Dumas Combustion Method as a Replacement for Kjeldahl – Examples for Non-Protein Nitrogen and Non-Casein Nitrogen Determination
As outlined in our previous blog post, the Kjeldahl method of protein determination is being replaced by the Dumas combustion method due to its longer analysis times, higher costs, safety concerns, and issues with recovery. Our initial blog post demonstrated that both methods offer comparable accuracy and precision.
In the second part of this blog series, we will detail a comparison test for determining non-protein nitrogen (NPN) and non-casein nitrogen (NCN).
The NPN was determined using ISO 27871. This method measures nitrogenous fractions in cheese and processed cheese from cow‘s milk. To perform this procedure, mix 50 mL of milk with 24% trichloroacetic acid (TCA) in a 100 mL flask until the concentration is about 12% TCA. After mixing, allow precipitation, filter the sample, and analyze the filtrate. For analysis with the FP828, approximately 1 gram of the filtrate should be accurately weighed into a large tin capsule.
The following table presents results from a recent comparison test conducted in 2024 by a large dairy products producer in France. The test utilized various dairy samples and demonstrated a high correlation between the Kjeldahl and Dumas methods, with an R² value exceeding 0.97.
The Dumas method exhibits precision and accuracy comparable to the Kjeldahl method in NPN analyses on Emmental cheese and other dairy products. The maximum deviation between two measurements of a measurement pair does not surpass the reproducibility of the Kjeldahl method.
The NCN (Non-casein nitrogen), or soluble nitrogen, was determined following the ISO 27871 standard. In this procedure, 10 mL of 1N hydrochloric acid is added to 50 mL of cheese milk. The mixture is transferred to a 100 mL flask and diluted with deionized water. After filtration, the solution is analyzed using the FP828 with approximately 1 gram of sample mass weighed into a large tin capsule.
As shown in Table 2, the Dumas method is as precise and accurate as the Kjeldahl method for NCN analyses on Emmental cheese and other dairy products.
The results indicate that the precision and accuracy are comparable to those obtained using the Kjeldahl method. This applies to both milk/milk powders (previous blog post) as well as NPN and NPC samples.
In conclusion, it is important to note some of the highly advantageous features of the LECO Dumas combustion analyzers that make your lab run smoother:
- High throughput with an analysis cycle time of 2.8 minutes for the FP828 model
- Simple method with linear calibration, applicable to nearly all food samples without requiring specific information about the sample
- Solid and liquid samples can be analyzed with the same setup, removing the need for a liquid sampler or mixing milk with absorbent material
- Macro capabilities allow for the analysis of up to 1 g of milk (FP828) or more than 1 g for the FP928 model
- High instrument uptime, no need for oxidizing reagents, and low costs per analysis due to the ballast aliquot principle
To learn more about this topic, register for our upcoming webinar!
