Abstract
‘Wonderful’ pomegranate (Punica granatum L.) peel contains a wide range of phytochemicals including vitamins, dietary fibre, phenolic compounds, and antioxidant properties. Yet, it is often used as animal feed or discarded in landfills, which is not the best eco-friendly way to utilize this phenolic-rich bioresource. Finding novel ways of utilizing pomegranate peel waste could prove a more profitable and eco-friendlier alternative that is far more beneficial to the economy. Adding a blanching pre-treatment step at optimal conditions prior to processing of pomegranate peel aids in the inactivation of quality changing enzymes such as polyphenol oxidase (PPO) and peroxidase (POD), which are accountable for the degradation reactions that cause breakdown of nutrients and phytochemicals. This study aimed to determine the effect of blanching at 80 °C for 3 min on the yield, polyphenol content, antioxidant properties, enzyme inactivation, and antibacterial activity of ‘Wonderful’ pomegranate peel ethanolic extracts from three different harvest maturities (unripe, ripe, and over ripe), including a comprehensive characterization and quantification using liquid chromatography-mass spectrometry (LC-MS). The blanched unripe peel extracts exhibited the highest total phenolic content, total tannin content, 2,2-diphenyl-1-picryl hydrazyl (DPPH) antioxidant activity, 2,2-azino-bis (3-ethylbenzothiazoline-6-sulphonic acid (ABTS) radical scavenging activity and ferric ion reducing antioxidant power (FRAP) at 14.0 mg gallic acid equivalent (GAE)/g dry mass (DM), 1.0 mg GAE/g DM, 359.1 µmol Trolox/g DM, 912.2 µmol Trolox/g DM and 802.5 µmol Trolox/g DM, respectively. There was significant (p < 0.05) decrease in PPO and POD activity of all blanched pomegranate peel extracts. The blanched unripe peel extracts had the lowest PPO activity at 0.2 U/g fresh weight (FW), with a 70% PPO inactivation compared to ripe and over ripe harvest, whereas the highest POD inactivation was recorded at 67% in over ripe peel extracts. All blanched peel extracts, irrespective of harvest maturity, had minimum inhibitory concentration (MIC) values at 160 µg/mL against all four bacteria strains tested, which included two Gram-positive bacterial strains (Bacillus subtilis ATCC 6051 and Staphylococcus aureus ATCC 12600) and two Gram-negative bacteria (Escherichia coli 11775 and Klebsiella pneumonia ATCC 13883). A total of 25 metabolites including phenolic acids (4), organic acids (1), flavonoids (4), ellagitannins (13), and other polyphenols (3) in all three pomegranate peel samples were tentatively identified after LC-MS profiling. The blanched unripe peel extracts showed significantly higher punicalin α and β, β punicalagin, catechin, epicatechin content at 414 mg/g, and 678 mg/g, 151 mg/g, 229 mg/g, respectively, compared to peel extracts from other harvest maturities. This study provides supportive information for the commercial utilization of pomegranate fruit peel as source of value-added ingredients for the development of novel food, cosmetics, and pharmacological products.
Highlights
The pomegranate (Punica granatum L.; Lythraceae) is cultivated in many places around the world such as North America, South America, Asia, North Africa, Australia, Israel, the Middle East, and, most recently, South Africa [1,2,3]
We demonstrate for the first time that blanching at 80 ◦C for 3 min of pomegranate peel significantly (p < 0.05) increased the extract yield of ‘Wonderful’ pomegranate peel at different harvest maturities (Table 1)
The highest phytochemical and antioxidant activity was recorded in unripe peel extracts and changes in activity were generally reduced with increasing harvest maturity stage
Summary
The pomegranate (Punica granatum L.; Lythraceae) is cultivated in many places around the world such as North America, South America, Asia, North Africa, Australia, Israel, the Middle East, and, most recently, South Africa [1,2,3]. There are over a thousand cultivars world-wide but ‘Wonderful’ is the most widely cultivated pomegranate cultivar in South Africa due to its high quality and yields [6] This particular cultivar has the highest cultivar distribution of 76% [7]. The peel has the highest phytochemical content and strong biological activity [3,8] It is a good source of nutrients, dietary fibre, phenolic compounds which include flavonoids (anthocyanins and flavonols), phenolic acids (ellagic acid, gallic acid caffeic acids, ferulic acids), hydrolysable tannins (ellagitannins and gallotannins), and condensed tannins (proanthocyanadins), which are produced through shikimic acid metabolism [3,9,10,11,12]. The global demand for natural health promoting products presents an opportunity to turn pomegranate peel waste to commercial products
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