Abstract
BackgroundThis study evaluated, using in vitro assays, the antibacterial, antioxidant, and tyrosinase-inhibition activities of methanolic extracts from peels of seven commercially grown pomegranate cultivars.MethodsAntibacterial activity was tested on Gram-positive (Bacillus subtilis and Staphylococcus aureus) and Gram-negative bacteria (Escherichia coli and Klebsiella pneumonia) using a microdilution method. Several potential antioxidant activities, including radical-scavenging ability (RSA), ferrous ion chelating (FIC) and ferric ion reducing antioxidant power (FRAP), were evaluated. Tyrosinase enzyme inhibition was investigated against monophenolase (tyrosine) and diphenolase (DOPA), with arbutin and kojic acid as positive controls. Furthermore, phenolic contents including total flavonoid content (TFC), gallotannin content (GTC) and total anthocyanin content (TAC) were determined using colourimetric methods. HPLC-ESI/MSn analysis of phenolic composition of methanolic extracts was also performed.ResultsMethanolic peel extracts showed strong broad-spectrum activity against Gram-positive and Gram-negative bacteria, with the minimum inhibitory concentrations (MIC) ranging from 0.2 to 0.78 mg/ml. At the highest concentration tested (1000 μg/ml), radical scavenging activities were significantly higher in Arakta (83.54%), Ganesh (83.56%), and Ruby (83.34%) cultivars (P< 0.05). Dose dependent FIC and FRAP activities were exhibited by all the peel extracts. All extracts also exhibited high inhibition (>50%) against monophenolase and diphenolase activities at the highest screening concentration. The most active peel extract was the Bhagwa cultivar against monophenolase and the Arakta cultivar against diphenolase with IC50 values of 3.66 μg/ml and 15.88 μg/ml, respectively. High amounts of phenolic compounds were found in peel extracts with the highest and lowest total phenolic contents of 295.5 (Ganesh) and 179.3 mg/g dry extract (Molla de Elche), respectively. Catechin, epicatechin, ellagic acid and gallic acid were found in all cultivars, of which ellagic acid was the most abundant comprising of more than 50% of total phenolic compounds detected in each cultivar.ConclusionsThe present study showed that the tested pomegranate peels exhibited strong antibacterial, antioxidant and tyrosinase-inhibition activities. These results suggest that pomegranate fruit peel could be exploited as a potential source of natural antimicrobial and antioxidant agents as well as tyrosinase inhibitors.
Highlights
This study evaluated, using in vitro assays, the antibacterial, antioxidant, and tyrosinase-inhibition activities of methanolic extracts from peels of seven commercially grown pomegranate cultivars
In the quest to promote the development of functional foods with health-benefiting properties, we investigated the antibacterial, antioxidant, and tyrosinase-inhibition activities of extracts from peels, using in vitro assays, of seven commercially pomegranate cultivars grown in the Western Cape, South Africa
Similar to the findings reported by Opara et al [42] on peels of pomegranates grown in Oman, all peel extracts of the investigated fruit cultivars showed activity against the Gram negative and positive bacteria used
Summary
Plant material The studies were performed on peels of seven pomegranate fruit cultivars (Arakta, Bhagwa, Ganesh, Herskawitz, Molla de Elche, Ruby, and Wonderful) which are commercially grown in South Africa. A blank containing methanol instead of the test sample or ascorbic acid was included under the same condition. Ð1Þ where Atest is the absorbance of the reaction mixture containing the standard antioxidant or extract, and Ablank is the absorbance of the blank test. A blank test containing methanol instead of the test sample or ascorbic acid was included under the same condition. Ð2Þ where Atest is the absorbance of the reaction mixture containing extract or ascorbic acid and Ablank is the absorbance of the blank test. A blank test containing 80% methanol instead of extract was included, while trolox at 10 μg/ml served as the positive control under the same condition. The absorbance was read at 520 nm (room temperature) against a blank test that contained aqueous methanol instead of the sample after 15 min incubation. The IC50 values for the tyrosinase assay were calculated from the logarithmic non-linear regression curve derived from the plotted data using GraphPad Prism software version 4.03 (GraphPad Software, Inc., San Diego, USA)
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