2.1. Peels and Pulps Color
The fig peels and pulps color showed significant differences among cultivars at
p < 0.001, with the exception of the pulp lightness coordinate (L*) (
Table 1). Therefore, peels’ chromatic coordinates present more accurate discrimination between cultivars than pulp color coordinates. Overall, peel color varied from bright yellow color (high and positive values of L* and c*) to atypical dark and blue purple color (negative L* and c* and high values of the hue). While in pulp, the color varied from pale pink (high values of L*, positive values of a* and b*) to dark red (low L* and c* and high positive a*). The cultivar “Trojana” had the brightest peels with the coordinates L* and c* recorded the highest values (73.15 and 50.94, respectively), whereas “INRA 1301” had the darkest colored figs (L* = 25.72 and c* = 22.09). Regarding pulp samples, the cultivars “Fassi” and “Breval Blanca” had the darkest color, where L* recorded the lowest values (18.6 and 19.05, respectively). All cultivars were classified based on their fruits’ peels and pulps characterization using principal component analysis (
Figure 1). Inspection of scatterplots showed that peels color displayed outlying subsets more than the pulp. Hence, the total variance obtained with peels data was of 91.51%, while pulps characterization accounted about 78.54%. In fact, the principal component analysis (PCA) scatterplot for peels’ chromatic coordinates splits the samples into two main groups describing blue-purple and light-colored cultivars. Having the lowest chroma value, the local cultivar “Fassi” was largely distinguished from the other subsets. However, a pulp samples scatterplot showed low discrimination resolution. Therefore, peel and pulp color evaluation using these coordinates is of great importance in fruits quality assessment. Several studies highlighted the importance of these descriptors to explore potential correlations between them and some antioxidant compounds, mainly phenols (anthocanins, tanins, catechins, etc.) and carotenoids (lycopene, beta-carotene, etc.) [
22,
23].
2.2. Spectrophotometric Assays
Total phenols (TPC), total flavonoids (TFC), total anthocyanins (TAC) and total proanthcyanidins content (TPAC) showed highly significant differences among cultivars, depending on their fruit parts (
p < 0.001) (
Table 1). These compounds were more than two times higher in fruits peels compared to their pulps, as observed in other fruits such as quince [
24] and apricot [
25]. This may suggest that peels are responsible of the higher level of figs total phenolics. A wide range of concentrations were obtained in both fruit parts except for TPAC, which showed a narrowed concentration interval (
Table 1).
In peels, TPC varied between 370 and 3162.86 mg GAE/100 g dw, while TFC were in the range of 188.57 and 2013.57 mg CE/100 g dw. TAC was highly abundant in dark samples and ranged between 4.14 and 192.5 mg cyanidin-3-rutinoside/100 g dw. In pulps extracts, TPC, TFC and TAC were in the range of 105.71–1255.71 mg GAE/100 g dw, 13.57–331.43 mg CE/100 g dw and 2.27–19.44 mg cyanidin/100 g dw, respectively. For both fruit parts, TPAC varied within a narrow interval of 0.2–3.09 and 0.2–1.06 mg cyanidin/100 g dw. Generally, they were present in high amounts in purple pulps when compared to light-colored ones. It is noteworthy that proanthocyanidins are quantified in all pulps samples as the same as the peels, which is probably due to the fact that they are the key determinant for red color in pulp and purple and blue colors skin fruits as well as anthocyanins [
26].
The local cultivar “INRA 1301” combined the highest levels of TPC, TFC, TAC and TPCA in its peels, where the mean values were of 2860.48 mg GAE/100 g dw, 1944.52 mg CE/100 g dw, 192.23 mg cyanidin-3-rutinoside/100 g dw and 2.59 mg cyanidin/100 g dw, respectively (
Table 2;
Table 3). Regarding the pulps, the local cultivar “Ghoudan” combined the highest amounts of TPC and TFC, where the mean concentrations were of 1186.67 mg GAE/100 g dw and 271.90 mg CE/100 g dw (
Table 2 and
Table 3). It is noteworthy that these compounds were found to be more abundant in dark-colored peels compared to light-colored ones, which is not always in the same sense regarding the fig pulps.
These results are consistent with those of Çalişkan and Polat. [
27], who reported that purple and black figs hold higher phenolic amounts than the green and yellow ones. The same observation was reported with Italian figs by Del Caro and Piga [
28] and Turkish ones where dark-colored fruits were mentioned to have higher levels of total phenols, flavonoids and anthocyanins than the light-colored ones, and those amounts were mainly concentrated in the peels [
29]. The significant difference between cultivars and their fruits peels’ and pulps’ phenolics contents has also been previously found by Harzallah et al. [
30] in three fig varieties growing in Tunisia and by Palmeira et al. [
31] in the Portuguese variety “Pingo de Mel”. These authors reported that the amounts of phytochemicals compounds are usually dependent not only on the variety but also differ significantly from one fruit part to the other. According to the same authors, the fig antioxidant potency seemed also to be mainly related to the peel part compared to the pulp part. The same result was reported in other consumed fruits, such as apricots [
25], quinces [
24], nectarines, plums and peaches [
32] and was mainly related to the genetic factor.
In the industrial processing of figs, the pulp is used, whilst the peel is usually discarded [
33], which generates a significant volume of byproducts consisting mainly of peels. In the studies conducted by Viuda-Martos et al. [
34] and Buenrostro-Figueroa et al. [
31], it was proven that these byproducts have abundant phytochemical compounds, which suggests their valorization and exploitation as nutraceuticals.
2.3. In Vitro Antioxidant Activity
Results of the free-radical-scavenging effect of figs’ peel and pulp extracts on DPPH• and ABTS•+ radicals and lipid peroxidation inhibition are summarized in
Table 1,
Table 2 and
Table 3. They are expressed as Trolox equivalent per g of dry weight and by the antioxidant concentration required for a 50% of radical reduction (IC50), so that a lower value of IC50 indicated a higher antioxidant activity and vice versa. These methods were combined to obtain an overview of figs antioxidant capacity, since no single assay can fully characterize the profile of each sample [
9]. Both the peel and pulp samples were proven to have antioxidant activities with significant differences (
p < 0.001) among all cultivars (
Table 1). In the DPPH assay, the values ranged from 21.23 to 367.26 mMol TE/g dw for peel samples, which is at least two times higher than the scavenging capacity exhibited by pulp samples, where the average concentrations ranged between 13.92 and 151.24 mMol TE/g dw (
Table 2). Regarding peel samples, the variety “Cuello Dama Blanca” recorded the highest antioxidant activity (AA) followed by “Fassi”, where the average values were of 333.99 and 332.13 mMol TE/g dw, respectively (
Table 3). Whereas “Trojana” and “INRA 2304′ exhibited the lowest AA (5.27 and 16.62 mMol TE/g dw, respectively). The pulp extracts present the low DPPH• scavenging activity, where “White Adriati” and “Chetoui” showed the highest values (121.65 and 104.73 mMol TE/g dw, respectively) (
Table 3).
The ABTS assay showed a wide range of variation for both peel and pulp antiradical capacity (7.57–563.53 and 6.59–207.49 mMol TE/g dw, respectively) (
Table 2). Peels of the cultivars “Chaari” and “Fassi” showed the highest AA (527.25 and 493.69 mMol TE/g, respectively), while “Cuello Dama Blanca” and “Snowden” fig pulps exhibited the highest AA, where the values were 204.68 and 160.43 mMol TE/g, respectively (
Table 2 and
Table 3).
The lipid peroxidation inhibitory effects of both fig parts were significantly different among cultivars and, generally, showed a narrow interval of variation compared to the other assays. Hence, in peels, the lipid peroxidation inhibition capacity (LPIC) was in the range of 139.17 and 353.11 mMol TE/g dw, whereas in pulps, it ranged between 42.89 and 226.88 mMol TE/g dw, respectively (
Table 2). Peels of “Kadota” and “Ghoudan” exhibited the highest LPIC (301.76 and 289.63 mMol TE/g dw, respectively), while “Bioudie” and “White Adriatic” had the lowest values (154.84 and 156.27 mMol TE/g dw, respectively). Similarly, pulps extracts displayed low LPIC compared to the peels, where “INRA 1305” and “Bioudie” showed the highest values (189.08 and 147.81 mMol TE/g dw, respectively), while “Sarilop” and “Ournaksi” recorded the lowest ones (57.85 and 67.12 mMol TE/g dw, respectively) (
Table 2 and
Table 3). To conclude, among all assays, figs’ peels seem to be the main contributors to the antioxidant capacity comparing to their pulps. In addition, dark-colored peels exhibited the highest antioxidant capacity compared to the light-colored ones. These results were similar to those reported by Solomon et al. [
29], Pande and Akoh, Ammar et al., Konak et al. [
35,
36,
37], where several methodologies have been employed to assess the in vitro antioxidant capacity of different fig parts. It is noteworthy that such in vitro antioxidant assays are semi-quantitative and do not always represent the in vivo antioxidant capacity [
38].
2.4. The Half Maximal Inhibitory Concentration (I50)
The IC50 is a variable that reflects the quality of radical scavenging for each of the antioxidant tests. The antioxidant potency, inversely proportional to the IC50 value, is more important when very small concentrations are required to scavenge half of the radicals [
13]. The IC50 results for both peel and pulp samples are summarized in
Table 1,
Table 2 and
Table 3. Indeed, significant divergences were spotted between sampled fruits following the cultivars and the fruit part investigated (
p > 0.001) (
Table 1). It is noteworthy that in all antioxidant assays, peels required very low concentrations to scavenge half of radicals, compared to pulp extracts. However, there are very few exceptions to this rule, where the pulps extracts had a higher IC50 values. In this case, “Breval Blanca”, “El Quoti Lbied” and “Kadota” exhibited higher DPPH IC50 values in their pulps compared to the peels’ extracts. Similarly, the local cultivars “Fassi” and “INRA 2201′ showed a higher LPIC IC50 in their pulps’ extracts than their peels. It should be noted that the first three cultivars have light-colored figs, whereas the last two give dark-colored fruits (
Table 2;
Table 3). A similar result was found by Harzallah et al. [
30], who reported that in DPPH assay, the IC50 purple pulps of some fig varieties were a little higher than their peels. It is probable that these differences are due to the partitioning of the phenolic compounds between both fruit parts and the radical scavenging potency of each compound [
39]. Among the 25 cultivars, the dark-colored peels of “INRA 2304” combined the lowest IC50 values for both DPPH (2.12 µg/mL) and ABTS (21.84 µg/mL) assays, which means that its peels required very low concentrations to scavenge 50% of free radicals. Taking all the assays together, the local cultivar “INRA 1302” peels combined the most promising IC 50 values, where the concentrations were of 3.97, 56.49 and 76.19 µg/mL, respectively, for DPPH, ABTS and LPIC assays (
Table 2). However, no cultivar had a similar combination for the pulps’ extracts. It is noteworthy that among all antioxidant assays, DPPH test had the lowest values of IC50, while ABTS showed the highest ones (
Table 2;
Table 3).
Even consumers usually prefer fruits with attractive appearance, especially the peels’ color, they tend, while eating the fruit, to remove the peel; however, this fruit part is evidently the major source of phenolic compounds that highly contribute to the antioxidant capacity and systematically protect against diseases related to oxidative stress. The consumption of the whole figs is clearly an important habit for promoting the health promoting diet in Mediterranean society [
30].
2.5. Polyphenolic Profile
High-performance liquid chromatography (HPLC) with a diode-array detector (DAD) analyses showed the presence of several phenolic compounds belonging to phenolic acids (hydroxycinnamic acid and hydroxybenzoic acid derivatives) and flavonoids (flavonols, flavones and anthocyanidins). Indeed, eight phenolic compounds, including: (+)-catechin, (−)-epicatechin, chlorogenic acid, quercetin-3-
O-rutinoside, quercetin-3-
O-glucoside, luteolin-7-
O-glucoside, cyanidin-3,5-diglucoside and cyanidin-3-
O-rutinoside, were detected in the pulp. While in peel extract, twelve compounds were isolated (gallic acid, (+)-catechin, (−)-epicatechin, chlorogenic acid, quercetin-3-
O-rutinoside, quercetin-3-
O-glucoside, luteolin-7-
O-glucoside, quercetin, apigenin, cyanidin-3,5-diglucoside, cyanidin-3-
O-rutinoside and pelargonidine-3-
O-rutinoside) (
Figure 2 and
Figure 3). These compounds showed significant differences among cultivars and fruits parts (
p < 0.001) (
Table 1). These results were similar to those reported by Vallejo et al. [
20], Viuda-Martos et al. [
34] and Harzallah et al. [
30].
Among all cultivars, the PCs’ concentrations were higher in peels compare to pulps extracts. Anthocyanins, particularly cyanidin-3,5-diglucoside and cyanidin-3-
O-rutinoside, were the predominant compounds in peels, where the mean concentrations were 75.902 ± 18.76 and 77.972 ± 18.95 µg/g dw, respectively. For flavonols, only (−)-epicatechin, quercetin-3-
O-rutinoside and quercetin-3-
O-glucoside were detected. Gallic acid and pelargonidin-3-O-rutinoside were only detected in the local cultivars “Chetoui” and “Nabout”, with the respective levels of 8.363 ± 1.88 and 6.731 ± 2.019 µg/g dw (
Table 4). These results agree with those reported for peels of the Portuguese variety “Pingo de Mel” by Palmeira et al. [
31]. The local cultivar “INRA 1301” presented the most interesting phenolic profile due to its very high levels of almost all detected PCs, especially (−)-epicatechin, quercetin-3-
O-rutinoside, quercetin-3-
O-glucoside, cyanidine-3,5-diglucoside and cyanidine-3-
O-rutinoside, with the main concentrations of 54.66, 141.08, 35.48, 494.08 and 478.66 µg/g dw, respectively (
Table 4). Likewise, the Spanish variety “Cuello Dama Blanca” combined the highest levels of chlorogenic acid, luteolin-7-
O-glucoside, quercetin and apigenin 8.76, 17.9, 59.52 and 4.84 µg/g dw, respectively.
In pulps extracts, (−)-epicatechin and cyanidin-3-
O-rutinoside were the major compounds. They were detected in all cultivars at high levels (5.23 ± 4.03 and 9.01 ± 5.67 µg/g dw, respectively). Cyanidin-3,5-diglucoside were the third predominant compound, that ranged from 0.81 to 28.45 µg/g dw, with a mean of 6.06 ± 6.71 µg/g dw, followed by (+)-catechin and chlorogenic acid (1.93 ± 1.29 and 1.01 ± 1.16 µg/g dw, respectively). However, luteolin-7-
O-glucoside was detected in only two cultivars, “Chetoui” and “Palmeras”, with respective concentrations of 0.75 ± 0.35 and 4.47 ± 0.04 µg/g dw (
Table 5). These results are generally in agreement with those of Del Caro and Piga. [
28], who used the same method on the Italian varieties “Mattalon” and “San Pietro”. These concentrations, mainly of (+)-Catechin, cyanidin-3-
O-rutinoside and luteolin-7-O-glucoside, are higher in comparison with bananas, pears and apples; however, they are similar to black grapes [
40].
In the study of Palmeira et al. [
31], rutin (quercetin-3-
O-rutinoside) was the predominant compound in fig skin, in contrast to our results, where cyanidine-3,5-diglucoside and cyanidine-3-
O-rutinoside were predominant. Several works on the species have shown that PCs are strongly dependent on the cultivar but is also influenced by other factors including the fraction analyzed (pulp, peel or juice), the ripening stage and the growing conditions [
15,
16,
30]. This is consistent with the results of Solomon et al. and Del Caro and Piga [
28,
29]. Finally, the PCs in fig pulp represent about 20% of the total concentration in the whole fruit. It is noteworthy that these are the first results reported regarding figs’ phenolic composition and their partitioning between the peel and the pulp of a large fig cultivar numbers growing under Moroccan climate, with respect to their antioxidant, chromatic coordinates. The herein reported findings are of great importance for efficient nutraceutical use of these raw materials.
2.6. Heat Map Analysis
Data visualization is an essential tool for biochemical data analysis, and dimensionality reduction methods, such as principal component analysis (PCA), are usually used to draw high dimensional data onto two- or three-dimensional space so it can be visualized. However, this transition is costly, often resulting in loss of the total variance. A hierarchically clustered heatmap is one of numerous analyses that does not need a dimensionality reduction to visualize data. It is a widely used technique to analyze complex biological data by displaying network connections in a symmetric adjacency matrix [
41].
Color-coded two-dimensional heatmaps for both fruit parts are formed with two clusters using Euclidean distance following Ward method; one is sample-oriented while the other is variable-oriented (
Figure 4). In this figure, weak correlations between studied variables are displayed in low color intensity, while stronger ones are shown with high color intensity. Cultivars and variables clustering as well as the correlations among dataset were quite different between fig peel and pulp. In pulp samples, the chromatic coordinates (L*, c*, h°) were clustered with LPIC and the IC50 of DPPH and ABTS assays, which are correlated to quercetin and apigenin. These compounds seem to have a large effect on the peel antioxidant potency (
Figure S1). These variables tend to be higher in the cultivars “Trojana”, “Breval Blnaca”, “Ournaksi”, “Bioudie” and “Nabout” that constitute, among others, a distinctive cluster. On the other hand, catechin, luteolin-7-O-glucoside, quercetin-3-
O-rutinoside, epicatechin and chlorogenic acid were clustered together and correlated to TPC, TFC and IC50 (LPIC). These compounds showed similar tendencies to be accumulated by the local cultivars “Chetoui”, “Noukali”, “INRA 2305′, “Ghoudan” and “Chaari”, which constitute a homogenous cluster. It is noteworthy that these cultivars are characterized by dark-colored figs, which are known to hold abundant amounts of these compounds. Pelargonidin-3-O-rutinoside, canidin-3,5-diglucoside and cyanidin-3-
O-rutinoside are anthocyanins that were clustered together with total proanthocyanins and revealed a strong correlation to the free radical scavenging capacity of peel extracts (
Figure S2). The pigments belong to the flavonoid class and seem to be the major contributors to the free radical scavenging process of fig peels. The local cultivar “INRA 1301” is clustered as a single branch and therefore largely distinguished from the other clusters. It combined the highest levels of flavonoids compounds and consequently showed high level of DPPH• and ABTS•+ radical scavenging capacity. This cultivar has dark-colored fruits, which is in accordance with several studies, which showed that fig skins have much higher amounts of phytochemical compounds, mainly flavonoids, which strongly contribute to the antioxidant capacity [
18,
29,
30,
35,
37].
The pulp heatmap showed a different spatial distribution of individuals and variables, where catechin, epicatechin, chlorogenic acid, cyanidine-3.5-diglucoside and cyanidine-3-O-rutinoside were the highly correlated variables, which were related to the free radical scavenging capacity. The cluster composed of the cultivars “INRA 2105”, “INRA 1302” and “White Adriatic” showed similar tendencies to accumulate these variables. “Chetoui”, the nearest neighbor to this cluster, combined the highest level of quercetin-3-O-rutinoside, quercetin-3-O-glucoside and TPAC. The other cultivars were essentially clustered based on their pulp chromatic coordinates that seemed moderately correlated to TAC and the antioxidant potential.