Essential Oils: Sources of Antimicrobials and Food Preservatives.

Aromatic and medicinal plants produce essential oils in the form of secondary metabolites. These essential oils can be used in diverse applications in food, perfume, and cosmetic industries. The use of essential oils as antimicrobials and food preservative agents is of concern because of several reported side effects of synthetic oils. Essential oils have the potential to be used as a food preservative for cereals, grains, pulses, fruits, and vegetables. In this review, we briefly describe the results in relevant literature and summarize the uses of essential oils with special emphasis on their antibacterial, bactericidal, antifungal, fungicidal, and food preservative properties. Essential oils have pronounced antimicrobial and food preservative properties because they consist of a variety of active constituents (e.g., terpenes, terpenoids, carotenoids, coumarins, curcumins) that have great significance in the food industry. Thus, the various properties of essential oils offer the possibility of using natural, safe, eco-friendly, cost-effective, renewable, and easily biodegradable antimicrobials for food commodity preservation in the near future.

Introduction

Since ancient times, commercial antimicrobial agents have been applied as a way to manage food deterioration or contamination. Nowadays, user concerns toward synthetic preservatives have resulted in increasing attention on various natural antimicrobials such as essential oils. Aromatic and medicinal plant essential oils and their components demonstrate antibacterial, antifungal, and food preservative activities against a wide range of microbial pathogens (Basim et al., 2000; Iacobellis et al., 2004; Tripathi and Kumar, 2007; Pandey et al., 2014b; Sonker et al., 2015; Gormez et al., 2016; Figure 1). These essential oils are hydrophobic liquids of aromatic compounds that are volatile and oily in nature and present in various plant parts such as twig, flower, leaf, bark, seed, and root. Many plant essential oils are useful as a flavor or aroma enhancer in cosmetics, food additives, soaps, plastics resins, and perfumes. Moreover, curiosity about essential oil applications that can act as antimicrobial agents is growing because of the broad range of activities, natural origins, and generally recognized as safe (GRAS) status of essential oils. Currently, essential oils are frequently studied for their antimicrobial (Cowan, 1999; Burt, 2004; Nedorostova et al., 2009), antifungal (Singh and Tripathi, 1999), antiulcer (Dordevic et al., 2007), antihelminthic (Inouye et al., 2001), antioxidant (Mimica-Dukic et al., 2003), anti-inflammatory (Singh et al., 1996), repellent, insecticidal, antifeedant (Isman et al., 1990; Pandey et al., 2014a), cytotoxic (Sylvestre et al., 2007), antiviral (Maurya et al., 2005), ovicidal (Pandey et al., 2011b), anesthetic (Ghelardini et al., 2001), molluscicidal (Fico et al., 2004), immunomodulatory (Mediratta et al., 2002), antinociceptive (Abdollahi et al., 2003), and larvicidal (Jantan et al., 2003) properties as well as for their use as food preservatives (Ukeh and Mordue, 2009; Pandey et al., 2014c).

Figure 1

Different activities and uses of essential oils.

Essential oils of aromatic and medicinal plants are reported to be effective against agents affecting stored products such as insects, human pathogenic fungi, and bacteria. Essential oils of Chenopodium ambrosioides, Clausena pentaphylla, Mentha arvensis, and Ocimum sanctum are contact-sensitive and act as fumigant toxicants against Callosobruchus chinensis and C. maculatus (Pandey et al., 2011a) associated with pigeon pea seeds. Similarly, the essential oil of Tanacetum nubigenum exhibit significant repellent and fumigant toxicity against Tribolium castaneum, which affects wheat during storage (Haider et al., 2015). Eucalyptus globulus essential oil has antibacterial activity against Escherichia coli and Staphylococcus aureus, thus, it is effective against both Gram-positive and Gram-negative bacteria (Bachir and Benali, 2012). In addition, other bacterial pathogens such as Haemophilus influenzae, S. aureus, S. pneumonia, and S. pyogenes were inhibited by Eucalyptus odorata essential oil under in vitro conditions (Posadzki et al., 2012). This review highlights the use of essential oils and their antifungal, fungicidal and food preservative properties in controlling fungi associated with food commodities. Additional emphasis has been given on the efficacy of essential oils against plant pathogenic bacteria as antibacterial and bactericidal.

Essential oils and functions of their active constituents

The majority of aromatic plants retain a volatile odoriferous mixture of compounds which can be extracted as an essential oil. Generally, aromatic and medicinal plants produce a wide range of secondary metabolites viz., terpenoids, alcoholic compounds (e.g., geraniol, menthol, linalool), acidic compounds (e.g., benzoic, cinnamic, myristic acids), aldehydes (e.g., citral, benzaldehyde, cinnamaldehyde, carvone camphor), ketonic bodies (e.g., thymol, eugenol), and phenols (e.g., ascaridole, anethole). Among those, terpenes (e.g., pinene, myrcene, limonene, terpinene, p-cymene), terpenoids (e.g., oxygen-containing hydrocarbons), and aromatic phenols (e.g., carvacrol, thymol, safrole, eugenol) are found to have major roles in the composition of various essential oils (Figure 2) (Koul et al., 2008). Derivatives of terpenoids and aromatic polyterpenoids are synthesized by the mevalonic acid and shikimic acid pathways, respectively (Bedi et al., 2008). Terpenoids are among an immense pool of secondary compounds produced by aromatic and medicinal plants, and they have an important role in providing resistance to pathogens. Monoterpenoids are antimicrobial in nature, result in disruptive multiplication and development of microorganisms, and interfere in physiological and biochemical processes of microorganisms (Burt, 2004). Some botanical constituents such as azadirachtin, carvone, menthol, ascaridol, methyl eugenol, toosendanin, and volkensin have reported potential to act against several bacterial and fungal pathogens as well as against insect pests (Isman, 2006; Pandey et al., 2012, 2016). Moreover, many of them have powerful bactericidal, fungicidal, and insecticidal activities and can be responsible for improved taste or toxic properties.

Figure 2

Actives compounds of essential oils. Figure as originally published in Hyldgaard et al. (2012).

Fungi such as Aspergillus flavus, Neurospora sitophila, and Penicillium digitatum are completely inhibited by Cymbopogon citratus essential oil (Shukla, 2009; Sonker et al., 2015). Essential oils from Nigella sativa, Cymbopogon citratus, and Pulicaria undulata inhibit the growth of Bacillus subtilis, Staphylococcus aureus, Pseudomonas aeruginosa and Escherichia coli (El-Kamali et al., 1998). Essential oils from Acorus, Artemisia, Chenopodium, Clausena, Curcuma, Cinnamon, Cymbopogon, Eupatorium, Foeniculum, Hyptis, Lippia, Ocimum, Putranjiva, Syzygium, and Vitex are known for their pronounced antimicrobial properties (Pandey et al., 2012, 2013b, 2014c; Sonker et al., 2015). The antibacterial properties of essential oils and their several active natural compounds against foodborne bacteria and their applications in food (Burt, 2004) could provide alternatives to conventional bactericides and fungicides (Perricone et al., 2015).

Potency of essential oils against phytopathogenic bacteria

In cereals, pulses, fruits, and vegetables, bacterial species can cause major loss of plant quality and quantity during cultivation, transit, and storage by 20–40% of the total harvest per year. The bacterial species responsible for many diseases and loss of crops include Clavibacter michiganensis, Pseudomonas syringae pv. tomato, P. solanacearum, P. cichorii, P. syringae pv. syringae, P. putida, Erwinia carotovora, E. amylovora, E. carotovora subsp. atroceptica, E. chrysanthemi, E. herbicola, Xanthomonas citri, X. campestris, X. axanopodis pv. malvacearum, X. axanopodis pv. vesicatoria, X. axanopodis pv. campestris, X. campestris pv. raphani, X. axanopodis pv. vitians, and X. campestris pv. zinnia. Such bacteria cause substantial losses in many crops of national and international significance (Agrios, 2005). There are many essential oils that have been evaluated for their potential for antibacterial activity against these phytopathogenic bacteria under in vitro and in vivo conditions (Dorman and Deans, 2000; Iscan et al., 2003; Kotan et al., 2013). The methods used to assess the actions of essential oils against phytopathogenic bacteria include disc diffusion, agar dilution, agar well, and broth dilution (Perricone et al., 2015). Antimicrobial studies of essential oil constituents and their mode of actions more have been extensively undertaken on bacteria; however, there is limited information available about their actions on yeasts and molds.

Generally, Gram-negative bacteria are less susceptible to essential oils than Gram-positive bacteria. The outer membrane of Gram-negative bacteria contains hydrophilic lipopolysaccharides (LPS) that acts as a barrier to macromolecules and hydrophobic compounds, thus providing increased tolerance to hydrophobic antimicrobial compounds such as those found in essential oils (Nikaido, 1994, 2003; Trombetta et al., 2005). Therefore, it is difficult to predict the susceptibility of microorganisms to essential oils due to the breadth of genetic variations among species. Antibacterial activities of essential oils against a variety of phytopathogenic bacteria are summarized in Table 1.

Table 1

Antibacterial investigations of essential oils against phytopathogenic bacteria.

Plant species (essential oil) tested	Plant pathogenic bacteria	Remarks	Investigators
1	2	3	4
Curcuma longa	Erwinia carotovora, Pseudomonas solanacearum, Xanthomons citri, Xa. malvacearum	Oil exhibited efficacy to all bacteria at 1:10 dilution than 1:1000 dilution.	Banerjee and Nigam, 1978
Carum copticum	5 Bacteria	Oils exhibited variable degree of efficacy to test bacteria. Dethymolysed oil of Carum copticum was found to be most potent bacteriotoxicant.	Pandey et al., 1981
Origanum vulgare, Satureja hortensis, Thymus vulgaris	Erwinia amylovora	The test bacterium was found to be susceptible toward all the tested oils.	Scortichini and Rossi, 1989
Ocimum basilicum	Pseudomonas putida	Resistant to the bacteria.	Lachowicz et al., 1998
Artemisia afra, Pteronia incana, Rosmarinus officinalis	Erwinia carotovora, Erw. chrysanthemi	All three oils showed variable range of zone of inhibition.	Mangena and Muyima, 1999
Thymbra spicata	6 Bacteria	The oil showed variable MIC and MBC values against all test bacteria in contact and volatile phase.	Basim et al., 2000
Thymus vulgaris, Pelarogonium graveolens	Erwinia carotovora	Thymus vulgaris showed highest zone of inhibition, while Pelarogonium graveolens showed no inhibition against test bacteria.	Dorman and Deans, 2000
Cinnamomum zeylanicum, Cymbopogon citratus	Erwinia amylovora, Erw. herbicola	Cinnamomum zeylanicum showed highest toxicity against both bacteria, while Cymbopogon citratus was found to be least toxic.	Vanneste and Boyd, 2002
Rosa damascena	Xanthomonas vesicatoria XV(88.5, 56, 97.2)	Remarkably inhibited tested strains of Xa. vesicatoria.	Basim and Basim, 2003
Satureja hortensis	Clavibacter michiganensis, Pseudomonas syringae pv. tomato, Xa. campestris subsp. campestris	Oil was effective against all three tested organisms.	Gulluce et al., 2003
Heracleum sphondylium sub sp. ternatum	5 Bacteria	The MIC of oil was least against 3 bacteria (Xa. compestris pv. phaseoli, Xa. compestris, Ps. syringae pv. syringae) while higher against Ps. pv. phasiolicola and Ps. syringae pv. tomato).	Iscan et al., 2003
Thymol oil, Palmerosa oil, Lemongrass oil, Tea tree oil	Ralstonia solanacearum	In all the oil tested, 3 oils remarkably inhibited the growth of the test bacteria except Tea tree oil.	Pradhanang et al., 2003
Rosa damascene, Thymbra spicata	Erwinia amylovora	R. damascene oil was least effective with MBC value 1386.5 μg/m than T. spicata oil (500μg/m).	Basim and Basim, 2004
Coriandrum sativum, Foeniculum vulgare	27 Bacteria	A significant antibacterial activity was observed by agar diffusion method with C. sativum oil whereas a much reduced effect was observed for F. vulgare oil.	Cantore et al., 2004
Coriandrum sativum, Foeniculum vulgare, Cuminum cyminum, Carum carvi	29 Bacteria	A significant antibacterial activity was observed against Gram+ and Gram –ve bacteria. A much reduced effect was observed for the wild fennel.	Iacobellis et al., 2004
Cuminum cyminum, Carum carvi	31 Bacteria	The activity was particularly high against the genera Clavibacter, Curtobacterium, Rhodococcus, Erwinia, Xanthomonas, Ralstonia and Agrobacterium while lower activity was observed against Pseudomonas sp.	Iacobellis et al., 2005
Thymol, Palmarosa oil	Ralstonia solanacearum	Both Thymol and Palmarosa oil during soil treatment reduced bacterial wilt significantly.	Ji et al., 2005
Artemisia absinthium, A. dracunculus, A. santonicum, A. spicigera	16 Bacteria	Among all the 4 oils, A. santonicum and A. spicigera oils showed antibacterial activities over a very wide spectrum, ineffective against Ps. syringae pv. populans.	Kordali et al., 2005
Ocimum gratissimum, Thymus vulgaris, Cymbopogon citratus, Zingiber officinale, Monodora myristica	5 Bacteria	The essential oils from Ocimum gratissimum and Thymus vulgaris were highly effective against all bacteria tested, Cymbopogon citratus and Zingiber officinale were moderate effective while Monodora myristica was least effective.	Nguefack et al., 2005
Artemisia turanica	Agrobacterium tumifacience	The oil did not exhibit any antibacterial effect on test strains.	Behravan et al., 2006
Thymbra spicata, Thymus kotschyanus, Origanum onites, Satureja hortensis	Clavibacter michiganensis sub. sp. michiganensis, Pseudomonas syringae pv. tomato, Xanthomonas campestris pv. malvacearum	All the essential oils exhibited antibacterial activity against all pathogens except Xa. campestris pv. malvacearum. Gram+ve bacteria were more sensitive than Gram –ve bacteria.	Kizil and Uyar, 2006
Ziziphora persica	5 Bacteria	The oil showed variable range of zone of inhibition against Ps. syringae and Erw. caratovora while ineffective against other three bacteria.	Ozturk and Ercisli, 2006
Origanum vulgare, Carum carvi	6 Bacteria	Origanum vulgare exhibited highest effectiveness against all tested bacteria, Xa. vesicatoria 67 was the most sensitive to the essential oil extracted from Carum carvi.	Vasinauskiene et al., 2006
Rosa brunonii	Xanthomonas campestris	20% dilution of essential oil was found to be most effective.	Jangwan et al., 2007
24 Plants	Xanthomonas axonopodis pv. vesicatoria	Of 24 plant samples, 7 essential oils were highly active showing inhibition zone in range of 22–46.3mm and MIC of 25–200μl/ml in range.	Kotan et al., 2007
Russowia sogdiana	Erwinia carotovora var. carotovora, Pseudomonas lachrymans, Xanthomonas vesicatoria, Agrobacterium tumifacience	The oil exhibited a broad spectrum of antibacterial activity against all test bacteria with MIC values ranging from 0.2 mg/ml to 0.8 mg/ml.	Tan et al., 2007
Satureja thymbra	Pseudomonas putida	The treatment resulted in log reduction at the level below the detection limit formed for either 60 or 18 min.	Chorianopoulos et al., 2008
Mentha arvensis, Citrus limonum, Tagetes bipinata, Lavandula latifolia	Clavibacter michiganensis sub. sp. sepedonicus (cms), C. michiganensis sub. sp. insidiosus (cmi)	Mentha arvensis and Citrus limonum were effective against cms and Tagetes bipinata and Lavandula latifolia were effective against cmi.	Pouvova et al., 2008
13 Plants	Agrobacterium tumefaciens, Erwinia carotovora var. carotovora	All the oils exhibited variable degree of toxicity toward both bacteria.	Saad et al., 2008
Teucrium montanum	5 Bacteria	Oil exhibited toxicity. The diameters of zone of inhibition ranged from (10–18 mm) with the highest zone of inhibition were observed against Azotobacter chlorococcum. Agrobacterium tumifaciens and Erwinia carotovora showed higher level of resistant.	Vukovic et al., 2008
Satureja hortensis	Erwinia amylovora	The MIC and MBC of oil against Erw. amylovora were found to be 0.025 and 0.05 μl/ml respectively.	Mihajilov-Krstev et al., 2009
Metasequoia glyptostroboides	Xanthomonas campestris pv. campestris KC94-17, Xa. campestris pv. vesicatoria YK93-4, Xa. oryzae pv. oryzae KX019, Xanthomonas sp. SK12	The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values of oil and the extract were ranged from 125 to 250 μl/ml and 125–500 μg/ml and 250–1000 μl/ml and 250–2000 μg/ml respectively.	Bajpai et al., 2010a
Cleistocalyx operculatus	Xanthomonas campestris pv. campestris KC94-17, Xa. campestris pv. vesicatoria YK93-4, Xa. oryzae pv. oryzae KX019, Xanthomonas sp. SK12	The MIC and MBC values of the oil and extract against the tested Xanthomonas spp. ranged from 31.25 to 125 μl/ml and 62.5 to 250 μg/ml respectively.	Bajpai et al., 2010b
Satureja hortensis, Thymus vulgaris	Erwinia amylovora	20 μl dose of both oil exhibited 25mm zone of inhibition.	Karami-Osboo et al., 2010
Origanum sp., Thymus sp., Mellisa sp., Mentha sp. and Nepeta sp	Erwinia amylovora, Pseudomonas syringae pv. syringae	1 μl dose of all the oils exhibited maximum efficacy and Origanum sp. oil was found to be more potent.	Kokoskova et al., 2011
Chenopodium ambrosioides	Erwinia herbicola, P. putida	The MIC and MBC of the oil were 0.25 and 2.0 μl/ml for Erw. herbicola, and 0.12 and 1.0 μl/ml for Ps. putida, respectively.	Pandey et al., 2012
Satureja hortensis	Clavibacter michiganensis ssp. michiganensis, Erwinia amylovora, Erw. carotovora subsp. atroceptica, Erw. chrysanthemi, Pseudomonas cichorii, Ps. syringae pv. syringae, Ps. syringae pv. tomato, Xanthomonas axanopodis pv. malvacearum, Xa. axanopodis pv. vesicatoria, Xa. axanopodis pv. campestris, Xa. campestris pv. raphani, Xa. axanopodis pv. vitians, Xa. campestris pv. zinnia, Xa. axanopodis pv. pelargonii	12.5 μl dose of essential oil gave maximum zone of inhibition against Xanthomonas axanopodis pv. vitians (50 mm) followed by Xanthomonas axanopodis pv. campestris (45 mm), Xanthomonas axanopodis pv. pelargonii, Xanthomonas axanopodis pv. malvacearum (42 mm), Erwinia carotovora subsp. atroceptica (41mm) while zone of inhibition was <40 mm for other bacterial species.	Kotan et al., 2013
Eupatorium adenophorum	Erw. herbicola Ps. putida	The MIC and MBC values were ranges of 0.25–4.0 μl/ml for the both bacterial species.	Pandey et al., 2014b
Nepeta hindostana	Erw. herbicola Ps. putida	The MIC and MBC values were 2 and 8 μl/ml for Erw. herbicola and 4 and >16 for Ps. putida.	Pandey et al., 2015
Origanum rotundifolium	20 plant pathogenic bacteria	The essential oil exhibits significant antibacterial effect against the test bacteria.	Gormez et al., 2016

Potency of essential oils against storage fungi

Fungi can act as major destroyers of food commodities, including cereals, pulses, fruits, and vegetables, through the production of mycotoxins and render food unhealthy for human consumption by adversely affecting their nutritional value (Paranagama et al., 2003; Pandey et al., 2016). During storage, spoilage of stored food commodities is a chronic problem in tropical hot and humid climates. According to the FAO, foodborne fungal pathogens and their toxic metabolites can produce qualitative and quantitative losses of up to 25% of total agricultural food commodities throughout the world (Agrios, 2005). Fungal infection in food commodities results in a reduction of food quality, color, and texture as well as a reduction in nutrients present and physiological properties of food commodities (Dhingra et al., 2001). During infection, fungi can also produce mycotoxins, which can lead to famines in developing countries (Wagacha and Muthomi, 2008). With regard to molds, food contamination by Alternaria, Aspergillus, Penicillium, Fusarium, and Rhizopus spp. is of great significance because of the related health hazards and foodborne infections (Pandey and Tripathi, 2011). Hence, during storage and transit, prevention of fungal growth by essential oils could be a cost-effective approach to combat food losses. In recent years, throughout the world, the antifungal potential of essential oils is being considered significantly important (Baruah et al., 1996; Arras and Usai, 2001; Lalitha and Raveesha, 2006; Bosquez-Molina et al., 2010). The antifungal activities of essential oils are related to the associated disintegration of fungal hyphae due to the mono- and sesquiterpene compounds present in the essential oils. Moreover, essential oils amplify membrane permeability; as such compounds can dissolve in cell membranes and cause membrane swelling, thereby reducing membrane function (Dorman and Deans, 2000). Additionally, the lipophilic property of essential oils is responsible for their antifungal activity as that property gives them the ability to penetrate cell walls and affect enzymes involved in cell-wall synthesis, thus altering the morphological characteristics of the fungi (Cox et al., 2000). The present account summarizes the investigations into essential oils tested for their antifungal activity against fungi affecting food storage (Table 2).

Table 2

Antifungal investigations of essential oils against fungi infecting food commodities during postharvest.

Plant species (essential oil) tested	Postharvest fungi	Remarks	Investigators
1	2	3	4
Raphanus sativus	Alternaria brassicae, Fusarium avenaceum, Phoma spp.	The oil was active at 1:250 to 1:1000000 dilutions.	Nehrash, 1961
Juniperus communis	Aspergillus niger	Exhibited toxicity.	Slavenas and Razinskaite, 1962
Eugenia bracteata, E. heyneana	Cephalosporium sacchri, Curvularia lunata, Fusarium moniliforme	Both the oils were toxic toward test fungi.	Rao and Joseph, 1971
Cymbopogon citratus, Mentha arvensis, Sweet basil	Penicillium italicum	Mentha oil was most toxic during both in vitro and in vivo testing.	Arora and Pandey, 1977
Curcuma aromatica, C. caesia, Myristica fragrans	15 Storage fungi	Myristica fragrans was most toxic.	Kher and Chaurasia, 1978
Trachyspermum ammi, Oenanthera stalonifera, Anethum graveolens, Apium graveolens, Parthenium histerophorus and Psoralea corylifolia	16 Fungal species	The essential oils exhibited significant antimycotic activity.	Sharma and Singh, 1979
Cestrum diurnum	39 Storage fungi	The oil inhibited mycelial growth of all test fungi at 0.7% concentration.	Renu et al., 1980
Ageratum conyzoides	Colletotricum capsici, Penicillium italicum	The oil was found to be toxic at its MIC of 0.5% and 0.2% against C. capsici and P. italicum respectively.	Chandra and Dixit, 1981
Ageratum conyzoides, Cymbopogon martini var. motia, Eupatorium capillifolium, Ocimum adscendens,	Helminthosporium oryzae	The oil of Ocimum adscendens was found most effective at 200 μl/l conc.	Asthana et al., 1982
Citrus medica, Ocimum canum	Aspergillus flavus, A. versicolor	Oils completely inhibited the mycelial growth of test fungi at 2000 ppm.	Dubey et al., 1983
Alpinia galanga	24 Storage fungi	The oil showed broad antifungal spectrum at 0.4% and 0.6% conc.	Tripathi et al., 1983
Lemon grass, Mentha sp., Palmarosa, Zingiber sp.	Aspergillus parasiticus	Mentha oil was most potent control the growth of A. parasiticus and aflatoxin production.	Kala et al., 1984
Citrus aurantifolia	20 Storage fungi	Fungitoxic at 2000 ppm.	Upadhyay et al., 1985
Ocimum adscendens	30 Storage fungi	Fungicidal at 400 ppm.	Asthana et al., 1986
Anisomeles ovata	Aspergillus flavus	Effective at 2000 ppm.	Upadhyay et al., 1987
Eucalyptus sp.	Aspergillus niger	Checked mycelial growth at 1000 ppm.	Tiwari et al., 1988
Thyme, Cumin, Clove, Caraway, Rosemary, Sage	Aspergillus parasiticus	All the oils exhibited broad range of fungitoxicity.	Farag et al., 1989
Amomum subulatum	A. flavus	Fungitoxic at 3000 ppm.	Mishra and Dubey, 1990
Daucus carota	A. flavus	Exhibited antifungal activity at 2000 ppm.	Dwivedi et al., 1991
Cinnamomum camphora	A. flavus	Fungitoxic at 400 ppm.	Mishra et al., 1991
Ocimum gratissimum	A. flavus, A. niger	MIC of oil was 100 ppm against A. flavus and 900 ppm against A. niger.	Dixit and Shukla, 1992
Hyptis suaveolens	21 Storage fungi	Exhibited mycelial inhibition at 2000 ppm.	Singh et al., 1992
Mixture of Apium graveolens and Cuminum cyminum	29 Storage fungi	Mixture showed antifungal activity at the conc. of 1:1 ratio.	Mishra et al., 1993
14 Plant essential oils	47 Storage fungi	Cymbopogon citratus oil was found to be fungistatic in nature at 1500ppm concentration against all test fungi.	Mishra and Dubey, 1994
Cinnamomum zeylanicum	35 Storage fungi	Showed strongest activity at 400 ppm.	Tiwari et al., 1994
Nardostachys jatmansi	A. flavus, A. niger, F. oxysporum	Completely inhibited mycelial growth of all test fungi at 1.0 × 1000 μl/l.	Mishra et al., 1995
Cymbopogon martini, Eucalyptus citriodora, Cinnamomum tamala, Mentha piperita	F. moniliforme	Mycotoxic activity of oils increased with increase concentration of oil.	Baruah et al., 1996
Monarda citriodora, Melaleuca alternifolia	15 Post-harvest fungi	Exhibited absolute toxicity.	Bishop and Thornton, 1997
Thymus vulgaris	Botrytis cinerea, Rhizopus stolonifer	Absolutely inhibited the mycelial growth of test fungi.	Reddy et al., 1998
Cedrus deodara, Tracchispermum ammi	A. niger, Curvularia ovoidea	The MIC of oils was found to be 1000 and 500 ppm respectively.	Singh and Tripathi, 1999
Ocimum gratissimum, Zingiber cassumunar and Cymbopogon citratus	A. flavus	Oils exhibited antimycotic activity at its MIC ranging from 500 to 1300 ppm.	Dubey et al., 2000
Thymus capitatus	Alternaria citri, Botrytis cinerea, Penicillium digitatum, P. italicum	Oil was effective against all test fungi at 250 ppm conc.	Arras and Usai, 2001
Caesulia axillaris	A. flavus, A. niger	MIC of oil against test fungi was 1000 ppm.	Dubey et al., 2002
Putranjiva roxburghii	15 Storage fungi	The MIC and MFC of the oil were 400 and 600 ppm respectively.	Kumar and Tripathi, 2002
Cinnamon and Clove oil	C. musae, F. proliferatum, Lasiodiplodia theobromae	The oils were effective at 500 ppm concentration.	Ranasinghe et al., 2002
Acorus calamus, Hedychium spicatum	Helminthosporium oryzae, F. moniliforme	The oil inhibited growth of test fungi at 0.5 × 103ml/l and 1.0 × 103ml/l respectively.	Mishra et al., 2003
Cymbopogon citratus	A. flavus	Oil was fungistatic and fungicidal nature at 0.60 and 1.0mg/ml concentration respectively.	Paranagama et al., 2003
Cymbopogon flexuosus	25 Fungi	Oil showed absolute toxicity against all fungi at 0.3 μl/ml conc.	Shahi et al., 2003
Curcuma longa	10 Storage fungi	The oil exhibited 10% toxicity at 3000 ppm.	Singh et al., 2003
Chrysanthemum viscidehirtum	Botrytis cinerea, Phytophthora citrophthora	Chrysanthemum viscidehirtum exhibited strong activity at 150 ppm.	Chebli et al., 2004
Mentha piperita	P. digitatum	M. piperita caused 100% inhibition at 1000 μg/ml conc.	Dhaliwal et al., 2004
9 Plant essential oils	P. expansum	All oils were found to be effective.	Neri et al., 2005
Thymus vulgaris and T. copticum	Alternaria citri, Penicillium italicum, P. digitatum	Out of 5 oils, Thymus vulgaris and T. copticum were absolutely fungitoxic at 500 mg/l conc.	Azizi et al., 2006
15 Plant essential oils	10 Fungi	13 essential oils were found to be effective inhibited mycelial growth of all test fungi at 3.0% (v/v).	Lalitha and Raveesha, 2006
Epicarp of Citrus sinensis	10 Post-harvest fungi	Oil exhibited absolute toxicity toward test fungi.	Sharma and Tripathi, 2006
Mentha arvensis	9 Post-harvest fungi	Oil exhibited absolute mycelial inhibition against Aspergillus flavus, A. fumigatus, Helminthosporium oryzae and Sclerotium rolfsii at 0.10 mg/ml.	Kumar et al., 2007
Satureja hortensis	A. flavus	Oil exhibited toxicity.	Dikbas et al., 2008
Lippia scaberrima	Botryosphaeria parva, C. gloeosporioides	Oil was found to be effective, absolutely inhibited the mycelial growth of test fungi.	Regnier et al., 2008
Mentha arvensis	A. flavus	The MIC of Mentha oil against A. flavus was recorded at 400μl/l and it exhibited broad fungitoxic activity against other 14 storage fungi.	Kumar et al., 2009
Satureja hortensis, Thymus vulgaris	Botrytis cinerea	Oil was found to be toxic and significantly inhibited the growth of test fungi	Abdollahi et al., 2010
Thyme and Mexican lime	C. gloeosporioides, Rhizopus stolonifer	0.060% concentration of thyme oil stopped the mycelial growth of both test fungi	Bosquez-Molina et al., 2010
Cinnamon oil	C. musae	0.4% concentration of oil suppressed mycelial growth	Maqbool et al., 2010
Mentha arvensis, Ocimum canum	A. flavus, A. ochraceus, A. niger, A. terreus	Both the oils exhibited significant growth of all the test fungi at 500 ppm concentration.	Pandey and Tripathi, 2011
Eucalyptus, Clove, Cinnamon, Nutmeg, Neem, Nirgudi, Karanj, Sesame	A. flavus, A. niger, A. terreus, A. oryzae, A. fumigatus, Fusarium moniliforme, F. solani and Penicillium sp.	At 50 μl concentration of Eucalyptus, Clove, Cinnamon, Nutmeg oils were more potent against these fungi and among these cinnamon was most potent where zone of inhibition observed was in range of 22.5–67.5 mm.	Shirurkar and Wahegaonkar, 2012
Clausena pentaphylla	A. flavus, A. ochraceus, A. niger, A. terreus	Oil exhibited absolute mycelial inhibition at 0.36 μl/ml and MIC and MFC values were 0.07 μl/ml against all the test fungi	Pandey et al., 2013a
Chenopodium ambrosioides	A. flavus, A. ochraceus, A. niger, A. terreus	Absolute mycelia inhibition for all the test fungi was found at 0.36 μl/ml	Pandey et al., 2013b
Cymbopogon citratus	A. flavus, A. niger, A. ochraceus	A 0.33 μl/ml dose of the oil caused 100% mycelial inhibition and MIC was reported to be 0.29 μl/ml against all the test fungi	Sonker et al., 2014
Artemisia nilagirica	A. flavus, A. niger, A. ochraceus	Oil showed absolute mycelial inhibition of all the test fungi at 0.33 μl/ml, and MIC and MFC reported were 0.29 and 0.58 μl/ml, respectively for all Aspergillus species.	Sonker et al., 2015
Lippia alba	A. flavus	Absolute mycelial inhibition was observed at 0.28 μl/ml. Oil was fungicidal (MIC) at 0.14 μl/ml, and fungistatic at 0.28 μl/ml.	Pandey et al., 2016

Potency of essential oils in food preservation

Research into the utility of essential oils in the preservation of food commodities in order to enhance shelf-life has been successfully carried out in recent years. Various investigators have used essential oils, either in pure or formulation forms, to enhance the shelf-life of food commodities in different storage containers such as those made of cardboard, tin, glass, polyethylene, or natural fabrics and have observed significant enhancement of shelf-life (Tripathi and Kumar, 2007; Pandey et al., 2014a). An earlier study reported that some essential oil constituents such as citral, citronella, citronellol, eugenol, farnesol, and nerol could protect chili seeds and fruits from fungal infection for up to 6 months (Tripathi et al., 1984). Essential oil from Ageratum conyzoides successfully controlled rotting of mandarins by blue mold and increased mandarin shelf-life by up to 30 days (Dixit et al., 1995). Anthony et al. (2003) investigated essential oils from Cymbopogon nardus, C. flexuosus, and Ocimum basilicum and observed that they could significantly control anthracnose in banana and increased banana shelf-life by up to 21 days. Cymbopogon flexuosus essential oil (20 μL/mL) is capable of protecting against rotting of Malus pumilo fruits for up to 3 weeks (Shahi et al., 2003). An fumigant application of essential oils from Putranjiva roxburghii was effective against A. flavus and A. niger infecting groundnuts during storage and enhanced the shelf-life of groundnut from fungal biodeterioration for up to 6 months (Tripathi and Kumar, 2007). The use of Cymbopogon pendulous essential oil as a fumigant increased groundnut shelf-life by 6–12 months (Shukla, 2009), thus proving to be more effective than P. roxburghii essential oil. These differences in efficacy of essential oils may be related to the use of oils from different plant species, as well as to their chemical composition, dose level, and storage container type.

Thyme (Thymus capitata) (0.1%) and maxican lime (Citrus aurantifolia) (0.5%) oil reduced disease incidence in papaya fruit (Bosquez-Molina et al., 2010), while cinnamon (0.3%) oil extended the storage life of banana by up to 28 days and reduced fungal disease incidence in banana (Maqbool et al., 2010). Seed dressing and fumigation of Ocimum cannum oil (1 μL/mL) enhanced the self-life of Bhuchanania (Singh et al., 2011). Clausena pentaphylla and Chenopodium ambrosioides oils, when used as fumigants in glass containers and natural fabric bags were able to protect pigeon pea seeds from A. flavus, A. niger, A. ochraceus, and A. terreus infection for up to 6 months (Pandey et al., 2013a,b). Powder-based formulations of C. pentaphylla and C. ambrosioides oils were also able to preserve pigeon pea seeds for up to 6 months (Pandey et al., 2014c). Artemisia nilagirica oil as a fumigant in cardboard improved the shelf-life of table grapes by up to 9 days (Sonker et al., 2015). Similarly, Lippia alba oil when used as an air dosage treatment in glass containers inhibited fungal proliferation and aflatoxin production in green gram (Vigna radiata) and enhanced its shelf-life by up to 6 months (Pandey et al., 2016).

Conclusion and future prospects

Worldwide investigations carried out on essential oils have motivated researchers to focus their interest toward the study of botanical antimicrobials. It is apparent that the use of essential oils and their derivatives has been widely described, and essential oils have been used against a wide range of pathogens. Accordingly, this review provides a brief overview of essential oils, their active constituents, and their potential as sources of antibacterials, antifungals, and food preservatives. The relevant literature summary shows that essential oils exhibit a diverse range of antimicrobial properties, and indicates their natural sustainability when used as potential biocontrol agents against fungal and bacterial pathogens. Hence, we conclude from this review that essential oils are potential sources of biocontrol products that should be further explored due to their potential to protect food commodities. Also, an essential oil-based fumigant having antimicrobial activity should have a promising GRAS status in mammalian systems. The LD50 values of some botanicals like azadirachtin and carvone are found to be high in rat and are reportedly nontoxic for human consumers. Additionally, several essential oils and their constituents (e.g., carvone, carvacrol, cinnamaldehyde, thymol, linalool, citral, limonene, eugenol, limonene, and menthol) are reported by the United States Food and Drug Administration to have a GRAS status and are approved as flavor or food additives.

Essential oil applications are evolving as a means of integrating pathogens into food containers; for example, fumigants that can be useful in natural fabric and cardboard containers, and even containers made of wooden boards. Some oils can be used as light sprays and integrated as a fumigant into the commodity itself. Many essential oils and their active constituents are active against bacteria and fungi, and they can be produced from commonly available raw materials; perhaps in many cases right at the site of use so as to be rather low-cost treatments. Based on this review, it can be summarized that it is possible to develop techniques for food commodity protection without the use, or with reduced use, of commercial bactericides and fungicides. Although the available literature indicates that essential oils are host specific, biodegradable, have limited effect on non-target organisms, have low levels of mammalian toxicity. There, sustainable and commercial uses have some drawbacks, such as their cost effectiveness. Regardless, there are innumerable potential uses of essential oils and more research is needed to meet the needs of a food industry shifting toward the use of green technology.

Author contributions

AP, PS, and NT conceived and designed the experiments. AP performed the experiments. AP and PK write the manuscript and PK and VB did the editing. All the authors read and approved the final manuscript.

Conflict of interest statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.