Raman spectral imaging for quantitative contaminant evaluation in skim milk powder

This study uses a point-scan Raman spectral imaging system for quantitative detection of melamine in milk powder. A sample depth of 2 mm and corresponding laser intensity of 200 mW were selected after evaluating the penetration of a 785 nm laser through milk powder. Horizontal and vertical spatial resolutions of 0.25 and 0.5 mm were selected for effective detection of melamine particles mixed with milk powder. The selected imaging parameters were used to quantitatively detect melamine concentration in milk powder. Ten different concentrations of melamine in milk (0.005, 0.01, 0.025, 0.05, 0.075, 0.1, 0.25, 0.5, 0.75, and 1 %- w/w) were prepared by mixing each mixture of melamine and milk powder in an acoustic mixer for 15 min, and seven sub-samples of each concentration were imaged. Each sub-sample image was acquired over a 45 mm × 45 mm surface area. Background-corrected Raman spectral images were converted into binary images and an intensity threshold value of 400 was applied to convert melamine pixels into black pixels and milk pixels into white pixels (background). At least one melamine pixel was detected in every sub-sample across all ten concentration levels except for one sub-sample at the 0.005 % concentration. It was found that the method developed in this study can detect melamine particles present at concentrations as low as 0.005 %. A linear relation was established between the detected melamine concentrations and the actual concentrations of the mixtures, with correlation coefficient of 0.99. The results show that the method developed in this study can be used for non-destructive quantitative prediction of melamine in milk powder to minimum concentration level of 0.005 %.