Detection and quantification of adulterants in milk powder using high-throughput Raman chemical imaging technique

Authors: Qin, Jianwei - Tony Qin; Kim, Moon; Chao, Kuanglin - Kevin Chao; DHAKAL, SAGAR - Us Forest Service (FS); LEE, HOONSOO - Us Forest Service (FS); CHO, BYOUNG-KWAN - Chungnam National University

Submitted to: Food Additives & Contaminants
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/5/2016
Publication Date: 2/1/2017
Citation: Qin, J., Kim, M.S., Chao, K., Dhakal, S., Lee, H., Cho, B. 2017. Detection and quantification of adulterants in milk powder using high-throughput Raman chemical imaging technique. Food Additives & Contaminants. 34(2):152-161.

Interpretive Summary: Milk adulteration has become a food safety concern since milk powder is a ubiquitous ingredient for many processed food products and thus vulnerable to economically motivated adulteration. This study presents a line-scan high-throughput Raman chemical imaging method for rapidly authenticating milk powder. Raman chemical images were generated to visualize the identification, spatial distribution, and morphological features of two chemical adulterants, melamine and urea, that were mixed into dry milk powder. Both melamine and urea could be detected in the images at concentrations as low as 50 parts per million (ppm), with high correlations found between the percentages of adulterant pixels in the images and the concentrations of melamine and urea. The results of this investigation show that this Raman imaging method can detect adulterants at concentrations even lower than those that occurred in actual incidents of milk product contamination. The high-throughput inspection method presented here will benefit regulatory agencies and food processors interested in authenticating milk powder for safety inspection.

Technical Abstract: Milk is a vulnerable target for economically motivated adulteration. In this study, a line-scan high-throughput Raman imaging system was used to authenticate milk powder. A 5 W 785 nm line laser (240 mm long and 1 mm wide) was used as a Raman excitation source. The system was used to acquire hyperspectral Raman images in a wavenumber range of 103–2881 cm-1 from the skim milk powder mixed with two nitrogen-rich adulterants (i.e., melamine and urea) at eight concentrations (w/w) from 50 to 10,000 ppm. The powdered samples were put in sample holders with a surface area of 150 mm×100 mm and a depth of 2 mm for push-broom image acquisition. Varying fluorescence signals from the milk powder were removed using a correction method based on adaptive iteratively reweighted penalized least squares. Image classifications were conducted using a simple thresholding method applied to single-band fluorescence-corrected images at unique Raman peaks selected for melamine (673 cm-1) and urea (1009 cm-1). Chemical images were generated by combining individual binary images of melamine and urea to visualize identification, spatial distribution, and morphological features of the two adulterant particles in the milk powder. Limits of detection for both melamine and urea were estimated in the order of 50 ppm. High correlations were found between pixel concentrations (i.e., percentages of the adulterant pixels in the chemical images) and mass concentrations of melamine and urea, demonstrating the potential of the high-throughput Raman chemical imaging method for detection and quantification of the adulterants in the milk powder.