The wastes generated during the olive oil extraction process, even if presenting a negative impact for the environment, contain several bioactive compounds that have considerable health benefits.
After suitable extraction and purification, these compounds can be used as food antioxidants or as active ingredients in nutraceutical and cosmetic products due to their interesting technological and pharmaceutical properties.
The aim of this review, after presenting general applications of the different types of wastes generated from this industry, is to focus on the olive pomace produced by the two-phase system and to explore the challenging applications of the main individual compounds present in this waste.
Hydroxytyrosol, tyrosol, oleuropein, oleuropein aglycone, and verbascoside are the most abundant bioactive compounds present in olive pomace.
Besides their antioxidant activity, these compounds also demonstrated other biological properties such as antimicrobial, anticancer, or anti-inflammatory, thus being used in formulations to produce pharmaceutical and cosmetic products or in the fortification of food.
Nevertheless, it is mandatory to involve both industries and researchers to create strategies to valorize these byproducts while maintaining environmental sustainability.
In the Mediterranean region, mainly in countries such as Spain, Italy, Portugal, Greece, Syria, Morocco, and Tunisia, the olive oil production is one of the most important industries for their economy. European Union countries produce about 69% of the world's olive oil, being the leading producer, consumer, and exporter of this product. In the last years, other countries such as Argentina, Australia, United States of America, and South Africa became olive oil producers (Dermeche et al., 2013; Roig et al., 2006) since the global consumption of olive oil has increased worldwide. As the olive oil benefits for human health are becoming widely recognized, the perspective is that of a continuous increase in its consumption (and consequent production) in the coming years.
The extraction of olive oil includes different processes such as olive washing, olive crushing, malaxing of the resulting pastes and the extraction itself (Roig et al., 2006; Zbakh & El Abbassi, 2012). The extraction of olive oil can be achieved through discontinuous (traditional pressing) or continuous (centrifugation) processes. Concerning the centrifugation processes, there are two possible systems, called three-phase and two-phase systems.
In the three-phase system, a solid cake and two liquids, olive oil and large amounts of an aqueous liquid known as olive mill wastewater (OMWW) are generated. In the two-phase system, less water is used during the process which means that the volume of OMWW produced is reduced in comparison with the other process. In this system, besides olive oil, a semisolid residue (wet pomace or olive pomace) constituted by olive husk and OMWW is generated (Caporaso et al., 2018; Rodrigues et al., 2015). Figure 1 summarizes the processes for olive oil extraction. The two-phase system is used in the modern units replacing the three-phase technology, in order to minimize the wastewater volume and energy requirements (Dermeche et al., 2013). In fact, Azbar et al. (2004) indicated that two-phase technology saves process water by 80% and energy up to 20%. Nevertheless, its residues have a negative impact on the environment when they are discharged without treatment, due to their high toxicity and the resistance to biological degradation (Al-Khatib et al., 2009; Fiorentino et al., 2003; Khdair et al., 2019; Pavlidou et al., 2014).
Although some papers are found describing the composition of olive wastes and their applications in different areas (Bhatnagar et al., 2014; Galanakis, 2018; Rodrigues et al., 2015), there is no review that evaluated and compiled the possible applications for each of its main bioactive compounds, individually. To our knowledge, the only review describing the use of hydroxytyrosol as a functional food ingredient, not only as pure compound but also in the form of hydroxytyrosol-rich extracts, was published by Silva et al. (2020). In the present review, the applications of the different wastes generated from the olive oil industry will be described. After that, the review will focus on the solid residue (olive pomace) produced by modern two-phase system. Phenolic characterization and quantification will be discussed and the challenging applications of the individual compounds will be carefully explored.
POTENTIAL APPLICATIONS OF OLIVE WASTE
Regardless of its negative environmental impacts, the potential added value of olive wastes for numerous sectors is well known. In the literature, there are a few reviews reporting the different applications for olive wastes (Dermeche et al., 2013; Roig et al., 2006).
Olive wastes have been used as soil amendment, to increase soil fertility and organic carbon stored in the agro-systems (Federici et al., 2017; Majbar et al., 2018; Regni et al., 2017). Fernández-Hernández et al. (2014) observed amended soils with higher content of nitrogen, phosphorus, potassium, and organic matter than the soil treated with inorganic fertilizer when using olive wastes composts mixed with different agro-industrial wastes applied to an olive grove in Spain. The increase of soil fertility produced an increase in the olive oil content of the fruits (Fernández-Hernández et al., 2014), although other researchers considered that recycling these wastes could promote soil phytotoxicity, considerable decline in soil germination capability, and necrosis of the olive leaves (Arvanitoyannis & Kassaveti, 2007).
Olive wastes can also be used as biomass to produce renewable fuels (Tayeh et al., 2014; Al Afif & Linke, 2019; Al-Addous et al., 2017; Messineo et al., 2020; Rincón et al., 2013; Romero-García et al., 2014; Serrano et al., 2017; Valenti et al., 2017). In a recent review paper, Messineo et al. (2020) reported the latest achievements in anaerobic digestion of olive mill residues in order to produce biofuels. In that review, the authors described not only the aspects of the process but also the existing pretreatments of olive wastes (Battista et al., 2016; Rincón et al., 2013; Siciliano et al., 2016) which are applied to induce the decomposition of the complex lignocellulosic structures before anaerobic digestion (Kumari & Singh, 2018).
Serrano et al. (2017) proposed a thermal pretreatment and a subsequent phenolic recovery before the anaerobic digestion step to improve methane production. Furthermore, some chemical and physical pretreatment methods, such as ultrasonic pretreatment, basic pretreatment with sodium hydroxide, calcium carbonate, and/or hydrogen peroxide were used to optimize hydrogen and bioethanol production from olive oil production residues (Battista et al., 2016; Siciliano et al., 2016).
Other way to valorize olive wastes is to convert them into inexpensive adsorbents for water pollution control (Anastopoulos et al., 2015), in particular, heavy metals (Abdelhadi et al., 2017; Fernández-González et al., 2019; Fernando et al., 2009; Martinez-Garcia et al., 2006; Martín-Lara et al., 2013; Pagnanelli et al., 2003). Martinez-Garcia et al. (2006) observed that olive wastes maintained their adsorptive capacity over 10 cycles. Moreover, the ability of these biosorbents to adsorb several metal ions may increase their potential application on industrial wastewater treatment (Abdelhadi et al., 2017; Martinez-Garcia et al., 2006).
Other interesting and valuable use of olive mill wastes is to replace fresh water in brick manufacturing which contributes to a reduction of the water consumption (De La Casa & Castro, 2014; de la Casa et al., 2009; Eliche-Quesada et al., 2014; Mekki et al., 2006; Mekki et al., 2008). Comparing their physical properties with control products using fresh water, promising results have been obtained showing a significant increase in the volume shrinkage (10%) and the water absorption (12%), while the tensile strength remained constant (Mekki et al., 2008). Similar results were observed by de la Casa et al. (2009) also with the improvement by 33% of the dry-bending strength when compared to the control bricks.
In order to contribute to a more sustainable food production, the wastes generated from olive industry can also be transformed into animal feed (Dunne, 2019; Estaún et al., 2014; Gerasopoulos, Stagos, Kokkas, et al., 2015; Gerasopoulos, Stagos, Petrotos, et al., 2015; Molina-Alcaide et al., 2010; Rojas-Cano et al., 2014), although it is necessary to pay attention to their digestibility, palatability, and safety (Rojas-Cano et al., 2014). Rojas-Cano et al. (2014) demonstrated that the inclusion of olive soap stocks in the diet of growing crossbred Iberian pigs did not affect the apparent digestibility of nutrients or body protein accretion but increased the energy value of the diet. Also, Serra et al. (2018) found that the swine diet was improved by the inclusion of olive wastes. Lipid oxidation slowed down in the sausages despite the higher and lower contents in polyunsaturated fatty acids (PUFAs) and saturated fatty acids, respectively, compared to the controls. In another study, it was observed that feed blocks containing olive pomace could improve the quality of milk compared with a conventional diet with no impact on the milk yield and reducing the feeding costs (Molina-Alcaide et al., 2010).
Similarly, for broiler chickens, the use of olive wastes extracts was effective in reducing the oxidative stress and led to higher antioxidant capacity in plasma and tissues (Gerasopoulos, Stagos, Kokkas, et al., 2015).
With increasing consumer demand for healthier food, the industry and the scientific community started to produce new functional ingredients for food and beverages. The recent pandemic situation also affected the food sector, mainly food safety and security, becoming more important in the development of sustainable and modern food systems (Galanakis, 2020; Galanakis et al., 2021). Thereby, the supplementation of consumers’ diets with bioactive ingredients (vitamins, peptides, polyphenols, and lipids) can be an important key for the prevention or recovery from COVID-19 disease (Galanakis et al., 2020). The addition of OMWW and olive paste, individually or combined, to bread and pasta was assessed by Cedola et al.
(2020) and Simonato et al. (2019). The results demonstrated that the enrichment of bread and pasta with OMWW slightly improved the chemical quality without compromising the sensory properties, whereas the enrichment with olive paste considerably improved both phenolic contents and antioxidant activity although the sensory acceptability was worse due to its bitter and spicy taste. The combination of the two byproducts in the fortification of bread and spaghetti increased the whole quality index, being higher for bread (Cedola, Cardinali, et al., 2020). The fortification of wheat pasta with olive pomace also enhanced the dietary fiber of the final product, while increasing its firmness and decreasing its cooking time (Simonato et al., 2019). In other study, the bread and rusks fortified with olive polyphenols (200 mg of polyphenols/kg) demonstrated higher antimicrobial activity and extended their shelf life from 10 to 15 days (Galanakis et al., 2018). Moreover, the addition of olive leaf extracts to poultry meat decreased the microbial growth and maintained both chemical quality and sensory attributes (Saleh et al., 2020), thus extending the shelf life of the meat when refrigerated for 15 days.
Olive byproducts are abundant sources of bioactive phenolic compounds (El-Abbassi et al., 2012; Madureira et al., 2020; Nunes et al., 2018; Yakhlef et al., 2018) that have promising potential as antioxidant, anti-inflammatory, and antimicrobial agents (Bulotta et al., 2014; Leouifoudi et al., 2014; Schaffer et al., 2010). Hence, the recovery of phenolic compounds from these wastes, after suitable purification, presents considerable interest for food and beverage (Araújo et al., 2015; Caporaso et al., 2018; Zbakh & El Abbassi, 2012), cosmetic (Galanakis et al., 2018; Rodrigues et al., 2017; Rodrigues et al., 2015), and nutraceutical (Vitali Čepo et al., 2018) industries, due to their interesting pharmaceutical properties.
In the last years, the extracted phenolic compounds have also been used to produce biodegradable packaging materials for various types of food products to replace the synthetic ones (de Moraes Crizel et al., 2018; Lammi et al., 2018). de Moraes Crizel et al. (2018) studied the incorporation of 30% olive pomace flour in chitosan based films, protecting nuts against oxidation during 31 days. Lammi et al. (2018) developed biodegradable olive pomace-based fillers with lower stress and elongation at break and reducing costs. More