# A 1064 nm Dispersive Raman Spectral Imaging System for Food Safety and Quality Evaluation
## 1 Introduction  
Incidents of foodborne illness outbreaks have necessitated the development of a reliable technique for food safety and quality evaluation. Advanced sensing techniques, such as optical methods, hold promise for food authentication [1; 2; 3]. Among the optical techniques, Raman spectroscopy has an advantage due to its higher specificity and sensitivity. Simple spectral measurement method and insensitivity to the presence of water in the sample (eliminating the need for the removal of water from the sample) are additional advantages of Raman spectroscopic technique. Unique fingerprints for chemicals in Raman spectra allow analyses of a wide range of chemicals. It has been widely used to evaluate the chemical properties of food samples [4; 5; 6]. Raman spectra can be used to identify and evaluate one or more chemicals in a mixture. Raman spectroscopy has a broad application in food processing [7; 8; 9; 10]. It has also been applied to food quality and safety evaluation [11; 12; 13; 14; 15; 16; 17; 18]. Commercially available backscattering Raman spectroscopy, transmission Raman spectroscopy, and Fourier transform Raman (FT-Raman) spectroscopy have been used to evaluate food samples. Backscattering Raman spectroscopy is suitable for surface analysis,such as the detection of food adulterants [19], determination of fatty acids in pork adipose tissue [20], and detection of pesticide on fruit surface [14]. Transmission Raman spectroscopy is used for analysis of internal layers of the sample by placing the sample in between the laser and the detector. Examples include evaluation of protein content in packed corn kernels [21] and the evaluation of protein and oil composition in single soybeans [22]. FT-Raman spectroscopy, using 1064 nm laser, has a negligible fluorescence effect. It has been used to detect foodborne microorganisms on food surface [23], screening deoxynivalenol toxin in cereals [24], and detecting chemical adulteration in food and feeds [25]. Commercial Raman spectroscopic systems use 532 nm, 633 nm or 785 nm laser and FT-Raman spectroscopy uses 1064 nm laser source for sample excitation [26]. In Raman spectroscopic systems, the sample is held and adjusted in the sample compartment to focus the laser beam on the sample surface for spectral measurement [27; 28; 29]. This technique is useful for sample measurement at the microscopic level only. In FT-Raman measurement, the sample is generally filled in NMR tube and held vertically in the sample compartment to adjust laser focus on the sample for measurement [30; 31]. This method requires sample adjustment for each measurement. FT-Raman technique is frequently used for analysis of liquid and paste [32; 33; 34; 35]. These commercial systems can measure food samples from selected spots only. Due to the necessity of adjusting the sample for each subsequent measurement, these systems cannot be used to measure the large surface area of the sample. The commercial Raman spectroscopic systems are designed for general use, and cannot be used for non-destructive subsurface detection by spatially offset Raman spectroscopic (SORS) technique. In SORS measurement, a series of spatially offset Raman spectra is collected as the sample is moved at a spatial distance away from the laser source [36]. Increasing the offset distance enhances the Raman signal of deep subsurface layer enabling detection of the inside material. Hyperspectral imaging system combines spectroscopic and imaging technique to acquire spectral image of the sample. [PERSON] et al. (2010) developed a 785 nm Raman spectral imaging system for food safety and quality evaluation [37]. The system consists of a 785 nm laser module for sample excitation. A dispersive Raman spectrograph, especially designed for a 785 nm laser excitation is mounted with a 16-bit charged couple device (CCD) camera for spectral imaging. The scattering light from sample is guided to reflection grating of the spectrograph through a narrow input slit. The reflection grating disperses the light beam into different wavelengths.