8/11/2019 SN 021_2010 Nitrogen Determination in Cheese

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SHORT NOTE

Nitrogen and Protein Determination in Cheese according to theKjeldahl Method

The determination of protein in food is a routine procedure for quality assurance and labelling. A simple and fast procedure forprotein determination in cheese, as described in the DIN EN ISO 8968:2001 appendix A regulations, is introduced below. Thesample is digested with sulfuric acid using the SpeedDigester K-436 or K-439, followed by distillation and titration with theKjeldahl Sampler System K-370/K-371. The determined protein contents correspond to the labelled values.

Introduction

Protein determination is one of the key analyses performedin the food industry. The samples require digestion withsulfuric acid to convert nitrogen into ammonium sulfate.After conversion to ammonia through the alkalinization with

sodium hydroxide, the sample is distilled into a boric acidreceiver by steam distillation, followed by a titration withsulphuric acid solution. The nitrogen content is multipliedby a sample-specific factor (6.38 for cheese) to obtain theprotein content.

Experimental

Instrumentation:SpeedDigester K-436, K-439,Kjeldahl Sampler System K-370/K-371

Samples: Cream cheese and Emmentaler cheese,grinded, labelled protein contents 6 % and 33 %,respectively.

Figure 1: Cheese samples

Determination:Approx. 0.5 - 2 g of the samples (dependson concentration of protein and organic matrix) were addeddirectly into a sample tube. A portion of 20 ml of sulfuricacid and 2 Kjeldahl tablets were added, and the digestionwas performed using the dairy products method (K-439),or the parameters specified in Table 1. After digestion, theammonia of the sample was distilled into a boric acidsolution by steam distillation and titrated with sulfuric acid(Table 2).

The method was verified by using 0.18 g tryptophan as thereference substance.

Table 1: Temperature profile for digestion with the K-436, K-439

K-439 K-436

Step Temp. [C] Time [min] Level Time [min]

Preheat 480 - 8.5 10

1 480 10 8.5 10

2 550 10 9.5 15

3 490 65 8.5 75

Cooling - 30 - 30

Table 2: Parameters for distillation and titration with the Kjeldahl SamplerSystem K-370/K-371

Distillation Titration

Water 80 ml Boric acid 4 % 50 ml

SodiumHydroxide

90 ml Titration solution H2SO40.1 mol/l

Reaction Time 5 s Min. TitrationTime

5 s

DistillationTime

300 s Max. TitrationVolume

40 ml

Steam Power 100 % Titration Method Standard

Sample Tube 500 ml Typ Endpoint

Stirrer Sp. Dist. 5 pH 4.65

Stirrer Sp. Titr. 7 Algorithm 1

Results

The tryptophan recoveries were 99.5 %, rsd 0.36 %

(K-439) and 99.4 %, rsd 0.41 % (K-436).

The determined protein contents are presented in Table 3.

Table 3: Determined protein contents in cheese (relative standard deviationin brackets, n=4)

Protein contentK-439 [%]

Protein contentK-436 [%]

Emmentaler cheese 33.31 (0.14 %) 33.34 (0.34 %)

Cream cheese 4.85 (0.20 %) 4.87 (0.19 %)

Conclusion

The determination of protein contents in cheese accordingto Kjeldahl using SpeedDigester K-436, K-439, andKjeldahl Sampler System K-370/K-371 provides reliableand reproducible results that correspond to the labelledvalues with low relative standard deviations. The totaldigestion time is approx. 85 min (K-439) or 100 min(K-436).

References

DIN EN ISO 8968:2001 appendix ALFBG 64 L01.00-10/1 appendix A

Operation manual SpeedDigester K-425 / K-436Operation manual SpeedDigester K-439Operation manual Kjeldahl Sampler System K-370/K-371

For more detailed information please refer to ApplicationNote 021/2010

021/2010SpeedDigester K-436 / K-439Kjeldahl Sampler System K-370/K-371