Plant Secondary Metabolites: an Opportunity for Circular Economy
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molecules Review Plant Secondary Metabolites: An Opportunity for Circular Economy Ilaria Chiocchio , Manuela Mandrone * , Paola Tomasi, Lorenzo Marincich and Ferruccio Poli Department of Pharmacy and Biotechnology, Alma Mater Studiorum—University of Bologna, Via Irnerio 42, 40126 Bologna, Italy; [email protected] (I.C.); [email protected] (P.T.); [email protected] (L.M.); [email protected] (F.P.) * Correspondence: [email protected]; Tel.: +39-0512091294 Abstract: Moving toward a more sustainable development, a pivotal role is played by circular economy and a smarter waste management. Industrial wastes from plants offer a wide spectrum of possibilities for their valorization, still being enriched in high added-value molecules, such as secondary metabolites (SMs). The current review provides an overview of the most common SM classes (chemical structures, classification, biological activities) present in different plant waste/by- products and their potential use in various fields. A bibliographic survey was carried out, taking into account 99 research articles (from 2006 to 2020), summarizing all the information about waste type, its plant source, industrial sector of provenience, contained SMs, reported bioactivities, and proposals for its valorization. This survey highlighted that a great deal of the current publications are focused on the exploitation of plant wastes in human healthcare and food (including cosmetic, pharmaceutical, nutraceutical and food additives). However, as summarized in this review, plant SMs also possess an enormous potential for further uses. Accordingly, an increasing number of investigations on Citation: Chiocchio, I.; Mandrone, neglected plant matrices and their use in areas such as veterinary science or agriculture are expected, M.; Tomasi, P.; Marincich, L.; Poli, F. considering also the need to implement “greener” practices in the latter sector. Plant Secondary Metabolites: An Opportunity for Circular Economy. Keywords: circular economy; waste valorization; plant by-products; plant secondary metabolites; Molecules 2021, 26, 495. natural products; bioactivities https://doi.org/10.3390/ molecules26020495 Academic Editors: 1. Introduction Domenico Trombetta and Antonella Smeriglio According to the United Nations [1], the global population is expected to increase Received: 29 December 2020 from 7.7 billion (2019) to 9.7 billion in 2050. This prospect raises several concerns about the Accepted: 15 January 2021 global consumption of biomass, fossil fuels, metals, and minerals, which should double [2], Published: 18 January 2021 and annual waste production, following the current trend, will increase by 70% in the next 40 years [3]. Undoubtedly, these premises challenge the move toward a more sustainable Publisher’s Note: MDPI stays neutral development. with regard to jurisdictional claims in In this scenario, wastes and by-products from the food and agricultural industries are published maps and institutional affil- gaining international attention, not only for the issues associated with pollution, but also iations. to overcome the paradox of 820 million people suffering from hunger and malnutrition while others are dealing with food over-consumption and related diseases, together with increasing food waste production [4,5]. Moreover, food waste and overproduction imply unnecessary exploitation of the environment and natural resources such as carbon, water, Copyright: © 2021 by the authors. and land. The land footprint estimated by the FAO (Food and Agriculture Organization of Licensee MDPI, Basel, Switzerland. the United Nations) in 2013 [6] revealed that almost 30% of the world’s agricultural lands This article is an open access article are used to produce food that is ultimately lost or wasted, determining additional pollution distributed under the terms and and greenhouse gases emissions to no actual purpose. Furthermore, food overproduction conditions of the Creative Commons and bad waste management also have a negative impact on the economy, with money loss Attribution (CC BY) license (https:// at different levels of the supply chain. Conversely, the reduction of food loss and waste creativecommons.org/licenses/by/ generation has the potential to generate considerable economic value. 4.0/). Molecules 2021, 26, 495. https://doi.org/10.3390/molecules26020495 https://www.mdpi.com/journal/molecules Molecules 2021, 26, x FOR PEER REVIEW 2 of 33 Molecules 2021, 26, 495 2 of 31 ReFED (Rethink Food waste through Economics and Data), a multi‐stakeholder non‐ ReFED (Rethink Food waste through Economics and Data), a multi-stakeholder non- profit committed to reducing food waste, identified 27 solutions, which were grouped profit committed to reducing food waste, identified 27 solutions, which were grouped intointo three three categories: categories: prevention, prevention, recovery, recovery, and and recycling recycling [7]. [ 7Although]. Although the theassociated associated in‐ comesincomes cannot cannot be generalized be generalized for forall countries, all countries, following following these these solutions, solutions, USD USD 100 100billion bil- islion expected is expected over 10 over years 10 in years U.S. in According U.S. According to this program, to this program, the solutions the solutions focused focusedon pre‐ ventingon preventing waste production waste production account account for over for 75% over of 75%the total, of the while total, 23% while relate 23% to relate waste to recovery,waste recovery, and only and 2% only to recycling 2% to recycling [5]. This proposal [5]. This is proposal consistent is consistentwith the EPA with (Environ the EPA‐ mental(Environmental Protection Protection Agency) “waste Agency) hierarchy,” “waste hierarchy,” which grades which waste grades prevention waste prevention as the most as preferablethe most preferable option (Figure option 1) (Figure[8]. 1)[8]. Figure 1. Scheme of the “waste hierarchy” proposed by the EPA [ 8]. Figure 1. Scheme of the “waste hierarchy” proposed by the EPA [8]. In this scenario, the idea of circular economy took shape, promoting the shift from a linear economic scheme of “take-make-use-dispose” to a circular model, employing reuse, In this scenario, the idea of circular economy took shape, promoting the shift from a sharing, repair, refurbishment, remanufacturing, and recycling to create a closed-loop linear economic scheme of “take‐make‐use‐dispose” to a circular model, employing reuse, system, attempting to minimize the use of resource inputs and the generation of waste, sharing, repair, refurbishment, remanufacturing, and recycling to create a closed‐loop sys‐ pollution, and carbon emissions [9,10]. In Europe, the most relevant strategy launched in tem, attempting to minimize the use of resource inputs and the generation of waste, pol‐ order to attain a sustainable development is the Circular Economy Action Plan, accord- lution,ing to and which carbon waste emissions materials [9,10]. and energy In Europe, become the most input relevant for other strategy industrial launched processes in or or‐ derregenerative to attain a resources sustainable for naturedevelopment (i.e., compost) is the Circular [11]. In orderEconomy to facilitate Action and Plan, stimulate according the toimplementation which waste materials of these guidelines, and energy the become scientific input community for other is industrial paying increasing processes attention or re‐ generativeto the valorization resources of for industrial nature (i.e., and compost) agricultural [11]. wastesIn order and to facilitate by-products, and stimulate in particular, the implementationthose derived from of these plants. guidelines, These neglected the scientific matrices community are often is manufacturedpaying increasing for biofuel atten‐ tionproduction, to the valorization such as methane of industrial or ethanol and [agricultural12]. However, wastes according and by to‐products, the proposed in particu “waste‐ lar,hierarchy” those derived (Figure from1), energy plants. recovery These neglected is a less preferable matrices are strategy often manufactured of waste management, for bio‐ fuelcompared production, to others. such Moreover,as methane although or ethanol biofuel [12]. isHowever, an alternative according to petroleum-derived to the proposed “wastefuels, its hierarchy” sustainability (Figure is quite 1), energy controversial recovery [13 is, 14a less]. preferable strategy of waste man‐ agement,Actually, compared the high to others. added-value Moreover, molecules although still biofuel contained is an alternative in plant wastes to petroleum and by-‐ derivedproducts fuels, offer its a widesustainability spectrum is of quite possibilities controversial for their [13,14]. valorization and reuse, as foreseen by circularActually, economy the high [ 15added,16].‐ Forvalue example, molecules from still agri-food contained wastes in plant it is wastes still possible and by to‐ productsextract macromolecules offer a wide spectrum such as of nucleic possibilities acids [ for17], their pectins valorization [18], cellulose and reuse, material as foreseen [19], and byenzymes circular such economy as bromelain, [15,16]. For which example, is derived from fromagri‐food pineapple wastes residues it is still and possible extensively to ex‐ tractused macromolecules as a pharmaceutical such as and nucleic meat acids tenderizer [17], pectins [20]. Primary [18], cellulose metabolites material (i.e., [19], organic and Molecules 2021, 26,