4.1. Artichoke Discards Proximal Characterization
The proximal composition of artichoke residues shows a significant content of mainly insoluble dietary fiber. This characteristic, and the low amount of fat observed (1.3%), make these dried byproducts useful and valuable for industrial manufacture as low fat, high-fiber food ingredients with potential health benefits, reducing the risk of severe disorders, such as obesity, cardiovascular diseases, diabetes, colon-rectal cancer and others [2
]. Even though the proximal characterization of artichoke discards has not been reported, when compared to those reported by Lutz et al. (2011) [31
] of artichoke heads, we observe a greater presence of ashes and total carbohydrates (dietary fiber and others: 75.53%) in agro-industrial discards. These differences may be due to the absence in our samples of the receptacle, which is very rich in soluble dietary fiber. Claus et al. (2015) [35
], studying artichoke bracts, observed a different content of fiber than that presented in Figure 1
, because only crude fiber may have been quantified, the value of which can approximate to the insoluble fiber content (mainly cellulose and lignin) of the materials under analysis. On the other hand, the highly recommended insoluble/soluble dietary fiber ratio presented a value of 2.3, which is much higher than that reported for artichoke heads (0.26: López et al., 1996 [36
Regarding inulin, Lattanzio et al. (2009) [4
] stated the content of inulin in artichoke ranges from 19% to 34% (dw) and may represent up to 75% of the total glucosidic content. In any case, the percentage content of inulin varies according to the variety and parts of the plant studied and even the geographical locations and harvesting times [2
]. Although Ruiz-Cano et al. (2014) [2
] obtained the highest proportion of inulin in inner bracts (thermally treated), Ceccarelli et al. (2010) [1
] reported flower heads to be particularly rich in this polysaccharide in European artichoke varieties. In contrast, this work was conducted on the most external parts of a South American (Chilean) variety.
The low content of inulin in artichoke industrial discards (7%) compared to those reported by the mentioned authors is explained by the lack of receptacle, heart and/or inner bracts of artichoke, fractions which are industrially processed. This result is in agreement with that reported by López-Molina et al. (2005) [19
] in artichoke discards, but lower compared to reports from other authors on the edible parts of the plant (heads of the Green Globe variety).
4.2. Total Phenolic Content (TPC) and Antioxidant Capacity (AOC)
The highest result obtained both for TPC and AOC—2461.89 mg GAE/100 g (dw) and 61,014.67 µmol TE/100 g (dw) respectively—was registered at 40 min and 60 °C of temperature. Good recoveries were also obtained in milder conditions—2155.75 mg GAE/100 g (dw) and 55,472.35 µmol TE/100 g (dw) at 40 °C and 10 min of reaction. These results are in line with those reported on European varieties of artichoke by Zuorro et al. (2014) [12
]: 2414–3571 mg GAE/100 g in outer bracts and stems, and those by Curadi et al. (2005) [30
], who reported 480–2980 mg GAE/100 g in the edible parts of the plant. On the other hand, Ninfali et al. (2005) [28
] reported an AOC of 6552 µmol TE/100 g (fresh weight (fw)), which would give a result higher than our values, considering the 86% moisture content in our raw material. For their part, Jiménez-Escrig et al. (2003) [37
] reported values of 5040 mg GAE/100 g in the edible parts of the plant, using more aggressive solvents and elaborated extraction protocols.
Regarding the phenolic content according to the part of the plant, some authors reported it to be higher in the heart and inner bracts of artichoke [4
], while others described the non-edible parts of the plant (outer bracts, leaves and stems) as being more enriched in polyphenolic compounds [6
]. The variety of the vegetable and time of harvesting are also important factors to take into account [1
] when comparing results from previous studies, mainly conducted on Mediterranean varieties. All in all, the TPC and AOC values of this study were higher than expected for Green Globe American varieties [1
]. Nevertheless, the wide range of factors determining the amount of phenolic compounds obtained (variety, time of harvesting, part of the plant studied, conditioning of raw sample, solvents and extraction techniques used) our results are consistent with those obtained from previous studies with similar characteristics [2
]. Obtained results show the high content of antioxidants in the extracts from artichoke discards of this study, observing ORAC values much higher than those reported on products considered “superfruits” [40
]. These results suggest the enormous potential application of these extracts as antioxidant ingredients and the benefits/suitability of utilizing industrial discards of artichokes.
Both for the recovery of phenolic compounds and the assessed antioxidant activity, it is possible to appreciate that the greatest effect of the operational conditions is related to the concentration of ethanol in the solution, followed by the temperature in the extraction process. In the first case, as expected, the values increased by increasing the concentration of ethanol, due to the polarity of the compounds present in the discards. It is important to remember that the dielectric constant of water and ethanol corresponds to 82 and 24, respectively, so mixtures of these solvents would allow both polar and non-polar compounds to be extracted. In addition to this, it was possible to determine that the pure solvent (ethanol) generated lower results than those obtained with hydroalcoholic mixtures, with the consequent joining and dragging effect of the phenolic compounds from the raw matter. These results contrast with studies by Zuorro et al. (2014) [12
], who described the time of extraction as the most affecting parameter in the obtaining of TPC and AOC from outer bracts of artichoke with an aqueous ethanol solution, followed by temperature and sample weight (solvent volume ratio parameters). These differences may be explained by the Italian variety of artichoke studied by this author, in contrast with the South American used in this study. In addition to this, the blanching procedure (85 °C in water, 15 min) implemented by Zuorro et al. (2014) [12
], could have contributed to the partial hydrolysis of the cell wall and subsequent release of polyphenols.
On the other hand, by increasing the process temperature between 30 °C and 40 °C an increase in the values of TPC and AOC was observed, due to the higher solubility of the compounds in the extraction solutions. Nevertheless, when increasing the temperature above these values, no improvement in the extraction performance was observed. Temperatures above 40 °C proved efficient to release polyphenols from the plant tissue due to the matrix softening in the hydroalcoholic extract, as previously reported by Lutz et al. (2011) [31
]. This process does not seem to happen at lower temperatures, such as 30 °C; hence the poor results of TPC and AOC registered at this temperature in all the concentrations of solvent and times of extraction studied. It is interesting to mention that artichokes possess endogenous enzymes that may affect the presence of phenolic compounds [2
], which can be inactivated at high process temperatures, such as in the bleaching process, but which also affect the polyphenol stability. In some cases, the loss in the recovery of polyphenolic compounds after the bleaching treatment was reported up to 46% [30
], while other authors reported that although some polyphenols were lost, some other types were observed [31
4.3. Artichoke Extract Characterization
The polyphenol profile shows a high presence of caffeoylquinic and dicaffeoylquinic acids, which is in agreement with studies from previous authors [1
In the case of dicaffeoylquinic acid (45.97%), however, this compound was not identified as cynarin specifically. Interestingly, the cynarin content in South American varieties of the artichoke is low, as previously reported by Silva da Costa et al. (2013) [3
] and Noldin et al. (2003) [41
] with Brazilian genotypes. In addition, the lack of blanching procedures could have affected the recovery of cynarin in its natural molecular structure, due to the potential presence of polyphenol oxidase enzyme, which is aggressive with ortho-dihydroxyphenolic compounds, as is the case with cynarin [12
]. The chlorogenic acid (3-O
-caffeoylquinic acid) identified represents a third of the TPC in the extract (around 33%), which is in line with that reported by Schutz et al. (2004) [33
] (33.3%) in artichoke heads and slightly lower than that reported by Lattanzio et al. (2009) [4
] (39%). Chlorogenic acid was 113.34 µg/mL, within the range of values described by Lombardo et al. (2010) [42
] in outer bracts of Italian varieties, and in a higher concentration than that obtained by Garbetta et al. (2014) [9
] using a methanolic solution in artichoke heads, although less than reported by Ceccarelli et al. (2010) [1
], who also focused on the edible parts of Mediterranean varieties of the vegetable.
Finally, the low concentration of flavones and their derivatives found, compared to previous studies from other authors, support the already stated great diversity of the polyphenolic composition amongst the worldwide varieties of artichoke, different parts of the plant, harvesting times and different extraction procedures [2
Another attractive component in the extract is inulin, in a 28% (dw) concentration.
The multiple reports on the functional properties of inulin would enhance its use as an ingredient to improve digestive deficiencies. Moreover, the presence of inulin in the extract would promote a protective effect on antioxidant polyphenols [44
It would, therefore, be interesting to study the synergic effects between inulin and phenolic antioxidants, which would enhance both their functional applications [45
4.4. Effect of Artichoke Extracts on the Viability of Cancer Cell Lines
Low concentrations of extract produced an increase of the viability of three cell lines, probably due to the presence of other compounds that could exert a positive effect on the cell growth, especially on fibroblasts. However, by increasing the concentration of the extract, the carcinogenic lines (Caco-2 and MCF-7) decreased their proliferation. A higher potential resistance of the fibroblasts to the presence of the extract was observed, requiring at least twice the concentration than that for the tumor cells to decrease cell proliferation.
These results are in agreement with those reported by Mileo et al. (2012) [46
]. They evaluated the anti-proliferative effect of an extract from the edible part of the artichoke on breast cancer cell lines, observing a dose-dependent effect on the cell line viability and growth, with no effect on healthy breast epithelial cells. However, the assays were conducted using extracts with concentrations up to 800 µM (equivalent to 283.5 mg/L chlorogenic acid).