Effect of drying on the bioactive compounds, antioxidant, antibacterial and antityrosinase activities of pomegranate peel

Abstract

Background: The use of pomegranate peel is highly associated with its rich phenolic concentration. Series of drying methods are recommended since bioactive compounds are highly sensitive to thermal degradation. The study was conducted to evaluate the effects of drying on the bioactive compounds, antioxidant as well as antibacterial and antityrosinase activities of pomegranate peel.

Methods: Dried pomegranate peels with the initial moisture content of 70.30 % wet basis were prepared by freeze and oven drying at 40, 50 and 60 °C. Difference in CIE-LAB, chroma (C*) and hue angle (h°) were determined using colorimeter. Individual polyphenol retention was determined using LC-MS and LC-MS(E) while total phenolics concentration (TPC), total flavonoid concentration (TFC), total tannins concentration (TTC) and vitamin C concentration were measured using colorimetric methods. The antioxidant activity was measured by radical scavenging activity (RSA) and ferric reducing antioxidant power (FRAP). Furthermore, the antibacterial activity of methanolic peel extracts were tested on Gram negative (Escherichia coli and Klebsiella pneumonia) and Gram positive bacteria (Staphylococcus aureus and Bacillus subtilis) using the in vitro microdilution assays. Tyrosinase enzyme inhibition was investigated against monophenolase (tyrosine) and diphenolase (DOPA), with arbutin as positive controls.

Results: Oven drying at 60 °C resulted in high punicalin concentration (888.04 ± 141.03 mg CE/kg dried matter) along with poor red coloration (high hue angle). Freeze dried peel contained higher catechin concentration (674.51 mg/kg drying matter) + catechin and -epicatechin (70.56 mg/kg drying matter) compared to oven dried peel. Furthermore, freeze dried peel had the highest total phenolic, tannin and flavonoid concentrations compared to oven dried peel over the temperature range studied. High concentration of vitamin C (31.19 μg AAE/g dried matter) was observed in the oven dried (40 °C) pomegranate peel. Drying at 50 °C showed the highest inhibitory activity with the MIC values of 0.10 mg/ml against Gram positive (Staphylococcus aureus and Bacillus subtili. Likewise, the extracts dried at 50 °C showed potent inhibitory activity concentration (22.95 mg/ml) against monophenolase. Principal component analysis showed that the peel colour characteristics and bioactive compounds isolated the investigated drying method.

Conclusions: The freeze and oven dried peel extracts exhibited a significant antibacterial and antioxidant activities. The freeze drying method had higher total phenolic, tannin and flavonoid concentration therefore can be explored as a feasible method for processing pomegranate peel to ensure retention of the maximum amount of their naturally occurring bioactive compounds.

Trial registration: Not relevant for this study.

Keywords: Freeze drying; Oven drying; Rutin; Total phenolics; Vitamin C.

Fig. 1

Effects of drying methods on concentrations of total phenolic (a), total tannins (b) and total flavonoid (c) of pomegranate peel. Bars with same letter are not significantly different (P < 0.05; Duncan’s multiple range test). Data represent the Mean ± SE (n = 3)

Fig. 2

Effects of drying methods on RSA (a), FRAP (b) and vitamin C concentration (c) of pomegranate peel. Bars with same letter are not significantly different (P < 0.05; Duncan’s multiple range test). Data represent the Mean ± SE (n = 3). RSA radical scavenging activity, FRAP ferric reducing antioxidant power, Vit C Vitamin C

Fig. 3

Principal component analysis of the first two factors (F1 and F2) based on colour attributes and bioactive compounds of pomegranate peel cv. Wonderful obtained from different drying methods. TCD total colour difference, TF total flavonoid, TP total phenolic, TT total tannin, RSA radical scavenging activity, Vit C Vitamin C, FRAP ferric reducing antioxidant power. L* lightness, C* chroma, H° hue angle, a* redness, RMC residual moisture content (dry basis %), MC moisture content (wet basis %)