in dairy productsplace 2 g celite 545 in the digestion vessel 4. add up the appropriate amout of...
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Proximate determination in dairy products
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BUCHI develops and offers efficient and reliable techniques for protein, fat, moisture, ash and carbohydrates determination. Explore the guidebook for proximate determination in dairy products and find the solution that fits you best.
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Fat determination in Yogurt
Application Note No. 184/2015Extraction Unit E-816 ECE: Fat Determination in Yogurt samples using Twisselmann and Soxhlet extraction
Nitrogen & protein determination in milk powder
Application Note No. 105/2013KjelDigester K-449, KjelMaster K-375 with Kjel-Sampler K-376: Accelerated Nitrogen and Protein Determination in Milk Powder with Kjeldahl Tablets and Hydrogen Peroxide
BUCHI NIR Applications
NIR Pre-calibrated applications for the Milk & Dairy Industry
Application NoteNo. 184/2015
Fat determination in Yogurt
Extraction Unit E-816 ECE:Fat Determination in Yogurt samples using Twisselmann and Soxhlet extraction
www.buchi.com Quality in your hands
Application Note 184/2015 March 2015 2/8
1. Introduction
An effective procedure for fat determination in food according to §64 LFGB L 01.00-20 is
presented [1]. The sample is hydrolyzed with the Hydrolysis Unit E-416. The Twisselmann
extraction is performed with the Extraction Unit E-816 ECE (Economic Continuous Extraction).
This allows the sample to be constantly kept in hot solvent vapor whilst being efficiently rinsed
with freshly distilled solvent. After the extract has been dried to a constant weight the total fat
content is determined gravimetrically.
The results from the Twisselmann extraction were compared to the results from the Soxhlet
extraction method.
2. Equipment
Extraction Unit E-816 ECE
Extraction Unit E-816 SOX
Hydrolysis Unit E-416
Analytical balance (accuracy ± 0.1 mg)
Microwave oven
Drying oven / Vacuum drying oven
3. Chemicals and Materials
Chemicals:
Quartz sand, particle size 0.3-0.9 mm, BUCHI (037689)
Celite 545, Macherey-Nagel (815560)
10 L of 4 M Hydrochloric acid (HCl) are prepared by dilution of 4 L HCl 32 % (Hänseler 20-
2000-5) to 10 L with deionised water
Petroleum ether, boiling range 40-60 °C, analytical grade, ACS, Merck (1.01775.2500)
For a safe handling please pay attention to all corresponding MSDS!
Samples:
Yogurt Mocha, declared fat content: 3.0 g/100g
Yogurt Vanilla, declared fat content: 3.0 g/100g
Yogurt Choco, declared fat content: 4.0 g/100g
Yogurt Greek with honey, declared fat content: 6.5 g/100g
Yogurt Fruits, declared fat content: 3.0 g/100g
Yogurt Apricot, declared fat content: 3.0 g/100g
Yogurt plain, declared fat content: 3.5 g/100g
Yogurt Strawberry, declared fat content: 3.0 g/100g
Yogurt Banana, declared fat content: 3.0 g/100g
Yogurt Greek, plain, declard fat content: 7.0 g/100g
The samples were purchased at a local supermarket.
Application Note 184/2015 March 2015 3/8
4. Procedure
The determination of fat includes the following steps:
Homogenization of the sample by stirring
Acid hydrolysis of the sample, using the Hydrolysis Unit E-416
Fat extraction of the sample following Twisselmann extraction and Soxhlet extraction, using
the Extraction Units E-816 ECE and E-816 SOX
Calculation of the fat content
4.1. Homogenization of the sample
The yogurt samples were only homogenized by stirring with a spatula before use
4.2. Acid hydrolysis of the sample, using the Hydrolysis Unit E-416
4.2.1. Preparation of the glass sample tubes
1. Add approx. 20 g of quartz sand to the glass sample tube and compact the sand by gently tapping the glass sample tube onto the table
2. Add approx. 2 g Celite 545 and spread it evenly using a spoon
The sand and the Celite layer should not be mixed together. Otherwise the Celite
phase may breakthrough the frit and affect the results either by increasing the
recovery or by blocking the frit.
4.2.2. Hydrolyzing the sample matrix
3. Place 2 g Celite 545 in the digestion vessel 4. Add up the appropriate amout of sample1 to the digestion vessel and note the accurate
weight of the sample 5. Add 50 mL hydrochloric acid (4 M) and form a suspension by gently swirling the tube 6. Add another 50 mL hydrochloric acid (4 M) making sure to rinse any remaining sample
off the glass wall 7. Preheat the Hydrolysis Unit for 10 min 8. Insert the samples into the unit and lower the digestion vessels 9. Insert the glass sample tubes into the Hydrolysis Unit 10. Connect the digestion tubes and the glass samples tubes with the aspiration tubes,
reduce the heat to level 3 and start the water-jet pump after boiling begins
Violent foaming can be prevented by adding 4 M hydrochloric acid dropwise.
The degree of foaming depends on the sample and on the preheating time of the
unit. Do not extend preheating excessively.
11. Hydrolyze the sample for 15 min after constant boiling is observed in each position 12. Add 100 mL of warm (40-50 °C) deionised water to each digestion vessel at the end of
the hydrolysis time 13. Switch off the heating and lift the digestion vessels to the top position in order to filter
the hydrolyzate 14. Wash each of the digestion vessels by gradually adding a total of at least 400 mL warm
deionised water, until a neutral pH is reached 15. Check the pH with a pH paper on the bottom of the frit
For maximum efficiency, aspirate all samples/rinsing water at the same time.
16. Stir the Celite layers (without touching the sand layer at the bottom) with a spatula to loosen the pulp
17. Carefully wipe off the spatula with a piece of tissue and add it on the top of the sample
1 The sample weight has to be chosen according to the approximate fat content of the sample. 80-100 %: 0.7-1 g 20-50 % 1.5-3.5 g <10 %: 7- 10 g 50-80 %: 1-1.5 g 10-20 % 3.5-7 g
Application Note 184/2015 March 2015 4/8
18. Dry the glass sample tubes in a vacuum oven (≤ 4 h at 100 °C/200 mbar), in a drying oven (≤ 8 h at 100 °C) or in a microwave oven
Using a microwave oven accelerates the drying process. However, its control is more delicate.
This is due to the fact that the sample can easily overheat (> 105 °C) if an inappropriate heating
power is chosen. The following suggestion is valid for the drying of six hydrolyzed samples at the
same time. First step: 15 min at 640 W, second step: 9 min at 480 W, power of microwave oven
800 W (the optimal parameters may depend on the model of microwave).
Faster drying at higher temperatures is not recommended because fat may
decompose at temperatures above 105 °C. Oxidized fat can result in an excessive
recovery.
19. Allow the glass sample tubes to cool down to room temperature in a desiccator 20. Add another layer of quartz sand (20 g). This prevents the Celite from being re-
suspended in the condensed solvent
4.3. Fat extraction of the sample following Twisselmann extraction and Soxhlet extraction, using the Extraction Units E-816 ECE and E-816 SOX
4.3.1. Preparation of the beakers
Always use dry and clean beakers for the Twisselmann extraction. Add a boiling aid (e.g. boiling
stones) to each beaker and dry them for at least 30 min at 102 °C. Let them cool down to ambient
temperature in a desiccator for at least 1 h. Record the exact weight prior to extraction.
4.3.2. Twisselmann extraction
Put the sample tubes into the extraction chamber using the pliers. See Figure 1.
Figure 1: Twisselmann extraction chamber with sample before start
Fill the solvent into the beakers and place them on their corresponding heating plate. Close the
safety shield and lower the rack. Activate the occupied positions, open the cooling water or switch
on the connected chiller and start the extraction according to the parameters listed in Table 1.
Table 1: Parameters for the extraction with the Extraction Unit E-816 ECE
Method parameters Extraction Unit E-816 ECE
Solvent Petroleum ether2
Extraction step 50 min (Heater 100 %)3
Drying step 10 min (Heater 100 %)4
Total time 60 min
Solvent volume 70 mL
2 Please select the solvent, which is the default in the menu. 3 Choose the heater power between 100 – 120 % so the boiling is sufficient. 4 Choose the same parameter for the heater power that was selected for the extraction step.
Application Note 184/2015 March 2015 5/8
4.3.3. Soxhlet extraction
To compare the fat contents determined with the Twisselmann extraction, the samples were also
extracted by the Soxhlet extraction method using the Extraction Unit E-816 SOX.
Put the sample tubes into the extraction chamber using the pliers and adjust the optical sensor to
the correct sample amount. Fill the beakers with solvent and place them on their corresponding
heating plate. Close the safety shield and lower the rack. Activate the occupied positions, open
the cooling water or switch on the connected chiller and start the extraction using the parameters
listed in Table 2.
Table 2: Parameters for the extraction using the Extraction Unit E-816 SOX
Method parameters Extraction Unit E-816 SOX
Solvent Petroleum ether5
Extraction step 120 min (Heater 100 %)6
Rinse step 5 min (Heater 100 %)7
Drying step 25 min (Heater 100 %)8
Total time 150 min
Solvent volume 120 mL
4.3.4. Drying of the extract
Dry the beakers containing the extract in a drying oven at 102 °C until a constant weight is
reached. Let the beakers cool down to ambient temperature for at least 1 h in a desiccator and
record the weight.
Make sure that the cooling down time of the beakers in the dessicator is the same
before and after extraction. Differences in beakers temperature falsify the results.
4.4. Calculation
The results are calculated as percentage of the fat according to equation (1).
%100)
m
m -(mFat %
Sample
BeakerTotal (1)
% Fat : Percentage of fat in the sample
mTotal : Beaker + extract [g]
mBeaker : Empty beaker weight with boiling aid [g]
mSample : Sample weight [g]
5. Results and Discussion
The fat contents of the yogurt samples obtained with the Twisselmann and Soxhlet extraction
method are lower than the declared values however, they are within the guidelines of the
laboratory from the Food control of the Canton of Zug, Switzerland. Importantly, the results
obtained by Twisselmann and Soxhlet extraction are well comparable with low relative standard
deviations.
5 Please select the solvent, which is the default in the menu. 6 Choose the heater power between 100 – 120 % so the boiling is sufficient. 7 Choose the same parameter for the heater power that was selected for the extraction step. 8 Choose the same parameter for the heater power that was selected for the extraction step.
Application Note 184/2015 March 2015 6/8
The results are summarized in Tables 3 - 12
Table 3: Yogurt Mocha (Declaration: 3.0 g/100g)
E-816 ECE E-816 SOX
Sample 1 2.20 2.45
Sample 2 2.47 2.49
Sample 3 2.41 2.50
Mean value [g/100g] 2.36 2.48
rsd [%] 6.0 1.1
Table 4: Yogurt Vanilla (Declaration: 3.0 g/100g)
E-816 ECE E-816 SOX
Sample 1 2.73 3.01
Sample 2 2.75 3.01
Sample 3 2.74 3.03
Mean value [g/100g] 2.74 3.01
rsd [%] 0.4 0.3
Table 5: Yogurt Choco (Declaration: 4.0 g/100g)
E-816 ECE E-816 SOX
Sample 1 3.21 3.35
Sample 2 3.26 3.30
Sample 3 3.21 3.33
Mean value [g/100g] 3.23 3.32
rsd [%] 0.9 0.8
Table 6: Yogurt Greek with honey (Declaration: 6.5 g/100g)
E-816 ECE E-816 SOX
Sample 1 5.75 5.86
Sample 2 5.85 5.98
Sample 3 5.86 5.90
Mean value [g/100g] 5.82 5.92
rsd [%] 1.1 1.0
Table 7: Yogurt Fruits (Declaration: 3.0 g/100g)
E-816 ECE E-816 SOX
Sample 1 1.93* 2.31
Sample 2 2.16 2.27
Sample 3 2.16 2.21
Mean value [g/100g] 2.16 2.26
rsd [%] 0.0 2.2
*outlier, not taken for calculation (Dean-Dixon test)
Application Note 184/2015 March 2015 7/8
Table 8: Yogurt Apricot (Declaration: 3.0 g/100g)
E-816 ECE E-816 SOX
Sample 1 2.34 2.42
Sample 2 2.42 2.37
Sample 3 2.20 2.45
Mean value [g/100g] 2.32 2.41
rsd [%] 4.7 1.7
Table 9: Yogurt Plain (Declaration: 3.5 g/100g)
E-816 ECE E-816 SOX
Sample 1 2.14 3.10
Sample 2 3.15 3.06
Sample 3 3.16 3.05
Mean value [g/100g] 3.15 3.07
rsd [%] 0.3 0.8
Table 10: Yogurt Strawberry (Declaration: 3.0 g/100g)
E-816 ECE E-816 SOX
Sample 1 2.36 2.29
Sample 2 2.35 2.25
Sample 3 2.36 2.24
Mean value [g/100g] 2.36 2.26
rsd [%] 0.1 1.2
Table 11: Yogurt Banana (Declaration: 3.0 g/100g)
E-816 ECE E-816 SOX
Sample 1 2.41 2.49
Sample 2 2.33 2.53
Sample 3 2.42 2.50
Mean value [g/100g] 2.39 2.50
rsd [%] 1.9 0.7
Table 12: Yogurt Greek, plain (Declaration: 7.0 g/100g)
E-816 ECE E-816 SOX
Sample 1 6.61 6.75
Sample 2 6.51 6.73
Sample 3 6.62 6.22*
Mean value [g/100g] 6.58 6.74
rsd [%] 0.9 0.2
*outlier, not taken for calculation (Dean-Dixon test)
6. Conclusion
The determination of fat in different yogurt samples using the Hydrolysis Unit E-416 and the
Extraction Unit E-816 ECE following Twisselmann extraction method provides results which were
comparable to the results received with the Soxhlet extraction method following Weibull-Stoldt.
The determined fat contents of the yogurt samples are within the acceptance criteria of the
Application Note 184/2015 March 2015 8/8
Association of cantonal chemists of Switzerland (Föderation der Schweizerischen
Nahrungsmittel-Industrie)9. The results show low relative standard deviations (rsd).
With the Extraction Unit E-816 ECE the time to results is significantly reduced (60 min) when
compared to the use of the Extraction Unit E-816 SOX (150 min).
7. Acknowledgements
We gratefully acknowledge Mr. Urs Aschwanden from the Food control, Canton of Zug,
Switzerland, for his support in the development of this application note.
8. References
[1] §64 LFGB L 01.00-20 Bestimmung des Gesamtfettgehaltes in Milch und Milchprodukte
Operation Manual of Hydrolysis Unit E-416
Operation Manual of Extraction Unit E-816 ECE
Operation Manual of Extraction Unit E-816 SOX
9 Acceptance criteria of the Association ofcantonal chemists, Switzerland:
Declared fat content: Max. acceptable deviation: ≤ 2 g +/- 50 % > 2 g ≤ 5 g +/- 40 % > 5 g ≤ 10 g +/- 30 % > 10 g ≤ 20 g +/- 25 % > 20 g ≤ 30 g +/- 20 % > 30 g +/- 15 %
Application Note No. 105/2013 Nitrogen & protein determination in milk powder KjelDigester K-449, KjelMaster K-375 with KjelSampler K-376:
Accelerated Nitrogen and Protein Determination in Milk Powder with Kjeldahl Tablets and
Hydrogen Peroxide
Application Note 105/2013 March 2013 2/8
Introduction1
An easy and reliable method for the determination of total nitrogen and protein in milk powder by
the use of hydrogen peroxide, according to ISO 8968-3, is introduced below. The samples are
digested using the KjelDigester K-449. The distillation and boric acid titration are performed with
the KjelMaster K-375 with KjelSampler K-376. The combination of the accelerated digestion
method, using the Kjeldahl Tablet Titanium in combination with hydrogen peroxide, and the
KjelMaster system K-375/K-376 increases the sample throughput.
Equipment2
KjelDigester K-449 (the parameters used are also valid for K-446)
Scrubber K-415 TripleScrub ECO
KjelMaster K-375 with KjelSampler K-376
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 105/2013 March 2013 3/8
Chemicals and Materials3
Chemicals:
Sulfuric acid conc 98 %, Merck (1007482500)
Titanium, BUCHI Kjeldahl Tablet (11057980)
Hydrogen peroxide, 30 %, Fluka (95302)
Sodium hydroxide 32 %, Brenntag (81980-452)
Boric acid 4 %, 400 g boric acid, Brenntag (80948-155) diluted to 10 L with deionized water,
pH adjusted to 4.65
Sulfuric acid 0.05 mol/L, Fluka (35358)
Neutralization solution for the Scrubber: 600 g sodium carbonate, calcined, technical,
Synopharm (0179420) about 2 mL ethanol and a spatula tip of bromthymol blue, Fluka (18460)
diluted to 3 L with distilled water
D/L tryptophan, assay 99 %, Fluka (162698)
For a safe handling please pay attention to all corresponding MSDS!
Samples:
Skimmed milk powder I with a protein content of 32 g/100 g
Skimmed milk powder II with a protein content of 32 g/100 g
Whole milk powder I with a protein content of 25 g/100 g
Whole milk powder II (reference material) with a declared protein content of 26.44 g/100 g
The reference material was purchased at LVU, Herbolzheim (Germany).
Procedure4
The determination of nitrogen and protein in milk powder includes the following steps:
Digestion of the sample, using K-449 (K-446 respectively)
Distillation and titration of the sample, using KjelMaster system K-375/K-376 4.1 Digestion method – tryptophan (verification of the method)
1. Start the KjelDigester K-449 according to the parameters listed in Table 1 2. Place 0.1 g tryptophan in a 300 mL sample tube 3. Add 1 Titanium tablet and 10 mL of sulfuric acid (conc. 98 %) 4. Prepare additional blanks, chemicals without sample 5. Connect the Scrubber K-415 to the K-449 for absorbing the acid fumes created during
digestion 6. Place the rack under the fume hood and affix the user protection shield
7. Slowly add 8 mL hydrogen 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 user protection shield and the samples into the cooling position and mount the suction module onto the samples, immediately start the digestion according to the parameters listed in Table 1.
9. Let the samples cool down when the digestion is completed.
Application Note 105/2013 March 2013 4/8
4.2 Digestion method – samples
1. Start the KjelDigester K-449 according to the parameters listed in Table 1 2. Place 0.25 g of each sample in a 300 mL sample tube 3. Add 1 Titanium tablet and 10 mL of sulfuric acid (conc. 98%) to each tube 4. Prepare additional blanks, chemicals 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 affix the user protection shield
7. Slowly add 8 mL hydrogen 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 user protection shield and the samples into the cooling position and mount the suction module onto the samples, immediately start the digestion according to the parameters listed in Table 1.
Table 1: Temperature profile for digestion with the K-449
Step Temperature [°C] Time [min]
1 330 0
2 420 60
Cooling – 25
NOTE: If the liquid inside the sample tube is not clear and blue-green, digest for additional 15 min at 420 °C. NOTE: 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.
9. Let the samples cool down when the digestion is completed.
Application Note 105/2013 March 2013 5/8
4.3 Distillation and titration Distill the samples according to the parameters listed in Table 2.
Table 2: Distillation and titration with the KjelMaster system K-375/K-376
Method parameters KjelMaster K-375
H2O volume 50 mL Titration solution H2SO4 0.05 mol/L
NaOH volume 45 mL Sensor type Potentiometric
Reaction time 5 s Titration mode Online
Distillation mode Fixed time Titration start time 90 s
Distillation time 180 s Measuring mode Endpoint pH
Stirrer speed distillation 5 Endpoint pH 4.65
Steam output 100 % Stirrer speed titration 7
Titration type Boric acid Titration start volume 0 mL
Receiving solution vol. 60 mL Titration algorithm Optimal
NOTE: The sample throughput for this application was increased by using the Titration mode “Online”.
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 tryptophan is considered in equation (4).
1000 m
Mfc z V-(Vw
Sample
NBlankSampleN
) (1)
%N = wN ∙ 100 % (2)
%P = wN ∙ PF ∙ 100 % (3)
P
100 •%N=%NTry (4)
wN : weight fraction of nitrogen
VSample : amount of titrant for the sample [mL]
VBlank : mean amount of titrant for the blank [mL]
z : molar valence factor (1 for HCl, 2 for H2SO4)
c : titrant concentration [mol/L]
f : titrant factor (for commercial solutions normally 1.000)
MN : molecular weight of nitrogen (14.007 g/mol)
mSample : sample weight [g]
1000 : conversion factor [mL/L]
%N : percentage of weight of nitrogen
%NTry : percentage of weight of nitrogen corrected for the purity of reference substance
tryptophan [%]
Application Note 105/2013 March 2013 6/8
%P : percentage of weight of protein
P : purity of the reference substance tryptophan [%]
PF : sample-specific protein factor (6.38 for dairy products)
Results5
5.1. Recovery of tryptophan
The results of nitrogen determination and recovery for tryptophan analysis (assay > 99 %) are presented in Table 3. The nominal value of tryptophan is 13.72 % nitrogen. The recoveries are within the specification of 98 – 100 % [1] Table 3: Results of the recovery of nitrogen in tryptophan
Tryptophan mSample [g] VSample [mL] %NTry Recovery [%]
Sample 1 0.0991 9.933 13.54 98.7
Sample 2 0.1075 10.704 13.50 98.4
Sample 3 0.1058 10.720 13.74 100.1
Sample 4 0.0988 9.923 13.57 98.9
Average [%] – – 13.58 99.0
Rsd [%] – – 0.8 0.8
The mean blank volume (VBlank) was 0.449 mL (n = 4).
5.2 Protein determination in dairy products The results of the determination of nitrogen and protein contents in different milk powders are presented in Tables 4 – 7. Table 4: Results of the determination of nitrogen and protein in skimmed milk powder(protein content 32 g/100 g)
Skimmed milk powder I
mSample [g] VSample [mL] %N %P
Sample 1 0.2470 9.242 4.986 31.8
Sample 2 0.2496 9.372 5.007 32.0
Sample 3 0.2523 9.398 4.982 31.7
Average [%] – – 4.987 31.8
Rsd [%] – – 0.4 0.4
The mean blank volume (VBlank) was 0.449 mL (n = 4). Table 5: Results of the determination of nitrogen and protein in skimmed milk powder (protein content 32 g/100 g)
Skimmed milk powder II
mSample [g] VSample [mL] %N %P
Sample 1 0.2519 8.180 5.067 32.3
Sample 2 0.2730 7.799 5.045 32.2
Sample 3 0.2456 7.706 5.057 32.3
Average [%] – – 5.056 32.3
Rsd [%] – – 0.2 0.2
The mean blank volume (VBlank) was 0.449 mL (n = 4).
Application Note 105/2013 March 2013 7/8
Table 6: Results of the determination of nitrogen and protein in whole milk powder (protein content 25.2 g/100 g [4])
Whole milk powder I
mSample [g] VSample [mL] %N %P
Sample 1 0.2483 7.602 4.035 25.7
Sample 2 0.2687 8.143 4.011 25.6
Sample 3 0.2576 7.823 4.010 25.6
Average [%] – – 4.019 25.6
Rsd [%] – – 0.4 0.4
The mean blank volume (VBlank) was 0.304 mL (n = 4). Table 7: Results of the determination of nitrogen and protein in whole milk powder (declared protein content 26.44 g/100 g)
Whole milk powder II
mSample [g] VSample [mL] %N %P
Sample 1 0.2625 8.180 4.125 26.32
Sample 2 0.2506 7.799 4.108 26.21
Sample 3 0.2456 7.706 4.139 26.41
Average [%] – – 4.124 26.31
Rsd [%] – – 0.4 0.4
The mean blank volume (VBlank) was 0.449 mL (n = 4). The whole milk powder was purchased at LVU, Herbolzheim. It has a declared protein content of 26.44 %, the protein content was verified by an interlaboratory test. Comparing the declared protein content with the one determined by this Kjeldahl method showed comparable results.
Comparison to ISO 8968-36
This application note is based on the standard method ISO 8968-3 with minor differences. These differences are shown in Table 8. Table 8: Differences to AOAC ISO 8968-3
Application note ISO 8968-3 Notes/Impact
Catalyst 1 × 3.7 g Tablets Composition 94.4 % K2SO4 2.8 % TiO2 2.8 % CuSO4*5H2O
2 × 3.7 g Tablets Composition 94.4 % K2SO4 2.8 % TiO2 2.8 % CuSO4*5H2O
Just one tablet to lower the risk of crystallisation
Hydrogen peroxide
8 mL (Conc. 30 %)
5 mL (Conc. 30 %)
Higher volume of hydrogen peroxide instead of two Titanium Tablets to speed up the digestion.
Boric acid 60 mL 50 mL In the conducted experiments a higher amount of boric acid was used then de-scribed in standard methods as the tip of the condenser outlet and the elec-trode hve to be immersed in the solution.
Application Note 105/2013 March 2013 8/8
Conclusion7
The determination of nitrogen and protein in milk powder using the KjelDigester K-449 and
Kjeldahl sampler system K-375/K-376 provides reliable and reproducible results. These results
correspond well to the labelled values of the different milk powders and with the results of the
standard Kjeldahl method Application Note 104/2013 with low relative standard deviations (rsd),
but the digestion time is reduced to 60 min. The recovery with tryptophan was 99.0 %
(rsd = 0.8 %), which was within the specification of 98 - 100 % [1].
In combination with the accelerated digestion method using the KjelDigester K-449, Kjeldahl
Tablet Titanium and hydrogen peroxide, the time needed for sample analysis is significantly
reduced and therefore throughput increased.
With the KjelDigester K-449 the digestion process (including preheating, digestion and cooling) is
very fast and is fully automated. Together with the fully-automatic KjelMaster system
K-375/K-376, the time to result is significantly reduced and it offers fully walk-away convenience.
References8
[1] ISO 8968-3 Milk-Determination of nitrogen content Part 3: Block-digestion method [2] Kessler, H.-G.: Lebensmittel- und Bioverfahrenstechnik, Molkereitechnologie, Verlag A. Kessler, Freising, 4. Auflage 1996 KjelCalc App Application Note 104/2013, Nitrogen and Proten Determination in Milk Powder according to Kjeldahl Method Operation Manual of KjelDigester K-446/K-449 Operation Manual of Scrubber K-415 Operation Manual of KjelMaster system K-375/K376
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Dairy products - I
Product
Yogurt and fresh
cheese
N555-505
Hard, semi-hard and
soft cheese
N555-507
Processed cheese
N555-506
Moisture [%] 14.6 – 93.5* 26.6 – 68.4*
Moisture in fat-free cheese [%] 39.7 – 90.0**
Dry matter [%] 44.1 – 61.0*
Fat [%] 0.02 – 48.60* 6.2 – 37.5*
Fat in dry matter [%] 13.6 – 72.7** 41.2 – 48.9*
Protein [%] 0.4 – 19.1* 4.5 – 51.1*
Lactose [%] 0.03 – 4.30**
Total sugar [%] 0.6 – 54.9* 0.3 – 7.4**
Ash [%] 0.2 – 2.0* 0.8 – 18.1**
pH 4.1 – 6.3* 4.1 – 6.4** 5.6 – 6.0*
Salt [%] 0.04 – 1.99** 0.1 – 4.0* 1.6 – 2.3*
Sample compatibility Yoghurt, quark, sour
cream, fresh cheeses,
mascarpone measured
with sample cup in
diffuse reflectance
mode
Cheeses like Emmental,
Gruyeres, St. Paulin,
Raclette, Appenzell,
Tilsiter and cream
cheese measured with
sample cup in diffuse
reflectance mode
Processed cheese
measured with sample
cup in diffuse reflectance
mode
* BUCHI Pre-calibrations; ** Pre-calibrations in developmentIf not indicated otherwise, the pre-calibrations listed are compatible with both the NIRFlex® Solids and the NIRMasterTM spectrometers
Milk
Pre-calibrated application
Milk transflectance
N555-509 Milk flow cell**
Dry matter [%] 7.8 – 15.5* 11.5 – 13.2
Fat [%] 0.05 – 9.80* 1.99 – 6.85
Protein [%] 1.1 – 6.5* 2.9 – 3.6
Lactose [%] 0.08 – 5.50** 4.22 – 4.98
Saturated fatty acids [%] 0.03 – 4.68**
Mono+ Poly unsaturated
fatty acids [%]0.01 – 2.34**
Casein 2.3 – 3.1
Sample compatibility Homogenized milk measured with
sample cup in transflectance mode
Raw milk measured after
homogenization with flow cell in
transflectance mode
Dairy products - II
Product Butter**
Milk creams
transflectance** Milk powders**
Moisture [%] 11.3 – 23.8 1.4 – 7.9
Dry matter [%] 17.5 – 50.9
Fat [%] 74.5 – 88.4 7.5 – 46.0 0.1 – 33.0
Protein [%] 2.0 – 4.5 1.7 – 91.2
Lactose [%] 36.5 – 55.0
Ash [%] 2.0 – 8.5
pH 3.7 – 7.1
Salt [%] 0.03 – 2.60
Sample compatibility Butter measured with
sample cup in diffuse
reflectance mode
Milk-based creams
measured with sample
cup in transflectance
mode
Full and skimmed milk
powder, whey powder
measured with sample
cup in diffuse reflectance
mode
*BUCHI Pre-calibrations; **Pre-calibrations in developmentIf not indicated otherwise, the pre-calibrations listed are compatible with both the NIRFlex® Solids and the NIRMasterTM spectrometers
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