Raman spectroscopy method for subsurface detection of food powders through plastic layers

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

Submitted to: Proceedings of SPIE
Publication Type: Proceedings
Publication Acceptance Date: 4/1/2017
Publication Date: 4/15/2017
Citation: Dhakal, S., Chao, K., Qin, J., Kim, M.S., Chan, D.E. 2017. Raman spectroscopy method for subsurface detection of food powders through plastic layers. Proceedings of SPIE, Sensing for Agriculture and Food Quality and Safety IX, 1021706.

Interpretive Summary: Turmeric is an herbaceous root commonly used for food seasoning and medicinal purposes. Curcumin, a yellow pigment in turmeric, is the major source of its medicinal value. Metanil yellow, a known carcinogen, has been found illegally added to turmeric powder to increase yellow color and product weight. The chemically contaminated turmeric powder, packaged in sealed containers such as plastic jars, can easily go undetected due to lack of effective techniques to nondestructively detect and analyze powders through surface packaging layers. Using a Raman optical instrument, this study measured spectra of turmeric powder and metanil yellow samples overlaid with surface layers of food grade plastic between 1 and 5 mm thick. The instrument was set up to maintain a fixed laser point on each sample, and to collect a series of measurements at 0.1 mm increments along a line from that the laser point (0 mm) to a point 10 mm away by moving the sample and laser point together away from the measurement probe. The contributions of the subsurface powder and the surface plastic changed with increasing distance, allowing for analysis of the data series to separate and identify each component. This method can benefit food technologists, processors, and regulatory agencies seeking to nondestructively inspect packaged food powders for potential counterfeiting or tampering, to help assure food safety and quality.

Technical Abstract: Turmeric (Curcuma longa L.) powder, valued for both medicinal properties and culinary use, has been subject to economically driven hazardous chemical adulteration by the similarly colored but toxic metanil yellow. Such adulterated food powders in sealed containers can often go undetected due to lack of effective technology for non-destructive subsurface food inspection through packaging materials. Development and use of methods for non-destructive detection and identification of chemicals through sealed containers can combat distribution and consumption of counterfeit food products. This study used a custom designed point-scan Raman spectroscopy system to acquire spectra of turmeric powder and metanil yellow overlaid with layers of plastic (HDPE, high-density polyethylene) cut from a food jar. The powders were each spread across a 50 x 50 mm square area in a 2 mm thickness, overlaid with 5 thicknesses of plastic (1 to 5mm), and placed on the motorized positioning platform. The laser focus unit was also fixed to the platform to produce a 45-degree incident laser angle to the sample surface, to maintain a fixed excitation spot on the sample surface for collection of a set of Raman spectra using a fixed position Raman probe while moving the positioning platform in one direction to achieve variable source-detector distance. A series of Raman spectra was collected for source-detector distance from 0 to 10 mm at increments of 0.1 mm, for a total of 101 spectra from each sample. Increasing the source-detector distance increased the laser penetration depth through the powder sample, retrieving Raman signals from the turmeric powder and metanil yellow through the plastic layers. Each set of sample spectra, containing mixed spectral information from the surface plastic and each subsurface powder, was decomposed into pure component spectra. Spectral information divergence values were then used to identify the pure component spectra as matches for plastic, turmeric, and metanil yellow. As demonstrated, this method can evaluate food powders through plastic (HDPE) packaging to detect counterfeit and tampered foods.