Evaluation of turmeric powder adulterated with metanil yellow using FT-Raman and FT-IR spectroscopy

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

Submitted to: Foods
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/20/2016
Publication Date: 6/17/2016
Citation: Dhakal, S., Chao, K., Schmidt, W.F., Qin, J., Kim, M.S., Chan, D.E. 2016. Evaluation of turmeric powder adulterated with metanil yellow using FT-Raman and FT-IR spectroscopy. Foods. 5(2):36-51.

Interpretive Summary: Turmeric (Curuma long L.) is a herbaceous root commonly used for food seasoning as well as for medicinal purposes. Turmeric is valued for medicinal properties arising from its natural content of curcumin, a yellow pigment with anti-inflammatory, anti-cancer, antioxidant, and wound healing attributes. However, economically driven adulteration of turmeric has occurred repeatedly, such as the addition of metanil yellow, a known carcinogen, to increase yellow color and product weight. This study utilized Fourier Transform Raman (FT-Raman) spectroscopy and Fourier Transform Infrared (FT-IR) spectroscopy technique for detection of metanil yellow in turmeric powder. FT-Raman and FT-IR spectra of metanil yellow, turmeric, and curcumin were acquired and analyzed. FT-Raman and FT-IR spectral analysis of sample mixtures of metanil yellow in turmeric at 8 different concentrations showed that the FT-Raman method was able to detect metanil yellow at 1% concentration, while detection by the FT-IR method was limited to 5% concentration. The results show that FT-Raman and FT-IR spectroscopy can be utilized for detection of chemical contaminants in food powders such as carcinogenic metanil yellow in turmeric. With the increasing popularity of turmeric as a health food additive, these techniques may be considered a potential tool for food safety inspection that could greatly benefit the food industry, safety regulators, and consumers worldwide.

Technical Abstract: Turmeric powder (Curcuma longa L.) is valued both for its medicinal properties and for its popular culinary use such as being a component in curry powder. Due to its high demand in international trade, turmeric powder has been subject to economically driven, hazardous chemical adulteration. This study utilized FT-Raman and FT-IR spectroscopy as separate but complementary methods for detecting metanil yellow adulteration of turmeric powder. Sample mixtures of turmeric powder and metanil yellow were prepared at concentrations of 30%, 25%, 20%, 15%, 10%, 5%, 1%, 0.01% (w/w). FT-Raman and FT-IR spectra were acquired for these mixture samples as well as for pure samples of turmeric powder and metanil yellow. Spectral analysis showed that the FT-IR method in this study could detect the metanil yellow at the 5% concentration, while the FT-Raman method appeared to be more sensitive and could detect the metanil yellow at the 1% concentration. Relationships between metanil yellow spectral peak intensities and metanil yellow concentration were established using representative peaks at FT-Raman 1406 cm-1 and FT-IR 1140 cm-1 with correlation coefficients of 0.93 and 0.95, respectively.