Exploring drying-induced separation of main constituents in melamine-blended milk infant formula using DCDR spectroscopy

Abstract

Drop coating deposition Raman (DCDR) spectroscopy was employed to investigate the melamine compound susceptibility to milk infant formula main constituents as lactose and lipids (specifically 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), and 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE)). DCDR approach is based on the deposition and drying of a small droplet (a few µl) of liquid sample on a hydrophobic substrate. After complete evaporation, the analyte is preconcentrated into the dried pattern from which the classical Raman spectra are acquired. A simple melamine-blended model reactions were tested (melamine-blended lactose, DOPC, and DOPE). From these, lactose was shown to be the most potent reaction partner for melamine, where a significant difference in dried pattern between pure and blended lactose was noticed, and the relevant Raman spectral changes were observed. Pure and melamine-blended infant formula solutions from two kinds of powdered infant formula purchased in the local market were then studied. For pure milk solutions, the drying process, as an integral part of the DCDR approach, led to the spatial separation of lipids and carbohydrates in the resulting pattern. Acquired Raman spectra revealed that the ring edge of the dried pattern was composed mainly of lipids, while the thin film in the central part contained mainly carbohydrates, especially lactose. For the melamine-blended formula, the DCDR approach identified that melamine was present only in the central part of the dried pattern together with carbohydrates. As a result, it was assumed that melamine has a higher susceptibility for carbohydrates than for lipid molecules, even in milk infant formula.