Melamine, Nitrogen, and Protein Determination: A History and Path Forward

From FCC e-Newsletter - September 2009

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Economically motivated adulteration (EMA) is defined as the fraudulent addition, removal, or substitution of non-authentic substances for economic gain without the knowledge of the buyer or user. Food proteins are particularly vulnerable to economic adulteration due to the volume of food protein consumed globally, its value in the human diet, the commercial use of proteins as food value indicators, and last but not least the use of nitrogen as a marker for the analytical determination of the amount of protein. This vulnerability was recently revealed by cases of the addition of melamine to mimic food protein in various foods.

The predominate protein measurement techniques are the total determination of nitrogen by Kjeldahl or Dumas methods. These techniques are now known to be vulnerable to adulteration with potentially devastating public health consequences because they lack selectivity/specificity for non-protein nitrogen and/or non-authentic protein. The recognition of nitrogen as an important constituent of protein dates back to the late 18th century. The method first described by Johan Kjeldahl in 1883 and its subsequent improvements has gained broad utility due to its high reproducibility and its applicability to a broad range of food and feed matrices.

Evaluating preventative solutions for the future necessitates a historical and chemical understanding of why Kjeldahl nitrogen has become the most commonly used determinant of total protein. Table 1 summarizes the historical and scientific reasons that led to the prevalent use of Kjeldahl nitrogen for total protein determination.

Table 1. History of Nitrogen, Protein Measurement, and Adulteration

Newer analytical strategies include adaptations of the Kjeldahl method to account for non-protein nitrogen, the use of methods more specific for proteins such as the Bradford method for aromatic amino acids, and the use of infrared spectroscopy. More recently developed techniques, which may find future utility, include the use of HPLC, immunoassays including multiplexed biosensors and microarrays, and mass spectrometry.

Scientific solutions to better protect the public health by preventing the adulteration of protein-based food ingredients exist but will require further analytical work to validate methods for individual matrices. This solution may include the use of reference or calibration standards to provide the reproducibility necessary to meet the wide range of needs for industry and regulators. Table 2 summarizes the issues that need to be considered for moving from a total nitrogen-based measurement for proteins to a more direct analysis of the proteins themselves.

Table 2. Path Forward for New Analytical Solutions

Conclusions:

• There is a need to move beyond total nitrogen-based and basic-IR methods for food protein measurement to protect the food supply from potentially harmful adulteration.
• Work is needed to further develop matrix-specific definitions, methods, and reference materials for food protein measurement.
• Screening methods to detect the presence of foreign substances by deviations from the unique and characteristic "signature" signal of an unadulterated sample should be developed to complement measurement techniques.

(Adapted from poster presented at the 2009 IFT Conference in Anaheim, CA: Melamine, nitrogen, and protein determination: A history and path forward. by Moore, J.C.; DeVries, J.W; Lipp, M.; Griffiths, J.C.)