Application Note No. 201/2015 Nitrogen & protein determination in starch and gluten KjelDigester K-449, KjelMaster K-375 with KjelSampler K-376 and

Transcription

1 Application Note No. 201/2015 Nitrogen & protein determination in starch and gluten KjelDigester K-449, KjelMaster K-375 with KjelSampler K-376 and DuMaster D-480

2 1 Introduction Gluten and starch are typical samples consisting of a high and a low protein content, respectively. Here, both sampels are analysed for their protein content according to Kjeldahl and Dumas. In general, the Kjeldahl method is applied to determine the organic nitrogen, whereas the organic and inorganic nitrogen content is measured using the Dumas method. Here we show the feasibility to analyse gluten and starch, both samples are expected to contain organic nitrogen only. The samples are digested using the KjelDigester K-449. The distillation and boric acid titration are performed with the KjelMaster System K-375 / K-376 and the measurements according to Dumas are performed with the DuMaster D Equipment For protein determination according to Kjeldahl: KjelDigester K-449 (the parameters used are also valid for the K-446) Scrubber K-415 TripleScrub ECO KjelMaster K-375 KjelSampler K-376 (the parameters used are also valid for the K-377) For protein determination according to Dumas: D-480 DuMaster Tin foil Manual pressing tool to form capsules ( ) Analytical balance (accuracy ± 0.1 mg) 3 Chemicals and materials Chemicals for protein determination according to Kjeldahl: Sulfuric acid conc 98 %, Merck ( ) Kjeldahl Tablet Titanium, BUCHI ( ) Sodium hydroxide 32 %, Brenntag ( ) Boric acid (H3BO3) 4 %, 400 g boric acid, Brenntag ( ) diluted to 10 L with deionized water, ph adjusted to 4.65 Boric acid (H3BO3) 2 %, 200 g boric acid, Brenntag ( ) diluted to 10 L with deionized water, ph adjusted to 4.65 Sulfuric acid 0.1 mol/l, Fluka (35357) Sulfuric acid 0.01 mol/l, VWR ( ) Neutralization solution for the Scrubber: 600 g sodium carbonate, calcined, technical, Synopharm ( ) about 2 ml ethanol and a spatula tip of bromthymol blue, Fluka (18460) diluted to 3 L with distilled water Tryptophan, Sigma Aldrich ( G) Application Note 201/2015 January /11

3 Chemicals for protein determination according to Dumas: L-Aspartic acid, reagent grade 98% (TLC), Sigma A G Starch, % nitrogen, used as standard Aspartic acid and starch are used to determine the daily factor For a safe handling please pay attention to all corresponding MSDS. Samples: Gluten powder: expected protein content: ~60 % Starch powder: expected protein content: <0.35 % The samples were provided from a customer for feed production. 4 Procedure for the Kjeldahl Method The digestion was done similar to the Application Note 110/2013 Nitrogen and protein determination in corn, flour and soy [1] which is based on ISO 20483:2006 [2]. To calculate optimal parameters the KjelOptimizer App was used [3]. The determination of nitrogen and protein in starch and gluten 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 Due to the broad range of the protein content in the samples different parameters for digestion, distillation and titration need to be choosen for each sample (Table 1). Table 1: Expected protein content, sample, tryptophan weight and acids concentrations Sample Exp. Protein [%] Weight [g] Tryptophan [g] H3BO3 [%] Starch < Gluten ~ Titrant H2SO4 [M] 4.1 Digestion method tryptophan (verification of the method) 1. Start the KjelDigester K-449 according to the parameters listed in Table 2 2. Place at least once 0.12 g and once 0.2 g tryptophan in a 300 ml sample tube 3. Add 2 Titanium Tablets and 15 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. Insert the rack with 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 2 7. Let the samples cool down when the digestion is completed 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 according to Table 1 3. Add two Titanium Tablets and 15 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. Insert the rack with 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 2 7. Let the samples cool down when the digestion is completed Application Note 201/2015 January /11

4 Table 2: Temperature profile for digestion with the K-449 Step Temperature [ C] Time [min] Cooling 35 NOTE: If the liquid inside the sample tube is not clear and blue-green, digest for additional 15 min at 420 C. 4.3 Distillation and titration Distill the samples according to the parameters listed in Table 3. Parameters were calculated with the KjelOptimizer App [3] which can be downloaded for free from itunes and GooglePlay. For gluten and starch the same parameters were used for distillation and titration, except the boric acid solution and titrant concentration as shown in Table 3. Table 3: Distillation and titration parametes applied with the KjelMaster System K-375 / K-376 H2O volume 80 ml Titration solution H2SO4 0.1 mol/l for gluten H2SO mol/l for starch NaOH volume 90 ml Sensor type Potentiometric Reaction time 5 s Titration mode Online Distillation mode Fixed time Titration start time 120 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 4 % for gluten 2 % for starch Titration start volume 0 ml Receiving solution vol. 60 ml Titration algorithm Optimal 4.4 Calculation The results are calculated as a percentage of nitrogen in the sample. 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. w N (V Sample - V m Blank Sample ) z c f MN (1) 1000 %N = wn 100 % (2) %PSample = wn PF 100 % (3) % Pr %P 100 Sample rec rate (4) %Preference wn VSample : weight fraction of nitrogen : amount of titrant for the sample [ml] Application Note 201/2015 January /11

5 VBlank z c : mean amount of titrant for the blank [ml] : molar valence factor (1 for HCl, 2 for H2SO4) : titrant concentration [mol/l] f : titrant factor (for commercial solutions normally 1.000) MN msample : molecular weight of nitrogen ( g/mol) : sample weight [g] 1000 : conversion factor [ml/l] %N : percentage of weight of nitrogen %Prrec rate : recovery rate [%] %Psample : percentage of weight of protein [%] PF : sample-specific protein factor (6.25 for starch and gluten) 5 Procedure for the Dumas Method The determination of nitrogen and protein in gluten and starch includes the following steps: Packaging of the samples Combustion and analysis by DuMaster D-480 The determination was done similar to the Application Note 159/2014 Nitrogen and protein determination in cereals and oil seeds [4] which is based on AOAC [5]. 5.1 Sample and system preparation 1. Prepare the tin foil or weighing paper, as shown in Figure 1. Figure 1: Preparation of the sample packaging 2. Place the recommended sample amount (Table 4) on the foil or paper and press it with the manual pressing tool (Figure 2) 3. Check the tightness of the packed sample visually. The tin foil or paper must be wraped completely around the whole sample 4. Start or wake-up the DuMaster D-480, check the maintance intervals of the instrument NOTE: Use the sleep-mode but do not switch off the instrument if the pause time is less than 5 days. NOTE: Paper creates less ash compared to tin foil. Therefore, the maintenance interval of the ash crucible can be increased. Application Note 201/2015 January /11

6 Table 4: Sample amount and method to be chosen Sample Weight [mg] Method Aspartic acid standard mgStandard Standard starch mgStarch Gluten sample mgStandard Starch sample mgStarch Figure 2: Manual pressing tool for tin foil packed samples 5.2 Determination 5. Check the parameters of the DuMaster D-480; all parameters (temperature, flow, pressure) must be green colored (Table 5) 6. Run two blanks (empty position), use the key word Blnk with the method Blank with O2 to check the purity of the gases and the performace of the instrument 7. Run one RunIn with the daily factor reference (typically mg aspartic acid) as conditioning using the method 250mgStandard 8. Run minimum two daily factor references each, mg aspartic acid and mg starch standard by using a keyword (e.g. aspartic acid and starch standard ) 9. Run the samples using the method 250mgStandard NOTE: Check the system if the deviation of one or several parameters is too high (red or blue colored). NOTE: The blank area should be <100. NOTE: The daily factor shoud be within for aspartic acid and for starch. Table 5: Parameters for the D-480 Parameter Accepted value Pressure Delta P < 400 mbar Flow Delta flow < 15 ml/min Application Note 201/2015 January /11

7 5.3 Method Use one of the default methods from the memory. For gluten, use the method 250mgStandard and for starch select the method 400mgStarch (Software Version 2.0.3). The method parameters are described in Table 6. Table 6: Parameters of the methods 250mgStandard and 400mgStarch Parameter Settings «250mgStandard» Settings «400mgStarch» O2 dosing time [s] O2 dosling flow [ml/min] Cut-off threshold [%] 15 0 Autozero delay [s] Peak anticipation [s] Results and Discussion 6.1 Results Kjeldahl The recovery rate of the tryptophan reference was % which is corresponding to % N. The used sample amount was g and the titrant comsumption ml. The measured nitrogen and protein contents in gluten and starch are shown in Tables 7, 8. Table 7: Measured nitrogen and protein contents in gluten Gluten msample [g] VTitrant [ml] %N %P Sample Sample Sample Sample Sample Average [%] Rsd [%] The mean blank volume (VBlank) was ml (n = 3). The measured protein content of % confirmed the expected content of ~60 % and the viability of the Kjeldahl method. Table 8: Measured nitrogen and protein contents in starch Starch msample [g] VTitrant [ml] %N %P Sample Sample Sample Sample Sample Average [%] Rsd [%] The mean blank volume (VBlank) was ml (n = 3). Application Note 201/2015 January /11

8 The measured protein content of 0.29 % corresponds well with the expected protein content of <0.35 %. 6.2 Results Dumas The daily factor is calculated by comparison of the actual and the theoretical nitrogen content of a high-purity standard. Due to sample determination of very low and high protein contents it`s necessary to determine two daily factors to cover the range. The results of the daily factor determinations are presented are listed in tables 9 and 10. Table 9: Results of the daily factor determination using L-aspartic acid Aspartic acid Weight [mg] %Ntheor Daily factor Sample Sample Sample Average Sd Rsd [%] Table 10: Results of the daily factor determination using reference starch Reference starch Weight [mg] %Ntheor Daily factor Sample Sample Sample Average Sd Rsd [%] The nitrogen content of the samples is multiplied with the corresponding daily factor to adjust the instrument to environmental conditions. NOTE: Repeat the daily factor determination after longer breaks (3-4 h) or 60 samples. NOTE: For aspartic acid the daily factor should be within For starch the daily factor is expected to be The results of the determination of nitrogen and protein content in gluten and starch are presented in Tables 11 and 12. To determine the protein content, the nitrogen content is multiplied with the sample-specific protein factor Table 11: Measured nitrogen and protein contents in gluten Gluten Weight [mg] % Nitrogen % Protein Sample Sample Sample Sample Sample Average [%] Sd Rsd [%] Application Note 201/2015 January /11

9 Table 12: Measured nitrogen and protein contents in starch Starch Weight [mg] % Nitrogen % Protein Sample Sample Sample Sample Sample Average [%] Sd RSD [%] Discussion For gluten a similar sample weight of 0.2 g was selected for Kjeldahl and Dumas determinations. The measured protein content as well the RSD correspond well with each other: Kjeldahl: % protein, 0.34 % RSD Dumas: % protein, 0.41 % RSD For starch the selected sample weight was about 0.38 g for Dumas and 2.0 g for Kjeldahl measurements. The results correspond well with each other. The RSD is low for Kjeldahl measurments due to the high sample weight. For Dumas the sample weight was lower, therefore the RSD is higher. Kjeldahl: 0.29% protein, 0.62 % RSD Dumas: 0.29 % protein, 7.01 % RSD The higher RSD for starch analyzed with Dumas results from the 10-times smaller sample weight used compared to Kjeldahl determination. To reduce the RSD obtained with the Dumas method, the sample weight can be increased. The maximum allowed sample weight is 1 g, depending on the sample matrix. Important is that the samples are packed tightly to avoid sample loss and that the sample fits into the autosampler. 7 Comparison to standard methods The Kjeldahl application is based on the standard method ISO 20483:2006 [2] and the Dumas application is based on AOAC [5] with minor differences.these differences are shown in Table 13 and 14. Table 13: Differences of the Kjeldahl application to ISO 20483:2006 [2] Application note ISO 20483:2006 Notes / Impact Digester Block digester with time and temperature control Block digester with adjustable temperature control Time / temperature control is more convienient and allows for full automation. Sample size g g No impact, consumption of the titration solution should be between 3-17 ml. Catalyst g Tablets Composition: 94.4 % K 2SO % TiO % CuSO 4* 5H 2O 10 g K 2SO g CuSO 4 * 5 H 2O 0.3 g TiO 2 The choice of catalyst does not influence the result. Digestion time is reduced using Titanium Tablets, see Application Note 078/2012. Sulfuric acid 15 ml 20 ml No impact, same ratio of sulfuric acid/catalyst. Digestion time Sodium hydroxide 125 min 2 h No impact 90 ml (Conc. 32 %) ml (Conc %) Water 80 ml 50 ml No impact No impact. Comparable ratio of sodium hydroxide/sulfuric acid. Application Note 201/2015 January /11

10 Boric acid solution Distillation time Titration solution Blanks with sucrose Titration 60 ml (Conc. 4 % or 2%) 50 ml (Conc. 4 %) Higher capacity to bind more NH seconds Not mentioned No impact. Distillation time must be sufficient to transfer all NH 3 into the receiving vessel. H 2SO 4 0.2N H 2SO N H 2SO 4 0.1N No impact, consumption of the titration solution should be between 3-17 ml. No sucrose With sucrose No significant difference observed between the blanks. Potentiometric (automatic titration) Colorimetric (visible color change) or potentiometric; to the first trace of pink or ph 4.6 Indicator No indicator Methyl red / bromocresol green 1:5 No impact, easier handling when working with automatic titration. No impact. Table 14: Differences of the Dumas application to AOAC [5] Application AOAC note Standards Aspartic acid Starch Nicotinic acid Lysine-HCl EDTA Notes / Impact Standard has to be selected depending on sample type that is going to be analyzed. Oxygen gas purity Less impurities when working with Conclusion The determination of nitrogen and protein in gluten and starch using the KjelDigester K-449 and KjelMaster System K-375 / K-376 provides reliable and reproducible results and is fully automated. The results correspond well to the expected values. Furthermore, the protein content determined with the D-480 provide also reliable and reproducible results and the system allows automated and unattented operation as well. Comparing the results of gluten and starch determined with both methods the values are as expected and identical. Therefore, both methods are equally suitable for the determination of nitrogen and protein in gluten and starch. Table 15: Comparison of Kjeldahl and Dumas Parameter Kjeldahl Dumas Steps Digestion Distillation - Titration Sample packing - combustion Results protein Gluten %, % RSD (n=5) Starch 0.29 %, % RSD (n=5) Gluten %, % RSD (n=5) Starch 0.29 %, % RSD (n=5) 9 References [1] Application Note 110/2013 Nitrogen and protein determination in corn, flour and soy according to Kjeldahl, BUCHI.*,** [2] ISO 20483:2006 Cereals and pulses Determination of the nitrogen content and calculation of the crude protein content Kjeldahl method. [3] KjelOpimizer App can be downloaded from itunes and GooglePlay. [4] Application Note 159/2014 Nitrogen and protein determination in cereals and oil seeds according to the Dumas method.*,** [5] AOAC Crude Protein Cereal Grains and Oilseeds, Generic Combustion Method, Application Note 201/2015 January /11

11 * All application notes can be downloaded from following link: **Information about the Kjeldahl and Dumas is available here: Application Note 201/2015 January /11