foods Review Bioactive Compounds in Waste By-Products from Olive Oil Production: Applications and Structural Characterization by Mass Spectrometry Techniques Ramona Abbattista 1, Giovanni Ventura 1 , Cosima Damiana Calvano 2,3,* , Tommaso R. I. Cataldi 1,2 and Ilario Losito 1,2,* 1 Chemistry Department, University of Bari Aldo Moro, via Orabona 4, 70126 Bari, Italy; [email protected] (R.A.); [email protected] (G.V.); [email protected] (T.R.I.C.) 2 Interdepartmental Centre SMART, University of Bari Aldo Moro, via Orabona 4, 70126 Bari, Italy 3 Pharmacy-Pharmaceutical Sciences, University of Bari Aldo Moro, via Orabona 4, 70126 Bari, Italy * Correspondence: [email protected] (C.D.C.); [email protected] (I.L.) Abstract: In recent years, a remarkable increase in olive oil consumption has occurred worldwide, favoured by its organoleptic properties and the growing awareness of its health benefits. Currently, olive oil production represents an important economic income for Mediterranean countries, where roughly 98% of the world production is located. Both the cultivation of olive trees and the production of industrial and table olive oil generate huge amounts of solid wastes and dark liquid effluents, including olive leaves and pomace and olive oil mill wastewaters. Besides representing an economic problem for producers, these by-products also pose serious environmental concerns, thus their partial reuse, like that of all agronomical production residues, represents a goal to pursue. This Citation: Abbattista, R.; Ventura, G.; aspect is particularly important since the cited by-products are rich in bioactive compounds, which, Calvano, C.D.; Cataldi, T.R.I.; Losito, once extracted, may represent ingredients with remarkable added value for food, cosmetic and I. Bioactive Compounds in Waste nutraceutical industries. Indeed, they contain considerable amounts of valuable organic acids, By-Products from Olive Oil carbohydrates, proteins, fibers, and above all, phenolic compounds, that are variably distributed Production: Applications and among the different wastes, depending on the employed production process of olive oils and table Structural Characterization by Mass olives and agronomical practices. Yet, extraction and recovery of bioactive components from selected Spectrometry Techniques. Foods 2021, 10, 1236. https://doi.org/10.3390/ by-products constitute a critical issue for their rational valorization and detailed identification and foods10061236 quantification are mandatory. The most used analytical methods adopted to identify and quantify bioactive compounds in olive oil by-products are based on the coupling between gas- (GC) or liquid Academic Editor: Enrico Valli chromatography (LC) and mass spectrometry (MS), with MS being the most useful and successful detection tool for providing structural information. Without derivatization, LC-MS with electrospray Received: 30 April 2021 (ESI) or atmospheric pressure chemical (APCI) ionization sources has become one of the most relevant Accepted: 27 May 2021 and versatile instrumental platforms for identifying phenolic bioactive compounds. In this review, Published: 29 May 2021 the major LC-MS accomplishments reported in the literature over the last two decades to investigate olive oil processing by-products, specifically olive leaves and pomace and olive oil mill wastewaters, Publisher’s Note: MDPI stays neutral are described, focusing on phenolics and related compounds. with regard to jurisdictional claims in published maps and institutional affil- Keywords: LC-ESI-MS; MALDI-MS; bioactive phenolics; olive oil; olive leaves; olive pomace; olive iations. oil mill wastewater Copyright: © 2021 by the authors. 1. Introduction Licensee MDPI, Basel, Switzerland. The production of high-quality olive oils implies the generation of vast quantities of This article is an open access article solid residues and/or wastewaters that may have a great impact on terrestrial and aquatic distributed under the terms and environments because of their high phytotoxicity [1]. Depending on the techniques used conditions of the Creative Commons for olive oil production, namely, on the type of horizontal centrifugation (two-phase or Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ three-phase), the process most adopted to separate olive oil from olive paste obtained after 4.0/). malaxation, different by-products are obtained (Figure1). Foods 2021, 10, 1236. https://doi.org/10.3390/foods10061236 https://www.mdpi.com/journal/foods Foods 2021, 10, x FOR PEER REVIEW 2 of 30 Foods 2021, 10, 1236 three-phase), the process most adopted to separate olive oil from olive paste obtained2 after of 28 malaxation, different by-products are obtained (Figure 1). Figure 1. Layout of the main steps of olive oil production involving two- or three-phase horizontal centrifugation and the Figure 1. Layout of the main steps of olive oil production involving two- or three-phase horizontal centrifugation and the main by-products obtained.obtained. When two-phase decanters are used for horizontal centrifugation, whereby water When two-phase decanters are used for horizontal centrifugation, whereby water is is not purposely added to the olive paste resulting from malaxation, a humid semi-solid not purposely added to the olive paste resulting from malaxation, a humid semi-solid pomace is obtained as by-product (wet olive pomace, wet OP, or alperujo), consisting pomace is obtained as by-product (wet olive pomace, wet OP, or alperujo), consisting in in olive pulp and husk, crushed olive stone and aqueous solution. In the case of three- olive pulp and husk, crushed olive stone and aqueous solution. In the case of three-phase phase centrifugal technology, solid OP and olive oil mill wastewater (OMWW) or alpechin centrifugal technology, solid OP and olive oil mill wastewater (OMWW) or alpechin are are the two major by-products. Notably, the modern two-phase centrifugal extraction the two major by-products. Notably, the modern two-phase centrifugal extraction tech- technology merges OP with OMWW to produce a single by-product named olive mill nology merges OP with OMWW to produce a single by-product named olive mill waste waste (OMW) [2]. OMW shows a broad spectrum of toxicity, antimicrobial activity, high (OMW) [2]. OMW shows a broad spectrum of toxicity, antimicrobial activity, high acidity acidity and phytotoxicity, that make its biological degradation challenging and its disposal and phytotoxicity, that make its biological degradation challenging and its disposal prob- problematic for major producing countries (all located in the Mediterranean basin), given lematicthe significant for major amounts producing of OMW countries generated (all located every in year. the ResearchMediterranean efforts basin), have thusgiven been the significantfocused on amounts valorizing of theOMW OMW generated in various every ways, year. such Research as using efforts it as have energy thus source been [fo-3] cusedor as fertilizer, on valorizing biomass, the OMW or additive in various in animal ways, feed such [ 4as]. using However, it as energy a large source quantity [3] of or by- as productsfertilizer, resultingbiomass, fromor additive oliveoil in productionanimal feed remains [4]. However, unexploited, a large with quantity their of potentially by-prod- uctshigh resulting biological from value olive being oil neglected.production Indeed, remains wastes unexploited, remaining with after their the potentially production high of biologicalolive oil are value a heterogeneous being neglected. mixture Indeed, of many wastes chemical remaining components, after the production such as metal of olive ions, oilcarbohydrates, are a heterogeneous and polyphenols, mixture of thatmany may chem exertical different components, biological such as activities, metal ions, primarily carbo- hydrates,acting as antioxidantsand polyphenols, [5]. Phenolic that may compounds exert different from biological olive fruits activities, are mostly primarily retained acting in the ascorresponding antioxidants waste,[5]. 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