Essential Oils: Extraction, Bioactivities, and Their Uses for Food Preservation

Essential Oils: Extraction, Bioactivities, and Their Uses for Food Preservation

R: Concise Reviews in Food Science Essential Oils: Extraction, Bioactivities, and Their Uses for Food Preservation Phakawat Tongnuanchan and Soottawat Benjakul Abstract: Essential oils are concentrated liquids of complex mixtures of volatile compounds and can be extracted from several plant organs. Essential oils are a good source of several bioactive compounds, which possess antioxidative and antimicrobial properties. In addition, some essential oils have been used as medicine. Furthermore, the uses of essential oils have received increasing attention as the natural additives for the shelf-life extension of food products, due to the risk in using synthetic preservatives.

Essential oils can be incorporated into packaging, in which they can provide multifunctions termed “active or smart packaging.” Those essential oils are able to modify the matrix of packaging materials, thereby rendering the improved properties. This review covers up-to-date literatures on essential oils including sources, chemical composition, extraction methods, bioactivities, and their applications, particularly with the emphasis on preservation and the shelf-life extension of food products.

Keywords: antimicrobial, antioxidant, biodegradable film, essential oil, food product, volatile compound Introduction Essential oils, also called volatile odoriferous oil, are aromatic oily liquids extracted from different parts of plants, for example, leaves, peels, barks, flowers, buds, seeds, and so on. They can be extracted from plant materials by several methods, steam distillation, expression, and so on. Among all methods, for example, steam distillation method has been widely used, especially for commercial scale production (Cassel and Vargas 2006; Di Leo Lira and others 2009). Essential oils have been widely used as food flavors (Burt 2004).

Essential oils found in many different plants, especially the aromatic plants, vary in odor and flavor, which are governed by the types and amount of constituents present in oils. Additionally, the amount of essential oil from different plants is different and this determines the price of essential oil. Apart from aromatic compounds, indigenous pigments contribute to varying colors of essential oil. This can affect the applications as the ingredient in some particular foods. Essential oils have been known to possess antioxidant and antimicrobial activities, thereby serving as natural additives in foods and food products.

It can be used as active compounds in packaging materials, in which the properties of those materials, particularly water vapor barrier property associated with hydrophobicity in nature of essential oils, can be improved. Almost any part of a plant may be the source of the oil, which could be extracted and fully exploited for food applications or others. Modern technologies have been continuously developed to conquer the limitation of conventional methods, and to enhance the extraction efficacy. Due to the increasing attention in natural additives, essential oils from several plants have been used more widely, especially in conjunction with other preservations under concept of “hurdle technology.” Thus, essential oils can serve as the alternative additives or processing aid as green technology.

MS 20131520 Submitted 10/23/2013, Accepted 4/9/2014. Authors are with Dept. of Food Technology, Faculty of Agro-Industry, Prince of Songkla Univ., 15 Kanchanawanish Road, Hat Yai, Songkhla, 90112, Thailand. Direct inquiries to author Benjakul (E-mail: Sources and Chemical Composition Several plants contain essential oils, however, parts of plants, which serve as the major source of essential oil can be different (Table 1). Those include roots, peels, leaves, seeds, fruits, barks, and so on. Plant essential oils are usually the complex mixture of natural compounds, both polar and nonpolar compounds (Masango 2005).

Dominant compounds in various essential oils are presented in Table 2. In general, the constituents in essential oils are terpenes (monoterpenes and sesquerpenes), aromatic compounds (aldehyde, alcohol, phenol, methoxy derivative, and so on), and terpenoids (isoprenoids) (Bakkali and others 2008; Mohamed and others 2010). Compounds and aroma of essential oils can be divided into 2 major groups: terpene hydrocarbons and oxygenated compounds.

  • Terpene hydrocarbons The hydrocarbons are the molecule, constituting of H and C atoms arranged in chains. These hydrocarbons may be acyclic, alicyclic (monocyclic, bicyclic, or tricyclic), or aromatic. Terpenes are the most common class of chemical compounds found in essential oils. Terpenes are made from isoprene units (several 5 carbon base units, C5), which are the combinations of 2 isoprene units, called a “terpene unit.” Essential oils consist of mainly monoterpenes (C10) and sesquiterpenes (C15), which are hydrocarbons with the general formula (C5H8)n. The diterpenes (C20), triterpenes (C30), and tetraterpenes (C40) exist in essential oils at low concentration (Mohamed and others 2010). Terpenoids (a terpene containing oxygen) is also found in essential oils (Burt 2004). Essential oils mostly contain monoterpenes and sesquiterpenes, which are C10H16(MW 136 amu) and C15H24 (MW 204 amu), respectively. Although sesquiterpenes are larger in molecules, structure and functional properties of sesquiterpenes are similar to the monoterpenes (Ruberto and Baratta 2000). For diterpenes, triterpenes, and tetraterpenes, they have the larger molecule than monoterpenes and sesquiterpenes, but they are present at very low concentration in essential oils (Bakkali and others 2008). Oxygenated compounds These compounds are the combination of C, H, and O, and there are a variety of compounds found in essential oils. C
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  • doi: 10.1111/1750-3841.12492 Vol. 79, Nr. 7, 2014 r Journal of Food Science R1231 Further reproduction without permission is prohibited
Essential Oils: Extraction, Bioactivities, and Their Uses for Food Preservation

R: Concise Reviews in Food Science Bioactivities and applications of essential oils . Table 1–Parts of plant material containing essential oils. Parts Plants Leaves Basil, bay leaf, cinnamon, common sage, eucalyptus, lemon grass, citronella, melaleuca, mint, oregano, patchouli, peppermint, pine, rosemary, spearmint, tea tree, thyme, wintergreen, kaffir lime, laurel, savory, tarragon, cajuput, lantana, lemon myrtle, lemon teatree, niaouli, may chang, petitgrain, laurel, cypress Seeds Almond, anise, cardamom, caraway, carrot celery, coriander, cumin, nutmeg, parsley, fennel Wood Amyris, atlas cedarwood, himalayan cedarwood, camphor, rosewood, sandalwood, myrtle, guaiac wood Bark Cassia, cinnamon, sassafras, katrafay Berries Allspice, juniper Resin Frankincense, myrrh Flowers Blue tansy, chamomile, clary sage, clove, cumin, geranium, helichrysum hyssop, jasmine, lavender, manuka, marjoram, orange, rose, baccharises, palmarosa, patchouli, rhododendron anthopogon, rosalina, ajowan, ylang-ylang, marjoram sylvestris, tarragon, immortelle, neroli Peel Bergamot, grapefruit, kaffir lime, lemon, lime, orange, tangerine, mandarin Root Ginger, plai, turmeric, valerian, vetiver, spikenard, angelica Fruits Xanthoxylum, nutmeg, black pepper Oxygenated compounds can be derived from the terpenes, in which they are termed “terpenoids.” Some oxygenated compounds prevalent in plant essential oils are shown as follows: - Phenols: thymol, eugenol, carvacrol, chavicol, thymol, and so on.

Alcohols: Monoterpene alcohol: borneol, isopulegol, lavanduol, αterpineol, and so on.

Sesquiterpenes alcohol: elemol, nerolidol, santalol, α-santalol, and so on. - Aldehydes: citral, myrtenal, cuminaldehyde, citronellal, cinnamaldehyde, benzaldehyde, and so on. - Ketones: carvone, menthone, pulegone, fenchone, camphor, thujone, verbenone, and so on. - Esters: bomyl acetate, linalyl acetate, citronellyl acetate, geranyl acetate, and so on. - Oxides: 1,8-cineole, bisabolone oxide, linalool oxide, sclareol oxide, and so on. - Lactones: bergaptene, nepetalactone, psoralen, aesculatine, citroptene, and so on.

Ethers: 1,8-cineole, anethole, elemicin, myristicin, and so on.

Different constituents in essential oils exhibit varying smell or flavor (Burt 2004). Also, the perception of individual volatile compounds depends on their threshold. Extraction of Essential Oils Essential oils can be extracted from several plants with different parts by various extraction methods. The manufacturing of essential oils, and the method used for essential oil extraction are normally dependent on botanical material used. State and form of material is another factor used for consideration. Extraction method is one of prime factors that determine the quality of essential oil. Inappropriate extraction procedure can lead to the damage or alter action of chemical signature of essential oil.

This results in the loss in bioactivity and natural characteristics. For severe case, discoloration, off-odor/flavor as well as physical change such as the increased viscosity can occur. Those changes in extracted essential oil must be avoided. Extraction of essential oils can be carried out by various means, as shown in Table 3. Distillation Steam distillation. Steam distillation is the most widely used method for plant essential oil extraction (Reverchon and Senatore 1992). The proportion of essential oils extracted by steam distillation is 93% and the remaining 7% can be further extracted by other methods (Masango 2005).

Basically, the plant sample is placed in boiling water or heated by steam (Figure 1). The heat applied is the main cause of burst and break down of cell structure of plant material. As a consequence, the aromatic compounds or essential oils from plant material are released (Perineau and others 1992; Babu and Kaul 2005). The temperature of heating must be enough to break down the plant material and release aromatic compound or essential oil. A new process design and operation for steam distillation of essential oils to increase oil yield and reduce the loss of polar compounds in wastewater was developed by Masango (2005).

The system consists of a packed bed of the plant materials, which sits above the steam source. Only steam passes through it and the boiling water is not mixed with plant material. Thus, the process requires the minimum amount of steam in the process and the amount of water in the distillate is reduced. Also, watersoluble compounds are dissolved into the aqueous fraction of the condensate at a lower extent (Masango 2005). Yildirim and others (2004) reported that the 2,2-diphenyl-1-picryl hydrazyl (DPPH) radical scavenging activities of essential oils from steam distillation process were markedly higher than those of oils extracted using hydrodistillation (HD).

Hydrodistillation. HD has become the standard method of essential oil extraction from plant material such as wood or flower, which is often used to isolate nonwater-soluble natural products with high boiling point. The process involves the complete immersion of plant materials in water, followed by boiling. This method protects the oils extracted to a certain degree since the surrounding water acts as a barrier to prevent it from overheating. The steam and essential oil vapor are condensed to an aqueous fraction (Figure 2). The advantage of this technique is that the required material can be distilled at a temperature below 100 °C.

Okoh and others (2010) studied the different extraction processes on yield and properties of essential oil from rosemary (Rosmarinus officinalis L.) by HD and solvent-free microwave extraction (SFME). The total yields of the volatile fractions obtained through HD and SFME were 0.31% and 0.39%, respectively. HD oil contained more monoterpene hydrocarbons (32.95%) than SFME-extracted oil (25.77%), while higher amounts of oxygenated monoterpenes (28.6%) were present in the oil extracted by SFME in comparison with HD (26.98%). Golmakani and Rezaei (2008) studied the microwave-assisted HD (MAHD), which is an advanced HD technique utilizing a microwave oven in the extraction process.

MAHD was superior in terms of saving energy and extraction time R1232 Journal of Food Science r Vol. 79, Nr. 7, 2014

Essential Oils: Extraction, Bioactivities, and Their Uses for Food Preservation
  • R: Concise Reviews in Food Science Bioactivities and applications of essential oils . Table 2–Major compounds in different plant essential oils. Monoterpene Oxygenated Sesquiterpene Oxygenated Essential oils hydrocarbons monoterpenes hydrocarbons sesquiterpenes Esters Others References Basil β-Pinene, β-Limonene, γ -Terpinene endo-5,5,6-Trimethyl-2- norbornanone β-Elemene, 2,6-Dimethyl-6- (4-methyl-3-pentenyl)- bicyclo[3.1.1]hept-2-ene, γ -Cadinene, γ -Muurolene Methyleugenol – Methylchavicol,3- Methoxycinnamaldehyde Teixeira and others (2013) Citronella S-3-Carene, Mentha-1,4,8-triene,
  • 2 -Carene, cis-2,6- Dimethyl-2,6octadiene, γ -Terpinene (−)-Isopulegol, β-Citronellal, βCitronellol β-Elemene, β-Selinene, α-Selinene, α-Muurolene, (+)-δ-Cadinene, Eremophilene, γ -Selinene, (+)-δ-Selinene, (−)-α-Amorphene (−)-Cedreanol m-(Trimethylsiloxy) -cinnamic acid methyl ester - Teixeira and others (2013) Clove – – trans-Caryophyllene, α-Humulene Methyleugenol Aceteugenol p-Eugenol Teixeira and others (2013) Garlic 1(7),5,8-o-Menthatriene trans-Limone oxide, endo-5,5,6-Trimethyl-2- norbornanone, – – – di-2-Propenyldisulfide, Dimethyl tetrasulphide, di- 2-Propenyltetrasulfide,3,3
  • - Thiobis-1-propene, Sulfur Teixeira and others (2013) Lemon α-Pinene, β-Pinene, Cymene, α-Limonene, α-Fellandrene – trans-Caryophyllene – – 1,2,3,5-Tetramethylbenzene,1-(1,5- Dimethylhexyl)-4- methylbenzene Teixeira and others (2013) Lemon α-Pinene, α-Fenchene, Limonene, Camphene Citronellal, cis-Carveol, α-Citral, Carvacol, Terpniol, Thymol, Carvacrol, Citral – – – Cyclohexane, Heptanal, Dihydroiso-pimaric, Dihydro-abitec Mohamed and others (2010) Lemongrass α-Pinene, 3-Carene, Camphene β-Citral, α-Citral, α-Cyclocitral, Terpineol,2,3-Dehydro- 1,8-cineole β-Caryophyllene – – m-Eugenol, Geranyl N-butyrate, Isogeraniol Leimann and others (2009) Mandarin α-Pinene, di-Limonene, Allo-Ocimene, Camphene, Sabinene Neo-Dihydrocaveol, cis-Limonene oxide, Linalool, Borneol, Limoneneglycol, Carvone Farnesene, α-Farnesene – – Linalyl acetate, Undecanoic acid, Methly-anthranilate, Benzaldehyde Mohamed and others (2010) Mint(Saturejacuneifolia) α-Pinene, Myrcene, Limonene, cisβ-Ocimene, p-Cymene, allo-Ocimene Thymol, Carvacrol, Camphor, Linalool, Terpinen-4-ol, Neral, α-Terpineol, Borneol, Geranial, Geraniol β-Bourbonene, β-Caryophyllene, Aromadendrene, β-Cubebene, δ-Cadinene, Caryophyllene oxide, aSpathulenol, Viridiflorol, – – Bezić and others (2005) Mintb(Satureja montana) α-Thujene, α-Pinene, Myrcene, α-Terpinene, γ -Terpinene, p-Cymene Linalool, α-Terpineol, Borneol, Thymol, Carvacrol β-Cubebene, δ-Cadinene Caryophyllene oxide, Spathulenol – 1-Octen-3-ol, Thymol methylether, Carvacrol methyl ether, Thymyl acetate Bezić and others (2005) Orange Myrcene, β-Phellandrene, α-Terpinolene, Menthatriene cis-Limoneneoxide, Decanal, Linalool, Verbenol, Carvone, Perilladehyde, cis-Carveol, Citronellol Farnesene – – Nonyl-aldehyde, Caprylic acid, Cinnamic-aldehyde, Heptadecanol Mohamed and others (2010) (Continued) Vol. 79, Nr. 7, 2014 r Journal of Food Science R1233
Essential Oils: Extraction, Bioactivities, and Their Uses for Food Preservation
  • R: Concise Reviews in Food Science Bioactivities and applications of essential oils . Table 2–Continued. Monoterpene Oxygenated Sesquiterpene Oxygenated Essential oils hydrocarbons monoterpenes hydrocarbons sesquiterpenes Esters Others References Oregano α-Terpinene, Limonene, γ -Terpinene 1,8-Cineole, Terpinen-4 -ol, α-Terpineol, Thymol, Carvacrol, β-Caryophyllene, cis-Hydrate sabinene, trans-Hydrate sabinene – – – Aguirre and others (2013) Plai-Dam (Zingiber ottensii) α-Pinene, β-Pinene, Sabinene, Myrcene, α-Terpinene, Limonene, E-β-Ocimene, p-Cymene, Terpinolene, γ -Terpinene 1,8-Cineole, Linalool, Terpinen-4-ol, cis-Menth-2-en-1-ol, Borneol, trans-Piperitol β-Elemene, β-Caryophyllene, Humulene Caryophyllene oxide, Humulene oxide, α-Eudesimol, β-Eudesimol, Zerumbone – Bornyl acetate, Sabinene hydrate, 4-phenylbutan-2-one Thubthimthed and others (2005) Rosemary α-Pinene, Camphene, β-Pinene, Cymene, α-Fellandrene, S-3-Carene, m-Cymene, Mentha-1,4,8-triene Eucalyptol, (E)-2,3-Epoxycarane,(−)- Camphor, endo-Borneol, endo-5,5,6-Trimethyl-2- norbornanone trans-Caryophyllene – (−)-Bornylacetate – Teixeira and others (2013) Sage α-Pinene, Camphene, β-Pinene, Cymene, α-Fellandrene, m-Cymene, Mentha-1,4,8-triene,
  • 2 -Carene,1,3,8-pMenthatriene, α-Terpinolene Eucalyptol, (E)-2,3-Epoxycarane,(−)- Camphor, endo-Borneol, endo-5,5,6-Trimethyl-2- norbornanone trans-Caryophyllene, β-Selinene, β-Bisabolene – (−)-Bornylacetate – Teixeira and others (2013) Tangerine α-Pinene, Limonene, α-Terpinene, transMenthadiene, trans-Ocimene, trans-Decalone Citronellal, Linalool, cis-Limonene oxide, trans-Carveol, Limonene dioxide, Perillyl alcohol – Ledol, Globulol – Aloxiprin, Heptadiene, Methylheptadiene, Cyclooctanone, Benzyldicarboxylic Mohamed and others (2010) Thyme Camphene, β-Pinene, Cymene, α-Fellandrene, m-Cymene Eucalyptol, (E)-2,3-Epoxycarane, endo-5,5,6-Trimethyl-mThymol, Carvacrol trans-Caryophyllene – – (3E,5E,8E)-3,7,11-Trimethyl- 1,3,5,8,10-dodecapentaene Teixeira and others (2013) Thymus longicaulis subsp.

longicaulis var. longicaulis α-Thujene, α-Pinene, Myrcene, Camphene, β-Pinene, α-Phellandrene, α-Terpinene, p-Cymene, (E)- β-Ocimene, γ -Terpinene, cis-Sabinene hydrate, Terpinolene Camphor, Borneol, Terpinen-4-ol, α-Terpineol, Thymol, Carvacrol, β-Caryophyllene α-Humulene, δ-Cadinene, Germacrene D – – – Sarikurkcu and others (2010) R1234 Journal of Food Science r Vol. 79, Nr. 7, 2014

R: Concise Reviews in Food Science Bioactivities and applications of essential oils . Table 3–Extraction of essential oils from various sources using several methods. Extraction methods Plants References Solvent extraction – Solvent sage (Salvia officinalis), apiaceae (Ptychotis verticillata), chasteberry (Vitexagnuscastus L.), lemon (Citrus x limon) Durling and others (2007); Matsingou and others (2003); El Ouariachi and others (2011); Sarikurkcu and others (2009); Koshima and others (2012) – Supercritical CO2 rosemary (Rosmarinus officinalis), fennel (Foeniculum vulgare), anise (Pimpinella anisum), cumin seed (Cuminum cyminum), sage (Salvia officinalis), lemon (Citrus x limon), carrot fruit (Daucus carrota L.), marjoram (Majorana hortensis Moench), catnip (Nepeta cataria L.), oregano (Origanum vulgare L.), lavender (Lavandula angustifolia Mill), thyme (Thymus vulgaris L.), hyssop (Hyssopus officinalis L.), anise hyssop (Lophantus anisatus Benth), patchouli (Pogostemon cablin), cumin (Cuminum cyminum), clove (Eugenia caryophyllata), coriander (Coriandrum sativum L.), chamomile (Matricaria chamomilla), baccharises (Baccharis uncinella, Baccharis anomala, and Baccharis dentata) Pereira and Meireles (2007); Reverchon and Senatore (1992); Eikani and others (1999); Djarmati and others (1991); Gironi and Maschietti (2008); Glišić and others (2007); Dapkevicius and others (1998); Donelian and others (2009); Li and others (2009); Guan and others (2007); Mhemdi and others (2011); Araus and others (2009); Xavier and others (2011) – Subcritical water fructus amomi, marjoram (Origanum majorana), olive (Olea europaea), coriander seeds (Coriandrum sativum L.) Deng and others (2005); Jimenez-Carmona and others (1999); Amarni and Kadi (2010); Eikani and others (2007) Distillation - Steam rose-scented geranium (Pelargonium sp.), thyme (Thymus kotschyanus), germander (Teucrium orientale), rosemary (Rosmarinus officinalis), fennel (Foeniculum vulgare), anise (Pimpinella anisum), eucalyptus (Eucalyptus citriodora), basil (Ocimum basilicum L.), lavender (Lavandula dentata L.), patchouli (Pogostemon cablin), clove (Eugenia caryophyllata), orange (Citrus sinensis) Babu and Kaul (2005); Sefidkon and others (1999); Yildirim and others (2004); Pereira and Meireles (2007); Rajeswara Rao and others (2003); Cassel and others (2009); Donelian and others (2009); Guan and others (2007); Farhat and others (2011) – Hydrodistillation rose-scented geranium (Pelargonium sp.), germander (Teucrium orientale), rosemary (Rosmarinus officinalis), lemon (Citrus x limon), oregano (Origanum vulgare L.), marjoram (Majorana hortensis Moench), catnip (Nepeta cataria L), lavender (Lavandula angustifolia Mill), hyssop (Hyssopus officinalis L.), anise hyssop (Lophantus anisatus Benth), sage (Salvia officinalis L), cumin (Cuminum cyminum), clove (Eugenia caryophyllata), caraway (Carum carvi), thyme (Thymus vulgaris L.), basil (Ocimum basilicum L.), garden mint (Mentha crispa L.) Babu and Kaul (2005); Yildirim and others (2004); Reverchon and Senatore (1992); Ferhat and others (2007); Bayramoglu and others (2008); Dapkevicius and others (1998); Li and others (2009); Guan and others (2007); Farhat and others (2010); Gavahian and others (2012) – Hydrodiffusion orange (Citrus sinensis), rosemary leaves (Rosmarinus officinalis) Farhat and others (2011); Bousbia and others (2009) Solvent-free microwave oregano (Origanum vulgare L.), fragrant fern (Dryopteris fragrans), rosemary (Rosmarinus officinalis), caraway (Carum carvi), 5 flavor berry (Schisandra chinensis), cumin (Cuminum cyminum L.), cardamom (Elletaria cardamomum L.), basil (Ocimum basilicum L.), garden mint (Mentha crispa L.), thyme (Thymus vulgaris L.), sea buckthorn (Hippophae rhamnoides L.), spearmint (Mentha spicata L.), pennyroyal (Mentha pulegium L.) Bayramoglu and others (2008); Li and others (2012); Okoh and others (2010); Farhat and others (2010); Ma and others (2012); Wang and others (2006); Lucchesi and others (2007); Lucchesi and others (2004); Michel and others (2011); Vian and others (2008) Combination methods - Solvent + Steam cumin (Cuminum cyminum), tobacco (Nicotiana tabacum) Li and others (2009), Zhang and others (2012) (75 min, compared to 4 h in HD).

Ohmic-assisted HD (OAHD) is another advanced HD technique (Gavahian and others 2012). OAHD method had the extraction time of 24.75 min, while HD took 1 h for extraction of essential oil from thyme. No changes in the compounds of the essential oils obtained by OAHD were found in comparison with HD.

Hydrodiffusion. Hydrodiffusion extraction is a type of steam distillation, which is only different in the inlet way of steam into the container of still. This method is used when the plant material has been dried and is not damaged at boiling temperature (Vian and others 2008). For hydrodiffusion, steam is applied from the top of plant material, whereas steam is entered from the bottom for steam distillation method. The process can also be operated under low pressure or vacuum and reduces the steam temperature to below 100 °C. Hydrodiffusion method is superior to steam distillation because of a shorter processing time and a higher oil yield with less steam used.

Bousbia and others (2009) compared the HD and innovative microwave hydrodiffusion and gravity (MHG) methods for their effectiveness in the isolation of essential oil from rosemary leaves (R. officinalis). The MHG method exhibits the excellent advantages over traditional alternatives including shorter isolation times (15 min against 3 h for HD), environmental impact (energy cost is fairly higher to perform HD than that required for rapid MHG isolation), cleaner features (no residue generation and no water or solvent used), increased antimicrobial and antioxidant activities. Farhat and others (2011) studied the microwave steam diffusion (MSDf), which is an advanced steam diffusion (SDf) technique utilizing microwave heating process for extraction of essential oils from by-products of orange peel.

The essential oils extracted by MSDf for 12 min had similar yield and aromatic profile to those obtained by SDf for 40 min. Solvent extraction Solvent. Conventional solvent extraction has been implemented for fragile or delicate flower materials, which are not tolerant to the heat of steam distillation. Different solvents including acetone, hexane, petroleum ether, methanol, or ethanol can be used for extraction (Areias and others 2000; Pizzale and others 2002; Kosar and others 2005). For general practice, the solvent is mixed with the plant material and then heated to extract the essential oil, followed by filtration.

Subsequently, the filtrate is concentrated by solvent evaporation. The concentrate is resin (resinoid), or concrete (a combination of wax, fragrance, and Vol. 79, Nr. 7, 2014 r Journal of Food Science R1235

R: Concise Reviews in Food Science Bioactivities and applications of essential oils . essential oil). From the concentrate, it is then mixed with pure alcohol to extract the oil and distilled at low temperatures. The alcohol absorbs the fragrance and when the alcohol is evaporated, the aromatic absolute oil is remained. However, this method is a relatively time-consuming process, thus making the oils more expensive than other methods (Li and others 2009). Essential oil with antioxidant activity from Ptychotisverticillata was extracted using solvent extraction method by El Ouariachi and others (2011).

The oil was dominated by phenolic compounds (48.0%) with carvacrol (44.6%) and thymol (3.4%) as the main compounds. Ozen and others (2011) studied the chemical composition and antioxidant activity of separated essential oils from Thymus praecox subsp. skorpilii var. skorpilii (TPS) extracted using different solvents. TPS essential oil was found to contain thymol (40.31%) and o-cymene (13.66%) as the major components. The ethanol, methanol, and water extracts exerted significant free-radical scavenging activity. The water extract has the highest total phenolics (6.211 mg gallic acid/g dry weight) and flavonoids (0.809 mg quercetin/g dry weight).

Moreover, Sarikurkcu and others (2009) reported that the water extract exhibited higher antioxidant activity than other extracts (hexane, dichloromethane, ethyl acetate, and methanol). However, solvent residue could be retained in the final product Figure 1–Diagrammatic illustration of steam distillation method. Figure 2–Diagrammatic illustration of hydrodistillation method.

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R: Concise Reviews in Food Science Bioactivities and applications of essential oils . due to incomplete removal. This may cause allergies, toxicity, and affect the immune system (Ferhat and others 2007a). Supercritical carbon dioxide. Conventional methods including solvent extraction and steam distillation have some shortcomings such as long preparation time and large amount of organic solvents (Deng and others 2005). Moreover, the losses of some volatile compounds, low extraction efficiency, degradation of unsaturated compounds, and toxic solvent residue in the extract may be encountered (Jimenez-Carmona and others 1999; Glišića and others 2007; Gironi and Maschietti 2008).

Therefore, supercritical fluids have been considered as an alternative medium for essential oil extraction. Carbon dioxide (CO2) is the most commonly used supercritical fluid because of its modest critical conditions (Hawthorne and others 1993; Jimenez-Carmona and others 1999; Senorans and others 2000). Under high-pressure condition, CO2 turns into liquid, which can be used as a very inert and safe medium to extract the aromatic molecules from raw material. No solvent residue remains in the final finished product since the liquid CO2 simply reverts to a gas and evaporates under normal atmospheric pressure and temperature.

Despite high solubilities of essential oil components in supercritical CO2, the extraction rates were relatively slow with pure CO2 (ca. 80% recovery after 90 min) (Hawthorne and others 1993). However, the combination methods by a 15-min static extraction with methylene chloride as a modifier followed by a 15-min dynamic extraction with pure CO2 yielded high recoveries. The extraction efficacy was equivalent to HD, which was performed for 4 h. The volatile compounds such as monoterpenes can be collected from the supercritical fluid extraction (SFE) effluent by >90%. SFE was able to recover some organic compounds that were not extracted by HD (Hawthorne and others 1993).

Pereira and Meireles (2007) showed that the supercritical fluid extraction is economically viable than steam distillation. This is mainly caused by the lower yield and the higher energy consumption of the latter.

Subcritical water. The subcritical water or pressurized hot water has been introduced as an extractant under dynamic conditions (pressure high enough to maintain water under liquid state and temperature in the range of 100 to 374 °C). Jimenez-Carmona and others (1999) reported that the efficiency (in terms of volume of essential oil/1 g of plant) of continuous subcritical water extraction was 5.1 times higher than HD method. This method is quicker (15 min compared with 3 h), provides a more valuable essential oil (with higher amounts of oxygenated compounds and no significant presence of terpenes), and allows substantial savings of costs, in terms of both energy and plant material.

Kubatova and others (2001) studied the subcritical water extraction of lactones from a kava (Piper methysticum) root, compared to a Soxhlet extraction with water. The extraction of ground samples with subcritical water at 100 °C took 2 h, but the shorter time (20 min) was required when extraction was carried out at 175 °C. Boiling for 2 h and extraction with Soxhlet apparatus for 6 h showed the lower yields by 40% to 60%, compared with that obtained using subcritical water.

Solvent-free microwave The disadvantages of conventional methods such as solvent or hydrodiffusion extraction are the losses of some volatile compounds, low extraction efficiency, long extraction time, degradation of unsaturated or ester compounds through thermal or hydrolytic effects, and toxic solvent residue in the extract (Pollien and others 1998; Luque de Castro and others 1999). These disadvantages have led to the consideration of the use of SFME. It is a rapid extraction of essential oils from aromatic herbs, spices, and dry seeds. SFME has several advantages, involving higher yield and selectivity, shorter time, and environmental friendly (Lopez-Avila and others 1994; Tomaniová and others 1998).

SFME is a combination of microwave heating and dry distillation, performed at atmospheric pressure without any solvent or water. Isolation and concentration of volatile compounds are performed by a single stage (Lucchesi and others 2004; Bayramoglu and others 2008). Using oregano as a raw material, SFME offered significantly higher essential oil yields (0.054 mL/g), compared to HD (0.048 mL/g) (Bayramoglu and others 2008). When microwave power at 662 W was used in SFME, process time was reduced by 80%, compared with conventional process. Ferhat and others (2007b) reported that microwave method offers the important advantages over traditional alternatives, such as shorter extraction times (30 min compared with 3 h for HD and 1 h for cold pressing [CP]); better yields (0.24% compared with 0.21% for HD and 0.05% for CP); environmental impact (energy cost is appreciably higher for performing HD and for mechanical motors (CP) than that required for rapid microwave extraction); cleaner features (as no residue generation and no water or solvent used); and high antimicrobial activities.

Farhat and others (2010) reported that essential oils of caraway seeds isolated by microwave dry-diffusion and gravity (MDG) exhibited the similar yield and aromatic profile to those obtained by HD, but MDG was better than HD in terms of shorter process time (45 min compared with 300 min), energy saving, and cleanliness. The present apparatus permits fast and efficient extraction, reduces waste, avoids water and solvent consumption, and allows substantial energy savings (Farhat and others 2010).

Role of Essential Oils as Food Additives Essential oils from plants have been known to act as natural additives, for example, antimicrobial agents, antioxidant, and so on. Their activities vary with source of plants, chemical composition, extraction methods, and so on. Due to the unique smell associated with the volatiles, this may limit the use of essential oil in some foods since it may alter the typical smell/flavor of foods. Antimicrobial activity The ability of plant essential oils to protect foods against pathogenic and spoilage microorganisms has been reported (Lis-Balchin and others 1998; Friedman 2006; Rojas-Graü and others 2007).

Among chemical components in several essential oils, carvacrol has been shown to exert a distinct antimicrobial action (Veldhuizen and others 2006). Carvacrol is the major component of essential oil from oregano (60% to 74% carvacrol) and thyme (45% carvacrol) (Lagouri and others 1993; Arrebola and others 1994). It has a broad spectrum of antimicrobial activity against most gram-positive and gram-negative bacteria (Friedman and others 2002). Carvacrol disintegrates the outer membrane of gram-negative bacteria, releasing lipopolysaccharides and increasing the permeability of the cytoplasmic membrane to ATP (Burt 2004).

For gram-positive bacteria, it is able to interact with the membranes of bacteria and alter the permeability for cations like H+ and K+ (Veldhuizen and others 2006). In general, the higher antimicrobial activity of essential oils is observed on gram-positive bacteria than gram-negative bacteria (Kokoska and others 2002; Okoh and others 2010). Lipophilic ends of lipoteichoic acids in cell membrane of gram positive bacteria may facilitate the penetration of hydrophobic compounds of essential oils (Cox and others 2000). On the other hand, the resistance of gram-negative Vol. 79, Nr. 7, 2014 r Journal of Food Science R1237

R: Concise Reviews in Food Science Bioactivities and applications of essential oils . bacteria to essential oils is associated with the protecting role of extrinsic membrane proteins or cell wall lipopolysaccharides, which limits the diffusion rate of hydrophobic compounds through the lipopolysaccharide layer (Burt 2004). The dissipation of ion gradients leads to impairment of essential processes in the cell and finally to cell death (Ultee and others 1999). The cytoplasmic membrane of bacteria generally has 2 principal functions: (i) barrier function and energy transduction, which allow the membrane to form ion gradients that can be used to drive various processes, and (ii) formation of a matrix for membrane-embedded proteins (such as the membrane-integrated F0 complex of ATP synthase) (Sikkema and others 1995; Hensel and others 1996).

Antimicrobial mechanism of essential oil is proposed as shown in Figure 3. The activity of the essential oils is related to composition, functional groups, and synergistic interactions between components (Dorman and Deans 2000). The removal of the aliphatic ring substituent of carvacrol slightly decreased the antimicrobial activity. 2-Amino-ρ-cymene has similar structure to cavacrol, except hydroxyl group (Figure 4). The lower activity by 3-fold of 2-aminoρ-cymene, as compared to carvacrol, indicates the essential role of hydroxyl group in antimicrobial activity of carvacrol (Veldhuizen and others 2006).

The hydroxyl group present in the structure of phenolic compounds confers antimicrobial activity and its relative position is very crucial for the effectiveness of these natural components; this can explain the superior antimicrobial activity of carvacrol, compared to other plant phenolics (Veldhuizen and others 2006).

Plant essential oils have been known as antimicrobial agents. Essential oil of rosemary (R. officinalis) exhibited both gram-positive (Staphylococcus aureus and Bacillus subtilis) and gram-negative (Escherichia coli and Klebsiella pneumoniae) bacteria (Okoh and others 2010). The major components of rosemary oil are monoterpenes such as α-pinene, β-pinene, myrcene 1,8-cineole, borneol, camphor, and verbinone (Santoyo and others 2005; Okoh and others 2010), which possess strong antimicrobial activity by the disruption of bacteria membrane integrity (Knobloch and others 1989). Aguirre and others (2013); Burt (2004); and Pelissari and others (2009) also reported that oregano essential oil had higher antimicrobial activity against the gram-positive bacteria (S.

aureus) than gram-negative (E. coli and Pseudomonas aeruginosa). The main constituents of oregano essential oil are thymol, carvacrol, γ -therpinene, and ρ-cymene (Lambert and others 2001; Burt 2004; Aguirre and others 2013). However, Pseudomonas putida was resistant to carrot seed and parsley essential oils (Teixeira and others 2013). E. coli and Salmonella typhimurium were also tolerant to carrot seed, grapefruit, lemon, onion, and parsley essential oils. The greater resistance of gram-negative bacteria toward essential oils may be attributed to the complexity of their double-layer cell membrane, compared with the single-layer membrane of grampositive bacteria (Hogg 2005).

Antimicrobial activity of Callistemon comboynensis essential oil was observed against gram-positive (B. subtilis and S. aureus), gramnegative (Proteus vulgaris and P. aeruginosa), and a pathogenic fungus Candida albicans. This might be associated with the high content of oxygenated constituents (Abdelhady and Aly 2012). Essential oil of C. comboynensis leave consisted of 1,8-cineole (53.03%), eugenol (12.1%), methyl eugenol (8.3%), α-terpineol (4.3%), and carveol (3.4%) (Abdelhady and Aly 2012). Teixeira and others (2013) found that the highest reduction (8.0 log CFU/mL) was obtained when coriander, origanum, and rosemary essential oils at a level of 20 μL were used to inhibit Listeria innocua.

Thyme essential oil (20 μL) was able to inhibit both L. innocua and Listeria monocytogenes. However, rosemary essential oil exhibited the highest MIC (90.8 mg/mL) against Brochothrix thermosphacta and S. typhimurium. Thus, essential oils from the selected plants can be used as antimicrobial agents for food applications as well as other purposes; however, their activity depends on types of essential oil used.

Figure 3–Schematic illustration for the effect of essential oils on bacteria cell. R1238 Journal of Food Science r Vol. 79, Nr. 7, 2014

R: Concise Reviews in Food Science Bioactivities and applications of essential oils . Antioxidant activity Several compounds in essential oils have the structure mimicking the well-known plant phenols with antioxidant activity. Among the major compounds available in the oil, thymol and carvacrol were reported to possess the highest antioxidant activity (Dapkevicius and others 1998). Essential oils have several modes of actions as antioxidant, such as prevention of chain initiation, free radical scavengers, reducing agents, termination of peroxides, prevention of continued hydrogen abstraction as well as quenchers of singlet oxygen formation and binding of transition metal ion catalysts (Yildirim and others 2000; Mao and others 2006).

With those functions, essential oils can serve as the potential natural antioxidants, which can be used to prevent lipid oxidation in food systems. Phenolics are organic compounds consisting of hydroxyl group (-OH) attached directly to a carbon atom that is a part of aromatic ring. The hydrogen atom of hydroxyl group can be donated to free radicals, thereby preventing other compounds to be oxidized (Nguyen and others 2003). Teixeira and others (2013) reported that the highest scavenging activity of DPPH radical was observed for clove and origanum essential oils with the EC50 values of 35.7 ± 1.2 and 46.8 ± 0.4 μg/mL, respectively.

Clove and origanum essential oils also showed the high ferric reducing power (Teixeira and others 2013). The antioxidant capability of phenolic compounds is mainly due to their redox properties, which permit them to act as hydrogen donors, reducing agents, singlet oxygen quenchers as well as metal chelators (Kumar and others 2005). The antioxidant activity is generally related with the major active compounds in essential oils such as eugenol in clove (Wei and Shibamoto 2010), carvacrol in origanum (Bounatirou and others 2007), m-thymol in thyme (Bozin and others 2006), and β-citronellol or β-citronellal in citronella (Ruberto and Baratta 2000).

However, the other antioxidant compounds in essential oils such as terpinene ) - camphor ) - bornylacetate, eucalyptol, and methylchavicol have been reported to exhibit antioxidant activity, but their amounts were probably too low to exhibit antioxidant activity (Ruberto and Baratta 2000; Mitić-Ćulafić and others 2009; Teixeira and others 2013). Antioxidant activity varies with source of essential oils. Tongnuanchan and others (2013a) reported that among essential oils from roots, plai essential oil showed the highest DPPH radical scavenging activity, followed by turmeric and ginger essential oil, respectively.

The highest 2,2-azino-bis (3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) radical scavenging activity was observed in turmeric essential oil, followed by plai and ginger essential oils. The differences in antioxidative activity of different essential oils were mostly due to the differences in types and amounts of antioxidative components present in essential oils (Burt 2004; Kordali and others 2005). Antioxidative activity of essential oil is also affected by extraction method or solvents used. Sarikurkcu and others (2010) reported that free radical scavenging activity (DPPH assay) and reducing power of essential oil from Thymus longicaulis subsp.

Longicaulis var. longicaulis extracted using HD method was lower than those extracted using methanol or water. Methanol extract of Salvia tomentosa exhibited superior radical scavenging activity to other extracts (IC50 = 18.7l μg/mL) (Tepe and others 2005). Nonpolar extracts showed less effective activities than polar extracts. Therefore, antioxidative activity of essential oil is strictly related with the polarities of their phytochemicals. The antioxidant activity of Figure 4–Structure of carvacrol and carvacrol-related compounds Source: Veldhuizen and others (2006). Vol. 79, Nr. 7, 2014 r Journal of Food Science R1239

R: Concise Reviews in Food Science Bioactivities and applications of essential oils . essential oil from T. longicaulis subsp. longicaulis var. longicaulis extracted by HD method at 2.0 mg/mL showed similar antioxidative activity to synthetic antioxidants butylated hydroxytoluene (BHT) and butylated hydroxyanisole (BHA) when tested by β-carotene– linoleic acid model system and was higher than those extracted with other solvents (Sarikurkcu and others 2010). In contrast, the inhibition of linoleic acid oxidation of model system by essential oil of S. tomentosa (Miller) was lower than those extracted using solvents with different polarities and BHT (Tepe and others 2005).

Abdelhady and Aly (2012) reported that C. comboynensis essential oil exhibited the antioxidant activity at a concentration of 1000 μg/mL (91.1 ± 0.3% inhibition), comparable to 100 μg/mL gallic acid (95.7 ± 2% inhibition). It has been reported that nonphenolic antioxidants of plant extracts might also contribute to the antioxidant activity (Newman and others 2002; Hassimotto and others 2005).

Additionally, the harvesting period of plant also determines the concentration of the major oil components such as phenolic compounds, which directly related with the antioxidant activity of essential oils (Malatova and others 2011; Zheljazkov and others 2012; Wu and others 2013). Active Packaging Containing Essential Oils and Applications Development of active packaging Nowadays, smart packaging has gained increasing attention, for example, antimicrobial packaging, which can be applied to extend the shelf life of food and products (Appendini and Hotchkiss 2002; Quintavalla and Vicini 2002).

To enhance the property of those packaging, antimicrobial compounds or extracts with the selected bioactivity are incorporated. Thus, several approaches have been introduced, not only for increasing bioactivity but also modifying the property of biomaterials used for packaging. Among biomaterials, proteins have gained attention, due to their variety in compositions, properties, as well as nutritive value. However, protein-based material for packaging is still encountering the poor property, especially poor barrier property toward water vapor. Chemical and enzyme treatment can be applied to modify polymer network through the cross-linking of the polymer chains to improve the properties of protein film (Mahmoud and Savello 1993; Yildirim and Hettiarachchy 1997; De Carvalho and Grosso 2004).

Hydrophobic plasticizer can be used to improve water vapor barrier property of films. However, it may yield films with different properties. The incorporation of hydrophobic substances such as lipid, fatty acid, wax, and so on, has been implemented to improve water vapor barrier property (Prodpran and others 2007; Limpisophon and others 2010; Soazo and others 2011). Hydrophobic materials such as essential oils have been incorporated to improve water vapor barrier property of protein-based films, for example, film from fish muscle protein, film from fish gelatin, and so on (Atarés and others 2010a; Tongnuanchan and others 2012, 2013a).

Tongnuanchan and others (2012) reported that water vapor permeability (WVP) of fish skin gelatin film decreased markedly from 3.11 to 1.88, 1.89, and 2.45 × 10−11 gm−1 s−1 Pa−1 (P < 0.05), when films were incorporated with ginger, turmeric, and plai essential oils, respectively, at a level of 100% based on protein. The incorporation of ginger, turmeric, and plai essential oils at the highest level (100% based on protein) reduced WVP of film by 39.54%, 39.22%, and 21.22%, respectively. The result suggested different hydrophobicity of compounds present in different essential oils used. Monoterpenes are highly hydrophobic substances found in essential oils, in which the content varied with types of essential oils (Turina and others 2006).

Hydrophobic essential oil could increase the hydrophobicity of films, thereby reducing the water vapor migration through the film. Essential oils with low density are separated and localized at the upper surface of film, thereby forming the bilayer microstructure. In general, there was no oil exudates on the film incorporated with low concentration (25%) of essential oil; however, at high concentration of essential oil (100%), some oil exudates were found at the surface of the films. The bilayer-morphological microstructure might contribute to lower WVP of essential-oil-incorporated gelatin films (Figure 5), compared with the control film.

Atarés and others (2010a) studied the mechanical properties of soy protein isolate incorporated with cinnamon and ginger essential oil at different concentrations (protein to oil mass ratios: 1 : 0.025, 1 : 0.050, 1 : 0.075, and 1 : 0.100). A slight decreasing trend of elastic modulus (EM) was observed as the oil content increased. The WVP was slightly reduced by both essential oils. The oil type significantly affected both tensile strength (resistance to elongation) and EM (capacity for stretching) (Atarés and others 2010a). Essential oils may cause some degree of rearrangement in the protein network, thus strengthening and increasing the film resistance to elongation.

Moreover, Pires and others (2011) studied the effect of thyme essential oil incorporated in hake protein film. The addition of thyme oil significantly reduced the WVP. Nevertheless, the addition of essential oil had impact on the transparency of film, depending on type and concentration of essential oils. The addition of thyme oil decreased the transparency value of hake proteins films (Pires and others 2011). Table 4 presents the properties of protein-based films containing various essential oils. The ability of plant essential oils to protect foods against pathogenic and spoilage microorganisms has been reported by several researchers (Lis-Balchin and others 1998; Friedman 2006; Rojas-Graü and others 2007).

Film or packaging incorporated with essential oils can be employed as active packaging due to their antimicrobial or antioxidant activities. Seydim and Sarikus (2006) evaluated antimicrobial activity of whey-protein isolate-based edible films incorporated with oregano essential oil. Oregano essential oil added films exhibited the larger inhibitory zone on S. aureus with increasing levels of essential oil added. Table 5 presents the antimicrobial activities of biopolymer films containing various types of essential oils.

Films added with essential oils are shown to possess antioxidant activities, which can vary with type and amount of essential oil incorporated. Gómez-Estaca and others (2009) reported that bovine-hide and tuna skin gelatin films supplemented with oregano and rosemary extracts exhibited the reducing ability and free-radical scavenging capacity. Antioxidant power was generally being proportional to the amount of added extract. Gelatin films incorporated with different essential oils containing 30% glycerol mostly had the higher antioxidant activity than those with 20% glycerol (P < 0.05) (Tongnuanchan and others 2012).

More loosen structure of film network found in film containing 30% glycerol favored the release of essential oils with antioxidative activity (Tongnuanchan and others 2012). Antioxidative activities of gelatin films incorporated with essential oils were lower than those of pure essential oil, regardless of type of essential oil used. The interaction between gelatin and antioxidative compounds in essential oil thus lowers the release of those compounds (Tongnuanchan and others 2013a). Antioxidant activities of protein-based films containing various essential oils are shown in Table 6.

However, film or packaging may have the smell of essential oils due to its volatilization. The smell intensity of essential oil in R1240 Journal of Food Science r Vol. 79, Nr. 7, 2014

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