Development of a Raman Chemical Imaging System for Food Safety Inspection

Raman chemical imaging technique combines Raman spectroscopy and digital imaging to visualize composition and structure of the target, and it offers great potential for food safety and quality research. In this study, a laboratory-based Raman chemical imaging platform was designed and developed. The imaging system utilizes a 785 nm spectrum stabilized laser as an excitation source to generate Raman scattering. The detection module mainly consists of a Raman signal detection probe, a reflection grating-based Raman imaging spectrometer, and a high performance spectroscopic CCD camera. The imaging system works in a point scanning mode. A Raman spectrum is obtained at a time for a single position in the scene. The specimens are carried by a two-axis motorized positioning table, and hyperspectral image data are accumulated as the samples move along two spatial dimensions. The parameterization and data-transfer interface software was developed using LabVIEW. Spectral and spatial calibration procedures were presented. The system covers a Raman shift range of 102.2-2538.1 cm-1 with a spectral resolution of 2.9 cm-1 and a spatial resolution as high as 0.1 mm. Performance of the system was demonstrated by an application example on detection of melamine in dry milk. Melamine was mixed into dry milk with concentrations (w/w) ranging from 0.2% to 10.0%. The system was able to create Raman chemical images that can be used to visualize quantity and spatial distribution of melamine particles in the mixtures. The developed system is versatile, and it will be used for safety and quality inspection of food and agricultural products.