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.
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.
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.
|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|
|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|
|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, Δ2‐Carene, cis‐2,6‐Dimethyl‐2,6octadiene, γ‐Terpinene||(−)‐Isopulegol, β‐Citronellal, β‐ Citronellol||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‐Tetramethyl‐benzene,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)|
|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, Δ2‐Carene,1,3,8‐p‐Menthatriene, 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, trans‐ Menthadiene, trans‐Ocimene, trans‐Decalone||Citronellal, Linalool, cis‐Limonene oxide, trans‐Carveol, Limonene dioxide, Perillyl alcohol||–||Ledol, Globulol||–||Aloxiprin, Heptadiene,Methyl‐ heptadiene,Cyclooctanone, Benzyl‐dicarboxylic||Mohamed and others (2010)|
|Thyme||Camphene, β‐Pinene, Cymene, α‐Fellandrene, m‐Cymene||Eucalyptol, (E)‐2,3‐Epoxycarane, endo‐5,5,6‐Trimethyl‐m‐Thymol, 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)|
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).
These compounds are the combination of C, H, and O, and there are a variety of compounds found in essential oils. 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, α‐terpineol, 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.
|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)|
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, water‐soluble 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).
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 (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 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.
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 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 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.
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.
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.
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 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‐amino‐ρ‐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 gram‐positive bacteria (Hogg 2005).
Antimicrobial activity of Callistemon comboynensis essential oil was observed against gram‐positive (B. subtilis and S. aureus), gram‐negative (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.
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 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−1s−1Pa−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.
|Protein type,||Plasticizer,||Essential oils,||WVP(×10−1 0||Transparency|
|concentration||concentration||concentration||Thickness(mm)||TS (MPa)||EAB (%)||g/m s Pa)||(%)||References|
|Hake muscle protein,1.5% (w/w) of FFS||Glycerol, 59% (w/w) of protein||Thyme (Thymus vulgaris L.), 0.025, 0.05, 0.1, and 0.25 mL oil/g protein||0.022 to 0.025||4.13 to 6.67,3.30 to 8.49 N (Breaking force)||111.2 to 129.8, 87.87 to 115.41 (Puncture deformation)||0.35 to 0.43||1.8 to 6.5||Pires and others (2011)|
|Soy protein isolate, 8% (w/w) of FFS||Glycerol, 30% (w/w) of protein||Cinnamon(Cinnamomum verum), 0.025, 0.05, 0.075, and 0.1 mL oil/g protein||–||11.0 to 17.6||3.4 to 7.5||0.46 to 0.64a||–||Atarés and others (2010a)|
|Ginger(Zingiber officinale), 0.025, 0.05, 0.075, and 0.1 mL oil/g protein||–||4 to 8||1.7 to 3||0.56 to 0.68a||–|
|Sodium caseinate, 8% (w/w) of FFS||Glycerol, 30% (w/w) of protein||Cinnamon (Cinnamomum verum), 0.025 and 0.075 mL oil/g protein||–||22 and 24b 10.2 and 11.4c||13 and 22b 67 and 76c||0.64 and 0.57d 2.14 and 1.7e||–||Atarés and others (2010b)|
|Ginger(Zingiber officinale), 0.025 and 0.075 mL oil/g protein||–||22 and 22b 10 and 11.6c||18 and 16b 57 and 72c||0.57 and 0.52d 2.1 and 1.8e||–|
|Sunflower protein concentrate, 5% (w/v) of FFS||Glycerol, 1.5% (w/v) of FFS||Clove(Syzygium aromaticum)||0.080 ± 0.01||2.5 ± 0.2||24.9 ± 1.7||1.16 ± 0.09aa||–||Salgado and others (2013)|
|Fish gelatin (tilapia), 3.5% (w/w) of FFS||Glycerol,20% and 30% (w/w) of protein||Bergamot (Citrus bergamia), 50% (w/w) of protein||0.047 and 0.048||42.42 and 36.52||15.29 and 19.19||3.15 and 3.22aaa||4.28 and 4.45||Tongnuanchan and others (2012)|
|Kaffir Lime Peel (Citrus hystrix DC)||0.048 and 0.047||36.87 and 34.22||31.43 and 30.93||2.95 and 3.38||5.48 and 5.56|
|Lemon, (Citrus limon)||0.048 and 0.048||32.82 and 31.06||39.06 and 52.66||2.81 and 2.85||5.46 and 5.31|
|Lime, (Citrus aurantifolia)||0.049 and 0.047||27.32 and 25.87||52.21 and 69.79||2.91 and 3.37||5.66 and 5.46|
|Fish gelatin (tilapia), 3.5% (w/w) of FFS||Glycerol, 30% (w/w) of protein||Ginger(Zingiber officinale), 25%, 50%, and 100% (w/w) of protein||0.041 to 0.057||18.58 to 35.73||41.70 to 72.03||1.88 to 2.61aaa||1.60 to 3.02||Tongnuanchan and others (2013a)|
|Tumeric(Curcuma longa)||0.041 to 0.053||23.34 to 34.04||42.96 to 72.80||1.89 to 2.48||1.45 to 1.63|
|Plai(Zingiber cassumunar roxb)||0.041 to 0.055||17.20 to 32.06||44.96 to 74.68||2.45 to 2.91||1.49 – 2.17|
|Fish gelatin (tilapia), 3.5% (w/w) of FFS||Glycerol, 30% (w/w) of protein||Lemongrass, (Cymbopogon citratus)||0.056 to 0.073||18.42 to 25.13||52.81 to 77.25||1.41 to 1.79†||2.48 to 3.24||Tongnuanchan and others (2013b)|
|Basil, (Ocimum sanctum)||0.054 to 0.084||18.70 to 21.37||46.53 to 85.06||1.20 to 2.11||2.18 to 3.26|
|Citronella, (Cymbopogon nardus)||0.068 to 0.080||17.39 to 21.85||44.63 to 97.29||1.07 to 1.42||3.67 to 4.41|
|Kaffir Lime Leaf, (Citrus hystrix DC)||0.066 to 0.081||25.07 to 26.21||43.95 to 95.08||1.03 to 1.59||4.25 to 6.08|
- *FFS = Film forming solution; WVP = water vapor permeability; a WVP unit (g mm/m2 h kPa); aa WVP unit (1010 g H2O/Pa m s); † WVP unit (1010 g H2O/Pa m s); b, c Final moisture content in the film: 5 and 10 g water/100 g film, respectively; d, e WVP of films tested at 25 °C and 2 range of relative humidity (RH) (33% to 53% and 53 to 75, respectively).
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.
|Film forming||Plasticizer,||Essential oils,|
|materials, concentration||concentration||concentration||Tested organisms||Inhibition effect||References|
|Soy protein isolate, 5%||Glycerol,||Oregano (Oreganum heracleoticum L.),||Staphylococcus aureus||27.50 to 49.50a||Emiroğlu and others (2010)|
|(w/v) of FFS||3.5% (w/v) of FFS||1%, 2%, 3%, 4%, and 5% (v/v) of FFS||Escherichia coli||32.00 to 45.50|
|Escherichia coli O157:H7||35.50 to 50.50|
|Pseudomanas aeruginosa||27.00 to 39.50|
|Lactobacillus plantarum||22.50 to 37.00|
|Thyme (Thymusvulgaris L.)||Staphylococcus aureus||30.00 to 49.50|
|Escherichia coli||36.50 to 49.00|
|Escherichia coli O157:H7||36.50 to 49.50|
|Pseudomanas aeruginosa||32.50 to 42.00|
|Lactobacillus plantarum||20.50 to 36.50|
|Bovine‐hide gelatin,||Sorbitol and glycerol, 0.15 and 0.15 g/g gelatin||Clove (Syzygium aromaticum L.)||Pseudomonas fluorescens||9.07 ± 0.13b||Gómez‐Estaca and others (2010)|
|8% (w/v) of FFS||0.75 ml/g biopolymer||Lactobacillus acidophilus||12.76 ± 2.51|
|Listeria innocua||7.46 ± 0.53|
|Escherichia coli||10.64 ± 1.37|
|Gelatin‐Chitosan,||Sorbitol and glycerol, 0.15 and 0.15 g/g gelatin||Clove (Syzygium aromaticum L.)||Pseudomonas fluorescens||9.51 ± 2.03b|
|6% of gelatin plus 2% of chitosan (w/v) of FFS||0.75 mL/g biopolymers||Lactobacillus acidophilus||12.60 ± 3.42||Gómez‐Estaca and others (2010)|
|Listeria innocua||6.42 ± 0.41|
|Escherichia coli||8.69 ± 0.42|
|Whey protein isolate, 5% (w/v) of FFS||Glycerol, 5% (w/v) of FFS||Oregano (Origanum minutiflorum) 1%, 2%, 3%, and 4% (v/v) of FFS||Escherichia coli O157:H7 Staphylococcus aureus Salmonella enteritidis Listeria monocytogenes Lactobacillus plantarum||0 to 37.09c0 to 43.07 0 to 40.59 0 to 41.650 to 13.45||Seydim and Sarikus (2006)|
|Rosemary (Rosmarinus officianalis L.)||Escherichia coli O157:H7 Staphylococcus aureus Salmonella enteritidis Listeria monocytogenes Lactobacillus plantarum||0 to 11.36 0 to 13.45 0 to 10.48 0 to 11.96 0 to 9.21|
|Garlic (Allium sativum L.),||Escherichia coli O157:H7 Staphylococcus aureus Salmonella enteritidis Listeria monocytogenes Lactobacillus plantarum||N.D. N.D. N.D. N.D. N.D.|
|Sunflower protein concentrate, 5% (w/v) of FFS||Glycerol, 1.5% (w/v) of FFS||Clove (Syzygium aromaticum), 0.75 mL/g biopolymer||Aeromonas hydrophila, Aspergillus niger, Bacillus cereus, Bacillus coagulans, Bifidobacterium animalis‐ subespecie lactis,||32.66 ± 17.59b 38.32 ± 11.24 25.61 ± 5.22 37.21 ± 5.13 25.52 ± 9.85||Salgado and others (2013)|
|Bifidobacterium bifidum, Brochothrix thermophacta, Citrobacter freundii, Clostridium perfringens, Debaryomyces hansenii, Enterococcus faecium, Escherichia coli, Lactobacillus acidophilus, Lactobacillus helveticus, Listeria innocua, Listeria monocytogenes, Penicilium expansum, Photobacterium phosphoreum, Pseudomonas aeruginosa, Pseudomonasfluorescens, Salmonella cholerasuis, Shewanella putrefaciens, Shigella sonnei, Staphylococcus aureus, Vibrio parahaemolyticus Yersinia enterocolítica||27.42 ± 14.04 40.20 ± 0.87 24.14 ± 5.54 21.15 ± 0.71 60.74 ± 9.90 22.07 ± 1.14 25.68 ± 1.43 22.00 ± 1.81 24.95 ± 5.64 31.04 ± 0.51 23.34 ± 4.13 50.34 ± 2.48 38.20 ± 6.43 27.09 ± 2.47 27.39 ± 8.13 26.40 ± 3.51 24.25 ± 4.99 22.04 ± 2.46 29.35 ± 3.52 30.77 ± 9.80 22.55 ± 4.79|
|Cassava starch‐Chitosan, 77% of starch plus 5% of chitosan||Glycerol, 18%||Oregano (Oreganum heracleoticum L.)0.1%, 0.5%, and 1% of FFS||Bacillus cereus Escherichia coli Salmonella enteritidis Staphylococcus aureus||6.28 to 19.50a 9.99 to 23.73 13.26 to 30.81 13.98 to 33.88||Pelissari and others (2009)|
|Hake protein, 1.5% (w/v) of FFS||Glycerol, 59% (w/w) of protein||Citronella (Pelargonium citrosum),0.25 mL oil/g protein||Brochothrix thermosphacta Escherichia coli Listeria innocua Listeria monocytogenes Pseudomonas putida Salmonella typhimurium Shewanella putrefaciens||48.7 ± 0.3d N.D. 40.6 ± 13.7 N.D. 71.6 ± 22.9 N.D. N.D.|
|Coriander (Coriandrum sativum)||Brochothrix thermosphacta Escherichia coli Listeria innocua Listeria monocytogenes Pseudomonas putida Salmonella typhimurium Shewanella putrefaciens||N.D. N.D. 32.0 ± 16.5 N.D. N.D. N.D. 77.5 ± 12.8||Pires and others (2013)|
|Tarragon (Artemisia dracunculus)||Brochothrix thermosphacta Escherichia coli Listeria innocua Listeria monocytogenes Pseudomonas putida Salmonella typhimurium Shewanella putrefaciens||79.8 ± 5.7 N.D. 14.1 ± 7.6 N.D. N.D. N.D. 80.5 ± 6.9|
|Thyme (Thymus vulgaris)||51.0 ± 5.3 N.D. 35.2 ± 5.3 N.D. N.D. 45.2 ± 18.99 6.9 ± 1.7|
|Triticale protein, 7.5% (w/v) of FFS||Glycerol, 20% (w/w) of protein||Oregano, 1% and 2% (w/v) of FFS||Escherichia coli Staphylococcus aureus Pseudomonas aeruginosa||10.81 and 21.53a 166.90 and 342.36 0.00 and 9.70||Aguirre and others (2013)|
- *FFS = Film forming solution; N.D. = inhibition not detected; a Inhibition unit (inhibition zone diameters (mm)); b Inhibition unit (percentage of inhibition (%) of the total plate surface); c Inhibition unit (inhibition zone (mm2)); d Inhibition unit (Macrodilution method (% reduction))
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.
|Film forming||Antioxidant activity|
|Fish gelatin (tilapia), 3.5% (w/w) of FFS||Glycerol, 20% and 30% (w/w) of protein||Bergamot(Citrus bergamia), 50% (w/w) of protein||0.25 and 0.42h||4.33 and 6.25i||–||1.17 and 2.15j||–||–||–||Tongnuanchan and others (2012)|
|Kaffir Lime(Citrus hystrix)||0.04 and 0.13||1.98 and 3.99||–||0.36 and 0.68||–||–||–|
|Lemon(Citrus limon)||0.15 and 0.27||28.01 and 31.18||–||2.76 and 3.42||–||–||–|
|Lime(Citrus latifolia)||0.02 and 0.09||2.54 and 4.24||–||0.30 and 0.46||–||–||–|
|Fish gelatin (tilapia), 3.5% (w/w) of FFS||Glycerol, 30% (w/w) of protein||Ginger(Zingiber officinale),25%, 50%, and 100% (w/w) of protein||0.26 to 1.30h||3.47 to 13.43i||–||–||–||–||–||Tongnuanchan and others (2013a)|
|Tumeric(Curcuma longa)||0.72 to 1.97||8.96 to 20.26||–||–||–||–||–|
|Plai(Zingiber cassumunar)||0.95 to 2.39||8.68 to 19.51||–||–||–||–||–|
|Fish gelatin (tilapia), 3.5% (w/w) of FFS||Glycerol, 30% (w/w) of protein||Lemongrass, (Cymbopogon citratus)||0.39 to 0.78h||5.40 to 7.66i||–||–||–||–||5.25 to 11.73k||Tongnuanchan and others (2013b)|
|Basil, (Ocimum sanctum)||9.31 to 12.40||51.42 to 123.72||–||–||–||–||0.29 to 7.65|
|Citronella, (Cymbopogon nardus)||1.98 to 3.32||6.71 to 28.72||–||–||–||–||1.38 to 12.78|
|Kaffir Lime Leaf, (Citrus hystrix DC)||0.52 to 0.63||4.25 to 5.41||–||–||–||–||3.10 to 13.74|
|Hake protein, 1.5% (w/v) of FFS||Glycerol, 59% (w/w) of protein||Thyme(Thymus vulgaris L.), 0.025, 0.05, 0.1, and 0.25 mL oil/g protein||18.5 to 40.1a||–||17.5 to 25.4b||–||–||–||–||Pires and others (2011)|
|Sunflower protein concentrate, 5% (w/v) of FFS||1.5% (w/v) of FFS||Clove(Syzygium aromaticum L.), 0.75 mL oil/g SPC||–||1194.1 ± 77.0 d||–||5733.3 ± 92.5e||229.0 ± 6.6 f||1767.0 ± 11.8g||–||Salgado and others (2013)|
- * FFS = Film forming solution; DPPH = 2,2‐diphenyl‐1‐picryl hydrazyl (DPPH) radical scavenging activity; ABTS = 2,2‐azino‐bis (3‐ethylbenzothiazoline‐6‐sulphonic acid) diammonium salt (ABTS) radical scavenging activity, FRAP = ferric reducing antioxidant power; PCL‐ACW = photochemiluminescence‐antiradical capacity of water‐soluble substances; PCL‐ACL = photochemiluminescence‐antiradical capacity of lipid‐soluble substances.
- a DPPH unit (%); b Reducing power unit (mg Ascorbic acid/g film); c Reducing power unit (μmol Ascorbic acid/g film); d ABTS unit (mg Ascorbic acid/g film); e FRAP unit (mmol FeSO4.7H2O equivalents/g film); f PCL‐ACW unit (μmol Ascorbic acid/g film); g PCL‐ACL unit (μmol Trolox/g film); h, i, j DPPH, ABTS, and FRAP units (μmol Trolox equivalents (TE)/g dried film); k Chelating activity unit (μmol EDTA equivalents/g dried film).
However, film or packaging may have the smell of essential oils due to its volatilization. The smell intensity of essential oil in films increased with increasing essential oil levels. This might limit the application of the film in food when it was incorporated at the high amount. However, smaller amount (25%) of essential oil added did not cause the detrimental effect on smell perception or unacceptability of the film (Tongnuanchan and others 2012).
Active film containing essential oil can be applied to extend the shelf life and maintain the quality of foods, such as meat, fish, and their products. Films can serve as carriers for various antimicrobial agent and antioxidant that can maintain fresh quality, extend product shelf life, and reduce the risk of pathogen growth. Table 7 presents the antimicrobial effect of active films containing various essential oils in food systems.
|Film forming||Essential oils,|
|Chitosan||Cinnamaldehyde, 1% (w/w) of FFS||Bologna, Regular cooked ham, Pastrami||Enterobacteriaceae, Lactobacillus sakei, Serratia liquefaciens, Lactic acid bacteria||Ouattara and others (2000)|
|Milk protein||Oregano (OR), Pimento (PI), Mixture (OR+PI, 1 : 1), 1% (w/v) of FFS||Whole beef muscle||Escherichia coli O157:H7, Pseudomonas spp.||Oussalah and others (2004)|
|Chitosan||Oregano, 1% and 2% of FFS||Bologna slices||Listeria monocytogenes, Escherichia coli O157:H7||Zivanovic and others (2005)|
|Pigskin gelatin||Oregano, Rosemary, 1.25% and 20% of FFS, respectively||Cold‐smoked sardine||Total viable bacteria, H2S‐producing microorganisms||Gomez‐Estaca and others (2007)|
|Whey protein isolate||Oregano, 1.5% (w/w) of FFS||Fresh beef||Total viable bacteria, Pseudomonas ssp., Lactic acid bacteria||Zinoviadou and others (2009)|
|Soy protein||Oregano (OR), Thyme (TH), Mixture (OR+TH, 1 : 1), 5% (v/v) of FFS||Fresh ground beef patties||Pseudomanas spp., Staphylococcus spp. Coliform||Emiroğlu and others (2010)|
|Bovine‐hide gelatin‐Chitosan||Clove, 0.75 mL/g biopolymer||Cod fillets||Total viable bacteria, H2S‐producing microorganisms, Lactic acid bacteria, Pseudomonas ssp., Enterobacteriaceae||Gómez‐Estaca and others (2010)|
|Sunflower protein concentrate||Clove, 0.75 mL/g biopolymer||Sardine patties||Total viable bacteria, Total mesophiles, H2S‐producing microorganisms, Luminescent colonies, Lactic bacteria, Pseudomonas spp. Enterobacteriaceae||Salgado and others (2013)|
Use of packaging for meat and meat products
Microorganisms are responsible for meat spoilage. Most essential oils are classified as generally recognized as safe (GRAS). However, their use as food preservatives is often limited due to flavoring considerations (Zinoviadou and others 2009). The effectiveness of bioactive films containing essential oils against the spoilage or pathogenic bacteria in food system has been studied. Zinoviadou and others (2009) studied the antibacterial effects of WPI film containing oregano oil (0.5% and 1.5% w/w of Film forming solution [FFS]) against total variable bacteria count, Pseudomonas spp. and lactic acid bacteria on beef cuts. The use of films containing the highest level of oregano oil (1.5% w/w of FFS) resulted in a significant reduction of total variable bacteria count and Pseudomonas spp. population during 12 d of refrigeration storage (5 °C). The total variable bacteria population of the samples wrapped with films containing the high essential oil level at day 8 was 5.1 log CFU/cm2, while the control had population of 8.4 log CFU/cm2. Since microbial loads higher than 107 CFU/cm2 are usually associated with off‐odors (Ercolini and others 2006), it may be suggested that the use of WPI films containing 1.5% (w/w) oregano oil could double the shelf life of fresh beef stored under refrigerated condition. Oussalah and others (2004) reported the application of milk protein films incorporated with essential oils (oregano, pimento, and mixed) on meat surfaces containing 103 CFU/cm2 of E. coli O157:H7 and Pseudomonas spp. Film containing oregano essential oil was the most effective in inhibition both bacteria, whereas film with pimento oils seemed to be the least effective against these 2 bacteria. The reduction of around 1 log unit of E. coli O157:H7 and Pseudomonas spp. was observed at the end of storage (day 7, at 4 °C) when film containing oregano essential oil was used, compared to samples without film coated. Ouattara and others (2000) reported that chitosan film incorporated with cinnamaldehyde reduced the growth of Lactobacillus sakei, Serratia liquefaciens, and Enterobacteriaceae, on the surface of meat products (bologna, cooked ham, and pastrami). However, the films had no effect or little effect on the numbers of lactic acid bacteria on bologna or pastrami, after 21 d of storage at 4 or 10 °C. Zivanovic and others (2005) tested the impact of chitosan film containing oregano essential oil (1% and 2% of FFS) on microbial growth of the inoculated bologna samples and stored for 5 d at 10 °C. The higher activity was obtained in films with 1% and 2% oregano essential oil, which decreased the numbers of L. monocytogenes by 3.6 to 4 logs and E. coli O157:H7 by 3 logs, whereas the pure chitosan films reduced L. monocytogenes by 2 logs.
Essential oils are able to extend shelf life of foods by lowering lipid oxidation (Oussalah and others 2004; Zivanovic and others 2005). Therefore, the incorporation of essential oils into the biodegradable films could provide antioxidant activity for resulting films. Oussalah and others (2004) reported that the incorporation of oregano essential oil into milk‐protein‐based film increased the ability to stabilize lipid oxidation in beef muscle samples during refrigerated storage. Moradi and others (2011) studied the antioxidant effects of chitosan film containing Zatariamultiflora Boiss essential oil (ZEO) wrapped on mortadella sausage during 21 d of refrigeration storage (4 °C). Lipid oxidation of samples decreased markedly at first 6 d when compared to samples wrapped with control film (without ZEO incorporated) and unwrapped samples up to the end of storage. The most effectiveness was observed when samples packed with film containing 10 g/kg ZEO and combination with 10 g/kg grape seed extract.
Use of packaging for fish and fish products
The antimicrobial effects of plant extracts including plant essential oils on a wide range of microorganisms have been described (Hammer and others 1999; Dorman and Deans 2000). As a consequence, plant extracts have been used to preserve meat and fish products due to their antimicrobial and antioxidant effects. Gómez‐Estaca and others (2010) reported that the complex gelatin–chitosan film incorporated with clove essential oil was applied to fish during chilled storage and the growth was drastically reduced for gram‐negative bacteria, especially enterobacteria, and corresponded with the delay in total volatile base (TVB) production. Lactic acid bacteria remained practically constant during 11 d of storage. H2S‐producing bacteria were also inhibited since their growth was interrupted with the application of the film. This microbial inhibition could be attributed to the hydrophobic nature of essential oil, which enable them and their components to partition in the lipids of the bacteria cell membrane and mitochondria while disturbing the structures and rendering it more permeable (Sikkema and others 1995). The intrinsic properties of the food (fat, protein, pH, and so on), as well as the environment in which the food is maintained (storage temperature, packaging, and so on), may influence the prevention effect of essential oils (Tassou and others 1995; Burt 2004). Low pH and storage temperature, decrease O2 concentrations, and high salt content enhances the antimicrobial effect of essential oils, while high levels of protein and fat and low water activity seem to protect bacteria from the inhibition by essential oils (Gómez‐Estaca and others 2010). However, soy protein film with oregano, thyme essential oil, and mixture of those did not have significant effects on total viable counts, lactic acid bacteria and Staphylococcus spp. when applied on ground beef patties. Nevertheless, the reduction in coliform and Pseudomonas spp. counts was observed. Gomez‐Estaca and others (2007) tested the antibacterial effects of gelatin‐based films added with an extract of oregano or rosemary against microbial spoilage in preserving cold‐smoked sardine. Coating the fish with films enriched with oregano or rosemary extract lowered the microbial growth by 1.99 and 1.54 log cycles, respectively, on day 16.
Salgado and others (2013) tested the antioxidant activity of sunflower protein films enriched with clove essential oil in preserving fish patties during 13 d of storage at 2°C. The rate of malonaldehyde production was lower in patties wrapped with clove containing films during the first 3 d of storage, indicating a noticeable delay in hydroperoxide (primary lipid oxidation products) degradation exerted by the clove essential oil components. This allowed thiobarbituric acid‐reactive substances (TBARS) remaining at the lowest values during storage. Use of natural plant extracts to prevent lipid oxidation in fish has been reported (Giménez and others 2004; Serdaroglu and Felekoglu 2005). Gomez‐Estaca and others (2007) developed gelatin‐based film enriched with oregano or rosemary essential oils to prevent lipid oxidation in cold‐smoked sardine during 20 d of storage at 5 °C. Coating the muscle with the films enriched with both essential oils, particularly oregano oil, lowered the lipid oxidation rate (as measured by the peroxide and TBARS indices) of the muscle. Therefore, the edible films with the added plant extracts could lower lipid oxidation levels in food systems.
In summary, essential oils from different sources can be exploited as the natural additives in foods. Essential oils with other bioactivities or functions from new sources should be further searched. New technology for lowering the unique and undesirable smell of essential oil, which can limit their use in foods, such as encapsulation, and so on, must be implemented. As a consequence, essential oil can be widely used without any negative effect on sensory property of foods. The development of release system for essential oil from packaging or fuming system inside packaging should be conducted to maximize the activity of active compounds in essential oils. Therefore, it can serve as the convenient packaging, which effectively extends the shelf life of foods.