Determination of ripeness stages of Mazafati variety of date fruit by Raman spectroscopy

Abstract

Introduction: The economical yield of date fruits depends on many factors (Al-Shahib and Marshall, 2003). One of them is harvesting in optimum stage. Generally, date fruits have four distinct stages of ripeness to satisfy different consumption requirements (e.g., fresh and processed). They are known throughout the world by their Arabic names which are Kimri, Khalal, Rutab and Tamr in order of ripeness (Imad and Abdul Wahab, 1995; Al-Shahib and Marshall, 2003; Sahari et al., 2007). Decreasing moisture content and increasing sugar content happens gradually while the date ripeness approaches to Tamr stage. From Kimri to Khalal stage, the size and acidity decreases when the color of Mazafati variety changes from green to red. The change in acidity continues from Rutab to Tamr stage while color transforms from brown to black. At the final stage of ripeness, Mazafati variety is soft and has a good storability (Al-Shahib and Marshall, 2003). The main Raman techniques commonly applied in agricultural product and food analyzing include dispersive Raman spectroscopy, Fourier Transform (FT), Raman spectroscopy, Surface-Enhanced Raman Spectroscopy (SERS) and Spatially Offset Raman Spectroscopy (SORS). Synytsya et al. (2003) illustrated that FT-Raman spectroscopy is a valuable tool in structural analysis of commercial citrus and sugar beet pectin. Yang and Irudayaraj (2003) employed an FT-Raman approach to detect and classify foodborne microorganisms on the whole apple surface for the first time. Schulz et al., (2005) revealed the potential of FT-Raman spectroscopy in natural carotenoid analysis. Also, many researchers have attempted to apply FT-Raman spectra on the whole fruits and vegetables. FT-Raman spectroscopy was used by Veraverbeke et al. (2005) to evaluate the natural, intact wax layers on the surface of whole fruits. Nikbakht et al. (2011) used a FT-Raman spectroscopy for qualitative and quantitative analysis of tomato ripeness parameters. The scope of this study was to evaluate the feasibility of a nondestructive method based on FT-Raman spectroscopy in distinction of Mazafati date fruits according to four mentioned ripeness stages. Materials and Methods: Sample preparation: Mazafati variety of date fruit was used for this study. During the harvest seasons of 2012 (July-August), the samples from each four stages of ripening namely Kimri, Khalal, Rutab and Tamr were collected from two different orchards in Bam, Kerman province, Iran. A number of 100 date samples were tested in this study, and the external features of the four stages are exemplified in Fig.1. To characterize the physical properties of studied samples, the selected physical properties such as initial moisture content, mass, geometric mean diameter, sphericity and density of studied samples were measured using represented methods by Mohsenin (1896), Jahromi et al. (2008) and Shakeri and Khodabakhshian (2011). At least, the samples were kept at 5C in a refrigerator for 7 days to distribute the moisture uniformly throughout the sample. Before spectral acquisition, the required quantities of date fruits in each ripeness stage was taken out of the frig and allowed to warm with room temperature for approximately 2 hr (Khodabakhshian et al., 2012). Chemical properties measurements: Tissue samples were cut from each fruit separately and were macerated with a commercial juice extractor, filtered and centrifuged. The supernatant juice was used for the determination of sugar content with a manual refractometer, and expressed as percent Brix in the juice. Dry weight percentage of samples (Between 3-5 g) was determined by weighing them first, then dried them at 105ºC in a forced-air oven for 4 h and finally reweighed. PH value of date fruits was determined by a pH meter. Raman spectroscopic set-up: FT-Raman spectra on the whole fruits in the region 200-2500 cm-1 were recorded using a Thermo Nicolet NEXUS 870 spectrometer (Thermo Electron Corp, Madison, Wis., U.S.A) equipped with a Deuterated Triglycine Sulfate (DTGS) detector and a solid substrate beam splitter. The spectra were collected with rapid scan software running under OMNIC (Nicolet, Madion, Wis., U.S.A) and a resolution of 4 cm-1 by coadding of 128 scans. FT-Raman has three main advantages over dispersive Raman systems: (1) reducing the laser-induced fluorescence that a number of samples exhibit; (2) easing the operation as with a Fourier transform infrared (FTIR) spectrometer; and (3) showing a high spectral resolution with a good wavelength accuracy (Yang and Ying, 2011). Furthermore, the Raman spectra of pure tannin were measured as a reference spectrum. The original data were used for further analysis only after subtracting dark current spectra. For obtaining dark current spectra, the laser was set to zero. Results and Discussion: Physical properties of date fruits: The results of some physical parameters of the studied date fruit are shown in Table1. The changes in the physical properties were dependent on the internal quality in different ripeness stages. This justification also was revealed for date fruits by Al-Hooti et al. (1995). The obtained relations between ripening stages and internal quality of studied samples are represented in the next part. Raman spectra of tannin: Raman features of the tannin in the wavelength range of 200-2500 cm-1 are shown in Figure 3. As shown in the figure, major Raman features of the tannin were observed in the spectral region of 600-1600 cm-1. Three main Raman peaks were identified in this region. The tannin showed its highest Raman intensity at 1590 cm-1, which was higher than that at 1357 cm-1. The other peak (650 cm-1) showed low intensity. As stated by many researchers (Shahidi and Naczk, 2004; Al-Farsi et al., 2005; Biglari et al., 2008), these bands are assigned to stretching C-C, C=C and C-H bonds which compose the structure of phytochemicals. Beyond 1600 cm-1, no notable Raman scattering signals were observed. Themain Raman features of tannin were revealed in the wavelength range of 600 to 1600 cm-1 since the main Raman features of tannin are in the wavelength range of 600-1600 cm-1, this region was used for calculating the spectral information divergence to evaluate the ripeness degree of the date fruits. Conclusions: This study reports the potential of FT Raman spectroscopy for nondestructive discriminating of Mazafati date fruits according to the four ripeness stages. The analysis of the Raman signal changes that happening during date ripening and its relationship with the ripeness degree of the date fruits was studied. In this regard, changes of pure tannin content in the wavelength range of 200-2500 cm-1 as a good ripeness index for date fruits was investigated. A modified polynomial, Self-Modeling mixture Analysis (SMA( and the Spectral Information Divergence (SID) was performed on different samples at four ripeness stages.

Keywords

• date fruit
• non-destructive evaluation
• raman spectroscopy
• quality factors