(Punica Granatum L.) Peel Waste: a Review
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molecules Review Processing Factors Affecting the Phytochemical and Nutritional Properties of Pomegranate (Punica granatum L.) Peel Waste: A Review Tandokazi Pamela Magangana 1,2 , Nokwanda Pearl Makunga 1 , Olaniyi Amos Fawole 3 and Umezuruike Linus Opara 2,* 1 Department of Botany and Zoology, Stellenbosch University, Private Bag X1, Matieland, Stellenbosch 7602, South Africa; [email protected] (T.P.M.); [email protected] (N.P.M.) 2 Postharvest Technology Research Laboratory, South African Research Chair in Postharvest Technology, Department of Horticultural Sciences, Faculty of AgriSciences, Stellenbosch University, Private Bag X1, Stellenbosch 7602, South Africa 3 Postharvest Research Laboratory, Department of Botany and Plant Biotechnology, University of Johannesburg, P.O. Box 524, Auckland Park, Johannesburg 2006, South Africa; [email protected] * Correspondence: [email protected]; Tel.: +27-21-808-4064 or +27-21-808-3743 Academic Editors: Ana Barros and Irene Gouvinhas Received: 5 September 2020; Accepted: 7 October 2020; Published: 14 October 2020 Abstract: Pomegranate peel has substantial amounts of phenolic compounds, such as hydrolysable tannins (punicalin, punicalagin, ellagic acid, and gallic acid), flavonoids (anthocyanins and catechins), and nutrients, which are responsible for its biological activity. However, during processing, the level of peel compounds can be significantly altered depending on the peel processing technique used, for example, ranging from 38.6 to 50.3 mg/g for punicalagins. This review focuses on the influence of postharvest processing factors on the pharmacological, phytochemical, and nutritional properties of pomegranate (Punica granatum L.) peel. Various peel drying strategies (sun drying, microwave drying, vacuum drying, and oven drying) and different extraction protocols (solvent, super-critical fluid, ultrasound-assisted, microwave-assisted, and pressurized liquid extractions) that are used to recover phytochemical compounds of the pomegranate peel are described. A total phenolic content of 40.8 mg gallic acid equivalent (GAE)/g DM was recorded when sun drying was used, but the recovery of the total phenolic content was higher at 264.3 mg TAE/g when pressurised liquid extraction was performed. However, pressurised liquid extraction is costly due to the high initial investment costs and the limited possibility of carrying out selective extractions of organic compounds from complex peel samples. The effects of these methods on the phytochemical profiles of pomegranate peel extracts are also influenced by the cultivar and conditions used, making it difficult to determine best practice. For example, oven drying at 60 ◦C resulted in higher levels of punicalin of 888.04 mg CE/kg DM compared to those obtained 40 ◦C of 768.11 mg CE/kg DM for the Wonderful cultivar. Processes that are easy to set up, cost-effective, and do not compromise the quality and safety aspects of the peel are, thus, more desirable. From the literature survey, we identified a lack of studies testing pretreatment protocols that may result in a lower loss of the valuable biological compounds of pomegranate peels to allow for full exploitation of their health-promoting properties in potentially new value-added products. Keywords: agriculture waste; antioxidant activity; horticultural processing; pomegranate peel; postharvest physiology; total phenolic content; value addition Molecules 2020, 25, 4690; doi:10.3390/molecules25204690 www.mdpi.com/journal/molecules Molecules 2020, 25, 4690 2 of 34 Molecules 2020, 25, x FOR PEER REVIEW 2 of 37 1. Introduction1. Introduction The pomegranateThe pomegranate (Punica (Punica granatum granatumL.) (Lythraceae; L.) (Lythraceae; formerly formerly belonging belonging to the to Punicaceaethe Punicaceae family) is a fruit-bearingfamily) is a deciduousfruit-bearing shrub deciduous or tree shrub [1]. Native or tree to [1] Iran. Native and Afghanistan,to Iran and Afghanistan, it is found it growing is found wild in thegrowing outer hills wild and in the warm outer valleys hills and of warm the Himalayas,valleys of the andHimalayas is often, and cultivated is often cultivated throughout throughout northern India,northern but now India, pomegranate but now pomegranate is grown commercially is grown commercially in various in various parts of parts the of world, the world including, including South Africa,South North Africa, Africa, North South Africa, America, South America, North America, North America, the Middle the Middle East, Israel,East, Israel, and and Australia Australia [2– [24].– 4]. The popularity of and demand for pomegranate are on the rise all over the world due to its The popularity of and demand for pomegranate are on the rise all over the world due to its multifunctionality and health-promoting effects in the human diet. The arils account for about 45–52% multifunctionality and health-promoting effects in the human diet. The arils account for about 45– of the weight52% of the of weight the whole of the fruit whole [5], fruit and [5] have, and long have beenlong been valued valued for theirfor their flavourful flavourful juice juice (Figure (Figure1 F). The peel1F). accountsThe peel accounts for about for 49about to 55%49 to of55% the of the weight weight of of the the fruit, fruit, depending on on the the cultivar cultivar [5]. [5]. The interiorThe interior of the of pomegranate the pomegranate fruit fruit (Figure (Figure1B) 1B) is is separated separated by by membranousmembranous walls walls (septum (septums)s) and and white spongywhite spongy tissue tissue chambers chambers (locules) (locules) packed packed with with arils arils containing containing soft soft or hardor hard seeds seeds and and edible edible juice, whichjuice, vary inwhich colour vary from in colo whiteur from to red, white depending to red, depending on the cultivar on the [ 6cultivar]. Pomegranate [6]. Pomegranate fruit has fruit numerous has seeds, whichnumerou accounts seeds, for which about account 18–20% for of about the fruit 18– weight20% of andthe fruit contain weight oil (Figureand contain1G). Pomegranateoil (Figure 1G). seed oil (PSO)Pomegranate accounts forseed about oil (PSO) 12–20% accounts of the for fruit about weight 12–20% and alsoof the has fruit many weight therapeutic and also propertieshas many [5]. therapeutic properties [5]. FigureFigure 1. Botanical 1. Botanical description description of Punica of Punica granatum granatumL. L. fruit: fruit: ((AA)) wholewhole fruit; fruit; (B (B) anatomical) anatomical description description of pomegranate fruit; (C) peel; (D) arils; (E) seeds; (F) juice; (G) oil. of pomegranate fruit; (C) peel; (D) arils; (E) seeds; (F) juice; (G) oil. The pomegranate peel, commonly considered as an agro-industrial waste, is a potential source The pomegranate peel, commonly considered as an agro-industrial waste, is a potential source of valuable secondary plant metabolites and nutrients. It has stronger biological activity than the pulp of valuable secondary plant metabolites and nutrients. It has stronger biological activity than the [7–9], yet often it is used as animal feed, whereby most is discarded, creating an environmental pulp [7problem–9], yet due often to the it is moisture used as content animal it feed,may carry. whereby The strong most isbiological discarded, activity creating is linked an to environmental its specific problemchemical due to composition, the moisture especially content theit secondary may carry. metabolites The strong, such biologicalas flavonoids activity and plant is linkedphenolics to its specific[5,10] chemical. Pomegranate composition, peel could especially prove theto be secondary more beneficial metabolites, in the suchform asof flavonoidsfood additives and or plant phenolicssupplements. [5,10]. Pomegranate By optimising peel food could waste prove recovery to be strategies more beneficial that could in be the used form in of the food food additives and or supplements.bioprocess industries, By optimising it also has food the wastepotential recovery to produce strategies innovative that products could bethat used can meet in the the foodneeds and bioprocessof the industries, growing health it also market. has the For potential instance, toin producethe area of innovative nanotechnology, products biosynthesis that can meetof biological the needs of the growingnanoparticles health using market. biological For agents instance, such in as the plant area-based of nanotechnology, extracts has gained biosynthesis attention in of the biological past decade [11]. Bio-based protocols for the synthesis of nanoparticles that are economical, nanoparticles using biological agents such as plant-based extracts has gained attention in the past environmentally friendly, and suitable for large-scale production under non-aseptic environments decade [11]. Bio-based protocols for the synthesis of nanoparticles that are economical, environmentally are in demand [12,13]. Pomegranate peel extracts are suggested to be good reducing agents that are friendly,used and in suitable the green for synthesis large-scale of productionsilver nanoparticles under non-aseptic, as this technology environments has potential are in demandfor creating [12, 13]. Pomegranatematerials, peel devices extracts, and are systems suggested with to fundamentally be good reducing new agentsfunctions that and are properties used in the [13 green–15]. synthesisSilver of silvernanoparticles nanoparticles, may asbe this utilised technology for a wide has potentialrange of app forlications, creating