Food Science and TechnologyVolume 37, Issue 1 p. 42-46 FeaturesFree Access Maximising olive oil by-products First published: 08 March 2023 https://doi.org/10.1002/fsat.3701_11.xAboutSectionsPDF ToolsExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Federica Flamminii and Carla Di Mattia outline how olive by-products could be used as functional ingredients and which technologies could enable their use in complex food formulations. The olive oil production chain generates vast amounts of waste and by-products, such as olive leaves, pomace, and olive mill wastewater (Figure 1). Cheap and available throughout the year, olive leaves (OL) are discarded during the extraction process and the pruning of the olive trees. This abundant vegetable material represents a significant producer cost due to its removal, storage, and disposal. An old approach with limited economic and environmental impact is their use in animal feeding. However, olive leaves contain considerable amount of phenolic compounds with beneficial health effects as antioxidant activity[1], anti-inflammatory[2], anti-tumoral[3], among others and functional properties[4]. Numerous studies have been focused on the evaluation of their chemical composition and, in particular, the content and pattern of bioactive compounds, mainly phenolic compounds, in which the secoiridoids group represents the major one. Oleuropein, which accounts near 60-90 mg g-1 of the dry weight, represent the most abundant bitter compound followed by other high value compounds, such as tyrosol, hydroxybenzoic acid, cinnamic acid, hydroxytyrosol, caffeic acid, syringic acid, elenolic acid, chlorogenic acid, ligstroside, verbascoside, among many others. Figure 1Open in figure viewerPowerPoint Olive oil production process and related by-products generate from different extraction operation (press, 2-phase, 3-phase). Nonetheless, olive leaves remain an undervalued by-product and, despite their high potential, additional aspects limit the use of olive leaves extracts in novel food formulations. The studies about the use of olive leaves extracts (OLEs) have significantly increased in the last few years but their applicability into food systems is currently bound to several aspects. Firstly, phenolic compounds show a very pungent and bitter taste generally not appreciated by consumers[5]. Secondly, the bioavailability of free phenolic compounds is very low, due to deleterious circumstances taking place during the digestion that determine significant degradation[6]. Thirdly, polyphenols can undergo instability phenomena under conditions encountered in food processing and storage (e.g. temperature, oxygen, light)[7]. Encapsulation technology as rising strategy Encapsulation is defined as the process that traps a core, i.e., the bioactive compound, within a secondary material, known as matrix or shell, to form a microcapsule or microsphere. The shell forms a protective coating around the core, isolating it from the surrounding environment; changes in the environment itself trigger the release of the encapsulated material[8]. In the food industry, microencapsulation is used for several purposes including: Protection of the bioactive material from environmental and matrix factors (e.g. heat, light, oxygen, moisture, pH, etc.); Release-control of the core material to the external environment; Modification of the physical properties of the original material for ease of handling; Masking the off-flavour or off-colour of core material; Prevent undesirable reactions with other components[9, 10]. Hydrogel encapsulation by ionic gelation Among the several methods for encapsulating active compounds, ionic gelation is an interesting technique, because of its relatively low cost and the use of simple equipment avoiding the use of high temperatures and organic solvents. Application of hydrocolloid gel particles is potentially useful in food, chemical, and pharmaceutical industries. Gelled particles are generally made of alginate. This is one of the most commonly used hydrocolloids as it is biocompatible, non-toxic, biodegradable, cheap and simple to produce, able to protect the active compound from factors such as heat and moisture thereby leading to enhanced stability. Extrusion/external gelation of alginate is one of the technologies available to encapsulate bioactive compounds, however the macroscopic particle size can limit its application. For instance, in foods, microgels with an average diameter of 30 μm are preferred in order to minimize the sensory perception of powdery or graininess in foods such as yoghurt and ice cream[11]. Methods based on in situ gelation of alginate, such as emulsification-internal gelation, have been explored to reduce the particle size. In such procedures, the alginate solution added with calcium in form of an insoluble salt is previously emulsified with a vegetable oil and gelation is achieved by a decrease in the pH (Figure 2-5)[12]. Figure 2Open in figure viewerPowerPoint Olive leaves extract (OLE) encapsulated by emulsion/internal gelation procedure Figure 3Open in figure viewerPowerPoint Use of olive leaves phenolic extracts as active ingredients in edible films and packaging. Figure 4Open in figure viewerPowerPoint Reformulation of foodstuffs with olive leaves extracts. Figure 5Open in figure viewerPowerPoint Consumer perception about olive by-products containing foods. From a technological and yield production point of view, in an encapsulation process, a high-trapping efficiency is desirable (Desai & Park, 2005). Capsules produced by ionic gelation are adopted for hydrophobic materials or those with low water solubility such as α-tocopherol. This could is widely used in the food industry for stabilizing fat based products[13] or the encapsulation of oils such as linseed oil[14], eucalyptus oil[15], rosemary essential oil[16]. On the contrary, the development of hydrogels for the encapsulation of hydrophilic actives of different nature is very challenging due to the molecular diffusion in and out of the porous matrix of the gel particles. The combination of alginate with other polymers (pectin, whey protein isolate, inulin, starch, chitosan), acting either as wall material or by reinforcing the structure of the hydrogel, increase the encapsulation efficiency of the hydrogel system. López Córdoba and co-authors (2013) encapsulated yerba mate polyphenols in calcium alginate beads filled with starch and found an improvement of encapsulation efficiency from 55% (control sample, without starch) to 65%[17]. Hosseini et al. (2014) also demonstrated that calcium alginate-starch beads had higher nisin encapsulation efficiency with a positive impact on both nisin slow release from the beads[18]. The interaction mechanisms between different wall materials and green tea polyphenols highlighted an improved encapsulation efficiency of total phenols from 66.1% (without proteins) to 77.2 and 76.5% when calcium caseinate or whey protein were respectively added. Moreover, coated hydrogel beads with an external layer of chitosan, enhanced the encapsulation efficiency and prolonged the release of hydrophilic compounds[19]. Encapsulation of olive leaf phenolic extracts From this point of view very few are the studies regarding encapsulation of olive leaves extract (OLE) into an hydrogel matrix; Belšćak-Cvitanović and co-authors (2011) encapsulated different medicinal plant extracts (i.e. raspberry leaf, hawthorn, ground ivy, yarrow, nettle and olive leaf) into alginate–chitosan microbeads obtained by electrostatic extrusion, while Flamminii and co-workers (2020, 2021) encapsulated OLE, combining alginate with pectin, whey protein isolate and caseinate, by emulsion/internal gelation technique with promising outcomes[20-22]. Other encapsulation approaches were studied. Olive leaves extract was trapped by inclusion complex with β-cyclodextrin (β-CD) upon mixing of the components in aqueous media and subsequent freeze-drying. Inclusion complex formation was confirmed by differential scanning calorimetry (DSC) which highlighted, under oxidative conditions, that the complex of olive leaf extract with β-CD was protected against oxidation, since it remained intact at temperatures where the free olive leaf extract was oxidized[23]. The same authors evaluated the extraction/recovery of phenolic compounds from olive leaves with inclusion trough β-CD mixed with green extraction solvents as water and glycerol; employing 2-hydroxypropyl-β-cyclodextrin (CD),the extractability of olive leaf polyphenols, by forming water soluble inclusion complexes, was improved. Spray-drying was deeply studied in order to optimize the best operating conditions to encapsulate olive leaves phenolic extract with maltodextrin[24] or inulin[25] and was successfully used also for the encapsulation of olive leaf extract (OLE) in chitosan microspheres[26]. Uses of olive leaf extracts as natural ingredients for food reformulation Nowadays, a growing trend in the food sector is the use of natural extracts in foods to formulate new products with potential positive effect on consumers’ health and improved nutritional profile as well as replacers of synthetic additives (i.e. BHT and BHA). Synthetic antioxidant compounds like BHT (butylate hydrixytoluene) and BHA (butylated hydroxyanisol) are cheap, effective and highly stable. However, the use of BHT and BHA in foods has decreased due to the suspected action as promoters of carcinogenesis as well as the general consumer rejection of synthetic food additives[27]. According to its antioxidant and antimicrobial activities, olive leaves extracts have been widely studied as food preservatives; however, their concrete exploitation into foodstuffs is still very limited and are generally sold more as dietary supplements than as food ingredient. Anyway, encouraging results are available from the literature. The enrichment with olive leaves extracts have indeed been tested in animal feeding as well as in food matrices with the aim of both improving the nutritional profile and enhancing the chemical stability towards lipid oxidation. Applications as either pure extract or encapsulated compounds have been explored in vegetable oils, table olives, functional smoothies, baked snacks, yogurt and meat products[4] as well as in edible films[28] or active packaging[29]. The incorporation of phenolic extracts in foodstuffs may therefore contribute to very important food-related aspects such as the extension of their shelf life and the exploitation of specific technological functionality when such ingredients are added in complex food formulations. Olive phenolic compounds have been proven to exert technological properties in formulated foods and in particular those containing oils and fats. Some investigations highlighted interesting surface properties that influenced the emulsification process in model systems[30, 31] as well as in complex food formulations as mayonnaise sauces[32, 33]. Olive phenolic compounds influenced the colloidal and gelling behavior of biopolymers like egg yolk proteins as well as the mechanical properties of ethylcellulose-based oleogels[34, 35]. Difonzo and co-authors (2019) studied the physico-chemical and sensory properties on a ready-to-use olive based pâtè enriched with olive leaves phenolic extract (OLE) in comparison with those stabilized with the synthetic antioxidant BHT (butylated hydroxytoluene)[36]. The results indicated the high potential of OLE as natural preservative in non-thermally stabilized olive-based pâté, since some spoilage-related microbial groups were negatively affected by the addition of OLE that, at the highest concentration, exhibited antimicrobial activity. Olive leaves were found to exhibit various technological functionalities, when added up to 150 μg phenols g-1 to minced beef meat, with improved stability against the oxidation of lipid and myoglobin during storage of raw and cooked meat and without affecting sensorial quality[37]. The enrichment of milk derivatives (e.g. yogurt) with olive polyphenols was investigated for innovative healthy products. The results of the study indicated that oleuropein was resistant upon milk heating and coagulation conditions (reduced pH), without adding any peculiar taste or flavour; furthermore, the oleuropein stability, during refrigeration and storage, enhanced yogurt rheological properties (higher firmness and viscosity and less syneresis)[38]. Olive leaf extract (OLE) was added at different concentrations, as natural preservatives, to an industrial sweet pepper paste. The findings revealed an optimum microbiological preservation of the pepper paste along 15 days of cover-open storage without experiencing physicochemical or color changes[39]. Studies on the applications of encapsulated olive leaves extracts as food ingredient has been poorly considered despite the high nutritional and technological potential. Mohammadi an co-workers (2016) evaluated the antioxidant activity of olive leave extract (OLE) encapsulated by nano-emulsions in soybean oil[40]. The antioxidant activity of the emulsions during storage, containing three concentrations of OLE (100, 200 and 300 mg), was evaluated, and compared with a control (not-encapsulated OLE) and synthetic antioxidant (TBHQ). Results showed an increased solubility and controlled release of olive leaf phenolic compounds through their nano-encapsulation and a higher antioxidant activity with respect of both not-encapsulated OLE and synthetic antioxidant. On the other hand, due to blockage of phenolic compounds within dispersed droplets of emulsions, the thermal stability of oil samples containing the encapsulated forms of olive leaf extract was lower than the free form. In a perspective of food reformulation, recent research studied OLE encapsulated in a double emulsion (W/O/W) used as fat-replacer in meat systems, with the aim of improving the lipid profile and the oxidative stability. The incorporation of OLE in the meat products, both in a non-encapsulated and encapsulated form, improved their oxidative stability during accelerated storage, showing a better performance for the entrapped one. The encapsulation of OLE in the double emulsion hindered oleuropein degradation, leading to meat systems with lower contents of both peroxide values and secondary oxidation products, as well as higher antioxidant capacity with respect to meat systems with non-encapsulated OLE. Furthermore, the replacement of pork backfat by olive:linseed:fish oil double emulsion with OLE, led to stable meat matrices with an healthier fatty acid profile, a noticeable reductions in the SFA content and higher proportions of ω3 PUFA[41]. Several food reformulation strategies have been proposed to make mayonnaise a healthier product: along with the reduction of fat, which is the most widespread approach used up to now, a more recent approach is based on the enrichment of emulsified sauces with beneficial ingredients that can respond to the health-related needs of people (antioxidant, prebiotics, and probiotics), with a potential improvement of the oxidative stability of the product allowing the replacement of synthetic and debated antioxidants. The results of a recent study highlighted an improvement of the physical properties in terms of both the dispersion degree of the oil droplets and lubricant properties of the enriched mayonnaise[33] along with an increased oxidative stability. Salad dressings, prepared as single and double emulsion systems, were enriched with olive leaves extracts both in pure and encapsulated form. The addition of encapsulated OLE extended the oxidation induction period of the emulsion, from 15-20 days to 50 days thanks to the sustained release of phenolic compound during storage, thus increasing product protection toward oxidation phenomena[42]. Biscuits are bakery products characterized by low levels of moisture content and aw, thus the limiting factor for their shelf-life is mostly ascribable to fat oxidation, which leads to rancidity and off-flavors. Free and alginate-pectin microencapsulated olive leaves extracts (OLE), proposed as natural antioxidant to prolong the stability of biscuits, showed higher radical scavenging activity and oxidative stability, mostly when the encapsulated ingredient was used[43]. How does the consumer perceive olive oil by-product as a food ingredient? Converting ingredients, that would otherwise go to waste into consumable food, may be an effective solution to minimize food waste and reduce the quantities of raw material used for food production with an important impact on the food innovation sector, to promote more efficient use of resources and answer to sustainability goal that is clearly a growing consumer concern. But, how do the consumers perceive foods that contain olive by-products? Consumers’ willingness to accept foods produced with olive by-product ingredients depends on the perception of different factors, mainly related to the general attitude of the consumer, rather than a product-specific choice. Indeed, information about the characteristics of olive by-products and the perception of the benefits from sustainable consumption can possibly offset the consumers’ choice with a positive association between the use of vegetable by-products and sustainable production and environmental responsibility[44]. Conclusion Olive by-products represent precious raw materials containing high valuable compounds such as phenolic compounds whose health and technological properties are well known. Thanks to encapsulation strategies, polyphenols instability issues can be overcome and the obtained microparticles can be exploited as functional ingredients for several end-users in the food, cosmetic and pharmaceutical industries. These applications represent a valid contribution in the wide panorama of functional foods and opens new opportunities for reformulating healthier and innovative foodstuffs while valorizing olive by-products. These aspects, combined with an appropriate communication of the overall benefits in recovering valuable compounds from food waste, can contribute to enhance consumers’ acceptance of foods made from by-products. Dr Federica Flamminii, Research Fellow, University of Chieti – Pescara and Dr Carla Di Mattia, Associate Professor, University of Teramo email federica.flamminii@unich.it cdimattia@unite.it References 1Scognamiglio, M., D'Abrosca, B., Pacifico, S., Fiumano, V., De Luca, P. F. et al. 2012. Polyphenol characterization and antioxidant evaluation of Olea europaea varieties cultivated in Cilento National Park (Italy). Food Research International 46: 264. 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