Capacity of Pomegranate Fruit Juice and Marc after In Vitro Digestion. O.A. Fawole1, U.L. Opara1 and L. Chen2. 1 South African Research Chair in Postharvest ...
Bioaccessibility of Total Phenolic Concentration and Antioxidant Capacity of Pomegranate Fruit Juice and Marc after In Vitro Digestion O.A. Fawole1, U.L. Opara1 and L. Chen2 1 South African Research Chair in Postharvest Technology, Faculty of AgriSciences, Stellenbosch University, Private Bag X1, Stellenbosch 7602, South Africa 2 School of Medical Instrument and Food Engineering, University of Shanghai for Science and Technology, No. 516, Jungong Rd., Shanghai 200093, China Keywords: acidic hydrolysis, processing, gastric phase, radical scavenging activity, value addition Abstract Processing of pomegranate fruit yields the juice during extraction but also generates pomegranate marc, a by-product made of seeds and peels which are usually used as cattle or pig feed or directly disposed as waste. The objective of this study was to investigate the effect of digestion on total phenolic concentration and antioxidant capacity of pomegranate juice and by-products. Pomegranate juice, peel and marc were extracted in water, 50% ethanol (50%EtOH) and absolute ethanol (100% EtOH) and analysed for total phenolic concentration (TPC) and total antioxidant capacity (via Ferric Reducing Antioxidant Power (FRAP), 2,2-diphenyl1-picrylhydrazyl (DPPH.) before and after in vitro digestion. Peel contained the highest TPC, between 91.85 and 97.81% of the TPC found in all fruit parts, with TPC order being peel > marc > juice. However, there was a significant (p0.05) increase in TPC in most of the extracts. This could be attributed to acidic hydrolysis of phenolics glycosides to their aglycons during gastric digestion (Bouayed et al., 2011). The most notable increase was in marc extracts with between 1.5- to 4.3-folds increase in gastric phase (Fig. 1). In contrast, after the duodenal, TPC decreased significantly compared to gastric phase, however, the trend peel > marc > juice was maintained. Decline in TPC could be attributed to degradation by the alkaline pH during the duodenal phase of digestion as phenolic compounds are highly sensitive to alkaline conditions (Bouayed et al., 2011). Total Antioxidant Capacity Prior to in vitro digestion, radical scavenging activity (RSA), in DPPH assay, reflected TPC in fruit parts, the order being peel > marc > juice. Overall, higher RSA was exhibited by 50% ethanol for juice and marc (Fig. 2). RSA decreased significantly in gastric phase of digestion for most of the extracts, this was especially more pronounced in peel extracts with between 7-10% decreases (Fig. 2). However, in duodenal phase RSA increased significantly in all the extracts compared to gastric phase, ranging between 5-18%. Activities exhibited in the duodenal phase were higher than those natively present in samples before in vitro digestion. The observed dynamics in RSA could be attributed to the dependency of phenolics on pH, as high pH values have been reported to significantly increase phenolics scavenging ability (Tagliazucchi et al., 2010). Specifically, RSA is mainly dependent on the number and position of hydrogen-donating hydroxyl groups on the aromatic rings of the phenolic compounds (Tagliazucchi et al., 2010). Reducing antioxidant power was consistent with the TPC in fruit parts prior to the in vitro digestion model (Fig. 3). Peel extracts showed between 5- to 30-fold reducing power, with activity order being peel > marc > juice among the investigated fruit fractions. Overall, 50% ethanol and water extracts showed higher reducing power than ethanol extracts (Fig. 3). FRAP values increased significantly in the gastric phase of digestion, perhaps as a result of the observed increase in phenolic concentration at this phase. However, reducing power decreased significantly by between 10 to 26% in the duodenal phase of in vitro digestion. Albeit, FRAP values remained relatively higher in the duodenal phase compared to those before digestion in most cases (Fig. 3). The overall loss in reducing power could be as a result of structural transformation of antioxidants by the alkaline pH in duodenal phase (Ryan and Prescott, 2010). Polyphenols are highly sensitive to alkaline conditions and do transform into different structural forms with different chemical properties (Bermudez-Soto et al., 2007). Initially, radical cation scavenging activity (RCSA) reflected the trend in TPC with the order being peel > marc > juice for all extracts with the exception of 100% EtOH extract (Fig. 4). In general, RCSA in peel was between 6- to 10-fold and 2.5- to 20-fold for juice and marc, respectively. Again, higher RCSA was exhibited by 50% EtOH before and after in vitro digestion. As observed in DPPH assay, RCSA decreased significantly in gastric phase of digestion in all the extracts with the exception of peel 50% EtOH and EtOH extracts. However, in the duodenal phase RCSA increased in all the extracts compared to the previous phases. The main highlight was observed in all marc extracts, which increased by 10-fold, 5-fold and 75-fold in water, 50% EtOH and EtOH extracts, 287
respectively (Fig. 4). The observed increase in radical cation scavenging activity by phenolics dependency on pH, with higher pH values in favour of deprotonation of hydroxyl moieties present on the phenolic compounds (Tagliazucchi et al., 2010). CONCLUSIONS There is large variation in the phenolic concentration and antioxidant capacities of the investigated fruit parts, with fruit waste products (peel and marc) showing great nutraceutical value. Results showed that changes in total phenolic concentrations during in vitro digestion phases influenced their antioxidant capacities. TPC and total antioxidant capacity after the duodenal phase of in vitro digestion were higher than the initial values (before digestion), suggesting the effect the environments of in vitro digestion on total phenolics in pomegranate fruit fractions. This study highlights the need to provide biologically relevant information on antioxidants by providing data concerning the bioaccessibility of antioxidants. ACKNOWLEDGEMENTS This work is based upon research supported by the South African Research Chairs Initiative of the Department of Science and Technology and National Research Foundation. O.A. Fawole is grateful for financial supports from the Claude Leon Foundation, Cape Town, and the National Research Foundation, South Africa. Literature Cited Bermudez-Soto, M.J., Tomas-Barberan, F.A. and Garcia-Conesa, M.T. 2007. Stability of polyphenols in chokeberry (Aronia melanocarpa) subjected to in vitro gastric and pancreatic digestion. Food Chem. 102:865-874. Bouayed, J., Hoffmann, L. and Bohn, T. 2011. Total phenolics, flavonoids, anthocyanins and antioxidant activity following simulated gastro-intestinal digestion and dialysis of apple varieties: bioaccessibility and potential uptake. Food Chem. 128:14-21. Gil, M.I., Tomas-Barberan, F.A., Hess-Pierce, B., Holcroft, D.M. and Kader, A.A. 2000. Antioxidant activity of pomegranate juice and its relationship with phenolic composition and processing. J. Agric. Food Chem. 48:4581-4589. Kamiloglu, S. and Capanoglu, E. 2013. Investigating the in vitro bioaccessibility of polyphenols in fresh and sun-dried figs (Ficus carica L.). Int. J. Food Sci. Technol. 48:2621-2629. Lansky, E.P. and Newman, R.A. 2007. Punica granatum (pomegranate) and its potential for prevention and treatment of inflammation and cancer. J. Ethnopharmacol. 109:177-206. Li, L., Shewry, P.R. and Ward, J.L. 2008. Phenolic acids in wheat varieties in the HEALTHGRAIN diversity screen. J. Agric. Food Chem. 56:9732-9739. Ryan, L. and Prescott, S.L. 2010. Stability of the antioxidant capacity of twenty-five commercially available fruit juices subjected to an in vitro digestion. Int. J. Food Sci. Technol. 45:1191-1197. Tagliazucchi, D., Verzelloni, E., Bertolini, D. and Conte, A. 2010. In vitro bioaccessibility and antioxidant activity of grape polyphenols. Food Chem. 120:599606.
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Fig. 1. Changes in total phenolic concentrations during in vitro digestion model of water, 50% ethanol and 100% ethanol extracts of pomegranate peel, marc and juice. Average values (±S.E) are presented. Bars with different letter(s), per fruit fraction and extract, are statistically significant different (p
