Effect of sun-drying on polyphenols and in vitro bioavailability of sarilop and Bursa siyahi figs (Ficus carica L.)

Figs have been consumed as a part of Mediterranean diet for centuries either as fresh or dried fruit. Many epidemiological studies suggest that regular consumption of fruits and vegetables, including fig fruit, can play an important role in preventing cancer and cardiovascular diseases. The nutritional value of figs created an interest in the effect of processing on its valuable compounds showing antioxidant activity.

In this study, in order to investigate the effect of sun-drying on the health-related constituents as well as bioavailability of figs; total phenolics, flavonoids, proanthocyanidins, anthocyanins, lycopene, antioxidant capacity, major phenolic compounds and color properties were determined for two commercial fig varieties (Sarilop and Bursa siyahi) differing in color (yellow and dark purple) along with simulation of in vitro gastrointestinal (GI) digestion. For each variety, four fractions were prepared, skin, pulp, and whole and dried fruit and all samples were obtained as three replicates. Major phenolic compounds were determined using high- performance liquid chromatography (HPLC) coupled with photodiode array (PDA) and fluorescence detectors.

The results showed that the moisture loss induced by processing fresh fig fruits into dried fruits was found to be 30% and 56% for Bursa siyahi and Sarilop varieties, respectively. In order to eliminate the differences in the moisture contents, all results are expressed on dry weight basis.

Fresh Sarilop skin showed lightness values (L*) of 63.4 and hue angles (H°) of 96.1, indicating a bright yellow color; whereas fresh Bursa siyahi skin showed lightness values (L*) of 33.4 and hue angles (H°) of 324.7, attesting to the typical dark purple color. Fresh fruit skins appeared to be brighter than dried ones, as reflected by 10% and 18% decrease in L* for Sarilop and Bursa siyahi, respectively.

Total phenolic contents were found to be 211.2 mg GAE/100 g dry weight and 492.9 mg GAE/100 g dry weight for fresh Sarilop and Bursa siyahi, respectively. Fruit skins had higher phenolic content than pulps, as expected. After drying, both the decrease by 8% in total phenolic content of Sarilop and the 15% decrease in case of Bursa siyahi were not statistically significant (p<0.05).

Total flavonoid contents of fresh Sarilop and Bursa siyahi was found to be 8.0 mg CE/100 g dry weight and 65.6 mg CE/100 g dry weight, respectively. Most flavonoids were located in the fruit skin (62.8 and 233.6 mg CE/100 g dry weight for Sarilop and Bursa siyahi, respectively). The decrease in total flavonoid content as a result of drying was found to be 21% in Bursa siyahi; on the other hand an increase by 70% was observed in Sarilop when dried. For Bursa siyahi, there was no significant difference between fresh and dried fruits, however, dried fruits of Sarilop were significantly higher than fresh fruits (p<0.05).

Total proanthocyanidin contents were determined as 6.5 mg cyanidin eq./100 g dry weight and 61.9 cyanidin eq./100 g dry weight for fresh Sarilop and Bursa siyahi, respectively. Drying resulted with a significant decrease by 75% in total proanthocyanidin content in Bursa siyahi figs whereas a statistically significant increase by 70% was observed in Sarilop (p<0.05).

Total anthocyanin contents were found to be 4.6 mg cyanidin-3-glucoside eq./100 g dry weight for Sarilop, and 83.4 mg cyanidin-3-glucoside eq./100 g dry weight for Bursa siyahi. Among two varities, Bursa siyahi showed the highest anthocyanin content, with most of the anthocyanins accumulated in the fruit skin (195.5 mg of cyanidin-3-glucoside eq./100 g dry weight). The highest loss as a result of drying was observed in total anthocyanin content (82.6% in Bursa siyahi and 97.2% in Sarilop) which was statistically significant for both varieties (p<0.05).

Total lycopene content was determined only for pulps of Sarilop and Bursa siyahi and the results revealed that Sarilop (0.4 mg lycopene/100 g dry weight) contained slightly higher amounts of lycopene compared to Bursa siyahi (0.3 mg lycopene/100 g dry weight) which was not statistically significant (p<0.05).

Among all four antioxidant capacity methods performed in this study (ABTS, DPPH, FRAP and CUPRAC), CUPRAC assay resulted with highest antioxidant capacity for Sarilop (278.2 mg TEAC/100 g dry weight); whereas for Bursa siyahi ABTS assay gave the highest results (1066.1 mg TEAC/100 g dry weight). After drying total antioxidant capacity was decreased by 24-57% and 13-60% in Bursa siyahi and Sarilop variety, respectively. On the basis of the CUPRAC method, for both varieties total antioxidant capacity was not significantly different in dried fruit as compared to fresh fruit. In contrast, according to the DPPH and FRAP methods, total antioxidant capacity was decreased significantly after drying. As judged by the ABTS method, total antioxidant capacity value did not change significantly for Sarilop, while it decreased significantly for Bursa siyahi as a result of drying (p<0.05).

Prior to the analyses of individual compounds, correlation coefficients for spectrophotometric assays were calculated. Total phenolic content and total flavonoid content of fig fruits showed a linear correlation with a high correlation coefficient of 0.9432. Among all four total antioxidant capacity assays, the highest correlation was demonstrated between the total phenolic content and CUPRAC (R2=0.9690).

The analyses of individual compounds revealed 14 phenolic compounds in analyzed fig samples, belonging to five groups of phenolics including phenolic acids, flavonols, flavons, flavan-3-ols and anthocyanins.

In the group of phenolic acids; chlorogenic acid, ellagic acid, gallic acid and p- coumaric acid were determined. After drying chlorogenic acid and p-coumaric acid contents of both varieties were decreased (58% and 59% for Sarilop; 87% and 34% for Bursa siyahi, respectively). However, drying caused an increase by 67% in gallic acid for both Sarilop and Bursa siyahi. Furthermore, ellagic acid stayed unchanged for Sarilop, whereas 50% increase was observed for Bursa siyahi. In terms of chlorogenic acid and ellagic acid contents, there was no significant difference between the fresh and dried fruits of both varieties. Gallic acid and p-coumaric acid contents of fresh and dried fruits of Sarilop were found to be significantly different, but no significant difference was observed between the fruits of Bursa siyahi (p<0.05).

In both varieties, rutin was confirmed as the major flavonol. Besides rutin, the following flavonols were determined: kaempferol-rutinoside, quercetin-3-glucoside and two different quercetin derivatives. The drying process caused a decrease in all flavonols of Sarilop (24% for rutin, 33% for kaempferol-rutinoside and quercetin-3- glucoside, and 50% for each quercetin derivative). However, there was no significant difference between the fresh and dried fruits (p<0.05). For Bursa siyahi, the loss in quercetin-3-glucoside, quercetin derivative 1 and quercetin derivative 2 as a result of drying was found to be 42%, 82% and 71%. On the other hand, drying resulted with 16% and 200% higher contents of rutin and kaempferol-rutinoside, respectively. Drying caused no significant difference between the fresh and dried fruits of Bursa siyahi (p<0.05).

From the group of flavons only apigenin was determined. The amount of apigenin was 0.6 mg/100 g dry weight for Sarilop, and 1.9 mg/100 g dry weight for Bursa siyahi. Drying reduced the apigenin content of Sarilop by 17% which was not statistically significant (p<0.05). On the other hand, for Bursa siyahi, apigenin content was increased significantly (68%) as a result of drying (p<0.05).

Both (+)-catechin and (-)-epicatechin from the group of flavan-3-ols were determined. For Sarilop, drying resulted with 35% and 45% lower contents of catechin and epicatechin, while the losses for Bursa siyahi were 45% and 68%, respectively. After drying, catechin contents of Bursa siyahi and epicatechin contents of Sarilop were found to be significantly different, but no significant difference was observed between the catechin contents of Sarilop fruits and epicatechin contents of Bursa siyahi fruits (p<0.05).

Cyanidin-3-glucoside and cyanidin-3-rutinoside were identified from the group of anthocyanins. Cyanidin-3-glucoside was not detected in the skin of Sarilop and cyanidin-3-rutinoside, was found to be 35.5 mg/100 g dry weight for Bursa siyahi, and 2.1 mg/100 g dry weight for Sarilop. For Bursa siyahi, drying caused a significant decrease for both anthocyanins (98% for cyanidin-3-glucoside and 96% for cyanidin-3-rutinoside) (p<0.05).

After the simulation of in vitro gastrointestinal digestion system, total phenolic content, total flavonoid content and total anthocyanin content as well as the total antioxidant capacity of fig skins, fig pulps and whole and dried fruits were evaluated. In order to determine the effect of in vitro gastrointestinal digestion on the total antioxidant capacity, CUPRAC assay was performed, because among all four antioxidant capacity assays the highest correlation was observed between either total phenolics or total flavonoids and CUPRAC assays.

Results of in vitro gastrointestinal digestion showed that 22% and 32% of the total phenolic compounds were retained in the dialyzed fraction for Sarilop and Bursa siyahi, respectively. The decrease in the dialyzed fraction as a result of drying was found to be 28% for Bursa siyahi; on the other hand an increase by 30% was observed for Sarilop. For all fractions (skin, pulp, whole and dried fruit) in each variety, dialyzed fractions were significantly lower compared to their initial total phenolic content (p<0.05).

The dialyzed flavonoid fraction after in vitro gastrointestinal digestion represented 17% and 32% of the initial total flavonoid values for Sarilop and Bursa siyahi, respectively. For Bursa siyahi, the loss in dialyzed fraction as a result of drying was found to be 67%. However, drying resulted with 181% higher total flavonoid value for the dialyzed fraction of Sarilop variety.

After the simulation of in vitro gastrointestinal digestion system, 80% and 86% of the antioxidant compounds were present in the dialyzed fraction for Sarilop and Bursa siyahi, respectively. Drying caused a decrease in the dialyzed fractions for both varieties (17% and 30% for Sarilop and Bursa siyahi, respectively).

In order to identify the influence of simulated in vitro gastrointestinal digestion on total anthocyanins, only Bursa siyahi samples were analyzed. The dialyzed anthocyanin fraction of the whole fruit represented only 9% of the initial total anthocyanin value. The decrease in dialyzed fraction of total anthocyanins as a result of drying was found to be 75%.

For each sample, percent recovery of total phenolics and total flavonoids after in vitro GI digestion were not significantly different from each other whereas antioxidant capacity was found to be significantly higher (p<0.05).