# A 1064 nm Dispersive Raman Spectral Imaging System for Food Safety and Quality Evaluation
## 3 Application to Authenticate Turmeric Powder
### Subsurface Detection of Turmeric Powder and Metanil Yellow
Figure 8: Spatially offset Raman spectroscopic technique for subsurface detection. Figure 10 shows the spatially offset Raman spectra from capsule-turmeric sample at two-, three-, four-, and five-capsule layer at no offset, 0.1 mm, 0.5 mm, 1.0 mm, 1.5 mm, 2.0 mm, 2.5 mm, and 3.0 mm offset. Steep increase in the 1599 cm\({}^{-1}\) and 1631 cm\({}^{-1}\) intensity with increasing offset distance is readily visible in Figure 10a for two-layer capsule-turmeric sample. Similarly, for three-, four-, and five-layer capsule samples, enhancement of 1599 cm\({}^{-1}\) and 1631 cm\({}^{-1}\) peak is evident as the offset distance is increased (Figure 11b-d). At no offset, the turmeric peaks evident in one- and two-layer capsule samples are attenuated as the capsule layer is increased (Figure 10c,d) because the capsule layer suppresses the deep subsurface turmeric signal. In the five-layer capsule sample (Figure 10d), 1599 cm\({}^{-1}\) and 1631 cm\({}^{-1}\) peaks are not evident at no offset. This prevents identification of turmeric inside the five-layer capsule. The no offset measurement measures backscattering Raman signal. This indicates that the backscattering Raman measurement technique is not useful to analyze the subsurface material through the thick top surface layer. However, increasing the offset distance enhanced the 1599 cm\({}^{-1}\) and 1631 cm\({}^{-1}\) peaks (Figure 10d), allowing for the identification of subsurface turmeric despite the presence of thick gelatin capsule layers. Figure 9: Spatially offset Raman spectra of turmeric powder packaged inside one-layer capsule. The effectiveness of the SORS method was further validated by identifying metanil yellow and turmeric powder packed inside one up to five-layer of capsule. Turmeric-metanil yellow mixture sample at 50% concentration (_w/w_) was prepared and packed inside gelatin capsule to obtain one-, two-, three-, four-, and five-layer samples. A series of spatially offset Raman spectra were collected from no offset to 3 mm offset using a 0.1 mm increment along the longitudinal flat surface of capsule sample. Figure 11 shows the SORS spectra from the one-layer capsule sample. The 994 cm\({}^{-1}\), 1147 cm\({}^{-1}\), 1189 cm\({}^{-1}\), 1433 cm\({}^{-1}\), and 1602 cm\({}^{-1}\) are metanil yellow peaks. The 1631 cm\({}^{-1}\) is a turmeric peak. The turmeric peak at 1599 cm\({}^{-1}\) is not resolved from the 1602 cm\({}^{-1}\) peak of metanil yellow. Metanil yellow peaks at 1147 cm\({}^{-1}\), 1189 cm\({}^{-1}\), and 1433 cm\({}^{-1}\) are evident at no offset, and are enhanced by increasing the offset distance. The 994 cm\({}^{-1}\) metanil yellow peak is not evident at no offset. Increasing  
Figure 10: Spatially offset Raman spectra of turmeric powder packaged inside: two-layer (**a**); three-layer (**b**); four-layer (**c**); and, five-layer (**d**) capsule. the offset distance enhanced the 994 cm\({}^{-1}\) peak. The 1631 cm\({}^{-1}\) turmeric peak is not evident at no offset. Increasing the offset distance gradually increased the 1631 cm\({}^{-1}\) peak intensity. Figure 12 shows the SORS spectra from two-, three-, four-, and five-layer capsule samples at no offset, 0.1 mm, 0.5 mm, 1.0 mm, 1.5 mm, 2.0 mm, 2.5 mm, and 3.0 mm offset. Increasing the capsule layer attenuated the metanil yellow and turmeric peaks at no offset. The metanil yellow peaks at 1147 cm\({}^{-1}\) and 1433 cm\({}^{-1}\) were evident at no offset in the two- and three-layer capsule samples (Figure 12a,b). The 1147 cm\({}^{-1}\) peak is not evident at no offset in the four- and five-layer capsule samples (Figure 12c,d). Increasing the offset distance enhanced the 1147 cm\({}^{-1}\) peak (Figure 12c,d). The 1433 cm\({}^{-1}\) peak is evident at no offset in the two-, three-, and four-layer capsule samples (Figure 12a-c). An increase in the peak intensity at 1433 cm\({}^{-1}\) was observed from no offset to 1.0 mm offset in two-, three-, and four-layer capsule samples (Figure 12a-c). After 1.0 mm offset, the 1433 cm\({}^{-1}\) peak intensity gradually decreased.