Milk protein consists of approximately 20% whey protein and 80% caseins, and contains nine essential amino acids necessary for synthesis of body proteins and other important nitrogen-containing compounds including hormones and neurotransmitters. Determining milk product protein content is important for nutrition, but is also important to help farmers determine the value of their product, and to support the international trade of dairy products. There are currently two international standards for protein determination in milk and milk products, the Kjeldahl and Dumas methods. While the Kjeldahl method is considered the reference method, and has been widely used and accepted, the Dumas method is also highly accurate, and offers several advantages including lower cost and more rapid analysis, and is a more environmentally friendly method that produces no hazardous waste.
All About the Chemistry
Approximately 95 percent of the nitrogen found in milk is considered protein, while the remaining five percent is non-protein bound. As such, nitrogen determination provides an accurate measure of protein content. Nitrogen content is used to ascertain protein content using a conversion factor of 6.38. The Kjeldahl method is based on wet chemistry, and requires sample digestion at a high temperature in concentrated sulfuric acid, with the addition of a catalyst to elevate the acid’s boiling point. Sodium hydroxide is added, and ammonia is distilled and introduced to a boric acid solution to produce ammonium borate, which is then titrated. Alternatively, the Dumas method is based on sample combustion in an oxygen-enriched environment. The nitrogen compounds are converted into nitrogen oxides, and reduced to nitrogen gas, which is quantified with a thermal conductivity conductor.
As a reference method, the Kjeldahl approach has been widely used. However, this method is time-consuming, requires extensive sample preparation and costly acids, and produces hazardous waste. As such, the routine Dumas method is becoming more widely adopted, as it overcomes many of these challenges and inefficiencies.
A Routine Method Offers Several Advantages
Every laboratory benefits from reduced operating costs, which can be achieved with cost-effective instrumentation. While an instrument’s purchase price is an important consideration, high operating costs can sometimes exceed any initial savings. Operating costs encompass everything from chemicals and consumables, to waste disposal, and the time required for skilled personnel to process samples. From this perspective, the Dumas method offers significant cost savings over time.
The Kjeldahl method is a form of wet chemistry requiring sulfuric acid. Such hazardous chemicals require proper disposal by trained individuals and hazardous waste companies, adding to the cost of analysis. The Dumas method is based on combustion, and generates no hazardous waste, thereby eliminating the added cost of waste disposal.
The Dumas method is also less laborious and time-consuming than the Kjeldahl approach. Employing the Dumas method, a technician simply weighs a small sample volume directly into reusable steel crucibles or tin foil and uses software to initiate sample analysis while continuing to prepare additional samples. This method eliminates the sample digestion, distillation, and titration steps required for Kjeldahl analysis, and supports greater throughput as analysis is achieved in four to eight minutes (compared to 100 minutes or more required for Kjeldhal analysis). Further, the Kjeldahl method requires tubes to be washed and dried between each use, whereas the Dumas method employs steel crucibles that can be quickly emptied and re-used. As a result, technicians can perform rapid, efficient nitrogen analysis leaving them with more time to attend to other responsibilities.
Accuracy Is Critical
Accurate nitrogen analysis is critical to determine accurate protein content for pricing milk products. While the Dumas method offers many advantages, including reduced cost per sample and greater efficiency, these benefits are insufficient to support its use if they come at the expense of accuracy. Hesitation in adopting the Dumas method may stem from reports that for some food matrices, it produces higher nitrogen values than the Kjeldahl method due to the conversion of non-protein nitrogen to elemental nitrogen. Here, we report findings from a case study in which various milk products were analyzed via the Kjeldahl method (AOAC 991.20), and the Dumas method (ISO 14891) using the Elementar rapid MAX N exceed N/Protein analyzer (rMAXNex-He). The nitrogen con-tent of various dairy products achieved by Kjeldahl and Dumas analysis is shown in Figure 1. Results demonstrate no bias between the Kjeldahl and Dumas methods for nitrogen determination in dairy products.
Combustion-based Dumas technology enables accu-rate nitrogen analysis for protein determination in milk and milk products. Compared to the Kjeldahl method, Dumas technology offers a similar degree of accuracy, and enables more rapid, cost-effective, and environmentally friendly analyses. The Elementar rapid N exceed, and rapid MAX N exceed for larger sample volumes, are ideal solutions for dairy product protein determination.