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published: 09 February 2015 doi: 10.3389/fmicb.2015.00076
Bioactivity of essential oils: a review on their interaction with food components
Marianne Perricone, Ersilia Arace, Maria R. Corbo, Milena Sinigaglia and Antonio Bevilacqua*
Department of the Sciences of Agriculture, Food and Environment, University of Foggia, Foggia, Italy
Edited by:
Eva-Guadalupe Lizárraga-Pauiín, Instituto Tecnológico y de Estudios Superiores de Monterrey Campus Estado de México, Mexico Reviewed by:
Anushree Malik, Indian Institute of Technology Delhi, India Eleni Skaltsa, National and Kapodistrian University of Athens, Greece
*Correspondence:
Antonio Bevilacqua, Department of the Sciences of Agriculture, Food and Environment, University of Foggia, Via Napoli 25, 71122 Foggia, Italy e-mail: antonio.bevilacqua@unifg.it
Essential oils (EOs) are liquid preparations, produced from plant materials. Although EOs showed a promising bioactivity in vitro, they could interact in foods with some components (fats, proteins, carbohydrates) and pH, thus many authors have reported that a significant effect of EOs toward spoiling and pathogenic microorganisms could be achieved in vivo by using higher amounts of oils. Different methods can be used to assess the bioactivity of EOs (disk diffusion and agar or broth dilution methods); however, there is not a standardized test and researchers propose and use different protocols (evaluating the Minimal Inhibitory Concentration, studying the survival curves, analysis through the scanning electron microscopy, etc.). Thereafter, the scope of this review is a focus on interactions of EOs with proteins, carbohydrates, oils, NaCl, and pH, as well as a brief description on the different protocols to assess their bioactivity both under in vivo and in vitro conditions.
Keywords: essential oils, antibacterial, bioactivity, food composition, methods
INTRODUCTION
Synthetic antimicrobial agents and chemical food preservatives have been used since ancient times as an effective method for controlling food spoilage. Nowadays, consumer concerns toward chemical preservatives determine an increasing interest on some natural antimicrobials, like essential oils (EOs). EOs are liquid preparations produced from plant materials (flowers, buds, seeds, leaves, twigs, bark, herbs, wood, fruits, and roots) of temperate to warm countries, like Mediterranean and tropical areas. Only few of them are solid or resinous at room temperature; they are limpid, soluble in lipids or in organic solvents, with a generally lower density than that of water, with a pale yellow to emerald green or blue to dark brownish red color (Burt, 2004; Gutierrez etal., 2008).
These extracts were referred as EOs by Paracelsus von Hohenheim in 16th, who used the term "Quinta essentia" to design the active component of a drug and from the Latin essentia comes the term "essential" (Guenther, 1948).
EOs play a major role in plants and act as antibacterials, antivirals, antifungals, insecticides, and protect the plants from herbivores. It is possible to list ca. 3000 EOs, but only 300 of them are used in perfumes and make-up products (creams, soaps, etc.), sanitary products, dentistry, agriculture, as preservatives, and flavor additives for foods, as fragrances for household cleaning products and industrial solvents and as natural remedies (as mixtures with vegetal oil in massages or in baths, in aromatherapy, etc.; Burt, 2004).
Essential oils can be produced by expression, fermentation, enfleurage, or extraction, although hydro-distillation is the most common method (Speranza and Corbo, 2010). EOs and their active components possess antiviral, antimycotic, antitoxigenic, and insecticidal properties. Table 1 reports the most important EOs, their aroma notes, and the target bacteria.
Even though several studies were performed in vitro to assess antibacterial and antifungal properties of EOs, only few studies reported on their bioactivity in vivo; food components (fats, carbohydrates, proteins, salts) and pH could reduce the antimicrobial effects of EOs in food systems. In fact, the same effect observed in vitro is achieved in food matrix only with higher concentrations (Tyagi etal., 2014).
The scope of this review is to highlight the interactions of EOs with proteins, carbohydrates, NaCl, and pH as a preliminary step to optimize food applications; the last section deals with the different protocols to assess their bioactivity in vivo and in vitro.
CHEMICAL COMPOSITION AND MECHANISM OF ACTION OF EOs
Essential oils are mixtures of 20-60 components at quite different concentrations, with some compounds at fairly high concentrations (20-70%), and others in trace amounts. The components at high concentrations (terpenes, terpenoids, molecules with an aromatic ring) play a major role in the antimicrobial/biological effect of EOs (Bakkali et al., 2008).
Some important compounds of EOs are mono and sesquiter-penes, carbohydrates, phenols, alcohols, ethers, aldehydes, and ketones (Speranza and Corbo, 2010). Phenolic compounds have also been recognized as bioactive components (Tabassum and Vidyasagar, 2013).
Essential oils with aldehydes or phenols as major components (cinnamaldehyde, citral, carvacrol, eugenol, or thymol) are the most effective, followed by EOs containing terpene alcohols (Bassole and Juliani, 2012). EOs with ketones or esters (P-myrcene, a-thujone, or geranyl acetate) possess a lower activity (Dormans and Deans, 2000; Barros et al., 2009).
Although the major components of EOs are very important for their biological activity, minor components play a
Table 1 | Antimicrobial and aroma characteristics of essential oils (EOs; modified from Ayala-Zavala etal., 2009).
Essential oil
Major volatile constituents
Antimicrobial effect against
Aroma notes
Garlic root Methyl disulfide, allyl sulfide, allyl disulfide,
(Allium sativum) allyl trisulfide, trimethylene trisulfide, allyl
tetrasulfide
Bacillus cereus, Escherichia coli, Shigella spp., Vibrio parahaemolyticus, Yersinia enterolitica, Salmonella enterica serovars Enteritidis, Infantis, Typhimurium, B. subtilis, Enterococcus faecalis Alternaria alternata
Pungent, spice
Cinnamon leaf (Cinnamomum zeylanicum)
Cinnamaldehyde, eugenol, copaene, ß-caryophyllene
E. coli, Pseudomonas aeruginosa, Ent. faecalis, Staphylococcus aureus, Staph. epidermidis, methicillin-resistant Staph. aureus, Klebsiella pneumoniae, Salmonella sp., Vibrio parahaemolyticus
Sweet, wood, spice
Thyme Thymol, p-cymene, y-terpinene, linalool
(Thymus vulgaris)
B. cereus, Clostridium botulinum, Ent. faecalis, E. coli, Spice, citrus, Staph. aureus, Listeria monocytogenes, Aspergillus wood
flavus, A. niger, K. pneumoniae, Ps. aeruginosa, Salmonella sp.
Oregano Sabinyl monoterpenes, terpinen-4-ol,
(Origanum vulgare) y-terpinene, carvacrol, thymol
B. cereus, B. subtilis, C. botulinum, Ent. faecalis, E. coli, Spice, herb Staph. aureus, A. niger, L. monocytogenes, K. pneumoniae, Ps. aeruginosa, Salmonella sp.
Clove Eugenol, eugenyl acetate, caryophyllene
(Syzygium
aromaticum)
B. brevis, B. subtilis, Cl. botulinum, Ent. faecalis, Candida Sweet, spice, spp., A. flavus, A. niger, E. coli, K. pneumoniae, Ps. wood
aeruginosa, Staph. aureus, Salmonella spp., L. monocytogenes
Basil (Ocimum basilicum) Linalool, methylchalvicol, eugenol, methyl eugenol, methyl cinnamate, 1,8-cineole, caryophyllene B. brevis, E. coli, A. flavus, A. niger, Ent. faecalis, E. coli, K. pneumoniae, Ps. aeruginosa, Staph. aureus Fresh, sweet, herb, spice
Coriander (Coriandrum sativum) 2(E)-decanal, 2(E)dodecenal, linalool E. coli, L. monocytogenes, Lactobacillus plantarum, Staph. aureus Sweet, flower, spice, citrus
Citrus peel (Citrus sp.) Limonene, linalool, citral A. niger, A. flavus, Penicillium verrucosum, P chrysogenum Sweet, citrus
Laurel (Laurus nobilis) 1,8-cineole, a-terpinyl acetate, linalool, methyl eugenol Staph. aureus, B. cereus, Ent. faecalis Fresh, herb, spice
Ginger (Zingiber officinale) p-sesquiphellandrene, zingiberene A. flavus, A. niger, Ent. faecalis, E. coli, K. pneumoniae, Ps. aeruginosa, Staph. aureus Pungent, spice
Rosemary (Rosmarinus officinalis) Borneol, verbenone, camphor, a-pinene, 1,8-cineole A. flavus, A. niger, Ent faecalis, E. coli, K. pneumoniae, Ps. aeruginosa, Staph. aureus, L. monocytogenes, Lb. plantarum, Salmonella spp., B. cereus Fresh, herb, resinous
Peppermint (Mentha piperita) Menthol, menthone, menthyl acetate, menthofurane B. brevis, Staph. aureus, Vibrio cholerae, Ent. faecalis, E. coli, K. pneumoniae, Ps. aeruginosa, A. flavus, A. niger Fresh, herb
significant role, as they can strengthen the effects of major components, though antagonistic, and additive effects have also been observed (Bassole and Juliani, 2012). Table 2 reports some examples of combination of EOs toward a wide range of bacteria.
The composition of EOs relies upon the harvesting seasons and the geographical sources (Burt, 2004), as well as from the part of plant, e.g., EO from the seeds of coriander (Coriandrum sativum L.) shows a different composition from EO of cilantro, produced from immature leaves (Delaquis et al., 2002).
Table 2 | Combination of components and EOs and their antimicrobial interactions against several microorganisms (modified from Bassole and Juliani, 2012).
Pair Organism Methods Interaction
Thymol/carvacrol Staph aureus, Ps. aeruginosa Half dilution Additive
E. coli Checkerboard Synergism
S. aureus, B. cereus, E. coli Checkerboard Antagonism
Staph. aureus, Ps. aeruginosa Mixture Additive
E. coli Checkerboard Additive
Salmonella Typhimurium Mixture Synergism
Thymol/eugenol E. coli Checkerboard Synergism
Carvacrol/eugenol E. coli Checkerboard Synergism
Staph. aureus, B. cereus, E. coli Checkerboard Antagonism
Carvacrol/Cymene B. cereus Mixture Synergism
Carvacrol/linalool L. monocytogenes Checkerboard Synergism
Menthol/GeraniolMenthol/Thymol Staph. aureus, B. cereus Synergism
Cinnamaldehyde/Carvacrol E. coli Checkerboard Additive
Salmonella Typhimurium Mixture Synergism
Cinnamaldehyde/Thymol E. coli Checkerboard Synergism
Salmonella Typhimurium Mixture Synergism
Cinnamaldehyde/Eugenol Staphylococcus spp., Mixture Additive
Micrococcus spp., Bacillus spp., Enterobacter spp.
S. aromaticum/R. officinalis Staph. epidermidis, Staph. aureus, B. subtilis, E. coli, Proteus Mixture Additive
vulgaris, Ps. aeruginosa
O. vulgare/O. basilicum B. cereus, E. coli, Ps. aeruginosa Checkerboard Additive
O. vulgare/T. vulgaris Ent. cloacae, Checkerboard Additive
Ps. fluorescens,
L. innocua
Cymbopogon citratus/C. giganteus E. coli, L. monocytogenes, Sh. dysenteriae, Staph. aureus, Checkerboard Synergism,;
Salmonella Typhimurium
Essential Oils are lipophiles, thus they can easily enter cells, disrupt the membrane and/or permeabilize it. The most important signs of membrane permeabilization are the loss of ions and the reduction of potential, the collapse of proton pump and the depletion of ATP pool (Bakkali et al., 2008).
In eukaryotic cells, EOs cause depolarisation of mitochondrial membranes, influence Ca2+ channels and reduce pH gradient, affecting the proton pump and the ATP pool (Bakkali etal., 2008). The membrane becomes abnormally permeable resulting in leakage of radicals, cytochrome c, calcium ions, and proteins. Permeabilization of outer and inner mitochondrial membranes causes apoptosis and necrosis and finally cell death (Armstrong, 2006; Speranza and Corbo, 2010); in addition, EOs can cause the coagulation of cytoplasm and some damages to lipids and proteins (Burt, 2004).
Intrinsic and extrinsic conditions can be responsible of susceptibility and resistance of pathogens (Bajpai etal., 2012). It is not possible to propose a general hit for the susceptibility/resistance to EOs; however, Speranza and Corbo (2010) suggested some milestones:
• Gram-negative bacteria appear more resistant. This higher resistance could be attributed to the outer membrane.
• Lactic acid bacteria (LAB) are the most resistant Gram-positive bacteria. This resistance was attributed to ATP generation by substrate level phosphorylation.
• Among the Gram-negative bacteria, pseudomonads show high resistance to these antimicrobials.
• Essential oils are generally more active toward yeasts.
ANTIBACTERIAL ACTIVITY OF EOs IN FOOD SYSTEMS
The bioactivity of EOs might be reduced by certain food components (fats, carbohydrates, proteins, water, salt, antioxidants, preservatives, other additives) and pH (Glass and Johnson, 2004; Gutierrez etal., 2008); moreover, some extrinsic factors (temperature, packaging in vacuum/gas/air, characteristics of microorganisms) play a crucial role (Skandamis and Nychas, 2000; Smith-Palmer etal., 2001). Different studies reported higher levels of bioactivity at acidic pHs, as at low pH EOs behave in a more hydrophobic way and enter more easily cells (Negi, 2012).
High concentrations of fats and/or proteins in foodstuffs may protect bacteria, as they could provide a protective layer and absorb EOs, thus decreasing their concentration and effectiveness in the aqueous phase; on the other hand, high water, and/or salt levels appear to facilitate the action of EOs (Smith-Palmer etal., 2001; Carson and Riley, 2003).
Gutierrez etal. (2008) studied the effect of food ingredients (potato starch-0, 1, 5, or 10%; beef extract-1.5, 3, 6, or 12%; sunflower oil-0, 1, 5, or 10%) and pH (4-7) on the antimicrobial efficacy of oregano and thyme. They focused on both the lag phase and the maximum specific growth rate of L. monocytogenes. Starch and sunflower oil exerted a negative effect on the biological activity of EOs, whilst proteins affected it in a positive way; finally, the highest activity was found at pH 5.
Cava et al. (2007) studied the antimicrobial activity of cinnamon and clove EOs against L. monocytogenes in milk and found that the biological activity was reduced by fat; these results are in agreement with the effects of EOs in full-fat and in low-fat soft cheeses (Smith-Palmer etal., 2001).
The effect of EOs could be reduced by increasing the amount of complex sugars (starch), whilst glucose and other simple sugars acted in a different way, thus EO application should be orientated to food products containing more simple sugars than complex carbohydrates (Gutierrez etal., 2008, 2009).
Another key factor for the biological activity of EOs is the physical structure of foods, which may limit and affect the antibacterial activity; e.g., Salmonella Typhimurium was inoculated in a broth and in a gelatine gel, both containing an EO. In the gel the effect of EO was reduced for its limited diffusion (Speranza and Corbo, 2010).
In many cases EO combinations showed additive effects, e.g., Gutierrez etal. (2008) combined oregano and basil or thyme toward Escherichia coli and Pseudomonas aeruginosa, with majo-ram toward E. coli, and majoram and thyme mixed with basil, rosemary or sage against L. monocytogenes. Moreover, Lambert et al. (2001) suggested that carvacrol and thymol acted as additive terms against Staphylococcus aureus and P. aeruginosa.
Some EOs, even at low concentrations, can have a negative impact on the sensory attributes, due to their low breakpoint for perception (Lvetal., 2011); therefore the need of higher concentrations in food is highly unfortunate and limits their application to spicy foods. An alternative approach is the use of EOs into active packaging, either encapsulated in polymers of edible and biodegradable coatings or entrapped in sachets able to slowly release the active compounds on food surface or in the headspace (Pelissari etal., 2009; Sánchez-González etal., 2011). Cerisuelo et al. (2014) tested some passive, active, and nanocomposite multilayer films; the performances of EVOH were low, as this matrix was not able to retain the active compounds. However, the inclusion of bentonite nanoparticles into EVOH active coatings increased the release rate and the retention ability.
In addition, another way to minimize the organoleptic effects of EOs is the preparation of nanoemulsions; this approach positively affects both the stability and the antimicrobial activity (Donsí et al., 2011).
Tyagi and Malik (2012) and Tyagi etal. (2012) proposed the use of EOs in the vapor phase, by combining bactericidal volatiles
and ionizing sources. Since active compounds of EOs are highly volatile, the presence in gaseous form facilitates the solubilization of lipophilic monoterpenes in cell membranes.
Some papers focused on the combination of EOs with other treatments as reported by Tyagi etal. (2012, 2013); they tested lemon grass and mentha oils in combination with mild thermal treatment (55° C). Hence, this strategy significantly reduces oil dose requirement, offers a very useful synergy, as the increase of the temperature increases the amount of oil in the vapor phase, thus it enhances its antimicrobial activity.
SOME CASE-STUDIES DEALING WITH THE APPLICATION OF
EOsIN FOODS
MEAT AND MEAT PRODUCTS
Eugenol and coriander, clove, oregano, and thyme oils were used to control pathogens and autochthonous spoilage flora in meat, as they caused a marked initial reduction in the viable cell number (Speranza and Corbo, 2010). As reported elsewhere fat reduced the bioactivity of EOs in meat products; in fact, some authors reported that thyme oil reduced significantly bacterial population of L. monocytogenes in zero and low-fat (90 g/Kg) beef hot-dogs, but not in full-fat hot-dogs (260 g/Kg; Lemay etal., 2002; Singh etal., 2004).
The new consumer preference toward hurdle technology suggests the potentiality of combining different elements to preserve foods; following this approach, Chouliara et al. (2007) combined oregano EO and modified atmosphere packaging (MAP) for the prolongation of the shelf life of fresh breast chicken meat, stored at 4°C. The effect of oregano EO (0.1 and 1% w/w) was evaluated in combination with two kinds of MAP [30:70 CO2:N2 (MAP1) and 70:30 CO2:N2 (MAP2)]. Samples treated with 1% oregano oil and packaged under both MAPs did not attain the critical level of cell count (7 log cfu/g) during a 25 day storage period.
SEAFOOD PRODUCTS
As reported for meat, fat reduced the bioactivity of EOs in fish. Speranza and Corbo (2010) reported that the effect of oregano oil at 0.05 % (v/w) toward Photobacterium phospho-reum was stronger on cod filets than on salmon (a fatty fish). Some authors (Corbo etal., 2008; Del Nobile etal., 2009a) proposed combinations of EOs to improve the microbial stability of fish burgers. A mix containing 0.11% of thymol, 0.10% of grapefruit seed extract (GFSE) and 0.12% of lemon extract was proposed, as it increased the shelf life of fish burgers (stored under refrigeration and packaged in air) by 40%. Moreover the combined effect of the EOs and MAP was evaluated; samples were packaged in air and in three different gas mix compositions: 30:40:30 O2:CO2:N2, 50:50 O2:CO2, and 5:95 O2:CO2. The proposed packaging strategies inhibited the growth of mesophilic bacteria.
VEGETABLES AND FRUITS
In vegetables the antimicrobial activity of EOs is enhanced by a decrease of storage temperature and pH (Smith-Palmer etal., 2001). The shelf life of unpasteurised fruit juices is limited by microbial enzymatic spoilage; moreover, these products could be contaminated by some pathogens. Some EOs could be used to
Table 3 | Different methods used to test the antimicrobial activity of EOs (Burt, 2004).
Purpose
Test method
Screening for antibacterial activity
Determination of the strength of antibacterial properties
Determination of rapidity and duration of antimicrobial activity
Evaluation of the physical effects
Disk diffusion (solid or vapor diffusion
assay)
Agar wells
Agar dilution method
Broth dilution (visible growth, optical
density/turbidity, absorbance, viable
count, colorimetric and
conductance/conductivity/impedance)
Time-kill analysis/survival curves
Scanning electron microscopy
prevent this kind of problem. Lemongrass and geraniol have been found effective against E. coli, Salmonella sp., and Listeria spp. in apple, pear, and melon juices (Raybaudi-Massilia etal., 2006).
Raybaudi-Massilia etal. (2009) used malic acid and EOs extracted from cinnamon, palmarosa, and lemongrass (0.3 and 0.7%) or their major compounds (eugenol, geraniol, and citral) to prolong the shelf life of fresh-cut "PieldeSapo" melon (Cucumis melo L.). EOs were entrapped into an alginate-based edible coating and used for a challenge test; Salmonella Enteritidis (108 cfu/ml) was used as the target microorganism. This system was able to control the growth of the pathogen for at least 21 days (Ayala-Zavalaet al.,2009). In addition, Rojas-Grau etal. (2007) entrapped lemongrass, oregano oil, and vanillin in an apple puree-alginate edible coating to prolong the shelf-life of fresh-cut Fuji apples. Vanillin (0.3%w/w) preserved sensory quality for at least 21 days at 4°C.
DAIRY PRODUCTS
Orange, lemon, grapefruit, madrine, terpeneless lime, orange, D-limonene, terpineol, and geraniol were tested against Salmonella Senftenberg, E. coli, S. aureus and Pseudomonas spp. in different types of milk. Terpineol was the most effective oil in vitro, thus it
was used in combination with orange oil for a validation in milk. The effect of terpineol oil was affected by fat content, showing a microbial reduction of 7 log cfu/ml in skimmed milk, 4 log cfu/ml in low butterfat milk and 3 log cfu/ml in whole milk (Fisher and Phillips, 2008).
Another approach was proposed by Bevilacqua etal. (2007) who studied the possibility of prolonging the shelf life of caprese salad using MAP (65:30:5 N2:CO2:O2) in combination with thymol. The combination of thymol dipping and MAP prolonged the shelf life by 8 days, without negative effects on the sensory quality and on the growth kinetics of LAB.
CEREAL-BASED PRODUCTS
Natural active compounds were also applied to fresh pasta. Del Nobile etal. (2009b) used thymol, lemon extract, chitosan, and GFSE at different concentrations (2000 and 4000 ppm) to improve the microbiological stability of refrigerated amaranth-based fresh pasta. The oils were tested against mesophilic and psychrotrophic bacteria, total coliforms, Staphylococcus spp., yeasts, and molds. Chitosan and GFSE were the most promising compounds, whereas lemon extract was the less effective.
METHODS TO ASSESS THE ANTIMICROBIAL ACTIVITY OF EOs
The methods to assess the antimicrobial activity of EOs could be grouped in three classes: diffusion, dilution, or auxographic methods (Rios etal., 1988). Tables 3 and 4 report an overview of the most common protocols used to test the bioactivity of EOs. The most widely used test is NCCLS method, generally designed to test antibiotics but modified for testing EOs (Hammer et al., 1999; NCCLS, 2000); a filter disk is impregnated with the antimicrobial agent, placed on the surface of inoculated agar plates and an inhibition of growth is observed after incubation. This test is generally used for screening purposes, although its results rely upon many factors, like the method used to extract the EO from plant material, the volume of inoculum, the physiological phase of the microorganism, the kind of culture medium, pH, incubation time, and temperature. A modification of the method is the use of wells instead of a paper disk.
However, many papers propose direct contact between microorganism and antimicrobial agent; whereas, an alternative method is the use of essential oil in the vapor phase (Lopez et al., 2005; Tyagi etal., 2012). In the vapor diffusion assays a filter disk
Table 4 | Terms used in antibacterial activity testing reported in literature (from different literature sources).
Definition presented in literature
Minimum inhibitory concentration (MIC)
Minimum bactericidal concentration (MBC)
Bacteriostatic concentration Bactericidal concentration
Lowest concentration Lowest concentration Lowest concentration Lowest concentration Concentration able to Lowest concentration Lowest concentration Lowest concentration
resulting in the maintenance or in the reduction of inoculum viability required for the complete inhibition up to 48 h inhibiting visible growth
resulting in a significant decrease in inoculum viability (>90%) kill at least the 99.9% of the target.
at which no growth is observed after subculturing into fresh broth. able to inhibit microbial growth, without killing the test organism able to kill/inactivate the test microorganism
is impregnated with the antimicrobial agent and placed on the medium-free cover of each Petri dish, while microorganism target are inoculated on agar surface; the Petri dishes were then sealed using sterile adhesive tape.
Other methods are the micro-dilution approaches (dilution in broth or in agar, evaluation of microbial growth by plate counting or by indirect indices).
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Conflict of Interest Statement: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Received: 25 April 2014; accepted: 21 January 2015; published online: 09 February 2015.
Citation: Perricone M, Arace E, Corbo MR, Sinigaglia M and Bevilacqua A (2015) Bioactivity of essential oils: a review on their interaction with food components. Front. Microbiol. 6:76. doi: 10.3389/fmicb.2015.00076
This article was submitted to Food Microbiology, a section of the journal Frontiers in Microbiology.
Copyright © 2015 Perricone, Arace, Corbo, Sinigaglia and Bevilacqua. This is an open-access article distributed under the terms ofthe Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
