kjelmaster k-375 with kjelsampler k-376 comparison of ... · three different types of kjeldahl...

077/2012

SpeedDigester K-436, K-439

KjelMaster K-375 with KjelSampler K-376

Comparison of different Kjeldahl Tablets for the Determination of Nitrogen and Protein in Meat Products according to the Kjeldahl Method

SHORT NOTE Comparison of different Kjeldahl Tablets for the Determination of

Nitrogen and Protein in Meat Products according to the Kjeldahl

Method

The following introduces a simple and fast procedure for determining protein contents in meat products according to the Kjeldahl method, as described in the ISO 937-1978 (E), LFBG §64 L06.00-7, and AOAC 928.08 regulations. This Short Note compares the behaviour of different types of Kjeldahl Tablets and summarizes their individual advantages. All the samples described in this Short Note were digested together with sulfuric acid and different types of Kjeldahl Tablets with the SpeedDigester K-439, and then distilled and titrated with the Kjeldahl sampler system K-375/K-376.

Introduction

BUCHI offers different types of catalyst tablets for performing the standard Kjeldahl method. The Kjeldahl Tablet “Titanium” contains copper sulfate, titanium oxide and potassium sulfate. The tablet “Missouri” contains copper sulfate and potassium sulfate and the tablet “ECO” consists of a very low content of copper sulfate and potassium sulfate [1]. For most of the experiments described in this Short Note, salami was selected as a very fatty sample material. The main focus of the experiments was to determine the individual behaviour of the different catalysts and to find out more about their influence on the total digestion time and the results. To see all results please refer to the Application Note 77/2012.

Experimental

Instrumentation: SpeedDigester K-439, KjelMaster K-375 with KjelSampler K-376

Samples: Salami with a labelled protein content of 25 % and a fat content of 32 %.

Determination: Approx. 1.5 g of the homogenized salami were placed inside a sample tube. For every experiment with the different types of Kjeldahl Tablets, 2 tablets and the corresponding volume of sulfuric acid were added. The digestion was performed according to the “meat products” method (K-439) with varying total digestion times of 60, 90 and 120 min. After the digestion, the ammonia of the sample was distilled into a boric acid solution by steam distillation and titrated together with sulfuric acid with the Kjeldahl sampler system K-375/K-376 (Table 1).

Table 1: Parameters for distillation and titration with the Kjeldahl sampler

system K-375/K-376

Method Parameters

H2O volume 60 ml Steam output 100%

NaOH volume 90 ml Receiving solution

50 ml H3BO3 4%

Reaction time 5 s Titration solution

H2SO4 0.25 mol/l

Dist. mode Fixed time

Endpoint pH 4.65

Dist. time 180 s Stirrer sp. titr. 7

Stirrer sp. dist. 5 Titr. algorithm Optimal

Results

Figure 1 presents the protein contents of salami determined with different types of tablets and after varying digestion times.

25

26

27

28

29

30

Titanium ECO Missouri

Kjeldahl Tablet

Pro

tein

(%

)

60 min 90 min 120 min

Figure 1: Results of the protein determination in salami (n=3) with different catalysts after 60, 90 and 120 min digestion time (Error bars = RSD)

Table 2 presents the time required by each catalyst for accomplishing a complete digestion.

Table 2: Comparison of the total digestion times for all catalysts


Digestion time [min] 90 120 120

Protein Content [%] 28.6 28.6 28.5

RSD [%] 1.0 1.0 1.0

Conclusion

Depending on the type of Kjeldahl Tablet used, samples with a high fat content can be digested within 90 – 120 min. The fastest option is the tablet “Titanium” which digests the sample in only 90 min. With a total of 120°min. it takes the tablet “Missouri” longer to completely digest the sample. This catalyst is mentioned in some official methods [2]. The tablet “ECO” contains the lowest copper content of all described tablets. It also takes this catalyst 120 min. to fully digest the sample.

References

[1] 070/2011 Technical Note [2] AOAC 928.08, Alternative II Operation manual SpeedDigester K-439 Operation manual Kjeldahl sampler system K-375/K-376 For more detailed information please refer to Application Note 077/2012.

077/2012 SpeedDigester K-436 / K-439 KjelMaster K-375 with KjelSampler K-376

Application Note 077/2012 Version 1, Copyright © 2010 Büchi Labortechnik AG 3/16

1 Introduction

An easy and reliable method for determining nitrogen and protein contents in meat products according to the Kjeldahl method, as described in the ISO 937-1978 (E), LFBG §64 L06.00-7, and AOAC 928.08 regulations, is introduced in chapter 8. All described samples were digested with the SpeedDigester K-436 and K-439. The distillation and boric acid titration were performed with the Kjeldahl sampler system K-375/K-376. In a first step, the new BUCHI Kjeldahl Tablets for performing standard Kjeldahl were compared with each other in order to learn more about the advantages each type of tablet has for digesting fatty samples. For the experiments glycine was selected as a reference substance, salami (with a fat content of 32 %) and smoked turkey (with a fat content of 2 %) were chosen as sample products.

2 Equipment

− SpeedDigester K-436, K-439 (the parameters used for the K-436 are also valid for the SpeedDigester K-425)

− Scrubber K-415 TripleScrub ECO

− KjelMaster K-375 with KjelSampler K-376 − Mixer, Retsch Grindomix GM200 − Analytical balance (accuracy ± 0.1 mg)

3 Kjeldahl Tablets

Three different types of Kjeldahl Tablets are available for performing standard Kjeldahl. Due to the different weight of the tablets, different volumes of sulfuric acid had to be used during the experiments in order to obtain the optimal ratio of catalyst to sulfuric acid. The composition of each tablet, their weight and the used volume of sulfuric acid are summarized in Table 1.

Table 1: Composition of the Kjeldahl Tablets Titanium, ECO and Missouri and ratio for sulfuric acid

Name Weight [g]

Potassium sulfate [%]

Copper sulfate penta-hydrate [%]

Titanium (IV) oxide [%]

Sulfuric acid [mL]

Titanium 3.7 94.3 2.8 2.8 15

ECO 4.0 99.94 0.06 - 16

Missouri 5.0 99.6 0.4 - 20

4 Chemicals and Materials

− Sulfuric acid conc 98 %, Fluka (84727) − Titanium, BUCHI Kjeldahl Tablet (11057980) − Missouri, BUCHI Kjeldahl Tablet (11057982) − ECO, BUCHI Kjeldahl Tablet (11057983) − Sodium hydroxide 32 %, Brenntag (81980-452) − Boric acid 4 %, 200 g of boric acid, Brenntag (80948-155) diluted to 5 l with

deionized water, pH adjusted to 4.65 − Sulfuric acid 0.25 mol/l, Fluka (35355) standard solution − Neutralization solution for the Scrubber: 600 g of sodium carbonate, calcined,

technical, Synopharm (0179420) about 2 ml of ethanol and a spatula tip of bromthymol blue, Fluka (18460) diluted to 3 l with distilled water

− Glycine, Merck (1.04201.0100)


5 Samples

− Salami, with a declared protein content of 25 %, and a fat content of 32 % − Smoked turkey, with a declared protein content of 21 %, and a fat content of 2 % The samples were purchased at a local supermarket.

6 Procedure

The determination of nitrogen and protein contents in salami and smoked turkey includes the following steps:

− Homogenization of the samples − Digestion of the samples, with the SpeedDigester K-439 or K-436 − Distillation and titration of the samples, with the

Kjeldahl sampler system K-375/K-376

6.1 Homogenization of the samples − Chop up the sample material with a knife − Deep-freeze the sample material − Mix the samples in a mixer using short intervals and repeat the mixing process

until the sample is homogenized

6.2 Digestion method - glycine (verification of the method) − Place approx. 0.2 g of glycine into a 300 ml sample tube − Add two tablets of the to be tested catalyst and the corresponding volume of

sulfuric acid (98 %) described in Table 1 − Prepare additional blanks, chemicals without sample − Carefully suspend the sample by gently swirling the tube − Connect the Scrubber K-415 to the SpeedDigester K-436 or K-439 for absorbing

the acid fumes created during the digestion − Insert the rack containing the samples into the preheated unit − Digest the samples according to the parameters listed in Table 2 or 14,

depending on the catalyst type

6.3 Digestion method - samples − Place approx. 1.5 – 2 g of the sample (depending on the concentration of the

protein and organic matrix) into a 300 ml sample tube − Add two tablets of the to be tested catalyst and the corresponding volume of

sulfuric acid (98 %) described in Table 1 − Prepare additional blanks, chemicals without sample − Carefully suspend the sample by gently swirling the tube − Connect the Scrubber K-415 to the SpeedDigester K-436 or K-439 for absorbing

the acid fumes created during the digestion − Insert the rack containing the samples into the preheated unit − Digest the samples according to the parameters listed in Table 2 or 14,

depending on the catalyst type


Table 2: Temperature profile for the comparison of the Kjeldahl Tablets digested with the K-439

K-439

Step Temperature [°C]

Time [min]

Preheating 350 -

1 350 10

2 550 15

3 490 variable 1

Cooling - 30

1 In order to be able to determine the progression of the digestion, different digestion times were applied in the third digestion step (490°C), with varying time periods of 35 min, 65 min and 95 min. In total the digestion time, of step 1 to 3, was of 60 min, 90 min and 120 min.

6.4 Distillation and titration Distill the samples according to the parameters listed in Table 3

Table 3: Distillation and titration with the Kjeldahl sampler system K-375/K-376

Method parameters KjelMaster K-375

H2O volume 60 ml Titration solution H2SO4 0.25 mol/l

NaOH volume 90 ml2 / 60 ml3 Sensor type Potentiometric

Reaction time 5 s Titration mode Standard

Distillation mode Fixed time Measuring mode Endpoint pH

Distillation time 180 s Endpoint pH 4.65

Stirrer speed distillation 5 Stirrer speed titration 7

Steam output 100 % Titration start volume 0 ml

Titration type Boric acid Titration algorithm Optimal

Receiving solution vol. 50 ml 2 Volume of sodium hydroxide used for the comparison of the Kjeldahl Tablets 3 Volume of sodium hydroxide used for the Application Note, chapter 8


6.5 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 glycine is considered in equation (4).

1000 m

Mfc z V-(Vw

Sample

NBlankSampleN

⋅

⋅⋅⋅⋅

=

) (1)

%N = wN · 100 % (2)

%P = wN · PF · 100 % (3)

P100 %N

%NGly⋅

= (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

%P : percentage of weight of protein

PF : sample-specific protein factor (6.25 for meat products)

%NGly : percentage of weight of nitrogen corrected for the purity of reference substance glycine (%)

P : purity of the reference substance glycine (%)


7 Results of the comparison of the different types of Kjeldahl Tablets

7.1 Recovery of glycine The results of the nitrogen determination and the recovery in glycine (assay ≥ 99.7 %) after a total digestion time of 60, 90 and 120 min are presented in Table 4-6. The nominal value of glycine is 18.66 % of nitrogen.

Table 4: Results of the recovery of nitrogen in glycine with K-439 after 60 min

Glycine mSample [g]

VSample [ml]

%NGly Recovery [%]

Average [%]

RSD [%]

Titaniuma 0.2053 5.514 18.574 99.5

Titaniuma 0.2006 5.418 18.673 100.1

Titaniuma 0.2146 5.780 18.639 99.9

99.8 0.3

ECOb 0.2162 5.847 18.704 100.2

ECOb 0.2070 5.603 18.707 100.3

ECOb 0.2051 5.532 18.637 99.9

100.1 0.3

Missouric 0.2118 5.706 18.601 99.7

Missouric 0.2253 6.063 18.599 99.7

Missouric 0.2005 5.407 18.601 99.7

99.7 0.0

a Blank 0.086 ml (n=3) b Blank 0.090 ml (n=3) c Blank 0.098 ml (n=3) Table 5: Results of the recovery of nitrogen in glycine with K-439 after 90 min

Glycine mSample [g]

VSample [ml]

%NGly Recovery [%]

Average [%]

RSD [%]

Titaniuma 0.2123 5.744 18.688 100.2

Titaniuma 0.2140 5.777 18.648 99.9

Titaniuma 0.2440 6.603 18.733 100.4

100.2 0.2

ECOb 0.2169 5.847 18.607 99.7

ECOb 0.2163 5.845 18.652 100.0

ECOb 0.2127 5.745 18.638 99.9

99.9 0.1

Missouric 0.2183 5.879 18.606 99.7

Missouric 0.2167 5.829 18.581 99.6

Missouric 0.2190 5.880 18.549 99.4

99.6 0.2

a Blank 0.096 ml (n=3) b Blank 0.102 ml (n=3) c Blank 0.097 ml (n=3)


Table 6: Results of the recovery of nitrogen in glycine with K-439 after 120 min

Glycine mSample [g]

VSample [ml]

%NGly Recovery [%]

Average [%]

RSD [%]

ECOb 0.2156 5.802 18.567 99.6

ECOb 0.2019 5.455 18.618 99.8

ECOb 0.1980 5.379 18.577 99.6

99.9 0.6

Missouric 0.2281 6.133 18.580 99.6

Missouric 0.2139 5.769 18.618 99.8

Missouric 0.2132 5.738 18.577 99.6

99.6 0.1

b Blank 0.103 ml (n=3) c Blank 0.100 ml (n=3)

7.2 Protein determination in meat products Smoked turkey and salami were selected as sample materials in order to determine the influence of the fat content when using the different Kjeldahl Tablets. Smoked turkey has a fat content of only 2 %. The results of the determination of nitrogen and protein contents in smoked turkey after a total digestion time of 60, 90 and 120 min are presented in Tables 7-9.

Table 7: Results of the recovery of nitrogen and protein in smoked turkey with the K-439 after 60 min

Turkey mSample [g]

VSample [ml]

%N Protein [%]

Average [%]

RSD [%]

Titaniuma 1.9026 8.919 3.252 20.3

Titaniuma 1.8417 8.661 3.261 20.4

Titaniuma 1.9140 9.082 3.292 20.6

20.4 0.6

ECOb 1.7817 8.303 3.228 20.2

ECOb 1.9474 9.122 3.248 20.3

ECOb 1.8263 8.539 3.240 20.3

20.2 0.3

Missouric 1.9787 9.291 3.254 20.3

Missouric 1.8208 8.595 3.268 20.4

Missouric 1.8701 8.812 3.264 20.4

20.4 0.2

a Blank 0.086 ml (n=3) b Blank 0.090 ml (n=3) c Blank 0.098 ml (n=3)


Table 8: Results of the recovery of nitrogen and protein in smoked turkey with the K-439 after 90 min

Turkey mSample [g]

VSample [ml]

%N Protein [%]

Average [%]

RSD [%]

Titaniuma 1.9406 9.336 3.335 20.8

Titaniuma 1.9340 9.298 3.332 20.8

Titaniuma 1.7625 8.467 3.326 20.8

20.8 0.1

ECOb 1.8721 8.868 3.279 20.5

ECOb 1.8311 8.663 3.274 20.5

ECOb 1.8928 8.980 3.285 20.5

20.5 0.2

Missouric 1.9086 9.081 3.297 20.60

Missouric 1.8679 8.894 3.298 20.61

Missouric 1.8826 8.969 3.300 20.63

20.6 0.1

a Blank 0.096 ml (n=3) b Blank 0.102 ml (n=3) c Blank 0.097 ml (n=3) Table 9: Results of the recovery of nitrogen and protein in smoked turkey with the K-439 after 120 min

Turkey mSample [g]

VSample [ml]

%N Protein [%]

Average [%]

RSD [%]

ECOb 1.7302 8.35 3.338 20.9

ECOb 1.8896 9.163 3.358 21.0

ECOb 1.8453 8.946 3.356 21.0

20.9 0.3

Missouric 1.7688 8.563 3.351 20.9

Missouric 1.9104 9.229 3.347 20.9

Missouric 1.9353 9.329 3.340 20.8

20.9 0.2


Salami is a sample with a very high fat content of 32 %. The results of the determination of nitrogen and protein contents in salami after a total digestion time of 60, 90 and 120 min, are presented in Tables 10-12.


Table 10: Results of the recovery of nitrogen and protein in salami with the K-439 after 60 min

Salami mSample [g]

VSample [ml]

%N Protein [%]

Average [%]

RSD [%]

Titaniuma 1.4638 9.384 4.449 27.8

Titaniuma 1.5078 9.616 4.429 27.7

Titaniuma 1.4372 9.216 4.449 27.8

27.8 0.3

ECOb 1.4734 9.274 4.365 27.3

ECOb 1.5791 9.892 4.347 27.2

ECOb 1.4623 9.054 4.336 27.1

28.2 0.3

Missouric 1.5578 9.969 4.438 27.7

Missouric 1.4534 9.323 4.445 27.8

Missouric 1.5855 10.072 4.449 27.8

27.8 0.1

a Blank 0.086 ml (n=3) b Blank 0.090 ml (n=3) c Blank 0.098 ml (n=3) Table 11: Results of the recovery of nitrogen and protein in salami with the K-439 after 90 min

Salami mSample [g]

VSample [ml]

%N Protein [%]

Average [%]

RSD [%]

Titaniuma 1.4776 9.663 4.535 28.3

Titaniuma 1.5488 10.314 4.620 28.9

Titaniuma 1.4630 9.620 4.559 28.5

28.6 1.0

ECOb 1.5351 9.619 4.342 27.1

ECOb 1.5183 9.894 4.517 28.2

ECOb 1.5298 9.751 4.418 27.6

27.7 2.0

Missouric 1.4729 9.466 4.501 28.1

Missouric 1.4849 9.514 4.487 28.1

Missouric 1.5120 9.846 4.561 28.5

28.2 0.9

a Blank 0.096 ml (n=3) b Blank 0.102 ml (n=3) c Blank 0.097 ml (n=3) Table 12: Results of the recovery of nitrogen and protein in salami with the K-439 after 120 min

Salami mSample [g]

VSample [ml]

%N Protein [%]

Average [%]

RSD [%]

ECOb 1.5115 10.073 4.628 28.9

ECOb 1.5750 10.284 4.535 28.3

ECOb 1.4110 9.295 4.571 28.6

28.6 1.0

Missouric 1.5678 10.407 4.604 28.8

Missouric 1.5320 10.056 4.552 28.5

Missouric 1.4963 9.737 4.511 28.2

28.5 1.0



7.3 Conclusions drawn from the comparison of the different types of Kjeldahl Tablets

No major differences between the individual catalysts could be determined during the experiments in which glycine was used as a reference material. All recoveries after the varying total digestion times and after using the different types of Kjeldahl tablets were within the specification of 98-102 % [1].

A pronounced difference on the other hand can be seen when analysing the real sample materials smoked turkey and salami (Figure 1).

25

26

27

28

29

30


Kjeldahl Tablet

Pro

tein

(%

)


19

20

21

22


Kjeldahl Tablet

Pro

tein

(%

)


Figure 1: Up: Protein content (%) of smoked turkey after using different types of Kjeldahl Tablets and varying digestion times; Below: Protein content (%) of salami after using different types of Kjeldahl Tablets and varying digestion times. The error bars represent the RSD (n=3).

With the Kjeldahl Tablet “Titanium” a complete digestion can be accomplished within 90 min. This tablet is the only catalyst that contains titanium(IV)oxide, moreover it has the highest copper sulfate content of all described Kjeldahl Tablets. The tablet “Missouri” has the second highest copper sulfate content and it takes this catalyst a total time of 120 min. to accomplish complete digestion. This catalyst is described in some official methods for the determination of protein in meat [2]. The copper sulfate content of the catalyst “ECO” has been reduced to an absolute minimum, nevertheless it takes this tablet the same amount of time to complete the digestion as it takes the catalyst “Missouri”. The experiments prove that samples which have a high amount of organic material that needs to be digested, require a higher amount of catalytic active components such as copper sulfate or titanium(IV)oxide in order to speed up the digestion. The following Table 13 summarizes the most prominent advantage of each catalyst.

Table 13: Comparison of the Kjeldahl Tablets

Kjeldahl Tablet Total digestion time [min] Advantage

Titanium 90 Shortest total digestion time

ECO 120 Lowermost copper content

Missouri 120 Mentioned in AOAC 928.08 Alternative II


For the digestion with the K-439 and the K-436, the fastest Kjeldahl Tablet, “Titanium”, was used for the following application.

8 Results of the digestion conducted with the SpeedDigester K-439 and K-436

The following chapter describes the digestion of the meat samples smoked turkey and salami which were conducted with the SpeedDigester K-436 and K-439. The tablet “Titanium” was the only catalyst used for this experiment, the total digestion time was of 90 min.

The preparation of the samples and the calculation of the results are similar to the ones described in Chapter 7. Two tablets “Titanium” and 15 ml of sulfuric acid (98 %) were used (Table 1) for the experiment. The total time of the digestion conducted with the K-436 and K-439, was of 90 min. All parameters are specified in Table 14.

Table 14: Parameters for the digestion with the K-439 und K-436

K-439 K-436

Step Temperature [°C]

Time [min]

Heating Level Time [min]

Preheating 350 - 6.0 10

1 350 10 6.0 10

2 550 15 9.5 15

3 490 65 8.5 65

Cooling - 30 - 30

− If the liquid inside the sample tube is not clear and of a blue-green color after the digestion, digest for an additional 30 min as described in step 3

− Let the samples cool down to room temperature

NOTE: If the samples are placed in the cooling position it takes them approx. 30 min to cool down; if they are left in the heating chamber it takes at least 60 min.

8.1 Digestion with the Kjeldahl Tablet Titanium and the K-439

8.1.1 Recovery of glycine

The results of the nitrogen determination and the recovery in glycine (assay ≥ 99.7 %) are presented in Table 15. The nominal value of glycine is 18.66 % of nitrogen. All recoveries are within the specification of 98 - 102% [1].

Table 15: Results of the recovery of nitrogen in glycine with the K-439

Glycine mSample [g] VSample [ml] %NGly Recovery [%]

Sample 1 0.2123 5.744 18.69 100.2

Sample 2 0.2140 5.777 18.65 99.9

Sample 3 0.2440 6.603 18.73 100.4

Average - - 18.69 100.2

Rsd [%] - - 0.2 0.2

The mean blank volume for this sample was 0.096 ml (n = 3).


8.1.2 Protein determination in meat products

The results of the determination of nitrogen contents in smoked turkey and salami are presented in Tables 16 and 17.

Table 16: Results of the determination of nitrogen and protein in smoked turkey with the K-439 (declared protein content 21 %)

Turkey mSample [g] VSample [ml] %N %P

Sample 1 1.9406 9.336 3.335 20.8

Sample 2 1.9340 9.298 3.332 20.8

Sample 3 1.7625 8.467 3.326 20.8

Average - - 3.331 20.8

Rsd [%] - - 0.1 0.1


Table 17: Results of the determination of nitrogen and protein in salami with the K-439 (declared protein content 25 %)

Salami mSample [g] VSample [ml] %N %P

Sample 1 1.4776 9.663 4.535 28.3

Sample 2 1.5488 10.314 4.620 28.9

Sample 3 1.4630 9.620 4.559 28.5

Average - - 4.571 28.6

Rsd [%] - - 1.0 1.0


8.2 Digestion with the Kjeldahl Tablet Titanium and the K-436

8.2.1 Recovery of glycine

The results of the recovery of nitrogen in glycine (assay ≥ 99.7 %) are presented in Table 18. The nominal value of glycine is 18.66 % of nitrogen. All determined values are within the specification of 98 - 102% [1].

Table 18: Results of the recovery of nitrogen and protein in glycine with the K-436

Glycine mSample [g] VSample [ml] %NGly Recovery [%]

Sample 1 0.2287 6.159 18.61 99.7

Sample 2 0.2151 5.818 18.67 100.0

Sample 3 0.2415 6.512 18.65 99.9

Average - - 18.64 99.9

Rsd [%] - - 0.2 0.2



8.2.2 Protein determination in meat products

The results of the determination of nitrogen contents in smoked turkey and salami are presented in Tables 19 and 20.

Table 19: Results of the determination of nitrogen and protein in smoked turkey with the K-436 (declared protein content 21 %)

Turkey mSample [g] VSample [ml] %N %P

Sample 1 1.8835 9.018 3.315 20.7

Sample 2 1.9658 9.496 3.347 20.9

Sample 3 1.8142 8.718 3.326 20.8

Average - - 3.330 20.8

Rsd [%] - - 0.5 0.5


Table 20: Results of the determination of nitrogen and protein in salami with the K-436 (declared protein content 25 %)

Salami mSample [g] VSample [ml] %N %P

Sample 1 1.4534 9.576 4.565 28.5

Sample 2 1.4611 9.636 4.570 28.6

Sample 3 1.4901 9.768 4.543 28.4

Average - - 4.560 28.5

Rsd [%] - - 0.3 0.3



9 Comparison to Standard Methods

This application note is based on the standard method ISO 937-1978 (E). AOAC 928.08 and LFGB §64 L06.00-7 are comparable with minor differences. These differences are shown in Table 21.

Table 21: Differentiation to the standard methods

Application note Standard methods Notes/Impact

Catalyst 2 x 3.7 g Tablets cont. - 94.4 % K2SO4 - 2.8 % TiO2 - 2.8 % CuSO4*5H2O

ISO: 15 g K2SO4 + 0.5 g CuSO4 x 5 H2O

LFBG: 5–10 g Catalyst

AOAC: 15 g K2SO4 + 0.45 g CuSO4

Easy to handle, especially in routine analytics. The result is not influenced by the different types of catalyst.

Sulfuric acid 15 ml LFBG: 20 ml

AOAC: 40 ml

No impact. Same ratio of sulfuric acid / catalyst

Water 60 ml ISO: 50 ml

LFBG: 50 – 100 ml

AOAC: 200 ml

The K-375 generates steam in a separate vessel; it is therefore not necessary to add as much water to the digested sample as described in the standard methods.

Sodium hydroxide

60 ml (Conc.: 32 %)

AOAC: 120 ml (Conc.: 50 %)

No impact. Same ratio of sodium hydroxide / sulfuric acid. Sodium hydroxide with a concentration of 32% is gentler to the pump than alkali with a higher concentration.

Boric acid 50 ml LFBG: 25 ml In the conducted experiments a higher amount of boric acid was used then described in standard methods because the tip of the condenser and the electrode had to be immersed in the solution.

Blank with sucrose for determination of samples

no AOAC: 2 g of sucrose for blank in addition to the chemicals.

No differences could be observed between blanks with and blanks without sucrose.

Titration Boric acid titration AOAC: back titration No impact. Boric acid titration and back titration achieved the same results.


10 Conclusion

The determination of nitrogen and protein contents in smoked turkey and salami with the help of the different types of Kjeldahl Tablets provides reliable and reproducible results which correspond to the labelled values of the sample products with low relative standard deviations. The total digestion time can vary between 90 and 120 min depending on the type of Kjeldahl Tablets and digestion unit used and can therefore be adapted to meet the individual needs. The experiments conducted revealed no differences between the results obtained with the K–436 and the ones obtained with the K-439. Two important characteristics of the latest version of the fully-automatic Kjeldahl sampler system, KjelMaster K-375 and KjelSampler K-376, are the short processing time as well as the reduced need for user attendance. In combination with the unique SpeedDigester models, the time needed for sample analysis is significantly reduced and the throughput is therefore increased.

11 References

[1] Application Note 001-437_370-03C: Operational Quality Check Procedure [2] AOAC 928.08 ISO 937-1978 (E) LFGB §64 L06.00-7 Operation manual of SpeedDigester K-425 / K-436 Operation manual of SpeedDigester K-439 Operation manual of Scrubber K-415 Operation manual of Kjeldahl sampler system K-375/K-376

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kjelmaster k-375 with kjelsampler k-376 comparison of ... · three different types of kjeldahl...

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