Nitrogen/protein determination in food

eFOOD-Lab_International_01_2016

Quality Management Nitrogen/protein determination in food Optimizing a total nitrogen/protein analyzer for maximum sample throughput and lowest cost-per-analysis Author: Michael Jakob, LECO Europe B.V. 6161 AG Geleen, The Netherlands www.leco-europe.com, michael_jakob@leco.com 1/2016 eFOOD-Lab international 19 The protein content of food is normally not determined directly, but calculated from the Nitrogen content of the food and feedstuff. This is done with several protein factors that multiply the Nitrogen content and calculate the Protein content. The most common ones used are 6.25 (generally), 6.38 (for diaries) and some others. Generally there are several technologies to determine Nitrogen in food and feed. Since more than 100 years the Kjeldahl method is used frequently in the food and feed control. This method is based on classical wet chemical digestion and titration, long used and widely accepted. Several hundred mg- to gram-sized sample is digested in concentrated sulfuric acid with a catalyst; nitrogen is converted to ammonium sulfate, concentrated sodium hydroxide (NaOH) is added to form NH3, which is distilled into standard acid for quantification by titration. Despite being well known throughout analytical history, this method has a lot of disadvantages and inconveniences. Long analysis times, low sample throughput, wet chemistry handling of hazardous materials (hot acids and bases) and high running costs were the most crucial points to look for an alternative. It was found by the so called Dumas method. The Dumas method is a combustion method that is transforming the samples Nitrogen into N2 or NOx. These gases are reduced (mostly with hot Copper or sometimes with Tungsten) to N2 and this is measured with a thermoconductivity cell versus a reference flow of the carrier gas. As carrier gases normally Helium or Argon are used. The Dumas method has several advantages vs. Kjeldahl. The analysis time is very fast – some minutes vs- hours, no toxic waste, no real safety issues and finally much lower operation costs. All these advantages made the decision easy, to use Dumas method also as a reference method beside the Kjeldahl method. This is true since the late 1990s for most food and feed products. (AOAC, ISO, etc…) There are several possible setups for the Dumas method. One is a gas chromatographic approach, another possibility is a trap and purge technology. With both methods some issues are pending. The first one is that all gases released Picture 1: LECO FP 628


eFOOD-Lab_International_01_2016
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