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Application Note No. 197/2015 Nitrogen and protein determination in dairy products Nitrogen determination in sodium nitrate KjelDigester K-449, KjelMaster K-375 with KjelSampler K-376: Accelerated Nitrogen and Protein Determination in Dairy Products According to the Kjeldahl Method by Digestion with Kjeldahl Tablets and Hydrogen Peroxide Followed by Colorimetric Titration
1. Introduction A reliable method for the determination of total nitrogen and protein in dairy products using hydrogen peroxide for digestion acceleration, according to ISO 8968-3 and AOAC 991.20 is presented [1,2]. The samples are digested by the KjelDigester K-449. The distillation and boric acid titration are performed using the KjelMaster system K-375 / K-376. Equivalent to the international normative procedures [1,2], the measuring method of the boric acid titration is colorimetric. Therefore, a mixed indicator accoding to Sher is added to the boric acid solution and the KjelMaster K-375 is equipped with a colorimetric sensor. The combination of the accelerated digestion method, using the Kjeldahl Tablet Titanium in combination with hydrogen peroxide, the KjelMaster system K-375 / K-376 increases the sample throughput to up to 120 samples per workday (9 h).
2. Equipment ⋅ ⋅ ⋅ ⋅ ⋅
KjelDigester K-449 (the parameters used are also valid for K-446) Scrubber K-415 TripleScrub ECO KjelMaster K-375 with KjelSampler K-376 Mixer, Retsch Grindomix GM200 Analytical balance (accuracy ± 0.1 mg) Safety accessories: ⋅ User protection shield, BUCHI (11057889) ⋅ Hirschmann bottle top dispenser ceramus® 5-30 mL, VWR (613-3243) with ceramus® discharge tube, spiral-shaped, VWR (612-0917)
Figure 1: Safety accessories for the digestion with hydrogen peroxide and Kjeldahl Tablet Titanium
Application Note 197/2015
3. Chemicals and Materials Chemicals: ⋅ ⋅ ⋅ ⋅ ⋅
Sulfuric acid, conc. 98 %, analytical reagent, Beijing Chemical Works Titanium, BUCHI Kjeldahl Tablets (11057980) Hydrogen peroxide, 30 %, analytical reagent, Beijing Chemical Works Sodium hydroxide 32 %, analytical reagent, Sinopharm chemical reagent Boric acid 4 %, 200 g boric acid (analytical reagent, Tianjin Guangfu Fine Chemical Research Institute) diluted to 5 L with deionized water, 12.5 mL of mixed Sher indicator BUCHI (003512) are added to the solution, adjust pH to 4.65 Sulfuric acid 0.05 mol/L, Aladdin (S112273-1L) Neutralization solution for the Scrubber: 600 g sodium carbonate (analytical reagent, Sinopharm chemical reagent) about 2 mL ethanol and a spatula tip of bromthymol blue (analytical reagent, Tianjin Kemiou chemical reagent) diluted to 3 L with distilled water L(+) Glutamic acid, assay 99.5 %, Aladdin (G103979-100g)
For a safe handling please pay attention to all corresponding MSDS. Samples: Table 1: The labelled protein and fat contents of the samples
3.4 g/100 mL
0 g/100 mL
2.2 g/100 g
36.0 g/100 g
Shake milk drink
1.5 g/100 mL
1.7 g/100 mL
33.0 g/100 g
38.4 g/100 g
The samples were purchased at a local supermarket.
4. Procedure The determination of nitrogen and protein in dairy products includes the following steps: ⋅ ⋅ ⋅
Homogenization of the Skimmed milk, Whipping cream and Shake milk drink by shaking and the Parmesan cheese by grinding Digestion of the sample, using the K-449 (K-446 respectively) Distillation and colorimetric titration of the sample, using KjelMaster system K-375 / K-376
4.1. Digestion method – glutamic acid (verification of the method) Start the KjelDigester K-449 according to the parameters listed in Table 2 1. 2. 3. 4. 5. 6. 7. 8.
Preheat the KjelDigester K-449 to 330 °C Place 0.12 g glutamic acid in a 300 mL sample tube Add 1 Titanium tablet and 10 mL of sulfuric acid (conc. 98 %) to each tube Prepare additional blanks, chemicals without sample Connect the Scrubber K-415 to the K-449 for absorbing the acid fumes created during digestion Place the rack under the fume hood and attach the protection shield Dropwise add 8 mL hygrogen peroxide (30 %) with the dispenser down the glass wall of the sample tube, wait until the fuming stops and the reaction subsides Insert the rack containing the protection shield and the samples into the cooling position of the KjelDigester and immediately start the digestion according to the parameters listed in Table 2 Let the samples cool down when the digestion is completed.
Application Note 197/2015
Table 2: Temperature profile for digestion with the K-449
4.2. Digestion method – samples 1. Start the KjelDigester K-449 according to the parameters listed in Table 2 2. Place each sample in a 300 mL sample tube as described in Table 3 Table 3: Weight for each sample
Shake milk drink
3. Add one Titanium tablet and 10 mL of sulfuric acid (conc. 98 %) to each tube 4. Prepare additional blanks, i.e. add 10 mL sulfuric acid and one Titanium tablet without sample 5. Connect the Scrubber K-415 to the K-449 for absorbing acid fumes created during digestion 6. Place the rack under the fume hood and attch the protection shield 7. Dropwise add 8 mL hygrogen peroxide (30 %) with the dispenser down the glass wall of the sample tube, wait until the fuming stops and the reaction subsides. 8. Insert the rack containing the protection shield and the samples into the cooling position of the KjelDigester and immediately start the digestion according to the parameters listed in Table 2. 9. Let the samples cool down when the digestion is completed. NOTE: If the liquid inside the sample tube is not clear and blue-green, digest for additional 15 min at 420 °C. The samples should be clear-green immediately after the digestion. A darkening of the clear liquid samples during the cooling down process is possible but does not affect the results. NOTE: Due to the use of hydrogen peroxide, the indicator bromthymol blue in the neutralisation solution of the K-415 could be decomposed faster compared to a classical Kjeldahl digestion, hence, a fading of the blue color is possible.
Application Note 197/2015
4.3. Distillation and titration For colorimetric titration it is necessary to determine the setpoint of the boric acid solution in advance to the blank and sample determinations. It is necessary to determine the setpoint every day before starting sample determinations, and when the method is changed or fresh chemicals are used to adjust the device to the current conditions. The detailed procedure, including the preparation of the sensor, is described in the Technical Note 179/2015 “Colorimetric titration procedure using Sher indicator” . The setpoint was measured three times. ⋅ ⋅ ⋅
1st setpoint 2nd setpoint 3rd setpoint
preheating 1st measurement 2nd measurement, confirms the 1st measurement
The last setpoint measurement is used as endpoint for all following determinations including priming, blanks and samples. 1. Determine the setpoint and check it’s range and deviation: Select all parameters for the setpoint determination according to Table 4. Table 4: Parameters for setpoint determination
Preheating before setpoint
Select the same method as for sample determination
NOTE: The selected method, boric acid and indicator for setpoint determination must be identical to the method used for sample determination. 2. Check the setpoint range and deviation ⋅ The determined setpoints should be in a range of 700 – 900 mV ⋅ The deviation between the two last measured setpoints should be ≤ 20 mV 3. Perform a priming to remove all residues 4. Determine blanks according to the parameters listed in Table 6 5. Determine samples according to the parameters listed in Table 6 Table 5: Setpoint measurements and deviation
Application Note 197/2015
Table 6: Distillation and titration with the KjelMaster system K-375 / K-376
H2SO4 0.05 mol/L
Titration start time
Stirrer speed distillation
Stirrer speed titration 10
Titration start volume 0 mL
Receiving solution vol.
NOTE: The sample throughput for this application was increased by using the “Online” titration mode: By applying the “Online” titration the time for the distillation and titration process is reduced to about 5 minutes per analysis because titration starts during the distillation is still in progress. 4.4. Calculation The results are calculated as a percentage of nitrogen. In order to calculate the protein content of the sample, the nitrogen content is multiplied with a sample-specific protein factor. The following equations (1), (2), and (3) are used to calculate the results. For the reference substance, the purity of the glutamic acid is considered in equation (4).
(VSample - VBlank ) ⋅ z ⋅ c ⋅ f ⋅ MN
m Sample ⋅ 1000
%N = wN · 100 %
%P = wN · PF · 100 %
%N Glu =
wN VSample VBlank z c f MN mSample 1000 %N %NGlu %P P PF
%N • 100 P
: weight fraction of nitrogen : amount of titrant for the sample [mL] : mean amount of titrant for the blank [mL] : molar valence factor (1 for HCl, 2 for H2SO4) : titrant concentration [mol/L] : titrant factor (for commercial solutions normally 1.000) : molecular weight of nitrogen (14.007 g/mol) : sample weight [g] : conversion factor [mL/L] : percentage of weight of nitrogen : percentage of weight of nitrogen corrected for the purity of reference substance : percentage of weight of protein : purity of the reference substance glutamic acid [%] : sample-specific protein factor (6.38 for dairy products)
Application Note 197/2015
5.1. Recovery of glutamic acid The results of nitrogen determination and recovery for glutamic acid analysis (assay ≥ 99.5%) are presented in Table 7. The nominal value of glutamic acid is 9.47 % nitrogen. The recoveries are within the specification between the 98 % and 100 % [1-3]. Table 7: Results of the recovery of nitrogen in glutamic acid
The mean blank volume (VBlank) was 0.649 mL (n = 4). 5.2. Protein determination in dairy products The results of the determination of nitrogen and protein contents in dairy products are presented in Tables 8-11. Table 8: Results of the determination of nitrogen and protein in skimmed milk (declared protein content 3.4 g/100 mL)
Skimmed milk mSample [g]
VSample [mL] %N
The mean blank volume (VBlank) was 0.649 mL (n = 4). The experimental protein content [%] was re-calculated taking the density (1.032 g/mL) into account in order to obtain the protein content as g/100 mL for milk. Table 9: Results of the determination of nitrogen and protein in whipping cream (declared protein content 2.2 g/100 g)
The mean blank volume (VBlank) was 0.554 mL (n = 3). Table 10: Results of the determination of nitrogen and protein in shake milk drink (declared protein cont. 1.5 g/100 mL)
Shake milk drink mSample [g]
The mean blank volume (VBlank) was 0.649 mL (n = 4). Application Note 197/2015
Table 11: Results of the nitrogen and protein determination in parmesan cheese (declared protein content 33 g/100 g)
Parmesan cheese mSample [g]
%P [g/100 g]
The mean blank volume (VBlank) was 0.609 mL (n = 3).
6. Comparison to Standard Methods This application note is based on the standard method ISO 8968-3 and AOAC 991.20 with minor differences. These differences are shown in Table 11. Table 11: Differences to ISO 8968-3 and AOAC 991.20
12 g K2SO4 + 1 mL CuSO4 Solution 5g CuSO4 x 5 H2O in 100 mL water
The choice of catalyst does not influence the result. Digestion time is reduced using Titanium Tablets, see Application Note 078/2012.
No impact, same ration of sulfuric acid/catalyst.
No impact, hydrogen peroxide can reduce the digestion time and foaming.
45 mL (Conc. 32 %)
40 mL (Conc. 40 %)
65 mL (Conc. 40 %)
No impact, same ratio of sodium hydroxide/sulfuric acid.
No impact consumption of the titration solution should be between 3-17 mL.
Colorimetric or potentiometric
No impact, the results are equal
1.75 – 3 h
1.75 - 2.5 h
Digestion time 60 min Indicator
Sher mixed indicator Methyl red / Methyl red / No impact. With the Sher bromocresol green bromocresol green mixed indicator the color 1:5 1:5 changes more sharply at pH 4.65 than methyl red / bromocresol green indicator.
7. Conclusion The determination of nitrogen and protein in dairy products using the KjelDigester K-449 and KjelMaster system K-375 / K-376 by colorimetric titration provides reliable and well reproducible results. By applying accelerated digestion using hydrogen peroxide, the process is intensified. These results correspond well to the labelled values of the dairy products. In addition, the protein content determined is in accordance the tolerance level as controlled by the Swiss Cantonal Chemists .
Application Note 197/2015
With the KjelDigester K-449 the digestion process (including preheating, digestion and cooling) is very fast and is fully automated. The combination with the fully-automatic KjelMaster system K-375 / K-376, allows unattended operation and highest sample throughput of about 120 samples per 9 hours working day.
8. References  ISO 8968-3:2007 Milk-Determination of nitrogen content Part 3:Block-digestion method (Semi-micro rapid routine method)  AOAC 991.20 Nitrogen (Total) in Milk  Technical Note No.179/2015 Colorimetric titration procedure using Sher indicator  FIAL, Verband der Kantonschemiker der Schweiz; Empfehlung: Genauigkeit der Angaben bei der Nährwertkennzeichnung, 3. Ausgabe 2008.