Effects of the Adulteration Technique on the Near-Infrared Detection of Melamine in Milk Powder.

The United States Pharmacopeial Convention has led an international collaborative project to develop a toolbox of screening methods and reference standards for the detection of milk powder adulteration. During the development of adulterated milk powder reference standards, blending methods used to combine melamine and milk had unanticipated strong effects on the near-infrared (NIR) spectrum of melamine. The prominent absorbance band at 1468 nm of melamine was retained when it was dry-blended with skim milk powder but disappeared in wet-blended mixtures, where spray-dried milk powder samples were prepared from solution. Analyses using polarized light microscopy, Raman spectroscopy, dielectric relaxation spectroscopy, X-ray diffraction, and mass spectrometry indicated that wet blending promoted reversible and early Maillard reactions with lactose that are responsible for differences in melamine NIR spectra between wet- and dry-blended samples. Targeted detection estimates based solely on dry-blended reference standards are likely to overestimate NIR detection capabilities in wet-blended samples as a result of previously overlooked matrix effects arising from changes in melamine hydrogen-bonding status, covalent complexation with lactose, and the lower but more homogeneous melamine local concentration distribution produced in wet-blended samples. Techniques used to incorporate potential adulterants can determine the suitability of milk reference standards for use with rapid detection methods.

Resonance Raman evidence for protein-induced out-of-plane distortion of the heme prosthetic group of mammalian lactoperoxidase.

Resonance Raman spectra have been acquired for resting state mammalian lactoperoxidase, LPO(N), and its six-coordinate, low-spin (6CLS) cyanide complex, LPO(CN), as well as for various heme l containing fragments resulting from partial or complete proteolytic digestion. These proteolytic fragments provide a useful set of reference compounds for analysis of the LPO(N) and LPO(CN) enzymes, using various ligands to generate well-defined five-coordinate and six-coordinate high-spin (5CHS and 6CHS) species. In addition, these model compounds, which contain zero, one, or two covalently attached ester linkages to polypeptide chains, are quite useful for determining the extent to which the presence of the ester linkages at the heme periphery affects the characteristic heme resonance Raman marker bands. The spectral results not only provide strong evidence for the formulation of the resting state enzyme as a 6CHS species, but also confirm the previously documented anomalous intensities of several low-frequency resonance Raman bands, which are most reasonably interpreted to arise from a protein-induced out-of-plane distortion of the heme l macrocycle mediated by the covalent ester linkages to the associated polypeptide residues of the intact protein.