MALDI of milk

MALDI-TOF screening of dried milk samples for contaminants, adulterants, and species of origin. 

Michelle Joyce, Notre Dame Mass Spectrometry Facility

Milk is a nutritional staple across the globe, and the discovery of tainted milk and infant formula in recent years necessitates the need for rapid screening of milk products. The 2008, the melamine in milk scare in China brought to light the issue of milk contamination, and the recent production and exporting of tainted milk from New Zealand’s major dairy producer Fonterra confirms that the issue still exists.  Following a thorough study of food fraud over the past three decades, the U.S. Pharmacopeia started an open-access database of research and media articles pertaining to the detection of contaminants in food.[i] Contaminating species in milk range from small molecules, such as herbicides and mycotoxins, to proteins from other species introduced as additives.[ii] Given today’s global food supply chain, milk contamination is an international problem.  Food producers and consumers could benefit from a quick and easy screening method that requires little sample preparation.

Matrix-Assisted Laser Desorption Ionization Time-Of-Flight (MALDI-TOF) mass spectrometry is a rapid analytical technique capable of detecting both small and large molecules.  In a MALDI-TOF experiment, a laser is pulsed at a metal target onto which a small amount of analyte and UV-absorbing matrix compound are dried. Upon absorbing the energy, the matrix and analyte become charged and are accelerated into the flight tube where they separate based on the mass-to-charge ratio.  MALDI-TOF is a reliable tool for profiling milk standards, both pure and spiked with contaminants.[iii] A library can then be compiled from the profile spectra.  The high resolution of MALDI-TOF in the low molecular weight range enables the detection of small molecule contaminants, and the unique proteins in differing species of animal milk allow for fingerprinting of a milk source.[iv] Samples from various international locations can be analyzed and compared to the library for quick assessment of milk purity.

The compilation of a milk library and screening of milk samples is a project that is well-suited for an REU student who has completed an undergraduate analytical chemistry course. The student will work in the Mass Spectrometry and Proteomics Facility (MSPF) and will be trained on the MALDI-TOF theory and operation. The library of milk MALDI-TOF spectra can be assembled in the MSPF, and samples can be used to assess the screening method. The MSPF MALDI-TOF instrument can be accessed remotely, so the student can easily continue the project upon return to his/her school. The proposed project is intended to tackle a global problem and teach the REU student how to approach and perform analytical research.

[i] J.C. Moore, J. Spink, and M. Lipp, “Development and application of a database of food ingredient fraud and economically motivated adulteration from 1980 to 2010”, J. Food Sci., 2012, 77, 118-126.

[ii] A. Singh and V. Panchagnula, “High throughput quantitative analysis of melamine and triazines by MALDI-TOF MS”, Anal. Methods, 2011, 3, 2360-2366.

[iii] C.D. Calvano, C. De Ceglie, A. Monopoli, and C.G. Zambonin, “Detection of sheep and goat milk adulterations by direct MALDI-TOF MS analytis of milk tryptic digests”, J. Mass Spectrom., 2012, 47, 1141-1149.

[iv] S. Sabbadin, R. Seraglia, G. Allegri, A. Bertazzo, and P. Traldi, “Matrix-assisted Laser Desorption/Ionization Mass Spectrometry in Evaluation of Protein Profiles of Infant Formulae”, Rapid Commun. Mass Spectrom., 1999, 13, 1438-1443.