeast foods, and animal feeds. The sample is digested in H2SO4, and N is converted to NH3, which is distilled and titrated.

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46-18.01 Crude Protein, Calculated from Percentage of Total Nitrogen, in Feeds and Feedstuffs

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46-19.01 Crude Protein, Calculated from Percentage of Total Nitrogen, in Wheat and Flour

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46-23.01 Nitrogen Solubility Index

To determine dispersible nitrogen in ground soybeans, whole or ground full-fat or extracted flakes, full-fat and defatted soy flours and grits, and soybean meal. In contrast to the alternative fast-stir method for the Protein Dispersibility Index (Method 46-24), the slower-stirring technique used in this method will give generally lower results.

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46-24.01 Protein Dispersibility Index

To determine dispersible protein in ground soybeans, whole or ground full-fat or extracted flakes, full-fat and defatted soy flours and grits, and soybean meal. In contrast to alternative slow-stir method for Nitrogen Solubility Index (Method 46-23), the faster-stirring technique used in this method will give generally higher results.

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46-30.01 Crude Protein—Combustion Method

Nitrogen, freed by pyrolysis and subsequent combustion at high temperature in pure oxygen, is quantified by thermal conductivity detection. This generic combustion method is applicable to all flours, cereal grains, oilseeds, and animal feeds. Different conversion factors are used for various cereal grains and oilseeds.

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46-50.01 Urea and Ammoniacal Nitrogen

This method determines urea and ammoniacal nitrogen in feeds and feedstuffs.

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