Αntioxidant activity of Cynara scolymus L. and Cynara cardunculus L. extracts obtained by different extraction techniques

Abstract Extracts of different parts (heads, bracts and stems) of Cynara cardunculus L. (cardoon) and Cynara scolymus L. (globe artichoke), obtained by two different extraction techniques (Ultrasound-Assisted Extraction (UAE) and classical extraction (CE)) were examined and compared for their total phenolic content (TPC) and their antioxidant activity. Moreover, infusions of the plant’s parts were also analysed and compared to aforementioned samples. Results showed that cardoon’s heads extract (obtained by Ultrasound-Assisted Extraction) displayed the highest TPC values (1.57 mg Gallic Acid Equivalents (GAE) g−1 fresh weight (fw)), the highest DPPH• scavenging activity (IC50; 0.91 mg ml−1) and the highest ABTS•+ radical scavenging capacity (2.08 mg Trolox Equivalents (TE) g−1 fw) compared to infusions and other extracts studied. Moreover, Ultrasound-Assisted Extraction technique proved to be more appropriate and effective for the extraction of antiradical and phenolic compounds. Graphical Abstract


Introduction
Cynara is a genus belonging to the Asteraceae (sunflower) family; it comprises of many species native to the Mediterranean basin which includes the wild artichoke (cardoon) Cynara cardunculus L. and the cultivated globe artichoke Cynara scolymus L. (Lanteri & Portis 2008). C. cardunculus L. and C. scolymus L. are edible vegetable widely consumed in the Mediterranean diet (Coinu et al. 2007) and the parts that usually consumed are mostly the heads of the plant (inner part of the flower).
Many studies have demonstrated that artichoke has major medicinal properties, including antioxidative, anticarcinogenic, antigenotoxic, cholesterol-lowering, hepatoprotective, bile-expelling, diuretic and anti-inflammatory, as well as antifungal, anti-HIV and antibacterial effects (Agarwal & Mukhtar 1996;Gebhardt 1997;Miadokova et al. 2008). Nutritional and pharmaceutical properties of artichoke are linked to their special chemical composition, which includes high levels of polyphenolic compounds and inulin. Caffeic acid derivatives are the main phenolic compounds in artichoke. Other phenolics such as the flavonoids, apigenin and luteolin as well as different cyanidin caffeoylglucoside derivatives and chlorogenic acid have been identified in artichoke tissues while recently apigenin-7-rutinoside and narirutin, were found to be unique to artichoke heads , Wang et al. 2003;Zhu et al. 2004;Lattanzio et al. 2009). It is also reported that artichokes contain cynarin  and are rich in fibre and low in fat. Hence its health benefits regard high fibre content (Emanuel et al. 2011).
The aim of this work was to study and compare the antioxidant activity of the extracts of the different parts (heads, bracts, stems) of globe and wild artichoke (cardoon) by applying different extraction techniques (Ultrasound-Assisted Extraction, Classical Extraction). Infusions were also prepared and compared to aforementioned extracts.

Total phenolic content
Results from Folin-Ciocalteu assay showed that between the three different extraction techniques there is significant difference (p < 0.05) as far as the TPCs of globe artichoke and cardoon is concerned. Specifically the ultrasound-assisted extraction technique, seems to be more effective for the extraction of the phenolic compounds (p < 0.05). Moreover, heads of globe artichoke and cardoon contained higher amounts of TPC compared to bracts and stems. Finally between the two plants, cardoon (C. cardunculus L.) had considerably higher amounts of TPC (p < 0.05) ( Table 1).

Antioxidant activity
Antioxidant activity of artichokes' extracts was determined by DPPH and TEAC assays.
DPPH assay: Statistical analysis of the radical scavenging activity (RSA) values of both kinds of artichoke extracts; showed again that the UAE seems to be better extraction technique compared to CE and infusions. Furthermore, there is no significant difference between the RSA of the extracts of artichoke and cardoon different parts. Considering the overall results, cardoon's heads extract, obtained by UAE technique, exhibited the higher RSA (lowest IC 50 ) compared to all the other extracts (Table 2). TEAC assay: Statistical analysis showed that there are some significant differences (p > 0.05) between the different extraction techniques, for both species of artichoke. Specifically heads of cardoon showed better results as far as the concentration of the antiradical compounds is concerned compared to all other extracts ( Table 3).
The above results lead to the conclusion that the extraction techniques (different conditions of temperature/extraction time) exhibited a straight effect on the antioxidant properties and the TPCs of the extracts. Almost in all cases UAE showed better results, probably because of the cavitation bubbles that created by the sound waves near the sample tissue and release cell contents by breaking down the cell walls (Khoddami et al. 2013). In the literature, differences have been reported concerning the phenolic content and the antioxidant activity of artichoke species and their organs. (Falleh et al. 2008;Soumaya et al. 2013). Previously Falleh et al. (2008) observed that the TPC values of C. cardunculus L. leaf were two times higher than those in heads while Wang et al. (2003) showed that the leaves of Table 1. tPc values of cardoon and globe artichoke extracts (heads, bracts, stems) from different extraction techniques (uae: ultrasound-assisted extraction, ce: classical extraction and hI: herbal Infusion).
Note: Means in the same row bearing different letters differ significantly (p < 0.05). Means in the same column bearing different numbers differ significantly (p < 0.05). results represent means ± sd (N = 3 separate samples).

Cardoon
Globe artichoke  Table 3. teac assay: values expressed as mg te g −1 FW for cardoon and globe artichoke extracts (heads, bracts and stems) from different extraction techniques (uae: ultrasound-assisted extraction, ce: classical extraction and hI: herbal Infusion).
Note: Means in the same row bearing different letters differ significantly (p < 0.05). Means in the same column bearing different numbers differ significantly (p < 0.05). results represent means ± sd (N = 3 separate samples).

Cardoon Globe artichoke
Heads Bracts Stems Heads Bracts Stems uae 2.08 ± 0.01 a C. scolymus L. contained the highest concentration of total phenols and that the 'younger' heads have higher phenol contents than the mature heads. Moreover, a study about the phenolic content and in vitro antioxidant activity of the bracts, leaves and floral stems of two Tunisian globe artichoke cultivars revealed that the leaves were associated with more antioxidant activity than the bracts or the floral stems (Dabbou et al. 2015). However, these differences are probably due to the different origin, genetic background, climatic/environment conditions, cultivation conditions and harvest time. However, these differences are probably due to the different origin, climatic conditions, cultivation conditions and harvest time. In the present study, cardoon's heads extract obtained from UAE, showed maximum TPC values (1.57 mg Gallic Acid Equivalents (GAE) g −1 fresh weight (fw)), the highest DPPH • scavenging activity (IC 50 ; 0.91 mg ml −1 ) and the highest ABTS •+ radical scavenging capacity (2.08 mg Trolox Equivalents (TE) g −1 fw) in comparison with all the other extracts.

Conclusion
Ultrasound-Assisted Extraction technique proved to be more appropriate and effective for the extraction of antiradical and phenolic compounds from the two kinds of artichoke. Cardoon's heads UAE extracts displayed significant differences for TPC values (1.57 mg Gallic Acid Equivalents (GAE) g −1 fresh weight (fw)), DPPH • scavenging activity (IC 50 ; 0.91 mg ml −1 ) and ABTS •+ radical scavenging capacity (2.08 mg Trolox Equivalents (TE) g −1 fw) compared to all other parts of both plants (cardoon and globe artichoke) under CE and HI techniques. Certainly more research is needed concerning the chemical characterisation of extracts and infusion to understand which compounds confer to the antioxidant activity that proved in the current research.

Disclosure statement
No potential conflict of interest was reported by the authors.

Funding
This work was supported by the National and Kapodistrian University of Athens [grant number 11400].