Nitrogen and Protein Determination in Meat Products by Accelerated Digestion with Hydrogen Peroxide and Sulfuric Acid

Transcription

1 071/2011 SpeedDigester K-436, K-439 Kjeldahl Sampler System K-370/K-371 Nitrogen and Protein Determination in Meat Products by Accelerated Digestion with Hydrogen Peroxide and Sulfuric Acid

2 071/2011 SpeedDigester K-436 / K-439 Kjeldahl Sampler System K-370/K-371 SHORT NOTE Nitrogen and Protein Determination in Meat Products by Accelerated Digestion with Hydrogen Peroxide and Sulfuric Acid A simple and very fast procedure for protein determination in meat products is introduced below. The sample is digested with hydrogen peroxide and sulfuric acid using the SpeedDigester K-436 or K-439, followed by distillation and titration with the Kjeldahl Sampler System K-370/K-371. The determination was tested by using reference material with declared protein content verified by a interlaboratory test. Introduction The digestion with hydrogen peroxide 30% instead of Kjeldahl tablets for nitrogen and protein determination [1] is a very fast (35 min instead of 90 min (K-439, K-436)) and environmentally friendly (free of any heavy metal) alternative to the classical Kjeldahl method. Experimental Instrumentation: SpeedDigester K-436, K-439, with H 2O 2 suction module, Kjeldahl Sampler System K-370/K-371 Samples: Boiled sausage and raw sausage (Salami) labeled protein contents g/100 g (boiled sausage) and 25 g/100 g (Salami). The boiled sausage was reference material from the DRRR (German Reference Office for Food Profiecy Testing and Reference Materials). The protein content was verified by a interlaboratory test. Determination: Approx. 1 g of the mixed boiled sausage and 0.75 g of the mixed salami, were weighed directly into a sample tube. A portion of 15 ml of sulfuric acid was added, and the digestion was started using the parameters specified in Table 1. Initially, 10 ml of hydrogen peroxide 30 % was added to the capillary funnel immediately after the start of the digestion. Then, another 10 ml of hydrogen peroxide 30% was added to the capillary funnel 6 minutes after the start of the digestion. The method was verified by using 0.15 g glycine as a reference substance. Table 1: Temperature profile for digestion with the K-436, K-439 Step Temp [ C] K-439 K-436 Time [min] Heating Level Preheat Cooling Time [min] After the digestion, the ammonia of the sample was distilled into a boric acid solution by steam distillation and then titrated with sulfuric acid using the parameters specified in Table 2. Table 2: Parameters for distillation and titration using the Kjeldahl Sampler System K-370/K-371 Distillation Titration Water 50 ml Boric acid 4% 50 ml Sodium hydroxide 60 ml Titration solution H 2SO mol/l Reaction time 5 s Min. titration time 5 s Distillation time 240 s Max. titration vol. 40 ml Steam power 100 % Titration method Standard Sample tube 500 ml Type Endpoint Stirrer sp. dist. 5 ph 4.65 Stirrer sp. titr. 7 Algorithm 1 Results The glycine recoveries (n = 4) were 99.8 %, rsd 0.2 % (K-439) and 99.8 %, rsd 0.3 % (K-436). The determined protein contents are presented in Table 3. Table 3: Determined protein contents in meat product samples (relative standard deviation in brackets, n = 4) Protein content K-439 [g/100g] Protein content K-436 [g/100g] Boiled sausage 13.6 (0.9 %) 13.6 (1.2 %) Raw sausage 25.9 (0.4 %) 25.8 (0.4 %) Conclusion The determination of the protein contents in meat products according to the hydrogen peroxide method using SpeedDigester K-436, K-439 and Kjeldahl Sampler System K-370/K-371 is very fast method and leads to reliable and reproducible results. References [1] Rossi et al., Comparison between the Kjeldahl and the Hach methods, J. Argentine Chemical Society, Vol. 92, No 4/6, (2004) Operation manual SpeedDigester K-425 / K-436 Operation manual SpeedDigester K-439 Operation manual Kjeldahl Sampler System K-370/K-371 For more detailed information please refer to Application Note 071/ Quality in your hands

3 1 Introduction An easy, fast, and reliable method for the determination of nitrogen and protein in meat product samples by digestion with hydrogen peroxide and sulfuric acid is introduced below. In literature both methods were compared and revealed comparable results [1]. The main differences to the standard methods [2], [3] [4] are as follows: - Considerably shorter digestion time (35 min instead of 90 min) [5] - More environmentally friendly (H 2 O 2 is used instead of heavy metal catalysts) The samples are digested using the SpeedDigester K-436 and K-439. The distillation and boric acid titration are performed with the Kjeldahl Sampler System K-370/K Equipment Mixer, Retsch Grindomix GM200 SpeedDigester K-436, K-439 (the parameters used for K-436 are also valid for SpeedDigester K-425) Suction module for H 2 O 2 digestion, BUCHI ( ) Stand with drip tray, BUCHI ( ) Scrubber B-414 with condenser Kjeldahl Sampler System K-370/K-371 (or any other BUCHI Kjeldahl Distillation Unit) Hirschmann bottle top dispenser ceramus 5-30 ml, with ceramus discharge tube, spiral-shaped Analytical balance (accuracy ± 0.1 mg) 3 Chemicals and Materials Sulfuric acid conc 98 %, Fluka (84727) Hydrogen peroxide 30 %, Fluka (95302); always store the hydrogen peroxide in a refrigerator with a temperature range of 4 8 C to prevent degradation Sodium hydroxide 32 %, Brenntag ( ) Boric acid 4 %, 400 g boric acid, Brenntag ( ), diluted to 10 l with deionized water, ph adjusted to 4.65 Sulfuric acid 0.1 mol/l, Fluka (35357), standard solution Neutralization solution without indicator (H 2 O 2 vapor will destroy the indicator) for the Scrubber: 600 g sodium carbonate, calcined, technical, Synopharm ( ), diluted to 3 l with distilled water. Glycine, Merck ( ; assay: > 99.7 %) 4 Samples Boiled sausage (DRRR reference material, labeled protein content %) Raw sausage (salami, labeled protein content: 25 %) The raw sausage was purchased at a local supermarket, the reference material boiled sausage was purchased at DRRR, Kempten. Application Note 071/2011 Version 1, Copyright 2011 Büchi Labortechnik AG 3/10

4 5 Procedure The determination of nitrogen and protein in meat products includes the following steps: Homogenization of the sample with the mixer Digestion of the sample using SpeedDigester K-436, K-439 Distillation and titration of the sample using Kjeldahl Sampler System K-370/K Homogenization of the sample Cut the sample material with a knife Deep freeze the sample material Mix the samples with the mixer, choose short intervals and repeat the mixing until the sample is homogenized 5.2 Digestion method - glycine (verification of the method) Place approx g glycine in a 300 ml sample tube Add a portion of 15 ml of sulfuric acid (98 %) Prepare additional blanks, chemicals without sample Carefully suspend the sample by gently swirling the tube Connect the Scrubber B-414 to the SpeedDigester K-436 or K-439 for absorbing the acid fumes created during digestion Insert the rack containing the samples into the preheated unit Digest the samples according to the parameters listed in Table 1 The capillary funnel must be mounted correctly (see Figure 1); ensure that the capillary funnel is free of fat or dust, as fat/dust can block the capillary Add 10 ml of hydrogen peroxide 30 % to the capillary funnel immediately after the start of the digestion Then, add another 10 ml of hydrogen peroxide 30 % to the capillary funnel 6 minutes after the start of the digestion Figure 1: Correct mounting of the capillary funnel: the hydrogen peroxide drops have to travel down the glass wall. If the drops drip directly into the digestion solution, boiling retardation may occur. Application Note 071/2011 Version 1, Copyright 2011 Büchi Labortechnik AG 4/10

5 5.3 Digestion method - samples Place approx. 1 g (boiled sausage) or 0.75 g (raw sausage) of the sample in a 300 ml sample tube (depending on the fat content of the sample material [6]) Add a portion of 15 ml of sulfuric acid (98 %) Prepare additional blanks, chemicals without sample Carefully suspend the sample by gently swirling the tube Connect the Scrubber B-414 to the SpeedDigester K-436 or K-439 for absorbing the acid fumes created during digestion Insert the rack containing the samples into the preheated unit Digest the samples according to the parameters listed in Table 1 The capillary funnel must be mounted correctly (see Figure 1); ensure that the capillary funnel is free of fat or dust, as fat/dust can block the capillary Add 10 ml of hydrogen peroxide 30 % to the capillary funnel immediately after the start of the digestion Then, add another 10 ml of hydrogen peroxide 30 % to the capillary funnel 6 minutes after the start of the digestion Table 1: Temperature profile for digestion with the K-439, K-436 K-439 K-436 Step Temperature [ C] Time [min] Heating level Time [min] Preheating Cooling If the liquid inside the sample tube is not clear and colorless after the digestion, digest for an additional 10 min as described in step 3 Let the samples cool down to ambient temperature NOTE: When the samples are placed in the cooling position it takes approx. 30 min to cool them down: when they are left in the heating chamber it takes at least 60 min. NOTE: If the preheat temperature or the temperature at step 1 is too high, H 2 O 2 evaporates too fast => insufficient digestion. If not all positions are used, the suction circuit has to be sealed by placing rubber plugs (Büchi No ) on top of the suction module, and glass caps (Büchi No ) have to be used instead of sample tubes. Install insulation caps (Büchi No ) in unused positions of the insulation plate to ensure even heat distribution inside the heating chamber. Unused positions should always be located at the rear end of the rack. For easy and safe handling and storage of the suction module and its funnels, the use of the stand with drip tray (Büchi No ) is recommended. Application Note 071/2011 Version 1, Copyright 2011 Büchi Labortechnik AG 5/10

6 5.4 Distillation and titration Distill the samples according to the parameters listed in Table 2. Table 2: Distillation and titration with the Kjeldahl Sampler System K-370/K-371 Distillation Titration Water 50 ml Boric acid 4% 60 ml Sodium hydroxide 60 ml Titration solution H 2 SO mol/l Reaction time 5 s Min. titration time 5 s Distillation time 240 s Max. titration volume 40 ml Steam power 100% Titration method Standard Sample tube 500 ml Type Endpoint Stirrer speed dist. 5 ph 4.65 Stirrer speed titr. 7 Algorithm 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). w N (V = Sample - V m Blank Sample ) z c f M 1000 N (1) %N = w N 100 % (2) %P = w N PF 100 % (3) %NGly %N 100 P = (4) w N V Sample V Blank : weight fraction of nitrogen : amount of titrant for the sample [ml] : mean amount of titrant for the blank [ml] z : molar valence factor (1 for HCl, 2 for H 2 SO 4 ) c : titrant concentration [mol/l] f : titrant factor (for commercial solutions normally 1.000) M N m Sample : molecular weight of nitrogen ( g/mol) : sample weight [g] 1000 : conversion factor [ml/l] %N : percentage of weight of nitrogen %N Gly : percentage of weight of nitrogen corrected for the purity of reference substance glycine (%) Application Note 071/2011 Version 1, Copyright 2011 Büchi Labortechnik AG 6/10

7 %P : percentage of weight of protein P : purity of the reference substance glycine (%) PF : sample-specific protein factor (6.25 for meat products) 6 Results 6.1 Digestion with SpeedDigester K Recovery of glycine The results of the nitrogen determination and recovery in glycine (assay 99.7 %) are presented in Table 3. The nominal value of glycine is 18.66% nitrogen. The recoveries are within the specification of > 98% [7]. Table 3: Results for the recovery of nitrogen in glycine with K-439 Glycine m Sample [g] V Sample [ml] %N Gly Recovery [%] Sample Sample Sample Sample Average Rsd [%] The mean blank volume for this sample was ml (n = 3) Protein determination in meat products The results of the determination of nitrogen and protein in meat product samples are presented in Tables 4 and 5. Table 4: Results for the determination of nitrogen in boiled sausage with K-439 Boiled sausage m Sample [g] V Sample [ml] %N %P Sample Sample Sample Sample Sample Average Rsd [%] The mean blank volume for this sample was ml (n = 3). The sample material was purchased at the DRRR, Kempten. It has a declared protein content of 13.62%. The Protein content was verified by a ring test. Comparing the declared protein content with the one digested by the hydrogen peroxide method (13.62 %) showed comparable results. Application Note 071/2011 Version 1, Copyright 2011 Büchi Labortechnik AG 7/10

8 Table 5: Results for the determination of nitrogen in raw sausage with K-439 Salami m Sample [g] V Sample [ml] %N %P Sample Sample Sample Sample Sample Average Rsd [%] The mean blank volume for this sample was ml (n = 2). The declared protein content for salami (raw sausage) was 25 %. The determined protein content with K-439 was %. The determination showed a good relative standard deviation of 0.37 %. 6.2 Digestion with SpeedDigester K Recovery of glycine The results of the nitrogen determination and recovery in glycine (assay 99.7 %) are presented in Table 6. The nominal value of glycine is % nitrogen. The recoveries are within the specification of > 98 % [7]. Table 6: Results for the recovery of nitrogen in glycine with K-436 Glycine m Sample [g] V Sample [ml] %N Recovery [%] Sample Sample Sample Sample Average Rsd [%] The mean blank volume for this sample was ml (n = 3). Application Note 071/2011 Version 1, Copyright 2011 Büchi Labortechnik AG 8/10

9 6.2.2 Protein determination in meat products The results of the determination of nitrogen and protein in meat products, boiled and raw sausage samples are presented in Tables 7 to 8. Table 7: Results for the determination of nitrogen in boiled sausage with K-436 Boiled Sausage m Sample [g] V Sample [ml] %N %P Sample Sample Sample Sample Sample Average Rsd [%] The mean blank volume for this sample was ml (n = 2). The sample material was purchased at the DRRR, Kempten. It has a declared protein content of 13.62%. The Protein content was verified by a interlaboratory test. Comparing the declared protein content with the one determined by the hydrogen peroxide method (13.58 %) showed comparable results. Table 8: Results for the determination of nitrogen in raw sausage with K-436 Salami m Sample [g] V Sample [ml] %N %P Sample Sample Sample Sample Sample Average Rsd [%] The mean blank volume for this sample was ml (n = 3). The declared protein content for salami (raw sausage) was 25 %. The determined protein content with K-436 was %. The determination showed a good relative standard deviation of 0.44 %. Application Note 071/2011 Version 1, Copyright 2011 Büchi Labortechnik AG 9/10

10 7 Conclusion The determination of nitrogen and protein in meat products according to the hydrogen peroxide method using the SpeedDigester K-436, K-439 and Kjeldahl Sampler System K-370/K-371 provides reliable and reproducible results. The digestion was tested with the reference material boiled sausage from DRRR, Kempten. The declared protein content of % and the determined protein content using the hydrogen peroxide method with K-439 (13.62 %) and K-436 (13.58 %) were comparable. Also the results for raw sausage (salami) were similar, for the K-439, % (rsd = 0.37 %) and for the K-436, % (rsd = 0.44 %). The digestion time is very fast: 35 min for both the K-439 and the K-436. The recoveries with glycine were 99.8%, rsd 0.18 % (K-439), and 99.8 %, rsd 0.27 % (K-436), respectively and are within the specification of > 98 % [7]. 8 References [1] Rossi et al., Comparison between the Kjeldahl and the Hach methods, J. Argentine Chemical Society, Vol. 92, No 4/6, (2004) [2] LFGB 64 L [3] AOAC [4] ISO (E) [5] AN 023/2010 Nitrogen and Protein Determination in Salami and Sausage according to the Kjeldahl Method [6] SLMB Kapitel 11 Fleisch- und Fleischerzeugnisse, (Stand 1999) [7] AN _370-03C Operational Quality Check Procedure Operation manual of SpeedDigester K-425 / K-436 Operation manual of SpeedDigester K-439 Operation manual of Scrubber B-414 Operation manual of Kjeldahl Sampler System K-370/K-371 BÜCHI Labortechnik AG CH-9230 Flawil 1/Switzerland T F Quality in your hands Application Note 071/2011 Version 1, Copyright 2011 Büchi Labortechnik AG 10/10