Raman spectral imaging for quantitative contaminant evaluation in skim milk powder

Authors: DHAKAL, SAGAR - Us Forest Service (FS); Chao, Kuanglin - Kevin Chao; Qin, Jianwei - Tony Qin; Kim, Moon; Chan, Diane

Submitted to: Journal of Food Measurement and Characterization
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/19/2016
Publication Date: 5/20/2016
Citation: Dhakal, S., Chao, K., Qin, J., Kim, M.S., Chan, D.E. 2016. Raman spectral imaging for quantitative contaminant evaluation in skim milk powder. Journal of Food Measurement and Characterization. 10(2):374-386.

Interpretive Summary: The consumption of food products adulterated out of economically driven greed, such as melamine in milk products, has been shown capable of causing illness and even death. Such incidents have illustrated an urgent need for rapid and non-destructive methods to screen foods for chemical adulterants. Qualitative detection of adulterants such as melamine in milk has been demonstrated using Raman spectroscopic methods, but rapid and non-destructive quantitative measurement of such adulterants remains under investigation. Ten dry mixtures of melamine in milk were prepared at concentrations of 0.005%, 0.01%, 0.025%, 0.05%, 0.075%, 0.1%, 0.25%, 0.5%, 0.75%, and 1%- w/w, using an acoustic mixer. Seven sub-samples of each concentration mixture, each presented over a square 45 mm x 45 mm surface area, were imaged using a point-scan Raman imaging system and melamine concentrations were calculated from the number of detected melamine pixels. A linear relationship was established between the detected melamine concentrations and the actual concentrations of the mixtures, with a correlation coefficient of 0.99. These results provide the basis for a quantitative detection model, using Raman spectral imaging, by which melamine can be detected in milk at concentrations as low as 0.005%. With further development, this method can be refined for quantitative measurement of melamine in milk and of chemicals in other food ingredients, for rapid and non-destructive detection of low-concentration adulterants. This research benefits food technologists, the food processing industry, and regulators interested in improving food safety and security related to contamination and adulteration.

Technical Abstract: 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 mm 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 minutes, and seven sub-samples of each concentration were imaged. Each sub-sample image was acquired over a 45 mm x 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%.