Food Eng Rev DOI 10.1007/s12393-014-9099-2
REVIEW ARTICLE
Essential Oils: Antimicrobial Activities, Extraction Methods, and Their Modeling Fatima Reyes-Jurado • Avelina Franco-Vega Nelly Ramı´rez-Corona • Enrique Palou • Aurelio Lo´pez-Malo
•
Received: 2 June 2014 / Accepted: 25 October 2014 Springer Science+Business Media New York 2014
Abstract Worldwide there is a wide variety of plants and spices that have different uses according to the culture and traditions of each region. Essential oils are gaining interest from the academic and industrial communities since they have been associated with possible antimicrobial activity against a wide range of microorganisms. Essential oil extraction can be made by traditional or emergent methods; nowadays, mathematical models are being developed for these methods in order to design efficient industrial processes. Although the majority of the essential oils are classified as GRAS, their use in foods as preservatives is often limited due to flavor considerations, since effective antimicrobial doses may exceed sensory acceptable levels. The antimicrobial effect of each essential oil mainly depends on the quality and quantity of their components, which are affected by different factors such as the environmental conditions of the growing season of the plant as well as the extraction method. The most common methods used to evaluate the antimicrobial activity of essential oils in vitro are diffusion and dilution (direct contact) or vaporphase (gaseous contact) methods. This review focuses on available methods for extraction of essential oils and their mathematical modeling, as well as their application as antimicrobial agents. Keywords Essential oils Extraction Mathematical modeling Antimicrobial activity In vitro evaluation
F. Reyes-Jurado A. Franco-Vega N. Ramı´rez-Corona E. Palou A. Lo´pez-Malo (&) Departamento de Ingenierı´a Quı´mica, Alimentos y Ambiental, Universidad de las Ame´ricas Puebla, Sta. Catarina Ma´rtir, 72810 Cholula, Puebla, Mexico e-mail:
[email protected]
Essential Oils Extraction Essential oils (EO’s) obtained from aromatic plants represent a diverse and unique source of natural products, which are widely used for bactericidal, fungicidal, antivirus, antiparasitical, insecticidal, medicinal, or cosmetic applications, especially in the pharmaceutical, sanitary, cosmetic, food, and agricultural industries [21, 37]. Driven by the growing interest of consumers for ingredients from natural sources and their concern about potentially harmful synthetic additives, the global demand for essential oils is increasing nowadays [54]. However, before EO’s can be used or analyzed, they have to be extracted from the plant matrix. Different methods can be used for this purpose, including the most common methods such as hydro-distillation (HD), steam distillation, cold pressing (CP), solvent extraction, and simultaneous distillation–extraction methods, among others [29]. Although these techniques have been used since many years for EO’s extraction, their application has shown a wide number of disadvantages like losses of some volatile compounds, low extraction efficiency, degradation of unsaturated, or ester compounds through thermal or hydrolytic effects, and possible toxic solvent residues in extracts or EO’s [135]. As a consequence of the increment in the energy cost and with the arrival of the ‘‘Green Era’’, the industries dedicated to EO’s extraction focused in the development of emergent extraction processes [18]. Due to the disadvantages that traditional methods of extraction represent, several new techniques are currently available for the extraction of EO’s from plants, including supercritical fluid extraction (SFE), pressurized liquid extraction, pressurized hot water extraction, membrane-assisted solvent extraction, solid-phase micro-extraction, microwave-assisted, and
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ultrasound-assisted extraction, among others [48]. These alternatives to conventional extraction procedures may increase production efficiency and contribute to the protection of the environment by reducing the use of solvents and fossil energy, or generation of hazardous substances [29]. The methods of extraction significantly affect the chemical constituents and composition of EO’s [95]. The most appropriate and convenient method to concentrate the targeted biologically active compound into the EO should be carefully selected; in the following sections, the most representative traditional and emergent methods of EO’s extraction are presented.
Traditional Methods Cold Pressing Cold pressing or expression is the most antique process for obtaining EO’s; it was practiced long before humans discovered the process of distillation. This method has the advantage of little or no heat generation during the process [146], but it gives low yields. It is mostly used for the isolation of peel oils of citrus fruits due to the relative thermal instability of the aldehydes contained in them [73]. The mechanical cold pressing method is used to extract essential oils from citrus peels by rupturing the oil glands using pressure or abrasion so that the oil is ejected and washed away with a water spray. A lot of equipment for oil extraction by CP is commercially available, being the in-line extractor from FMC (Food Machinery Corporation, Chicago, Illinois) the most popular [31]. The EO’s obtained by CP are not entirely volatile compounds, they also contain coumarins, plant pigments, and so on [73]; thus, when a pure essential oil is required, it is necessary to use distillation over diluted NaOH or a carbonyl-adduct agent [146]. In view of the disadvantages with regards to low yield extraction and low purity obtained by this method, pretreatments using enzymes have been researched to enhance the quality and amount of EO’s extracted. Soto et al. [130] used an enzymatic hydrolysis combined with CP for obtaining borage (Borago officinalis) seed oil, founding bigger yields compared to the control without the enzyme pre-treatment; Collao et al. [33] improved the yield in primrose (Oenothera biennis) oil extraction using an enzyme-assisted extraction process with CP; while Anwar et al. [7] proved the effect of different enzyme preparations on the yield of cold pressed flaxseed oil, obtaining a considerable higher yield from enzyme-treated cold pressed flaxseeds (38 %) compared to the control (32 %). Recently, CP has been utilized for the production of organic EO’s; these products are typically marketed as
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special products and command a premium price in the market [146]. Distillation The traditional way of isolating volatile compounds as EO’s from plant material is distillation. During distillation, fragrant plants exposed to boiling water or steam, release their EO’s through evaporation. Recovery of the essential oil is facilitated by distillation of two immiscible liquids, namely water and essential oil, based on the principle that, at the boiling temperature, the combined vapor pressure equals the ambient pressure. Thus, the essential oil ingredients, for which boiling points normally range from 100 to 300 C, are evaporated at a temperature close to that of water [29]. Indirect cooling with water condenses the vapor mixture of water and oil; from the condenser, distillate flows into a separator, where oil separates from water [62]. Considering the manner in which the contact between water and the original matrix is promoted, a terminology that distinguishes three types of distillation has been proposed: HD, steam distillation, and water/steam distillation [89]. Comparing these three distillation process with regards to oil yield, speed of distillation, loss of oxygenated constituents, and the susceptibility to hydrolysis of the obtained compounds, it can be considered that steam distillation presents higher yields than the other two methods and less susceptibility to hydrolysis, and inversely HD have the disadvantage of producing less oil and high susceptibility to hydrolysis but the process is faster. Meanwhile, water/steam distillation present intermediate values for the mentioned parameters [77]. In the HD process, the plant material is completely immersed in boiling water. The characteristic feature of this process is that there is direct contact between boiling water and the raw material [73]. In steam distillation, both water and steam are utilized, but the plant material is not in direct contact with water. The steam is produced in a boiler and blown through a pipe into the bottom of the container, where the plant material rests on a perforated tray. The condensed distillate consists in a mixture of water and oil, the oil is separated from the water by means of a Florentine flask, which separates them on the basis of their differing densities [123]. The typical features of this method are that steam is always fully saturated, wet, and never superheated, and the material is only in contact with steam, and not with boiling water. Water/steam distillation is similar to the preceding type; in this case, steam is generated at the bottom of the container below the perforated tray; while in the case of steam distillation, the steam comes from an external source. Each method of distillation can be carried out at a reduced pressure, atmospheric pressure, or even at high pressures [56].
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Among the different techniques of distillation, steam distillation is the most utilized for obtaining EO’s at large scale; however, with the increase in the cost of energy and the demand at industry for fast processes, this technique that used to be considered cheap and suitable for field operation in the past, nowadays, represents an expensive waste of energy and time. Furthermore, distillation sometimes causes degradation of heat-sensitive compounds.
Solvent Extraction The hydrophobic and nonpolar character of EO’s allows their extraction by organic solvents, especially fuel derived solvents [10]. Solvent extraction refers to the distribution of a solute between two immiscible liquid phases in contact with each other [34]. In this process, a substance is transferred from a matrix by the use of a liquid in which the substance is soluble. When the extractable component is a solid, as in the case of plant materials, the process is a form of leaching. The simplest of these processes is batch percolation at ambient temperature. Here, ground dried powder of the plant material to be extracted is filled in a vertical vessel with a false bottom. A cloth covers the bottom and prevents passage of fine particles along with oil’s micelles. The early extract will be richer, and this is taken for distillation to produce the EO’s. The following weaker micelle is directed to the next percolator filled with fresh ground plant material. Since the concentration of the solute is highest at this stage, exchange of solute with weak micelle will occur. Progressively weaker and weaker micelle passes through progressively partially extracted plant material. Finally, fresh solvent passes through nearly completely extracted material to complete the process [10]. In the Soxhlet extraction, plant powder is placed in a cellulose thimble in an extraction chamber, which is placed on top of a collecting flask beneath a reflux condenser. A suitable solvent is added to the flask, and the setup is heated under reflux. When a certain level of condensed solvent has accumulated in the thimble, it is siphoned into the flask beneath [121]. Solvent extraction is by far the simplest method for obtaining EO’s and is most often utilized in the laboratory. Its main drawback is contamination of the sample with the solvent (or impurities in the solvent) that must be completely removed either to characterize the olfactory qualities of the oil or to study its biological activity. Unfortunately, often many low molecular weight species are lost during solvent evaporation, thereby changing, in some cases very dramatically, the aroma balance of the essential oil [95]. Hence, extracts obtained by solvent extraction with different organic solvents may not be considered as true essential oils; however, often they
possess most aroma profiles that are almost identical to the raw material from which they have been extracted [73]. Another disadvantage regarding solvent extraction processes are the solvent extraction effluents, which may contain biochemically active substances posing ‘‘new’’ hazards to the environment [34]. Since 1990, the concept of green chemistry has been introduced due to the concern of the impact that chemical compounds generate in the environment; reduction, elimination, or use of solvents more environmentally friendly is the goal of green chemistry [75]. For this reason, new research is being performed for the development of new solvents, greener than the traditional ones within which are ionic liquids, supercritical carbon dioxide, and biomass-derived solvents, among others [2, 65, 81].
Emergent Methods Development of new separation techniques for the chemical, food, and pharmaceutical industries has lately received a lot of attention due to increasing energy costs and the drive to reduce CO2 emissions [18]. These green approaches have led to considering the use of new techniques for the extraction of EO’s, such as microwave-assisted extraction, ultrasound-assisted extraction and SFE. Some examples of extraction applications with these emergent methods are summarized in Table 1. Microwave-Assisted Extraction (MAE) In recent years, different researchers have applied microwaves for extraction of several EO’s [17, 18, 48] and reported that the EO’s obtained in 30 min or less were comparable, both from a qualitative and from a quantitative point of view, to those obtained after more of the double of the time with selected traditional techniques such as HD or Soxhlet extraction. MAE uses microwave radiation as the source of heating for the solvent–sample mixture. Due to the particular effects of microwaves on matter (namely dipole rotation and ionic conductance), heating with microwaves is instantaneous and occurs in the interior of the sample, leading to very fast extractions [23]. One advantage of microwave heating is the disruption of weak hydrogen bounds, promoted by the dipole rotation of the molecules. MAE extractive processes are different from those of conventional methods because the extraction occurs as the result of changes in the cell structure caused by electromagnetic waves. The application of microwaves dramatically reduces both the extraction time and the required solvent volume, which automatically helps in lowering environmental burden by diminishing CO2 to the
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Food Eng Rev Table 1 Essential oils extracted by emergent methods and their main components Raw material
Essential oil
Main component
Percentage (%)
Method of extraction
References
Lime
Citrus limon
Limonene
47.5
HD
Atti-Santos et al. [9]
48.9
SFE
Oregano
Origanum glandulosum Desf
Thymol
81.1
SFME
65.4
MAE
Bendahou et al. [17]
41.6
HD
Rosemary
Rosmarinus officinalis
1,8-Cineol
52.4
SD
Boutekedjiret et al. [19]
Wild cinnamon
Cinnamomum iners Reinw. ex Bl.
Linalool
31.9 35.6
HD HD
Phutdhawong et al. [110]
50.5
MAE
Basil
Ocimum basilicurn
Linalool
HD SD
62.8
SE
11.3
UAE
Alissandrakis et al. [4]
UAE
Liu et al. [82] Figueredo et al. [46]
Flowers of lemon
Citrus limon
Flowers of orange
Citrus sinensis
51.6
Flowers of sour orange
Citrus aurantium
80.6
Flowers of tangerine
Citrus tangerina
Japanese basil
Perilla frutesens
Perillaldehyde
Thyme-leaved savory
Satureja thymbra L.
Carvacrol
Salvia
Salvia mirzayanii c
Linalool
48.2 48.1
75.2
Linalyl acetate
38.7
MAE
34.0
HD
24.6
SFE
8.0
HD
71.9 71.2
SD SE
Ozel and Kaymaz [105] Wenqiang et al. [154]
Origanum onites
Carvacrol
Clove buds
Syzygium aromaticum
Eugenol
Aerva javanica seed
38.8
1,8-cineole Cretan oregano
Desert cotton
Charles and Simon [28]
Heptacosane
58.8
SFE
56.2
SD
48.8
HD
57.2
SE
25.4
HD
41.4
HD
Yamini et al. [158]
Samejo et al. [120]
HD hydro-distillation, SFE supercritical fluid extraction, SFME solvent-free microwave extraction, MAE microwave-assisted extraction, SD steam distillation, SE solvent extraction, UAE ultrasound-assisted extraction
atmosphere [107]. Process acceleration and high extraction yield may be the result of a synergistic combination of two transport phenomena: heat and mass gradients working in the same direction [147]. MAE is particularly suited for the extraction of thermolabile compounds, since during the process the temperature remains low [79]. Even though, the first publications that reported the efficiency of microwave heating for organic extraction appeared in 1986 [79] is until recently that MAE was considered to be particularly attractive due to fast heating of aqueous samples [48]. Several research reports mentioned that factors that affect organic compound extraction by MAE include microwave power output, exposure time, pressure, sample viscosity, matrix moisture and nature, as
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well as nature and volume of solvent [86]. Most of the time, the chosen solvent possesses a high dielectric constant and strongly absorbs microwave energy; however, extracting selectivity and ability of the medium to interact with microwaves can be modulated using mixtures of solvents [70]. Advances in microwave extraction have resulted in the development of several techniques such as microwaveassisted solvent extraction [140], vacuum microwave HD [55], microwave HD [51], solvent-free microwave extraction [17], microwave accelerated steam distillation [30], microwave hydro-diffusion and gravity (MHG) [148], and microwave-assisted simultaneous distillation-solvent extraction [45].
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Ultrasound-Assisted Extraction (UAE)
Supercritical Fluid Extraction
Ultrasound potential industrial application has been recognized for in the phyto-pharmaceutical extraction industry for a wide range of herbal extracts [149]. UAE is utilized for the isolation of the volatile compounds from natural products at room temperature with organic solvents [4] shortening processing time, decreasing the volume of solvent, and increasing the yield of the extract in comparison with conventional methods [111]. Its effect is much stronger at low frequencies (18–40 kHz) and practically negligible at 400–800 kHz [35]. The enhancement of extraction efficiency of organic compounds by ultrasound is attributed to the phenomenon of cavitation produced in the solvent by the passage of an ultrasonic wave. Cavitation bubbles are produced and compressed during the application of ultrasound. The increase in pressure and temperature caused by the compression leads to the bubble collapse, a resultant ‘‘shock wave’’ passes through the solvent enhancing the mixing [111]. Also, in the treatment of raw plant tissues, it has been shown that the cells containing essential oils possess a very thin skin that can be very easily destroyed by sonication [139], thereby the application of ultrasound facilitates the release of extractable compounds and enhances mass transport of solvent from the continuous phase into plant cell [80]. Therefore, efficient cell disruption and effective mass transfer are the two major factors leading to the enhancement of extraction with ultrasonic power [153]. For the extraction of EO’s at laboratory scale, equipments such as cleaning baths or probe systems are commonly used. In the case of cleaning baths, extraction can be performed by indirect or direct wave, in both cases it is preferable to use a mechanical stirrer and cool the extraction mixture since the use of ultrasound can raise the temperature [139]. For small extraction volumes, the probe system can be sufficient [150]. Many factors govern the action of ultrasound in order to obtain an efficient and effective ultrasound-assisted extraction, some of them are related to the plant characteristics (moisture content and particle size), some other factors include the solvent used for the extraction, as well as variables of the process (frequency, pressure, temperature, and sonication time) [153]. Increments in yields extraction have been reported when UAE was utilized to obtain EO’s when compared with traditional methods [64, 82, 139]. Toma et al. [139] reported that sometimes when ultrasound is employed, the extraction yield did not significantly increases; however, degradation of herb constituents is always diminished. Therefore, UAE represents a suitable process for sensible compounds.
SFE is a promising technique for industrial application [117]; it has been used to extract plant materials, especially lipids, flavors, and EO’s [152]. This emergent extraction technique is usually faster, more selective toward the compounds to be extracted, as well as more environmentally friendly when compared to traditional methods [90]. SFE is based on the use of solvents at their supercritical state, which means that they are subjected to temperatures and pressures above their critical points. Supercritical fluids (SCF) have unique properties, between those of gas and liquids, which depend on the pressure, temperature, and composition of the fluid. These fluids are heavy-like liquid but have the penetration power of a gas, qualities that make them effective and selective solvents [94]. CO2 is the supercritical solvent of choice for the extraction of plants compounds, since it is not toxic and allows supercritical operation at relatively low pressures and near room temperatures [159]. SFE process consists of two steps: extraction of the soluble components in a supercritical solvent and separation of the extracted solutes from the solvent. Separation of soluble compounds from the SCF can be performed modifying the thermodynamic properties of the supercritical solvent, changing the temperature or pressure of the system, or by an external agent by adsorption or absorption. In such cases, the desired effect is a reduction in a solvent power [106]. A typical SFE system consists of a high pressure pump that delivers the fluid and an extraction cell containing the sample and which is maintained at the selected pressure and temperature. An organic solvent (cosolvent) may be added to the fluid to enhance its solvating properties; this can be performed using premixed cylinders or an additional pump [23]. Changing the temperature and pressure, the selectivity of a SCF can change, making it available for the selective extraction of specific compounds from a mixture. Fractional extraction process (FEP), single stage extraction (SSE), and sequential depressurization (SD) are examples of procedures to selectively extract or separate specific compounds with SFE [125, 159]. SD is commonly utilized for the extraction of EO’s; in this process, the light and heavy fractions of the compounds are simultaneously extracted using high-density fluid, and then the SCF and the extract passes through multiple depressurization steps, allowing fractional separation. The extraction takes place at high pressures (40–60 MPa), and the EO’s are collected in the second step [106]. Most studies concerning SFE of EO’s focus on the optimization of extraction conditions to increase the yield of extraction or the recovery of a specific compound. The yield of SFE of EO from Salvia mirzayanii at 35 C and
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35.5 MPa was 9.7 % (w/w) while HD gave a yield of 2.2 % [158]. Atti-Santos et al. [9] compared the efficiency of extraction of supercritical carbon dioxide and HD for the extraction of EO from lime and found that the best results were obtained after 3 h with HD or after 30 min for the supercritical extraction. A highest amount of eugenol was reported in EO’s isolated by SFE (58.7 %) compared with HD (48.8 %), SD (56.2 %), and SE (57.2 %) [154]. The advantages of using SCF’s offer environmental benefits (due to the reduction of the use of environmentally damaging conventional organic solvents), health and safety benefits (supercritical CO2 and supercritical H2O are noncarcinogenic, non-toxic, non-mutagenic, nonflammable, and thermodynamically stable) and chemical benefits (these compounds possess high diffusivity, low viscosity, and a tunable density and dielectric constant) [72]. Optimization of SFE is a function of various independent parameters such as solvent flow rate, residence time, moisture content, particle size, particle size distribution, as well as supercritical pressures and temperatures [159].
Mathematical Models for Essential Oil Extraction Methods The low concentration of essential oils in plant materials requires high-performance extractions to achieve larger yields. Therefore, extraction methods and processing parameters need optimization for each crop as plant matrix, oil content, and constituents affect extraction kinetics. To optimize industrial processes for the extraction of essential oils, mathematical models are useful tools for simulating processes’ performance, since they allow assessment of several alternatives or operational scenarios in order to define the best processing conditions, without the necessity of carrying out a large number of experimental trials [88]. Since model development requires understanding of the physical processes involved during essential oil extraction, several approaches have been proposed. The response surface methodology has been typically used to determine the significant effects of different process variables on the interest responses, along with their possible interactions (Eq. 1). X X XX g ¼ b0 þ bi xi þ bii x2i þ bij xi xj ð1Þ i
i
i
j¼iþ1
where g is the response, xi the variable i and b0, bi, bii, bij the regression coefficients. For instance, this approach can be found in Rezzoug and Louka [118] for improving the thermo-mechanical process for EO’s extraction, in Allaf et al. [5] for the intensification of EO’s extraction via instant auto-vaporization, and in Farhat et al. [44] for the optimization of microwave steam extraction of essential oil
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from orange peel. This kind of models are based on the statistical analysis of the experimental design, most of them reported quadratic polynomial equations that were utilized to determine the best combination of the operation parameters. It is important to notice that their applicability is constrained to the experimental range and only represent studied variables’ effect on the process yield, but do not explain the physical mechanisms or pathways of the process. With the aim of taking into account physical mechanisms, some models based on mass transfer fundamentals have also been proposed. Sovova´ and Aleksovski [131] developed a model for simulating the HD process for EO’s extraction considering two different kinds of particles: spherical particles with homogeneously distributed oil or slabs with a part of essential oil deposited on their surface. They assumed equilibrium between regions of broken cells in boiling water and vapor phase; therefore, the only mass transfer process that had to be taken into account was the oil diffusion from the particle core to the region of broken cells. The analytical solution of their proposed model is described in Eq. 2, where Y is the oil yield and Y? is the asymptotic oil yield. t t Y ¼ Y1 1 f exp ð1 f Þ exp ð2Þ T1 T2 where: T2 [ T1 Parameters Y?, f, T1, and T2 were fitted from experimental runs; 1/T2 is proportional to the mass transfer coefficient, and 1/T1 is associated to the equilibrium constant. Then these physical properties were evaluated from the parameters obtained at different water/matrix ratios, mater flow rates, and particle sizes. Their model accuracy was evaluated comparing the obtained results with the literature data for EO’s distillated from thyme leaves and intact coriander seeds. Results indicated that essential oil content in thyme was 11.1 g/kg of dry leaves, 61 % of the oil was initially deposited in glandular trichomes, the maximum oil concentration in boiling water was 0.58 g/kg, and its concentration in the water flowing back to the still at 79 C was 0.03 g/kg. Sovova´ and Aleksovski [131] model underestimated the initial rate of distillation from intact coriander seeds. Nevertheless, it gave reliable estimates of the oil content in the seed (0.64 g/kg), the steadystate mass transfer coefficient (4.3 9 10-8 m/s), and the effective diffusivity in coriander seeds (1.7 9 10-11 m2/s). Cassel et al. [25] developed a mathematical model based on Fick’s law to predict essential oil recovery by steam distillation, which was used to simulate the oil extraction of rosemary (Rosmarinus officinalist), basil (Ocimum basilicum L.), or lavender (Lavandula dentate L.). It was assumed that diffusion in the particle is the controlling step and that soluble components’ concentration was
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homogeneous and constant for every particle; the obtained model was based on single plate particle description and required only one adjustable parameter to predict the experimental data. They proposed that model parameter evaluation could be a useful tool during the scale-up of the extraction process and/or during pilot or industrial operation in order to evaluate the extraction time required to obtain a given yield. The extraction curve followed the same behavior for rosemary, basil, and lavender EO’s. The maximum yield values obtained for rosemary, basil, and lavender oils were 0.51, 0.38, and 0.32 % (w/w), respectively. Xavier et al. [157] developed a model for steam distillation extraction, considered as fluidized bed and based on the existence of intact particles and broken particles; the oil located in the broken cells is rapidly extracted, while the oil deposited within the intact cells diffuses slowly to the surface. Therefore, the model considers the existence of two phases during the extraction process: The first one associated with an equilibrium process (Eq. 3) and the second one governed by the mass transfer mechanism (Eq. 4). qf e
oY oY þ qf u ¼ J ðx; Y Þ ot oz
qs ð1 eÞ
ox ¼ J ðx; Y Þ ot
ð3Þ ð4Þ
Along the first stage, the solute flux is defined as J(x, Y) = kfa0qf(Y* - Y) and during the second stage is defined as J(x, Y) = ksa0qsx. The proper solution of that model is given as: i 1 e qs h K2 ðtzeu Þ Y ðz; tÞ ¼ expK2 t x0 exp ð5Þ e qf K2 ¼
k s a0 ð1 eÞ
ð6Þ
where, qf is the solvent density, qs solid density, e bed void fraction, u superficial fluid velocity, Y solute mass fraction in fluid phase, x0 solute mass fraction in solid phase, kf fluid phase mass transfer coefficient, ks solid-phase mass transfer coefficient, a0 specific surface area per volume, Y* equilibrium fluid phase mass fraction, t extraction time, and z axial coordinate. This model was validated with data from the extraction of essential oils from different species of rosemary. Furthermore, since model selection depends on knowledge of the oil distribution in the plant, some models based on Langmuir adsorption isotherms have also been developed, as that reported by Babu and Singh [11] for the HD process of Eucalyptus cinerea oil (Eq. 7). y¼
Yt bþt
ð7Þ
In this model, y is the yield as function of time, Y is the yield after infinite extraction time, and Y/b is the initial slope of the yield. Parameters in Langmuir equation had no direct relationship with the solute–solvent transport properties and mechanism of extraction from the plant matrix. However, these model parameters can be used to correlate the trends of essential oil production kinetics, i.e., can help the essential oil producers to predict the oil yields before the actual distillation process is far away from its completion. Results inferred that the equilibrium had not formed instantaneously, which could be due to the mass transfer resistance caused by the waxy layer on the foliage surface. Several attempts for modeling SFE have also been reported, considering both empirical kinetic equations and mass transfer models, which reliability strongly depends on the quality of the experimental data, given the difficulty in obtaining them [116]. Most proposed models based on mass transfer fundamentals take into account the matrix structure, particle size, equilibrium processes, and internal mass transfer resistance. Reis-Vasco et al. [115] developed a model for the SFE of pennyroyal essential oil; from their experimental results, they established that the first part of the extraction was governed by equilibrium, associated to the desorption of essential oil located on the surface of leaves (70 % of the available EO is adsorbed on the surface). For the second stage, their model considered that internal mass transfer controls the process. In order to consider the axial dispersion, the proposed model was extended to take into account the effect of flow rate of the supercritical fluid, observing a slightly better fitting of experimental data. Mathematical models for emergent extraction methods, such as solvent-free microwave extraction (SFME) have been recently explored; Navarrete et al. [96] model considers the electromagnetic energy and its interaction with the material, through the Maxwell’s equation, obtaining an estimation of the heating and evaporating during SFME. Mass and heat transfer were considered for a packed bed, where the temperature profile was determined by means of Fourier law (Eq. 8), considering that generated heat is provided by the interaction of the microwaves, described by Eq. 9. oT ¼ r ðkrT Þ q000 ot 1 q000 ¼ ðr þ xe0 e00 ÞjEj2 2
qCP
ð8Þ ð9Þ
where q is the apparent density of the bed, CP lumpedspecific heat (J/(kg K)), q¢¢¢ heat source (W/m3), x angular frequency, e00 dielectric loss factor, e0 vacuum permittivity, r conductivity (A/(Vm)), and E is the electric field (V/m).
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Food Eng Rev Table 2 Main components of essential oils obtained from selected plants Component
Examples
Plants rich in these components
Terpenes
Pinene, camphene, phellandrene, limonene, myrcene
Conifers, umbellifers, citrus fruit
Sesquiterpenes
Cardinene, selinene, humulene, cedrene, chamazulene
Celery cloves, true chamomile
Aromatic phenols
Carvracol, eugenol, thymol, guaiacol
Cinnamon, rosemary, mints, fennel
Aromatic alcohols
Terpineol, menthol, cedrol, linaoe oil, carotol
Lemon, lavender, sandalwood
Aromatic oxides
1-8-cineole, linalool oxide, menthofurane, ascaridol, safrol
Cardamom, mint, eucalyptus
Aromatic ether
Methyl chavicol, methyl salicylate, methyl cinnamate, methyl eugenol, transanethol Linalyl acetate, bornyl acetate, cinnamic acetate, terpenyl acetate, neryl acetate
Basil, cloves, bay laurel
Aromatic and terpene aldehydes
Benzaldehyde, citrole, vainillin, neral, citral
Cumin, anise, fennel
Aromatic ketones
Carvone, thujone, camphone, verbenone, fenchone
Rosemary, artemisa, dill
Terpene esters and terpeneless esters
Peppermint, cloves, cinnamon
Adapted from Balz et al. [15]
As can be noticed, process modeling is an important tool for obtaining operational parameters useful for scaling up EO’s extraction methods. However, although this approach reduces the number of required experiments, mathematical model development implies a number of assumptions, such that models must always be validated before they are utilized.
Essential Oils Composition EO’s are complex mixtures of volatile substances generally present at low concentration in different plant materials including flowers, buds, bark, herbs, wood, fruits, roots seeds, leaves, and branches [138]. These compounds consist of relatively low molecular weight organic molecules containing carbon, hydrogen, oxygen, and occasionally nitrogen and sulfur; chlorine and bromine also may be found less frequently, particularly in seaweed volatiles [156]. In general, these components are \500 Da in molecular weight and contain only 1–3 oxygen atoms [36].
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The classification and nomenclature of EO’s compounds is complicated by the fact that many of them were isolated and studied before the establishment of systematic chemical nomenclature. Consequently, most of them are known by their non-systematic or common names. These are sometimes but not always based on their source, such as eucalyptol, limonene, and thymol [24]. EO’s are composed of many kinds of molecules including terpenoids and nonterpenoids, depending on the plant and the extraction method, among other factors [117, 119]. The non-terpenoid hydrocarbons are short chain alcohols, phenylpropanoids, acids, ketones, esters, and aldehydes formed by metabolic conversion or degradation of phospholipids and fatty acids. In addition, several nitrogen and sulfur containing compounds, which impart characteristic sensory attributes, are also important constituents of many EO’s [76]. Some examples of these compounds and plants with high content of them are presented in Table 2. Terpenes, also called isoprenoids, are by far the largest class of natural volatile chemicals found in plants. They are formed by head-to-tail condensation products of two or more isopropenes molecules. Oxygen-containing derivatives such as prenol and isovaleric acid are also hemiterpenes [56]. Terpenoids identified in EO’s include from hemiterpenes (5 carbons) to triterpenes (30 carbons). Sesquiterpenes are the second most common compounds in EO’s. They are formed by the combination of three isopropene units. These compounds are a structurally diverse group, all deriving from farnesyl pyrophosphate by various cyclization processes often followed by skeletal rearrangement [32, 122]. The composition of EO’s varies due to genetic and environmental factors that influence genetic expression [28]. Essential oil bearing plants possess a variation in the oil composition across the plant; different species may accumulate active principles distributed throughout the plant or concentrate them only in certain organs [77]. The presence and EO composition of secretory structures in the leaf, flower, root, and rhizome of Alpinia zerumbet were determined by Jezler et al. [63], the analyzed organs of this plant present essential oil, but the main compound in the leaf was 1,8-cineol while a-terpineol was the main compound in the flower and rhizome. The variety of plant is another intrinsically factor that affect its composition; Aligiannis et al. [3] compared the composition of aerial parts of Origanum scabrum and Origaum microphyllum. In their study, twenty-eight constituents were identified in O. scabrum, being carvacrol the major component (74.9 %). for O. microphyllum forty-one constituents were determined in the essential oil and cis-sabinene hydrate (31.1 %) was found as the major compound. Extrinsic factors that can affect EO’s composition can be related to the environment, method of extraction, or
Food Eng Rev
conditions of storage. In the case of environmental conditions, weather, water, sunlight, day length, pressure, nutrients, diurnal fluctuation, soil type, diseases, and insect damage among others, can have an effect on the composition of the oil [15]. Jorda´n et al. [67] monitored season variations in the composition of Spanish Thymus vulgaris EO, at five different phenological stages during the vegetative cycle. The major components quantified within the oil were 1,8-cineole, followed by terpenyl acetate, borneol, linalool, b-pinene, a-terpineol, and camphor. They found that mid-vegetative stage seems to be the most appropriate harvesting time for this species, since cineol, borneol, monoterpenic hydrocarbons, and camphor exhibited their maximum relative concentrations at this phenological stage. In contrast, terpenyl acetate, a-terpineol, and linalool, probably components that are associated with the fresh aroma in the oil, were mostly concentrated from full bloom to advanced fruit formation. Similar results have been reported for basil EO studied at four seasonal times by Hussain et al. [57]. Composition of EO’s is largely affected by extraction method. It has been found that traditional techniques used for extraction, that imply the use of high temperatures (distillation, steam distillation, HD, etc.) cause losses of some volatiles and degradation of unsaturated or ester compounds through thermal or hydrolytic effects; while with the use of solvent extraction, toxic solvent residue may be encountered [29, 65]. Once obtained, the volatile nature of EO’s make them susceptible to damage by many storage factors such as light, heat, oxidation, and hydration [68, 114]. In the case of citrus, EO’s that consist mainly of monoterpene hydrocarbons that possess high levels of unsaturation the use of airtight storage containers in the dark is needed to prevent compositional changes [29]. EO’s typically contain dozens of constituents with related, but distinct chemical structures [138]. Major components can constitute up to 85 % of the EO, whereas other components are present only as traces. Due to their considerable importance, many detection methods have been developed to determine the presence and amount of specific compounds. Among them the high-performance liquid chromatography (HPLC) and gas chromatography coupled with mass spectrometry (GC–MS) are the most often used [36, 136].
Chemical Preservative Agents Worldwide there is a trend to consume natural, safe, and high quality food. Antimicrobials are used in food for two main reasons: to control natural spoilage processes (food preservation) and to prevent growth of pathogens (food safety) [134]. Consumers’ preferences are moving toward
foods that contain lower levels of chemical preservatives and exhibit more fresh-like and natural characteristics. The salts of weak acids, such as sodium benzoate and potassium sorbate, can inhibit growth of several postharvest fungal and bacterial pathogens. Furthermore, nisin, monolaurin, and lactoperoxidase are examples of ‘‘natural’’ preservatives, but they have several limitations, which include limited spectra of activity, high application costs, the potential emergence of resistant strains, as well as their impact on foods’ sensory attributes. Application of the hurdle technology concept, i.e., deliberated combination of existing and novel preservation techniques in order to establish a series of microbial stress factors that any microorganism present should not be able to overcome [79].
Antimicrobial Activity of Essential Oils In nature, EO’s play an important role in the protection of plants functioning as antibacterials, antivirals, antifungals, insecticides, and also against herbivores by reducing their appetite for such plants. EO’s can be lethal to several organisms including bacteria, virus, fungi, protozoa, parasites, acarids, and insects, or they may simply inhibit the production of metabolites such as mycotoxins [49]. Considering the wide variety of chemical compounds present in EO’s, it is likely that their antimicrobial activity is not attributable to a specific mechanism of action, but the combined action of several of them on different parts of the microbial cell. Many researchers mention that EO’s antimicrobial activity depends mainly on three characteristics: the character of EO (hydrophilic or hydrophobic), its chemical components and the type of organism that must attack [47, 54, 68, 129]. As typical lipophiles, EO’s pass through the cell wall and cytoplasmic membrane, disrupt the structure of their different layers of polysaccharides, fatty acids and phospholipids and permeabilize those membranes [14]. In bacteria, the permeabilization of the membranes is associated with loss of ions and reduction of membrane potential, collapse of the proton pump, and depletion of the ATP pool. EO’s can coagulate the cytoplasm and damage lipids and proteins. Damage to the cell wall and membrane can lead to the leakage of macromolecules and to lysis. The antimicrobial activity of EO’s is mostly due to the presence of phenols, aldehydes, and alcohols [47, 54, 127]. EO’s are effective against molds, yeasts, and bacteria, however, has been observed more susceptibility to EO’s in Gram-negative bacteria in contrast to Gram-positive bacteria; these susceptibilities may be related to the outer membrane [68]. Fisher and Phillips [47] mentioned that the antimicrobial activity in Gram-negative bacteria involves a
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delay of the effect, so to achieve the same killing effect it would require a longer exposure to the EO’s. A good review of EO’s was recently written by Tongnuanchan and Benjakul [141].
Effect of Selected Extraction Methods for Essential Oils on Their Antibacterial and Antifungal Activities As previously stated, the essential oils of aromatic herbs could be obtained by different methods such as HD, solvent extraction, and microwave-assisted extraction among others [101, 140, 154]. According to the extraction method used, the energy and time required for the isolation of EO vary and, consequently, the yield could be or not improved. However, these parameters are not the only ones affected by the selected method, also the composition of the extracted oil may vary from one extraction method to another [28, 112]. During the past years, a great effort has been made to determine the effect of different extraction methods in the composition of the EO obtained, finding that the specific compounds that integrate an EO may be lost depending on the selected extraction method [17]. The most common method to obtain EO is distillation. However, the different traditional techniques based in distillation (HD, steam distillation, water–steam distillation) may induce different thermal degradations [154] and dissolve some EO constituents [64]. In the case of citrus essential oils obtained by distillation, it has been reported that they deteriorate easily and develop off-flavors due to the instability of the terpene hydrocarbons present during the process [9]. Boutekedjiret et al. [19] compared rosemary essential oils obtained by steam and HD from a qualitative and quantitative point of view; they reported that despite these two oils were both characterized by the presence of monoterpene hydrocarbons, oxygenated monoterpenes, and sesquiterpenes, a quantitative difference existed. The monoterpene hydrocarbons compounds were in small proportions in the hydro-distilled oil, due to chemical conversions in the presence of water, resulting from hydrolysis reactions of these components in monoterpene alcohol’s components. Solvent extraction as conventionally applied could result in severe losses of volatile materials because the liquid in which the oil is collected must be subsequently removed by evaporation [140]. Charles and Simon [28] evaluated HD, steam distillation, and solvent extraction using three basil species. By hydro- and steam distillation, 32–35 constituents were obtained, while with organic solvent extraction only 22 compounds were observed. Linalool, the major constituent, was 48.2 and 48.1 % in oils obtained by hydroand steam distillation, respectively, but 62.8 % by organic solvent extraction.
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In order to improve the quality of essential oils new techniques of isolation have been used, some of them are based in distillation but the time of process needed is less allowing minor degradation of sensible compounds or at least obtaining EO’s with the same quality but in a faster process [65, 82, 153]. Bousbia et al. [18] compared the composition of lime peel EO obtained by MHG with conventional methods such as HD and CP. They reported that essential oils of lime peels extracted either by MHG, HD, or CP were rather similar in their composition and contained the same dominant components and that the same number of volatile secondary metabolites was found in the essential oil isolated by MHG. However, it was necessary to use an appropriate microwave irradiation power such that to ensure that the essential oil will be extracted quickly, but not too fast that loss of volatile compounds would result. In the case of SFE, a limitation is that it often fails in quantitative extraction of polar compounds from solid matrices, because of the poor solvating power of this fluid as well as the insufficient interaction between supercritical CO2 and the matrix. S. mirzayanii EO’s extracted by SFE and HD was used for comparison [158], the recovery of some components in SFE was better than HD, but the number of the essential oil components extracted by the SFE (maximum of 20) were lower than those obtained by the HD method (34 components). It is known that essential oils have specific biological functions; among these, the antimicrobial activity of EO’s depends on their components which, due to their range of complexity, have the capacity to affect many biological systems [24]. Specifically, for the case of EO’s antimicrobial action, it has been reported that it is determined by more than one component and that the major component is not the only one responsible for the antimicrobial activity [16]. For this reason, it is expected that a reduction or loss of any compound present in an essential oil caused by the extraction method will affect its antimicrobial activity. However, most of the reports regarding the effect of extraction method in EO’s focused in the identification (qualitative and quantitative) of their composition [48, 65, 154, 158], while research performed with regards to assessment of their effect as antimicrobials depending of the method used for their extraction has not been thoroughly evaluated. The antimicrobial activity of Origanum majorana essential oil and extracts obtained with ethyl alcohol and supercritical carbon dioxide were investigated with microbiological tests against several food-borne fungi and bacteria [145]. In this study, the authors observed that extracts obtained by SFE at high pressure and temperature showed significantly stronger antimicrobial properties in comparison with the slight inhibitory effects of the ethanol
Food Eng Rev Table 3 Effect of selected methods of extraction (HD: hydro-distillation, SFME: solvent-free microwave extraction) on essential oils’ antimicrobial activity
References
Essential oil
Microorganism
MIC (lg/ml) HD
SFME
Bendahou et al. [17] Algerian oregano (Origanum glandosulum) Bacillus subtilis
87.5
59.5
Staphylococcus aureus
79.25
79.25
Listeria monocytogenes
78
58
E. coli (E1)
87
79.25
E. coli (E2)
87
79.25
E. coli (E3)
59.25
59.25
Klebsiella pneumoniae
59
59
Pseudomonas aeruginosa (P1)
120.5
120.5
P. aeruginosa (P2) Salmonella typhimurim
110 64
100 64.25
Candida albicans 444
36
36.25
C. albicans 9036
57
57
Fusarium oxysporum
56
56.5
Cladosporium herbarum
56.5
50.25
Botrytis cinerea
57.5
57.25
Aspergillus flavus
55.25
52.25
Okoh et al. [101] Rosemary (Rosmarinus officinalis)
MIC minimal inhibitory concentration
extract; these authors related the stronger antimicrobial activity of SFE extract with the higher concentration of volatile compounds being almost twice higher than in the ethanol extract. In Table 3, it can be observed the effect of SFME and HD in the biological activity of two essential oils used as antimicrobials. Bendahou et al. [17] compared the antimicrobial activity of Origanum glandulosum essential oils obtained by SFME and HD; for this purpose, they determined the minimum inhibitory concentration (MIC) against three Gram-positive bacteria, seven Gram-negative bacteria, two yeasts, and four molds. The antimicrobial nature of the EO’s investigated in this study was apparently related to the large amount of thymol present in SFME and HD (81.1 and 41.6 %, respectively) extracted oils, but the oil obtained by SFME showed better antifungal activity with lower MICs and higher inhibition zones than in the case of HD oil. Similarly, Okoh et al. [101] obtained MICs for Rosmarinus officinalis L. EO obtained by HD and SFME against two Gram-positive and two Gram-negative bacteria. The EO obtained by SFME proved higher antimicrobial activity due to their lower MIC values, the higher amount
Staphylococcus aureus
3,750
470
Bacillus subtilis
1,880
1,880
Escherichia coli
7,500
1,880
940
230
Klebsiella pneumoniae
of oxygenated monoterpenes such as borneol, camphor, terpene-4-ol, linalool, a-terpeneol (28.6 %) in comparison with the oil extracted by HD (26.9 %) is an explanation of its different antimicrobial activity. This is probably due to the reduction of thermal and hydrolytic effects compared with HD which uses a large quantity of water and is time and energy consuming.
Effect of Environmental Factors on Antimicrobial Activity of Essential Oils Another possible explanation for antimicrobial activity differences that are found in EO’s from the same plant may be associated to environmental factors, because the composition and content of essential oils varies within the same herb or spice depending on agricultural practices, geography, and climatic conditions during the growing season [80, 132]. According to Gil et al. [50], any environmental factor can affect the presence and amount of the components present in an essential oil. This happens because plants as a part of the ecosystem are affected by biotic and abiotic factors and as a result biosynthesis and metabolism
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of its components are altered, being these interrelationships governed by light, temperature, the fluctuations of both, the weather, the amount and quality of water and nutriments, among others [6]. It has been demonstrated that these factors associated to the growing process of the plant affect the composition of its EO; it is well known that the compounds available in an EO are the responsible for its antimicrobial activity; thereby any change in the composition could affect this biological function. For this reason, several researches have centered in identifying the variations on quality and quantity of the components present in EO’s obtained from the same species but varying one or more of the environmental factors mentioned above [13, 43, 108] as well as the impact that these variations have on the antimicrobial activity of the EO. Variation in the quantity, quality, and antibacterial activity of the essential oil of wild Thymus caramanicus at different phenological stages including vegetative, floral budding, flowering, and seed formation were reported for Ebrahimi et al. [40]. Antibacterial activity of the oil and their main compounds were tested against Gram-positive (Bacillus subtilis, Enterococcus faecalis, Staphylococcus aureus, and Staphylococcus epidermidis A) and Gramnegative bacteria (Escherichia coli, Pseudomonas aeruginosa, and Klebsiella pneumoniae) by disc diffusion assay and determining its MIC values. They reported that T. caramanicus EO have great potential due to its antibacterial activity against the seven bacteria tested, but also observed that low amounts of c-terpinene during the flowering phase were associated with low antimicrobial activity of the oil obtained during this period. Mu¨ller-Riebau et al. [93] determined variations in essential oil composition of the studied species throughout the time period investigated and estimated the best time of harvesting for their use as antifungal agents. They reported that the inhibitory effect of the essential oils was mainly due to the most abundant components and these components varied according to the season when the plant was collected. Hussain et al. [57] investigated the composition of the essential oil isolated from the aerial parts of basil (O. basilicum) as affected by different growing seasons along with their antioxidant and antimicrobial activities. Samples collected in winter were found to be richer in oxygenated monoterpenes (68.9 %), while those of summer were higher in sesquiterpene hydrocarbons (24.3 %). The contents of most of the chemical constituents varied significantly (p \ 0.05) between seasons. Antimicrobial activity of these essential oils against S. aureus, E. coli, B. subtilis, Pasteurella multocida and pathogenic fungi Aspergillus niger, Mucor mucedo, Fusarium solani, Botryodiplodia theobromae, Rhizopus solani was assessed by disc diffusion assay and determination of MICs were performed. Antimicrobial activities of the oils varied significantly
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(p \ 0.05) as seasons changed. In general, essential oils from winter and fall crops exhibited greater antimicrobial activity which might be attributed to the high contents of linalool and other oxygenated compounds in these samples. Hussain et al. [58] studied the variation in chemical composition, and antimicrobial and cytotoxic activities of essential oils from the leaves of four Mentha species as affected by harvesting season (summer or winter crops). The antimicrobial activities of Mentha essential oils and main components were assessed against human pathogenic and food-borne microorganisms; their results indicated that the essential oils from the four Mentha species exhibited excellent antimicrobial activity against tested microorganisms. Seasonal variation exerted notable effects on the antimicrobial activity of Mentha essential oils, and they attributed this to the differences in chemical composition of studied oils.
Antimicrobial Assays Antimicrobial activity has been studied in vitro for most of pathogenic microorganisms. The most common methods used to assay the antimicrobial activities are based on agar diffusion method (paper disc or well) and dilution method (agar or liquid broth). Testing and evaluating the antimicrobial activity of EO’s is difficult because of their volatility, water insolubility, and complexity. The specific properties of EO’s require some modifications to these methods, which have been developed for water soluble antimicrobial agents. EO’s are hydrophobic and with high viscosity. These properties may reduce the dilution capability or cause unequal distribution of the EO through the medium even if a proper dispersing or solubilizing agent is used. Furthermore, it has to be checked whether the applied concentrations of the emulsifier or solvent did not affect the growth and differentiation of tested microorganisms [68]. Essential oils are very complex mixtures of volatile components; so long incubation time may result in the evaporation or decomposition of some of the components during the testing period, thus specific experimental conditions should be set to avoid these phenomena. Besides, the effectiveness of each method could be affected by: source of the EO, volume of the inoculum, growth phase of the microorganism, culture medium used, incubation time, temperature, as well as pH and water activity [21]. The MIC is cited by most researchers as a measure of the antimicrobial activity of EO’s. The definition of MIC differs among publications and this is an obstacle to perform comparisons between studies. In most cases, MICs are defined as the lowest concentrations of EO’s required to slowdown the growth of microorganisms (bacteriostatic or
Food Eng Rev
fungistatic properties) [127] or the minimum lethal concentration to ensure reduction of 99.9 % of the population of tested organism (bactericidal or fungicidal) [21]. Moreover, the activity of EO’s against molds can also be assessed by monitoring the inhibition of sporulation or toxin production [68]. Furthermore, bactericidal or bacteriostatic effect of the EO’s can be determined by analyzing the time to death (survival curve) in a nutrient medium, where the number of viable cells in the medium after EO addition, is plotted against time [21]. Also, the damage to the cell wall and loss of cell contents can be analyzed by electron microscopy [74]. The most common methods used to evaluate the antimicrobial activity by determining the MIC of EO’s by direct contact or vapor contact are described below. Agar Dilution The agar dilution method is utilized to determine the lethality of EO’s against microorganisms. It is used with aerobic or micro-aerophilic microorganisms with variable growth rate. For this technique, different dilutions or concentrations of EO’s are prepared; then the dilutions are added to the agar and placed in Petri dishes to solidify. Finally, test microorganisms are inoculated in the plates and incubated at optimum temperature and time. For this method, the MIC is considered as the lowest concentration that inhibited growth. The main advantage of this method is that many organisms can be evaluated at once, contamination is readily detectable and the medium (agar) can contain opaque materials [85]. Broth Dilution The broth dilution or micro-dilution methods are performed in tubes or wells with liquid media (broth), which contain increasing concentrations of EO diluted in broth and in which a defined number of viable microbial cells was inoculated. After incubation, the presence of turbidity or sedimentation indicates growth of the microorganism. In this technique, the MIC is the lowest concentration that prevents visible growth of the microorganism. These methods are mostly used to test EO’s against bacteria and yeasts [85, 155]. Agar Diffusion In the agar diffusion method, the EO at different concentrations is added in agar and then poured into Petri dishes. On the surface of the agar, the test microorganism is inoculated and incubated at optimum temperature and time. The principle is based on the diffusion of the EO to the agar surface [68, 85].
Inverted Petri Dish In the technique of the inverted Petri dish, the agar inoculated with the test organism is separated from the EO (previously dissolved in a solvent and placed onto a filter paper) while maintaining a controlled temperature. The principle is based on the rapid volatilization of EO’s which come into contact with the microorganism [71, 133]. The MIC is the lowest concentration that inhibited growth of the microorganism. This technique is mostly used for bacteria, which have higher growth rates compared to yeast or molds [41].
Antibacterial Activity of Essential Oils Food spoilage can occur from farm (raw food materials) to table (processing and distribution). Fruits and vegetables are generally contaminated with soil originated microorganisms. However, while growing, during their harvesting and shipping, they also are in contact with insects, animals, and people which are known as important contamination sources. Fruits and vegetables might be contaminated by microorganisms during these processes; so many reports regarding outbreaks in minimally processed foods are available in the literature. Some pathogenic bacteria can be found in processed vegetables and fruits and they can survive in foods containing high amount of water. Most of the food-borne diseases are caused by E. coli O157:H7, Listeria monocytogenes, and Salmonella spp. found in vegetables and fruits [69]. It has been demonstrated that essential oils cause structural and functional damages to the bacterial cell membrane. EO’s are mostly used in the range of 0.05–0.5 % (500–5,000 ppm). However, some EO’s have stronger antimicrobial activity than others and can be effective at \500 ppm but some others require higher concentrations [134]. Table 4 lists different EO’s reported as antimicrobials against some pathogenic bacteria. Elgayyar et al. [42] examined the activity of essential oils of anise, angelica, basil, cardamom, carrot, celery, coriander, dill, fennel, parsley, oregano, and rosemary against L. monocytogenes, S. aureus, E. coli O:157:H7, Yersinia enterocolitica, P. aeruginosa, Lactobacillus plantarum, A. niger, Geotrichum, and Rhodotorula using the broth dilution method; in this study, the oregano essential oil showed the greatest inhibition for the tested bacteria and anise oil was not particularly inhibitory. Bagamboula et al. [12] reported antimicrobial activity of thyme and basil essential oil against Shigella sonnei and S. flexneri using the agar well diffusion method. Oussalah et al. [104] observed inhibitory effects of 22 EO’s against four pathogenic bacteria (E. coli O157:H7, L. monocytogenes 2812, Salmonella Typhimurium SL 1344, and S.
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Food Eng Rev Table 4 Plants’ essential oils with antimicrobial activities against several pathogenic bacteria Plant
Effective against
References
Cinnamon (Cinnamomum osmophloeum)
Escherichia coli, Pseudomonas aeruginosa, Enterococcus faecalis, Staphylococcus aureus, Klebsiella pneumoniae, Salmonella sp, and Vibrio parahemolyticus
Chang et al. [27]
Bay (Pimenta racemosa), Thyme (Thymus vulgaris), Oregano (Origanum vulgare) and Clove bud (Syzygium aromaticum)
Escherichia coli O157:H7
Burt and Reinders [22]
Mint (Mentha piperita) and Basil (Ocimum basilicum L)
Escherichia coli O157:H7 and Salmonella Typhimurium
Karago¨zlu¨ et al. [69]
Ginger (Zingiber officinale), Fingerroot (Boesenbergia pandurata) and Turmeric (Curcuma longa)
Listeria monocytogenes and Salmonella Typhimurium DT104
Thongson et al. [137]
Spanish oregano (Corydothymus capitatus), Cinnamon (Cinnamomum cassia), Greek oregano (Origanum heracleoticum), Savory wild (Satureja montana), and Cinnamon (Cinnamomum verum) barks
E. coli O157:H7, Salmonella Typhimurium, Staphylococcus aureus and Listeria monocytogenes
Oussalah et al. [104]
Eucalyptus (Eucalyptus globules), Tea tree (Melaleuca alternifolia), Rosemary (Rosmarinus officinalis), Mint (Mentha piperita),) Sweet basil (Ocimum basilicum), Clove (Syzygium aromaticum), Lemon (Citrus limonum) and Oregano (Origanum vulgare)
Escherichia coli O157:H7
Moreira et al. [92]
Basil (Ocimum basilicum L), Lemon balm (Melissa officinalis), Marjoram (Origanum mejorana), Oregano (Origanum vulgare), Rosemary (Rosmarinus officinalis L), Sage (Salvia officinalis L) and Thyme (Thymus vulgaris L.)
Bacillus cereus, Escherichia coli, Listeria monocytogenes and Pseudomonas aeruginosa
Gutierrez et al. [52]
Dill (Anethum graveolens L.), Coriander (seeds of Coriandrum sativum L.), Cilantro (leaves of immature C. sativum L.) and Eucalyptus (Eucalyptus dives)
Saccharomyces cerevisiae and Listeria monocytogenes
Delaquis et al. [39]
Thyme (Thymus vulgaris) and Aniseed (Pimpinella anisum seeds)
Staphylococcus aureus, Bacillus cereus, Escherichia coli, Proteus vulgaris, Proteus mirabilis, Salmonella typhi, Salmonella typhimurium, Klebsiella pneumoniae and Pseudomonas aeruginosa
Al-Bayati [1]
aureus), their results indicate that the most active essential oils against tested bacteria were Corydothymus capitatus, Cinnamomum cassia, Origanum heracleoticum, Satureja montana, and Cinnamomum verum (bark). Solomakos et al. [128] investigated the antimicrobial effect of thyme essential oil in combination with nisin against E. coli O157:H7 in minced beef during refrigerated storage, revealing that the combination of tested EO and nisin showed an additive effect against studied pathogen growth and survival. Nedorostova et al. [97] studied 27 EO’s in vapor phase against five food-borne bacteria (E. coli, L. monocytogenes, P. aeruginosa, Salmonella enteritidis, and S. aureus). Their results showed that Armoracia rusticana was the most effective against studied strains, followed by Allium sativum, Origanum vulgare, T. vulgaris, S.
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montana, Thymus pulegioides, Thymus serpyllum, O. majorana, Caryopteris 9 clandonensis, Hyssopus officinalis, Mentha villosa, Nepeta 9 faassenii, and O. basilicum var. Grant verte. The determination of the MIC differs among research reports and this is an obstacle to perform useful comparisons among studies. Table 5 summarizes some studies, which determined MICs of different EO’s. Bendahou et al. [17] examined the antibacterial activity of O. glandulosum EO using the dilution agar method against ten bacteria, 3 Gram-positive, and 7 Gram-negative. In that study obtained MICs between 59.0 and 120.5 lg/mL of the EO. Tyagi and Malik [143] evaluated the antibacterial activity of Eucalyptus globulus EO against six pathogenic bacteria in vapor phase using disc volatilization method (inverted
Food Eng Rev Table 5 Essential oils tested in vitro for antibacterial activities Plant
Common name
Origanum glandulosum
Oregano
Cinnamomum osmophloeum
Thymus vulgaris L.
Pseudocinnamomum
Thyme
Bacteria tested
MIC
References Bendahou et al. [17]
S. aureus
79.25 lg/mL
L. monocytogenes
58 lg/mL
E. coli
79.25 lg/mL
Salmonella sp
500 lg/mL
E. coli
250 lg/mL
S. aureus
250 lg/mL
S. aureus
0.03 % v/v
Hammer et al. [53]
E. coli
12.5 lg/L of air
Inouye et al. [60]
S. aureus
6.25 lg/L of air
S. aureus B. cereus
100 lg/L of air 17.5 lg/L of air
S. aureus
34.9 lg/L of air
Chang et al. [27]
E. coli Thymus zygis Rosmarinus officinalis Cinnamomum zeylanicum
White thyme Rosemary Cinnamon
Thymus vulgaris L.
Thyme
L. monocytogenes
26.2 lg/L of air
Origanum vulgare L.
Oregano
S. enteritidis
62.5 lg/L of air
Cinnamomum aromaticum Nees
Cassia cinnamon
S. aureus
250 lg/L of air
Cinnamomum cassia
Chinese cinnamon
L. monocytogenes
0.05 % p/p
Origanum majorana
Sweet marjoram
L. monocytogenes
\0.8 % p/p
Origanum compactum
Oregano
S. aureus
0.013 % p/p
Cinnamomum cassia
Chinese cinnamon
S. aureus
0.025 % p/p
Lo´pez et al. [83]
Kloucek et al. [71] Oussalah et al. [104]
MIC minimal inhibitory concentration
Petri dish); MICs varied from 2.25 to 9.00 mg/mL. They also reported significantly higher antimicrobial activity in the vapor phase than by direct contact; they argue this could be attributed to the variation in the relative composition of the oil and vapors. Antibacterial activities of cinnamaldehyde from Cinnamomum osmophloeum EO against nine bacteria were reported by Chang et al. [27]; MICs varied from 250 to 1,000 lg/mL; these authors reported cinnamaldehyde as a strong antimicrobial agent. Hammer et al. [53] used broth micro-dilution method to evaluate the antibacterial activity of thyme (T. vulgaris) EO against two bacteria, MIC was 0.03 % p/p for both microorganisms. Some in vitro experiments report that the use of the vapors generated by EO’s has an increased antimicrobial effect as compared to direct contact [47, 60, 61, 142]. This has particular impact against molds due to their surface growth; therefore, they are more susceptible to the vapors [41].
Antifungal Activity of EO’s Fungi are increasingly important causes of acute or chronic deep-seated human infections, especially recurrent mucosal, cutaneous, or nail infections that may be severe in debilitated or immune compromised individuals [26]. On
the other hand, several cereal, fruits, and other crops are susceptible to fungal attack either in the field or during storage. Fungi may produce as secondary metabolites a diverse group of chemical substances known as mycotoxins and they also share the ability of producing a large numbers of asexual spores. The possible presence of mycotoxigenic fungi in foods, pressure to diminish fungicide residues on cereals, legumes, vegetables, and fruits, are still current problems for the food industry around the world. Growers have to conform to regulations that limit undesirable biocide residues and choose treatments that will maintain the quality of the products. However, the health risks associated with the presence of fungi and their spores in human living environments is of increasing concern [113]. Thus, prevention of fungal growth is an effective way of preventing mycotoxin accumulation. However, it is important to note that partial inhibition of fungal growth, such as reduction of fungal growth rate, could enhance mycotoxin production as a response of the mold to stress [37]. Grains, vegetables and fruit crops are vulnerable to fungal contamination, mainly with Aspergillus and Fusarium followed by Penicillium and other phytopathogenic genera [87]. The most common Aspergillus toxins are aflatoxins that are potent carcinogens and affect the man and many animal species; Cyclopiazonic acid is a
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potent mycotoxin that produces focal necrosis in most vertebrate inner organs when in high concentrations; Cytochalasin E is a very toxic metabolite; Gliotoxin is a strongly immunosuppressive agent, but probably only in animal feeds; Ochratoxin A and Sterigmatocystin are probable human carcinogens. Fusarium toxins include butenolide that has been associated with cattle diseases; Culmorin has a low toxicity; Fumonisins are highly toxic compounds. Penicillium toxins include citrinin that is a nephorotoxin; Mycophenolic acid that may varied from low to acute toxicity; Patulin is very toxic for prokaryotes and eukaryotes; Rubratoxin is a potent hepatotoxin; and Verrucosidin cause mycotoxicosis in animals [49]. Fungal growth involves germination and hyphal extension, eventually forming visible mycelium. A product will be spoiled shortly after spores are germinated. Therefore, prevention of germination will prevent fungal growth, subsequent spoilage, and possible production of mycotoxins. It could be also highly desirable to reduce the initial load of viable spores by applying products that inactivate spores [38]. To better understand the process of fungi propagation, we need to understand the conidia formation process. Conidia are cellular propagules which commonly emerge from aerial hyphae at zones which lie behind the growing colony edge, and therefore, no longer participate in vegetative growth. Their purpose is to provide the fungal colony with a means of dispersal in a rapidly changing environment. Hence, conidial production (conidiation) typically relies on relatively simple cellular transformations that can be completed relatively swiftly in every aerial hypha, resulting in a concerted and massive production of spores. Conidiation has attracted interest in the food industry, since conidia can be used a biotransformation catalysts and as inoculum for fermentations. Fungal spores are well known for harboring mycotoxins [113]. The fungal spore is a resting phase and as such is not very reactive to antifungal compounds. Killing of spores with other methods than heat is a very difficult task; germination of spores (conidia) is a gradual development from resistant and not-responsive forms through a number of stages. Knowledge about sensitivity of the different phases to antifungal compounds is vital to evaluate their potential on fungal spores [113]. The presence of toxigenic fungi and mycotoxins in foods and grains stored for long periods of time presents a potential hazard to human and animal health. Some alternative biodegradable chemical control measures should be developed to replace synthetic pesticides for fungal and pest management without generating fungicide/pesticide pollution. Recently, in different parts of the world, attention has been posed in EO’s because of non-phytotoxicity, systemicity, easy biodegradability, and stimulatory nature of the host metabolism [91]. Also the possible use of EO’s to be effectively used against mycotoxin production [102].
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Although previous works have suggested that several EO’s exhibited important antifungal activities, the microorganisms (especially molds) in contact with plant extracts or EO’s may enhance the production of secondary metabolites, thus it is crucial to analyze each individual case. Partial inhibition of fungal growth cannot be correlated with inhibition of mycotoxin production because this fungistatic activity could trigger secondary metabolism as a response to stress [37]. Several published results regarding fungal inhibition by plant compounds (EO’s) are displayed in Table 6. In recent years, the potential of EO vapors to inhibit fungi has been studied. EO vapors have the ability to attack the lifecycle of molds at the germination stage, as well as in the hyphal growth and sporulation stages. Conidia inactivation in the air by EO’s vapors are key processes in the inhibition, since conidia (airborne) are stable to heat, light, and chemical compounds, being very difficult to remove [38]. Suhr and Nielsen [133] mention that this effect was observed only when in contact with EO’s vapors and not in liquid form. Aslan et al. [8] evaluated the activity of three EO’s (thyme, basil, and savory) against two infesting plants species Bemisia tabaci (whitefly) and Tetranychus urticae (mite); the three essential oils were toxigenic against tested organisms, showing that increasing the dose of the EO increased its antimicrobial effect. Tullio et al. [142] evaluated the antifungal activity of seven EO’s (thyme, sage, fennel, lemon, pine, lavender, and clove) and determined that the EO of thyme displayed the best antifungal activity, followed by fennel, and subsequently by pine. EO’s from lemon and lavender presented minor antifungal activity. Lopez et al. [83] conducted a study with three essential oils (cinnamon, thyme, and oregano), antibacterial and antifungal activity of EO’s was demonstrated. The EO of cinnamon had greater antifungal effect (very low MIC) against Aspergillus flavus. The MIC of oregano EO against Yersinia enterolitica was lower than for Penicillium islandicum with regards to the EO of thyme, MICs required to inhibit microbial growth were significantly higher than for EO’s from cinnamon and oregano. Omidbeygi et al. [102] tested the antifungal activity of EO’s of thyme, summer savory, and clove against A. flavus in a tomato paste; tested EO’s inhibited the growth of A. flavus, thyme and summer savory EO’s exhibited the strongest inhibitions at 350 and 500 ppm, respectively. Nguefack et al. [99] investigated the inhibitory effect of five EO’s extracted from Cymbopogon citratus, Monodora myristica, Ocimum gratissimum, T. vulgaris, and Zingiber officinale against three food spoilage and mycotoxin producing fungi, Fusarium moniliforme, A. flavus, and Aspergillus fumigatus. EO’s from O. gratissimum, T. vulgaris, and C. citratus were the most effective and prevented conidial germination
Food Eng Rev Table 6 Selected plant extracts and essential oils tested for their antifungal capacity Plant
Common name
Fungi tested
Adenocalymma alliaceum
Garlic creeper
Aspergillus niger, A. flavus, Cladosporium cladosporioides, Mucor spp., Dreschlera spp. and Fusarium roseum
Anthemis nobilis L.
Chamomile
A. candidus, A. niger, Penicillium spp., and F. culmorum
Cassia fistula
Cassia
Alternaria alternata
Cinnamomum zeylanicum
Cinnamon
A. niger, A. flavus, F. moniliforme, F. graminearum, Fusarium spp., P. citrinum and P. viridicatum
Citrus limon L.
Lemon
P. chrysogenum, P. verrucosum, A. niger and A. flavus
Citrus paradisi L.
Grapefruit
P. chrysogenum, P. verrucosum, A. niger and A. flavus P. chrysogenum, P. verrucosum, A. niger and A. flavus
Citrus reticulata L.
Mandarin
Citrus sinensis L.
Orange
P. chrysogenum, P. verrucosum, A. niger and A. flavus
Cuminum cyminum
Cumin
F. solani, F. oxysporum, F. oxysporum, F. verticillioides, F. poae, F. equiseti, R. solani, Botrytis cinerea and A. citri
Cymbopogon nardus
Citronella grass
F. oxysporum, F. verticillioides, P. expansum, P. brevicompactum, A. flavus and A. fumigatus Fusarium spp.
Cymbopogon spp
Lemongrass
Eucalyptus globulus
Eucalyptus
A. flavus and A. parasiticus
Foeniculum vulgare
Fennel
Phytophthora infestans
Lavandula stoechas subsp. stoechas
Lavender
Ph. infestans and B. cinerea A. candidus, A. niger, Penicillium spp., and F. culmorum
Malva sylvestris L.
Malva
Origanum syriacum var. bevanii
Oregano
Ph. infestans and B. cinerea
Pelargonium roseum
Geranium
F. oxysporum, F. verticillioides, P. expansum, P. brevicompactum, A. flavus and A. fumigatus
Rosmarinus officinalis
Rosemary
Ph. infestans and B. cinerea
Syzygium aromaticum
Clove
Fusarium spp.
Thymus vulgaris
Thyme
F. oxysporum, F. verticillioides, P. expansum, P. brevicompactum, A. flavus, A. alternata and A. fumigatus
Adapted from da Cruz-Cabral et al. [37]
and the growth of the three studied fungi at 800, 1,000, and 1,200 ppm, respectively. Moderate activity was observed for the EO from Z. officinale (between 800 and 2,500 ppm), while the EO from M. myristica was less inhibitory. Since antimicrobial effects of EO’s against different microorganisms are still being studied, there are few reports indicating their possible application in foods. However, some experiments have shown a positive effect when EO’s are used in active packaging processes [100, 124, 126]. Tzortzakis [144] evaluated the quality of strawberry and tomato, which were exposed to eucalyptus and cinnamon EO’s vapors. Both tested vegetables retained their quality. The author refers the potential use of EO’s in active packaging. Phillips et al. [109] reported the antifungal effect of a mixture of bergamot and orange EO’s exposed to tomatoes and beans, the mixture of EO’s did not affect sensory attributes. Jobling [66] reported that mushrooms packed in plastic bags with a modified atmosphere composed of carbon dioxide, oxygen, and vapors of eucalyptus EO had better color than control mushrooms; besides the EO exerted antimicrobial effect. Wang [151] tested the
antimicrobial effect of tea tree EO in active packaging for the preservation of raspberries using modified atmospheres; active packaging maintained the quality and delayed deterioration of raspberries when stored at 10 C. Serrano et al. [124] developed active packaging containing eugenol, menthol, thymol, and eucalyptol, for storing cherries. The growth of yeasts and molds was significantly reduced, besides cherries’ quality was improved; maintaining the color and fruit firmness and reducing fruit weight losses. These reported inhibitory effects are interesting in connection with the ones reporting prevention of mycotoxin contamination in several foods; therefore, EO’s could be used instead of synthetic antifungal preservatives. Further studies where sensory attributes of foods exposed to EO’s are evaluated would be an excellent resource to prove the success of EO’s as natural antimicrobials.
Interactions of EO’s with Other Preservation Factors Despite their use as flavoring and aromatic agents, nowadays, the interest in the use of EO’s has incremented due
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their capacity of acting as antioxidants and antimicrobials in food systems. The ability to exhibit antimicrobial activity depends on many factors, such as the volume of inoculum, growth phase, culture medium used, pH of the media, incubation time, and temperature [76]. Recent investigations have proved that factors that are independent of the microorganism include temperature, pH, water activity, and the presence of other antimicrobials. These could enhance the inactivation ability of EO’s by means of synergistic or additive action [84]. Combinations of treatments with EO’s and pH or water activity have been performed. In the case of pH, it has been observed that a decrease of this parameter in combination with EO improved inactivation of several bacteria and molds. Lo´pez-Malo et al. [84] investigated the interaction of pH and a natural extract (vainillin) in regards to the inhibition of A. flavus, A. niger, A. ochraceus, and A. parasiticus; their results demonstrated a synergistic effect at low pH (3.0), for equal concentrations of the compound the reduction of pH synergistically reduced mold radial growths. Similar effects were observed by Gutierrez et al. [52], which tested combinations of oregano, basil, lemon grass, marjoram rosemary, thyme, and sage EO’s against Gram-positive and Gram-negative bacteria at four levels of pH (4, 5, 6, or 7); pH had a synergistic effect in the inactivation of the tested microorganisms, being at pH 5 the most effective interactions. However, the observed action sometimes was attributed more to the action of pH and depended also on the microorganisms. The use of combinations among different values of water activity and EO’s has been successful for the inactivation of several bacteria; while the resistance of molds to EO’s could not been always altered changing the value of water activity of the medium, due to the capacity of these microorganisms to tolerate wide ranges of aw [103]. Solutes like sodium chloride have been shown a synergistic effect in combination with EO’s and their constituents for the inactivation of different bacteria; the synergism observed is attributed to an increase in the permeability of the cells by the EO, after which the solute inhibited growth by its action on intracellular enzymes [20]. The sensory impact of EO’s in food products currently limits their use thus mixtures of EO’s have been studied. Although the antimicrobial activity of EO’s is generally attributed to some particular compounds, the interaction among EO’s in a combination can have three different outcomes, synergistic, additive, or antagonistic. Synergy occurs when a blend of two antimicrobial compounds has an antimicrobial activity that is greater than the sum of the individual components; this phenomenon may result in a higher bioactivity compared to the single components. An additive effect is obtained when the combination of antimicrobials has a combined effect equal to the sum of the individual
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compounds. Antagonism occurs when a blend of antimicrobial compounds has a combined effect less than when applied separately [98]. Synergistic blends of commercial interest must be evaluated under the actual environmental conditions which reflect the food matrixes to which they will be applied, as interactions with food matrix ingredients could also decrease their activity (antagonism) [59].
Concluding Remarks Research on essential oils’ properties, activity, and composition, as well as the available methods for their extraction, has achieved great interest from the scientific community, as evidenced in this review. Antimicrobial activity of essential oils from different plants and spices has been explored in order to find the lowest concentration that allows delaying the growth of microorganisms or ensures their bactericidal or fungicidal effect, considering their implementation through direct or indirect methods. Although the use of these essential oils in the food, pharmaceutical, and cosmetic industries has been widely studied, extraction of essential oils’ from their natural sources continues to be a bottleneck, since the traditional methods for essential oils’ extraction produce low yields and/or need lengthy extraction times, which has motivated development of new extraction methods for essential oils. Furthermore, in order to optimize traditional and emerging extraction methods, mathematical models for process simulation have been also proposed, which allow to virtually assessing several processing conditions for essential oils’ extraction prior to conducting them at a laboratory or pilot plant scale. Possible applications of essential oils as antimicrobial agents are vast and the evidence presented in this review demonstrates that essential oils could replace traditional antimicrobials. Acknowledgments Authors acknowledge financial support from the National Council for Science and Technology (CONACyT) of Mexico and Universidad de las Ame´ricas Puebla (UDLAP). Authors Reyes-Jurado and Franco-Vega gratefully acknowledge financial support for their PhD studies from CONACyT and UDLAP.
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