The optimization of ultrasonic-assisted extraction of Centaurea sp. antioxidative phenolic compounds using response surface methodology
Introduction
The genus Centaurea is the fourth largest genus in the family Asteraceae and includes approximately 700 species of herbaceous thistle-like flowering plants essentially centered in the Mediterranean region and South-West Asia (Tekeli et al., 2010). In Algeria, it is represented by 45 species including 7 in the Sahara (Azzouzi et al., 2016). The species of the genus Centaurea have been subjected to numerous phytochemical examinations which have revealed their wealthy potent of bioactive secondary metabolites, and in particular phenolics (Albayrak et al., 2017). Several recent pharmacological studies have investigated various species of Centaurea. For example, Centaurea pulchella, C. drabifolia, and C. solstitialis are used to treat abscesses, hemorrhoids, peptic ulcers, microbial infections, cancer, and ulcers (Aktumsek et al., 2011). Studies have revealed the importance of these species for the food and pharmacological industries (Ayaz et al., 2017; Kumarasamy et al., 2003).
In Algeria, the Centaurea sp. is usually known as “Mers’gousse” (Fig. 1). Even though this plant has been employed in traditional medicine to treat skin burns and various deformities resulting from them, nevertheless, no uses or pharmacological studies have yet been reported for this Centaurea sp. (Hamadi et al., 2014). Although promising as a medicinal crop, this species has become threatened with extinction in Algeria as a result of extensive consumption by the local inhabitants and the absence of any specific measures for its conservation and commercialization.
Among others, polyphenolics have been categorized under nutraceutical optically active compound and have been proposed in the treatment of various diseases due to their anticarcinogenic, antimicrobial, antiviral and antioxidant properties, etc. (Teplova et al., 2018). Numerous studies have employed ultrasound-assisted extraction for polyphenolic compounds from various vegetal samples (Medina-Torres et al., 2017). This technique is also regarded as a green and environmental process (Irakli et al., 2018; Pandey et al., 2018). The extraction of different bioactive components from various plants with medicinal characteristics cannot follow a unique universal preset list of optimal conditions (Chirinos et al., 2007). Optimizations of extraction conditions are therefore essential. Ultrasonic extraction has been shown to be more, or at least similarly effective to other conventional extraction methods. The key advantages of this technique are a dramatically shorter effective extraction time, and reproducibility and applicability for wide sample sizes. The ultrasound waves generate a phenomenon known as “cavitation bubbles” when waves propagate trough a liquid material medium. Then, due to a series of cycles of rarefaction and compression, the bubbles collapse. As a result of this cavitation phenomena, the solvent will penetrate more deeply into the sample material, which may be explained by the larger surface area of contact between both solid and liquid phases, and thus in an improvement of the annual transfer process (Shirsath et al., 2012).
During the UAE, there are variables that are directly related to extraction efficiency and yield, such as the extraction conditions: time, temperature, solvent type and concentration used (Katsampa et al., 2015). Procedures involving optimization multivariate techniques are much used in the development of analytical procedures because they are more economical and effective, and allow more than one variable to be optimized simultaneously (Dias et al., 2013; Meira and de Souza Dias, 2017).
The current study aimed (a) first, to compare the most appropriate month of plant collection and to determine and the most efficient solvent among of four with different polarities (methanol, ethanol, ethyl-acetate and acetone) that maximize the UAE of the total phenolic compounds (TPC) content from Centaurea sp. leaves; (b) to determine the values of three UAE conditions variables (time, temperature and sample-to-solvent ratio) using appropriate solvent determined in the previous step that maximize the TPC yield. This screening is performed over seven consecutive months of collections (whole growing period) of Centaurea sp. leaves using the single-factor experiment analysis; (c) to optimize UAE extraction conditions (time, temperature and sample-to-solvent ratio) of that maximize both TPC content and the antioxidative capacity (using DPPH and FRAP assays) all together; and finally (d) to analyze and quantify the phenolic acids compounds using the High Performance Liquid Chromatography-Diode Array Detector (HPLC-DAD) technique under optimal determined conditions and to identify the most appropriate solvent and optimal month of collection for maximum phenolic compound yield.
Section snippets
Plant materials
Leaf samples of Centaurea sp. (Asteraceae) were collected from the Dellys region (36°54′48′′ North, longitude 3°45′51′′ East, altitude 200 m above sea level), in the Algerian province of Boumerdès in 2017. Voucher specimen under a reference code name of CSsp-Plv-BMRDS-Y17 was deposited in the herbarium of the Research Laboratory of Fundamental Sciences at Laghouat University. The collecting process was performed regularly at the same location spot for seven months, beginning with January and
Effect of solvent type on the yield of phenolics
The selection of a suitable solvent is a crucial step in the extraction process and directly affects the extraction efficiency. More precisely, the solvent system selection is based primarily on the extraction purpose, that involves the polarities of desired/undesired components, solvent safety and process costs (Yu et al., 2002). Additionally, the solubility of phenolic compounds in different solvents cannot be based only on their polarities as the solubility is related to different parameters
Conclusion
Prior to the optimization process, four solvents with different polarities (methanol, ethanol, ethyl-acetate and acetone), were employed and tested over several months of sample collection (whole growth season). Methanol was by far the most appropriate solvent for obtaining a considerable recovery amount of TPC content in April. This study determined the best conditions for the three variables, temperature, time and solvent volume, to obtain the best (optimal) results for TPC, DPPH and FRAP
Declaration of Competing Interest
None
Acknowledgments
The authors are most grateful to Dr. Mohamed Toumi (Bioactive Molecules Laboratory and Valorization of Biomass, Superior National School “École Normale Supérieure de Kouba”, Algiers, Algeria) for his valuable contributions to the plant identification. The authors also wish to acknowledge the DGRSDT (Direction Générale de la Recherche Scientifique et du Développement Technologique) of the MESRS (Ministère de l’Enseignement Supérieur et de la Recherche Scientifique) of Algeria for the regular
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