NIR spectroscopy-multivariate analysis for rapid authentication, detection and quantification of common plant adulterants in saffron (Crocus sativus L.) stigmas
Introduction
Among 85 known spices in the world, saffron, the dried stigmas of Crocus Sativus L. family. Iridaceae, is the most high-priced one. It also appears as a top member on the list of the most precious agricultural products, for which reasons, saffron is non-surprisingly known as Red Gold (Husaini et al., 2009). Saffron flowers, with only three stigmas each, are used as food additive due to their aroma, color, and bitter taste (Singh, Singh, & Verma, 2010). Saffron has a long history of use in folk medicine and Ayurvedic health system as a sedative, expectorant, anti-asthma, emmenagogue, adaptogenic agent, and in various preparations for pain relief (Schmidt, Betti, & Hensel, 2007).
Being highly expensive, opened the window for cheaters to adulterate saffron with cheaper adulterants, constituting danger for customers (Petrakis, Cagliani, Tarantilis, Polissiou, & Consonni, 2017). Historically, saffron stigmas have been commonly adulterated with other plant materials possessing similar appearance. There have been several reports demonstrating that dried Crocus sativus stamens, Carthamus tinctorius L. petals (safflower), Calendula officinalis L. petals (calendula), Curcuma longa L. rhizomes, Hibiscus sabdariffa calyces (hibiscus), Capsicum annum fruit (paprika), and Punica granatum fruit peel (pomegranate) in addition to exhaustively extracted saffron stigmas are habitually experienced to be detected as incorporated materials used to fraud saffron (Dowlatabadi et al., 2017; Guijarro-Díez, Castro-Puyana, Crego, & Marina, 2017; Heidarbeigi et al., 2015; Mohamad & Shakeel, 2015; Petrakis, Cagliani, Polissiou, & Consonni, 2015; Petrakis et al., 2017; Schumacher, Mayer, Sproll, Lachenmeier, & Kuballa, 2016, p. 13; Varliklioz Er, Eksi-Kocak, Yetim, & Boyaci, 2017).
The ISO 3632–2 for grading saffron reports a standard UV–vis spectrophotometric method which has been used to detect adulteration of saffron with some of its common adulterants like safflower, turmeric, or calendula, but with considerably very high adulteration levels of about 200 mg/g (Petrakis & Polissiou, 2017; Sabatino et al., 2011).
Molecular methods are now the most widely used techniques for detection of saffron adulteration by materials of plant origin (Babaei, Talebi, & Bahar, 2014; Gul, Nasrullah, Nissar, Saifi, & Abdin, 2018). The main pitfalls of such methods include the high cost of sample preparation and inability to detect adulteration practices with plant extracts or dried powders, most likely due to the lack of DNA material that can be recovered (Soffritti et al., 2016). Several other methods are reported for saffron adulteration detection including High performance liquid chromatography (HPLC) hyphenated to photodiode array (PDA) and electrospray ionization mass spectrometry (ESI-MS) (Sabatino et al., 2011), electronic nose system coupled to multivariate statistical analysis (Heidarbeigi et al., 2015) and finally 1H nuclear magnetic resonance (1H NMR) metabolomics (Petrakis et al., 2015). Infrared (IR) spectroscopy has emerged as a lucrative time saving and non-destructive technique for quality assessment of foods and herbal products (Chen et al., 2016; Wilde, Haughey, Galvin-King, & Elliott, 2019). Indeed, Fourier transform mid infrared spectroscopy was implemented for the characterization of crocins and apocarotenoids, (Biancolillo & Marini, 2018; Tarantilis, Beljebbar, Manfait, & Polissiou, 1998). Adulteration of saffron with six plant materials; Crocus sativus stamens, calendula, safflower, turmeric, buddleja, and gardenia could distinctly be identified and evaluated in another study by mid-range infrared spectroscopy (4000–600 cm−1) (Petrakis & Polissiou, 2017).
Meanwhile, the application of near infrared spectroscopy (NIRS) for quality control and detection of saffron adulteration is not yet well investigated. Previous reports include the application of NIRS for the determining of geographical origin as well as the chemical composition of saffron samples from the main producers' countries (Anastasaki et al., 2010; Zalacain et al., 2005). In another report, NIRS combined with multivariate analytical was utilized for quantitative determination of crocin I and II in saffron (Li et al., 2018). Previous utilization of two-dimensional infrared spectroscopy in conjunction with PCA was reported. However, near infrared (NIR) extracted data, together with micro-spectroscopic imaging and FT-IR were used only for evaluation of one common adulterant (i.e. safflower) (Chen, Zhou, & Sun, 2016).
In view of the above mentioned points, the aim of the presented work is to investigate the feasibility of applying near infrared spectroscopy combined to multivariate statistical analysis for rapid, and precise authentication of saffron as well as detection and quantification of adulteration by plant materials at low levels which is considered a challenging task. The study aims at designing a regularity quality control protocol that can be applied for the routine analysis of saffron stigmas.
Section snippets
Plant materials
A total of 50 saffron stigma samples were purchased from reputable companies and their botanical identity were confirmed by the Department of Industrial crops, Faculty of Agriculture, Alexandria University, Egypt. Details of the samples are listed in Table S1. The samples were identified through inspection of their macroscopical and microscopical features. Taxonomical validation through http://mpns.kew.org/mpns-portal/are available as supplementary material.
Exhaustively-extracted saffron
Results and discussion
A workflow of the establishment of the different multivariate models used in this work including SIMCA and PLS-DA classification models, PLSR models, pre-processing, variable selection, internal and external validation is given in Fig. 1.
Conclusion
The main useful target of this work is the development of a practical strategy for the authentication and detection of commercial adulteration of the expensive saffron stigmas by other cheaper plant material using NIR spectroscopy combined with multivariate data analysis. SIMCA class modeling technique was successfully applied for authentication of saffron while PLS-discriminant analysis could effectively discriminate authentic pure samples and intentionally adulterated ones as low as 10 mg/g
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
CRediT authorship contribution statement
Eman Shawky: Conceptualization, Validation, Formal analysis, Data curation, Investigation, Writing - review & editing, Visualization. Rasha M. Abu El-Khair: Validation, Data curation, Investigation, Writing - review & editing, Visualization. Dina A. Selim: Validation, Data curation, Investigation, Writing - review & editing, Visualization.
Declaration of competing interest
All authors of the manuscript entitled “NIR spectroscopy-multivariate analysis for rapid authentication, detection and quantification of common plant adulterants in saffron (Crocus sativus L.) stigmas “, declare that they have no conflict of interests.
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