INTRODUCTION
Many food products are perishable by nature and require protection from spoilage during their preparation, storage and distribution to give them desired shelf-life. Because food products are now often sold in areas of the world far distant from their production sites, the need for extended safe shelf-life for these products has also expanded. The development of food preservation processes has been driven by the need to extend the shelf-life of foods. Food preservation is a continuous fight against microorganisms spoiling the food or making it unsafe.
Several food preservation systems such as heating, refrigeration and addition of antimicrobial compounds can be used to reduce the risk of outbreaks of food poisoning; however, these techniques frequently have associated adverse changes in organoleptic characteristics and loss of nutrients. Within the disposable arsenal of preservation techniques, the food industry investigates more and more the replacement of traditional food preservation techniques by new preservation techniques due to the increased consumer demand for tasty, Received: 13 June, 2007.
Accepted: 18 July, 2007. nutritious, natural and easy-to-handle food products. Improvements in the cold distribution chain have made international trade of perishable foods possible, but refrigeration alone cannot assure the quality and safety of all perishable foods. The most common classical preservative agents are the weak organic acids, for example acetic, lactic, benzoic and sorbic acid. These molecules inhibit the outgrowth of both bacterial and fungal cells and sorbic acid is also reported to inhibit the germination and outgrowth of bacterial spores.
In the production of food it is crucial that proper measures are taken to ensure the safety and stability of the product during its whole shelf-life. In particular, modern consumer trends and food legislation have made the successful attainment of this objective much more of a challenge to the food industry. Firstly, consumers require more high quality, preservative-free, safe but mildly processed foods with extended shelf-life. For example, this may mean that foods have to be preserved at higher pH values and have to be treated at mild-pasteurization rather than sterilization temperatures.
As acidity and sterilization treatments are two Invited Review Food 1(2), 111-136 ©2007 Global Science Books crucial factors in the control of outgrowth of pathogenic spore-forming bacteria, such as Clostridium botulinum, addressing this consumer need calls for innovative approaches to ensure preservation of products. Secondly, legislation has restricted the use and permitted levels of some currently accepted preservatives in different foods. This has created problems for the industry because the susceptibility of some microorganisms to most currently used preservatives is falling.
An increasing number of consumers prefer minimally processed foods, prepared without chemical preservatives. Many of these ready-to-eat and novel food types represent new food systems with respect to health risks and spoilage association. Against this background, and relying on improved understanding and knowledge of the complexity of microbial interactions, recent approaches are increasingly directed towards possibilities offered by biological presservation.
Throughout the development of both Western and Eastern civilization, plants, plant parts, and derived oils and extracts have functioned as sources of food and medicine, symbolic articles in religious and social ceremonies, and remedies to modify behavior. Taste and aroma not only determine what we eat but often allow us to evaluate the quality of food and, in some cases, identify unwanted contaminants. The principle of self-limitation taken together with the long history of use of natural flavor complexes in food argues that these substances are safe under intended conditions of use.
Originally added to change or improve taste, spices and herbs can also enhance shelf-life because of their antimicrobial nature. Some of these same substances are also known to contribute to the self-defense of plants against infectious organisms (Kim et al. 2001). In spite of modern improvements in food production techniques, food safety is an increasingly important public health issue (WHO 2002a). It has been estimated that as many as 30% of people in industrialized countries suffer from a food borne disease each year and in 2000 at least two million people died from diarrhoeal disease worldwide (WHO 2002a).
There is therefore still a need for new methods of reducing or eliminating food borne pathogens, possibly in combination with existing methods. At the same time, Western society appears to be experiencing a trend of ‘green’ consumerism (Smid and Gorris 1999), desiring fewer synthetic food additives and products with a smaller impact on the environment. Furthermore, the World Health Organization has already called for a worldwide reduction in the consumption of salt in order to reduce the incidence of cardio-vascular disease (WHO 2002b).
If the level of salt in processed foods is reduced, it is possible that other additives will be needed to maintain the safety of foods. There is therefore scope for new methods of making food safe which have a natural or ‘green’ image. One such possibility is the use of essential oils (EOs) as antibacterial additives. Based on rich histories of use of selected plants and plant products that strongly impact the senses, it is not unexpected that society would bestow powers to heal, cure diseases, and spur desirable emotions, in the effort to improve the human condition.
The perception that these products are “natural” and have a long history of use has, in part, mitigated the public’s need to know whether these products work or are safe under conditions of intended use. Until recently, EOs have been studied most from the viewpoint of their flavor and fragrance only for flavoring foods, drinks and other goods. Actually, however, EOs and their components are gaining increasing interest because of their relatively safe status, their wide acceptance by consumers, and their exploitation for potential multi-purpose functional use (Ormancey 2001).
It has long been recognized that some EOs have antimicrobial properties (Boyle 1955) and these have been reviewed in the past (Shelef 1983; Nychas 1995) as have the antimicrobial properties of spices (Shelef 1983) but the relatively recent enhancement of interest in ‘green’ consumerism has lead to a renewal of 112 scientific interest in these substances (Tuley 1996).
Besides antibacterial properties (Mourey and Canillac 2002; Rasooli and Razzaghi 2004; Rasooli and Owlia 2005), EOs or their components have been shown to exhibit antiviral (Bishop 1995), antimycotic (Mari et al. 003), anti oxidative (Gachkar et al. 2006; Yadegarinia et al. 2006; Bektas et al. 2007a; Bektas et al. 2007b), antitoxigenic (Akgul et al. 1991; Juglal et al. 2002; Ultee and Smid 2001), antiparasitic (Pandey et al. 2000; Pessoa et al. 2002), and insecticidal (Karpouhtsis et al. 1998) properties. These characteristics are possibly related to the function of these compounds in plants (Mahmoud and Croteau 2002).
The antibacterial properties of EOs and their components are exploited in such diverse commercial products as dental root canal sealers (Manabe et al. 987), antiseptics (Cox et al. 2000) and feed supplements for lactating sows and weaned piglets (van Krimpen and Binnendijk 2001; Ilsley et al. 2002). It is therefore scientifically sound to evaluate the impact of EOs on food and food products safety. Natural flavor complexes (NFCs) are mixtures of mainly low molecular weight chemical substances separated from plants by physical means such as distillation, extraction, and cold pressing. The most common NFCs are EOs. The EO is typically obtained by steam distillation of the plant or plant parts.
With few exceptions, plants are dependent on their EO content for their unique aroma and gustatory profile. In other words, the volatile constituents of the plant isolated in the EO are primarily responsible for aroma and taste of the plant. For economic reasons, crude EOs are often produced via distillation at the source of the plant raw material and subsequently further processed at modern flavor facilities. EOs, also called volatile or ethereal oils, are aromatic oily liquids obtained from plant flowers, buds, seeds, leaves, twigs, bark, herbs, wood, fruits and roots.
They can be obtained by expression, fermentation, effleurage or extraction but the method of steam distillation is most commonly used for commercial production of EOs (van de Braak and Leijten 1999). The term ‘essential oil’ is thought to derive from the name coined in the 16th century by the Swiss reformer of medicine, Paracelsus von Hohenheim; he named the effective component of a drug Quinta essentia (Guenther 1948). An estimated 3000 EOs are known, of which about 300 are commercially important – destined chiefly for the flavors and fragrances market (van de Braak and Leijten 1999).
Distillation as a method of producing EOs was first used in the East (Egypt, India and Iran) (Guenther 1948) more than 2000 years ago and was improved in the 9th century by the Arabs. By the 13th century EOs were being made by pharmacies and their pharmacological effects were described in pharmacopoeias (Bauer et al. 2001). The greatest use of EOs in the European Union (EU) is in food (as flavorings), perfumes (fragrances and aftershaves) and pharmaceuticals (for their functional properties) (van de Braak and Leijten 1999). EOs and fractions are also formulated in shampoos, toothpaste, disinfectants, topical ointments and cosmetics.
However, when used in foods, highly volatile plant EOs are sometimes lost during processing operations. Microencapsulation technology is one way these losses of EOs by volatilization can be prevented. This technique is being widely used in the pharmaceutical industry for controlled delivery of drugs.
It is also currently used in the food industry for flavor stabilization. By encapsulating antimicrobial EOs, not only can they be protected from heat, but they also can be released in products at a controlled rate to deliver effective inhibitory concentrations over extended periods and thereby extend shelf-life. This review presents the current understanding of the mode of action of these compounds and their possible applications in food protection. Food biopreservation.
INFLUENCE OF CHEMICAL COMPOSITION OF ESSENTIAL OILS ON THEIR ANTIMICROBIAL ACTIVITIES
Due to their natural origin, environmental and genetic factors will influence the chemical composition of the plant EOs. Factors such as species and subspecies, geographical location, harvest time, plant part used and method of isolation all affect chemical composition of the crude material separated from the plant.
Steam distillation is the most commonly used method for producing EOs on a commercial basis. Extraction by means of liquid carbon dioxide under low temperature and high pressure produces a more natural organoleptic profile but is much more expensive (Moyler 1998). The difference in organoleptic profile indicates a difference in the composition of oils obtained by solvent extraction as opposed to distillation and this may also influence antimicrobial properties.
This would appear to be confirmed by the fact that herb EOs extracted by hexane have been shown to exhibit greater antimicrobial activity than the corresponding steam distilled EOs (Packiyasothy and Kyle 2002). EOs are volatile and therefore need to be stored in airtight containers in the dark in order to prevent compositional changes. The composition of EOs from a particular species of plant can differ between harvesting seasons and between geographical sources (Juliano et al. 2000; Faleiro et al. 2002).
It was postulated that individual components of EOs exhibit different degrees of activity against gram-positives and gram-negatives (Dorman and Deans 2000) and it is known that the chemical composition of EOs from a particular plant species can vary according to the geographical origin and harvesting period (vide supra). It is therefore possible that variation in composition between batches of EOs is sufficient to cause variability in the degree of susceptibility of Gram-negative and Gram-positive bacteria.
The inherent activity of oil can be expected to relate to the chemical configuration of the components, the proportions in which they are present and to interactions between them (Dorman and Deans 2000; Marino et al. 2001; Delaquis et al. 2002). An additive effect is observed when the combined effect is equal to the sum of the individual effects. Some studies have concluded that whole EOs have a greater antibacterial activity than the major components mixed (Gill et al. 2002; Mourey and Canillac 2002), which suggests that the minor components are critical to the activity and may have a synergistic effect or potentiating influence.
The two structurally similar major components of oregano EO, carvacrol and thymol, were found to give an additive effect when tested against S. aureus and P. aeruginosa (Lambert et al 2001). A mixture of cinnamaldehyde and eugenol at 250 and 500 ? g/ml, respectively inhibited growth of Staphylococcus sp. , Micrococcus sp. Bacillus sp. and Enterobacter sp. for more than 30 days completely, whereas the substrates applied individually did not inhibit growth (Moleyar and Narasimham 1992).
The oils with high levels of eugenol (allspice, clove bud and leaf, bay, and cinnamon leaf), cinnamamic aldehyde (cinnamon bark, assia oil) and citral are usually strong antimicrobials (Davidson and Naidu 2000). Activity of sage and rosemary is due to borneol and other phenolics in the terpene fraction.
The volatile terpenes carvacrol, p-cymene and thymol are probably responsible for the antimicrobial activity of oregano, thyme and savory. In sage, the terpene thejone and in rosemary a group of terpenes (borneol, camphor, 1,8 cineole, ? -pinene, camphone, verbenonone and bornyl acetate) is responsible (Davidson and Naidu 2000). Little information is available on interaction among constituents in EOs and the effects they have on antimicrobial activity.
Phenolic components are responsible for antimicrobial action and other constituents are believed to have little activity. Dependability of EOs as antimicrobials could be improved if their content of active agents should be standardized by distillation (Delaquis et al. 2002). As a general 113 observation, spice extracts are less antimicrobial than the whole spice but little quantitative data are available (Shelef 1983). Four studies are relevant. Lachowicz et al. (1998) found crude EO of basil more effective than components linalool and methyl chavicol either separately or together.
Vardar-Unlu et al. (2003) found similar results following fractionation of extracts from thyme. In aqueous extracts from oregano or thyme there was little antimicrobial activity. Thus there appear to be interactive effects among constituents not extractable in the water-soluble phase and these components do not appear to be the phenolics normally considered to show the major antimicrobial activities. In contrast with the above studies, Delaquis et al. (2002) found that individual fractions of cilantro and dill EOs had greater antimicrobial activity than did the whole oil.
In addition, they found cilantro fractions deficient in phenolics but enriched in long chain (C6–C10) alcohol and aldehydes that were particularly active against Gram-positive bacteria including L. monocytogenes. To broaden the antimicrobial spectrum, a fraction from cilantro oil with no activity against Gram-negative bacteria was combined with a eucalyptus fraction having broader activity. Additive or synergistic action was reported against all Gram-positive bacteria plus Yersinia enterocolitica and the mixture was antagonistic to P. fragi, E. coli O157:H7 and Salmonella typhimurium.
Bactericidal effects of cinnamaldehyde and thymol against B. cereus (Demo et al 2001; Kwon et al. 2003), as well as the development of synergistic effects between carvacrol or thymol and nisin have been also reported (Pol and Smid 1999; Periago and Moezelaar 2001; Periago et al. 2001). The effects of various concentrations of borneol, carvacrol, cinnamaldehyde, eugenol, menthol, thymol, and vanillin on the growth kinetics of activated Bacillus cereus INRA L2104 spores inoculated into tyndallized carrot broth were determined.
Five microliters of cinnamaldehyde, 15 ? of carvacrol, or 30 mg of thymol per 100 ml of inoculated carrot broth completely inhibited bacterial growth for more than 60 days at 16°C. Lower concentrations of the three antimicrobials prolonged the lag phase and reduced both the exponential growth rate and the final population densities of cultures. The study of the sensory characteristics of the supplemented broths suggested that low concentration of cinnamaldehyde enhanced the taste of carrot broth, and that it did not have any adverse effect on the taste and smell of carrot broth at concentrations less than 6 ? 100 ml? 1 (Valero and Giner 2006).
The major constituents of the oils of thyme and oregano species have been reported to be thymol, carvacrol and ? terpinene. Thyme EO and its ingredients have been shown to exhibit a range of biological activities. Since EOs of thyme and oregano possess strong antibacterial and antimicrobial activity they can be used to delay or inhibit the growth of pathogenic microorganisms. These activities are mostly attributable to the presence of phenolic compounds such as thymol and carvacrol, and to hydrocarbons like ? erpinene and p-cymene (Dorman and Deans 2000; Lambert et al. 2001; Aligiannis et al. 2001; Vardar-Unlu et al. 2003; Baydar et al. 2004).
Thymol and carvacrol can be used alone or in combination during the treatment of oral infectious diseases because of their inhibitory activity on oral bacteria (Ditry et al. 1994; Kohlert et al. 2002). Thyme and oregano were found to inhibit aflatoxin production (Vaughn et al. 1993). Antispasmodic and antiplatelet aggregation activities were also reported with thyme constituents (Meister et al. 999; Okazaki et al. 2002).
Monoterpenes are natural ten-carbon (C10) compounds constructed from two isoprene molecules (C5H8, or hemiterpene), the five-carbon building-block of all terpenes. They are found in edible, medicinal and aromatic plants and are the main chemical constituents of their EOs. Plant volatile oils as well as their monoterpenoid constituents have been widely used as flavorings additives in foods and beverages, as fragrances in cosmetics, and as intermediates in the manufacture of perfume chemicals. They have also been em-
Food 1(2), 111-136 ©2007 Global Science Books ployed as scent in household products (e. g. , detergents, soaps, room air-fresheners and insect repellents) and as active ingredients in some old drugs (Leung and Foster 2003). Pinene, for instance, is one of the main constituents of a mixture of six monoterpenes used to dissolve gallstones (Ellis et al. 1984), and ? -terpinene is one of the putative active ingredients of tea tree (Melaleuca alternifolia) oil, an antibacterial and antifungal remedy employed in both veterinary and human medicine (Dryden et al. 2004).
IN VITRO ANTIMICROBIAL ACTIVITIES OF ESSENTIAL OILS
A large number of studies have examined the in vitro antimicrobial activity of spices, herbs and naturally occurring compounds from other sources. Plant EOs have been widely tested against both Gram-positive and -negative bacteria. For example, Farag et al. (1989) examined the antimicrobial activity of the oils of sage, thyme and rosemary leaves, caraway fruits, clove flower buds, and cumin fruits against three Gram-negative bacteria (P. fluorescens, E. coli, and Serratia marcescens) and four Gram-positive bacteria (S. aureus, Micrococcus spp. Sarcina spp. , and B. subtilis). They found that the EOs from sage, cumin, rosemary and their principal components had no or very little effect against Gram-negative bacteria, but oil of caraway was moderately effective against this group.
Oils from clove and thyme were highly active at a concentration of 0. 75– 1. 5 mg /ml against S. aureus and Micrococcus spp. , while only small inhibition zones were reported for Gram-negative bacteria. In general, Gram-negative bacteria were more resistant to EOs than Gram-positive bacteria, with the oils being effective even at low concentration (0. 5–12 mg /ml) against the Gram-positive organisms.
In similar work it was also found that mint EO was more effective against Grampositive bacteria than against Gram-negative bacteria (Sivropoulou et al. 1995; Iscan et al. 2002). Delaquis et al. (2002) reported that Gram-positive bacteria were more sensitive to the EOs of dill, cilantro, coriander and eucalyptus than Gram-negative bacteria. It is well established that essential or volatile oils from plant sources have wide spectra of antimicrobial action (Alzoreky and Nakahara 2002; Packiyasothy and Kyle 2002).
The composition, structure as well as functional groups of the oils play an important role in determining their antimicrobial activity. Usually compounds with phenolic groups are most effective (Dorman and Deans 2000). Among these, the oils of clove, oregano, rosemary, thyme, sage and vanillin have been found to be most consistently effective against microorganisms. Most studies investigating the action of whole EOs against food spoilage organisms and food borne pathogens agree that, generally, EOs are generally more inhibitory against Grampositive than against Gram-negative bacteria (Marino et al. 001).
That gram-negative organisms are less susceptible to the action of antibacterials is perhaps to be expected, since they possess an outer membrane surrounding the cell wall (Ratledge and Wilkinson 1988), which restricts diffusion of hydrophobic compounds through its lipopolysaccharide covering (Vaara 1992). While this is true of many EOs, there are some such as oregano, clove, cinnamon and citral; which are effective against both groups (Skandamis et al. 2002).
However, not all studies on EOs have concluded that gram-positives are more susceptible (Wilkinson et al. 2003). There are also some non-phenolic constituents of oils such as allyl isothiocyanate, AIT; which are more effective (Ward et al. 1998) or quite effective against Gram-negative bacteria as in garlic oil (Yin and Cheng 2003). A study testing 50 commercially available EOs against 25 genera found no evidence for a difference in sensitivity between Gram-negative and Gram-positive organisms (Deans and Ritchie 1987).
However, a later study using the same test method and the same bacterial isolates but apparently using freshly distilled EOs, revealed that Gram-positive bacteria were indeed more susceptible to two of the EOs tested and equally sensitive to four other EOs than were Gram-negative species 114 (Dorman and Deans 2000). Of the Gram-negative bacteria, Pseudomonads, and in particular P. aeruginosa, appear to be least sensitive to the action of EOs (Ruberto et al. 2000; Senatore et al. 2000; Tsigarida et al. 2000; Dorman and Deans 2000; Pintore et al. 2002; Wilkinson et al. 2003).
Pseudomonads consistently show high or often the highest resistance to these antimicrobials such as linalool/chavicol (Smith-Palmer et al. 1998), terpenoids/carvacrol/thymol (Griffin et al. 1999), oregano (Skandamis et al. 2002), Capsicum or bell pepper (Careaga et al. 2003) and annatto, (Galindo-Cuspinera et al. 2003). Nonetheless, since pseudomonads are so frequently responsible for spoilage of food stored at low temperatures they have often been used as targets, and at high concentrations some EO components have been reported to be effective (Careaga et al. 2003).
Infections caused by Campylobacter in humans are considered to be the result of ingestion of contaminated foods of animal origin, mainly poultry products and raw milk, or untreated water (Moore et al. 2002; Park 2002). Successful steps to reduce the occurrence of Campylobacter on poultry could have a major effect on reduction of foodborne illness. In a recent study, a proprietary mixture of herbs (Protecta II) at 2% (w/v) was used in poultry chill water and reduced the numbers of both Campylobacter and E. coli by 2 log cfu/ml in carcass rinses (Dickens et al. 2000).
Friedman et al. 2002) evaluated 96 different naturally occurring plant oils and oil compounds against C. jejuni in iron-supplemented brucella agar. The oils of marigold taegetes, ginger root, jasmine, patchouli, and gardenia were most effective with bactericidal activity (BA) assessed as BA50’s (concentration of oil at which a 50% reduction of total cfu was observed) ranging from 0. 003% to 0. 007%. Like plant EOs and oil-derived compounds, garlic-derived organosulphur compounds have also shown antimicrobial activity. When evaluated against C. jejuni in ground beef, diallyl sulphide and diallyl disulphide at 20 ?
M showed a significant reduction with final viable numbers of 1. 63 log cfu /g and 1. 26 log cfu/g, respectively, compared to 7. 54 log cfu/g in untreated controls during 6 d storage at 15°C (Yin and Cheng 2003). Recently there has been significant interest in the development of secondary preservation steps that could reduce L. monocytogenes viability and growth in refrigerated readyto-eat foods (Rocourt et al. 2003). Four recent studies examined the effects of different natural antimicrobials on this organism in broth media.
Of the agents tested isoeugenol was most effective, giving a 4. log reduction of L. monocytogenes numbers at 100 ppm in conjunction with use of freeze thaw cycles at ? 20°C (Cressy et al. 2003). Cilantro oil was more effective than hydroxycinnamic acids, with MICs of cilantro against L. monocytogenes of 0. 02– 0. 07% (v/v) in Brain Heart Infusion (BHI) broth at 24°C (Gill et al. 2002) as compared with MICs of 0. 2–0. 27% (w/v) for 4 hydroxycinnamic acids (Wen et al. 2003). A GRAS carotenoid pigment used in butter and cheese, annatto, was least effective (MIC 1. 25% v/v) against this organism (Galindo-Cuspinera et al. 2003).
Differences in strain susceptibility were evident and cilantro oil was inefective against L. monocytogenes when used on the surface of inoculated ham at a concentration of 6% (v/v) of the enrobing gel (Gill et al. 2002). The antibacterial activity of EOs is influenced by the degree to which oxygen is available. This could be due to the fact that when little oxygen is present, fewer oxidative changes can take place in the EOs and/or that cells obtaining energy via anaerobic metabolism are more sensitive to the toxic action of EOs (Paster et al. 1990).
The antibacterial activity of oregano and thyme EOs was greatly enhanced against S. yphimurium and S. aureus at low oxygen levels (Paster et al. 1990). The use of vacuum packing in combination with oregano EO may have a synergistic effect on the inhibition of L. monocytogenes and spoilage flora on beef fillets; 0. 8% v/w oregano EO achieved a 2–3 log initial reduction in the microbial flora but was found to be even more effective in samples packed under vacuum in low- Food biopreservation. Iraj Rasooli permeability film when compared to aerobically stored samples and samples packaged under vacuum in highly permeable film (Tsigarida et al. 000).
Similarly, the lethal effect of clove and coriander EOs on A. hydrophila on pork loin steak stored at 2 and 10°C was more pronounced in vacuum packed pork than on samples stored in air (Stecchini et al. 1993). Oregano EO was found to delay microbial growth and to suppress final counts of spoilage microorganisms in minced beef under modified atmosphere packaging (MAP, 40% CO2, 30% N2 and 30% O2) when, in contrast, no pronounced inhibition was evident in beef packed under air (Skandamis and Nychas 2001).
Cinnamon oil and clove oil are both natural preservative and flavouring substances that are not harmful when consumed in food products. Soliman and Badeaa (2002) found that ? 500 ppm of cinnamon oil can inhibit A. flavus, A. parasiticus, A. ochraceus and Fusarium moniliforme on potato dextrose agar. Matan et al. (2006) tested mixtures of cinnamon and clove oils for inhibitory activity against important spoilage microorganism of intermediate moisture foods.
Four fungal species (Aspergillus flavus, Penicillium roqueforti, Mucor plumbeus and Eurotium sp. , four yeast species (Debaryomyces hansenii, Pichia membranaefaciens, Zygosaccharomyces rouxii and Candida lipolytica), and two bacteria species (Staphylococcus aureus and Pediococcus halophilus) inoculated separately on agar plates were sealed in a barrier pouch and exposed to EO volatiles under a modified atmosphere of low O2 (; 0. 05–10%) and high CO2 (20% or 40%), with the balance being N2. A. flavus and Eurotium sp. proved to be the most resistant microorganisms. Cinnamon and clove oils added between 1000 and 4000 ? L at a ratio of 1:1 were tested for minimum inhibitory volume (MIV) against molds and yeasts.
The gas phase above 1000 ? L of the oil mixture inhibited growth of C. lipolytica and P. membranaefaciens; 2000 ? L inhibited growth of A. flavus, P. roqueforti, M. plumbeus, Eurotium sp. , D. hansenii, and Z. rouxii, while inhibition of A. flavus required the addition of 4000 ? L. Higher ratios of cinnamon oil/clove oil were more effective for inhibiting the growth of A. flavus. Citrus EOs can have very pronounced antimicrobial activity, even if their complexity and variability make difficult to correlate their action to a specific component, also in relation to possible antagonistic and synergistic effects.
For this reason, Caccioni et al. (1998) proposed a holistic approach to explain the antimicrobial capabilities of such EOs, whose performances could be the result of a certain quantitative balance of various components. Citrus oxygenated monoterpenes are among the molecules with the highest antifungal activity (Caccioni and Guizzardi 1994) and citral was the most active compound against Penicillium digitatum and P. italicum (Caccioni et al. 1995). Origanum vulgare L. , Lamiaceae family, is being used in traditional medicine systems in many countries (Sagdic et al. 2002; Sahin et al. 2004).
Origanum vulgare L. has been known as having many therapeutic properties (diaphoretic, carminative, antispasmodic, antiseptic, tonic) and its antimicrobial activity has currently received a renewed interest. It has been widely used in agricultural, pharmaceutical and cosmetic industries as a culinary herb, flavoring substances in food products, alcoholic beverages and perfumery for its spicy fragrance (Dorman and Deans 2000; Novak et al. 2000; Aligianis et al. 2001). Some res
earchers have found antimicrobial activity in O. vulgare L. (Skandamis et al. 2002; Baydar et al. 2004; Chun et al. 2004; Nostro et al. 004). The oils extracted from plants of the genus Origanum have been shown to have antimicrobial activity in vitro and in food (Aligiannis et al. 2001). Souza et al. (2007) reported the effectiveness of O. vulgare L. EO to inhibit the growth/survival of various food spoiling yeasts. Anti-yeast activity was studied by determining the MIC by solid medium diffusion and microplate bioassay, as well as observing the effect of the EO MIC on the yeast cell viability. O. vulgare EO showed effectiveness to inhibit the growth of all assayed yeasts with MIC values for the most ones of 20 and 0. 6 ?
L/mL when determined, respectively, 115 by solid medium diffusion and microplate bioassay. Solid medium diffusion MIC presented statistically significant inhibitory effects (P ; 0. 05) on yeast cell viability, mainly when interacting with Candida albicans and Candida krusei. On the other hand, the microplate MIC just provided statistically significant inhibitory effects on the cell viability when interacting with C. krusei. It is well established that bacterial biofilms exhibit more resistance to antimicrobial treatments than the individual cells grown in suspension (Knowles and Roller 2001; Chavant et al. 004).
Lebert et al. (2007) investigated bactericide solutions effective on spoilage and pathogenic bacteria while preserving technological bacteria. Two compounds of EO (thymol and eugenol), one EO of Satureja thymbra and two industrial biocides (PE 270–30, Brillo) were tested on technological strains (Staphylococcus equorum, Staphylococcus succinus and Lactobacillus sakei) grown in monoculture biofilm and on a mixed biofilm of pathogenic bacteria (Staphylococcus aureus, Listeria monocytogenes) and spoilage bacteria (Pseudomonas fragi, Escherichia coli).
Biofilm cultures were performed in glass fibre filters for 24 h at 20°C before application of biocides. Thymol and eugenol had no effect on the mixed biofilm. S. thymbra (2%) was highly effective on spoilage strains (5 log reduction), and S. equorum (4 log reduction) was moderately effective on pathogens (2. 3 log reduction) and not effective on S. succinus and L. sakei (0. 5 log reduction). PE-270-30 with 10% Na2SO4 decreased spoilage bacteria (5. 1 log reduction), maintained the technological bacteria, but did not reduce the pathogens. The disinfectant Brillo (3%) killed all the strains.
Their results showed the difficulty in obtaining a biocide that is effective in destroying spoilage and pathogenic bacteria while preserving technological bacteria. Lebert et al. (2007) concluded that EOs could be a good alternative for eradicating spoilage bacteria in the food environment where they are often found at high levels. Antimicrobial packaging is a form of active packaging that could extend the shelf-life of product and provides microbial safety for consumers. It acts to reduce, inhibit, or retard the growth of pathogen microorganisms in packed foods and packaging material.
In order to control undesirable microorganisms on food surfaces: (1) volatile and nonvolatile antimicrobial agents can be incorporated into polymers or (2) coating or adsorbing antimicrobial onto polymer surfaces can be applied (Appendini and Hotchkiss 2002). Several compounds have been proposed for antimicrobial activity in food packaging, including organic acids, enzymes such as lysozyme, and fungicides such as benomyl, imazalil and natural antimicrobial compounds such as spices (Tharanathan 2003). These compounds carry mostly antimicrobial and some antioxidant properties.
Natural compounds, such as nisin and lysozyme, have been studied as potential food preservatives added to the edible films that are safe for human consumption (Padget et al. 2000; Hoffman et al. 2001; Dawson et al. 2002; Cagri et al. 2004; Min et al. 2005). Some spice EOs incorporated into packaging materials can control microbial contamination in beef muscle by reducing the growth of Escherichia coli O157:H7 and Pseudomonas spp. (Oussallah et al. 2004). The use of edible films to release antimicrobial constituents in food packaging is a form of active packaging.
Antimicrobial properties of whey protein isolate (WPI) films containing 1. 0– 4. 0% (wt/vol) ratios of oregano, rosemary and garlic EOs were tested against Escherichia coli O157:H7 (ATCC 35218), Staphylococcus aureus (ATCC 43300), Salmonella enteritidis (ATCC 13076), Listeria monocytogenes (NCTC 2167) and Lactobacillus plantarum (DSM 20174). Ten millilitres of molten hard agar was inoculated by 200 ? l of bacterial cultures (colony count of 1 ? 108 CFU/ml) grown overnight in appropriate medium.
Circular discs of WPI films containing spice extracts, prepared by casting method, were placed on a bacterial lawn. Zones of inhibition were measured after an incubation period. The film containing oregano EO was the most effective against these bacteria at 2% level than those containing garlic and rosemary extracts Food 1(2), 111-136 ©2007 Global Science Books (P ME ; WtF ; HxF according to the hydrogen peroxide-induced luminol chemiluminescence assay, and results were the same with the exception of the rank order of HxF and WtF according to the DPPH free radical-scavenging assay.
Eleven EOs, namely, Cananga odorata (Annonaceae), Cupressus sempervirens (Cupressaceae), Curcuma longa (Zingiberaceae), Cymbopogon citratus (Poaceae), Eucalyptus globulus (Myrtaceae), Pinus radiata (Pinaceae), Piper crassinervium (Piperaceae), Psidium guayava (Myrtaceae), Rosmarinus officinalis (Lamiaceae), Thymus ? citriodorus (Lamiaceae) and Zingiber officinale (Zingiberaceae), were evaluated for their food functional ingredient related properties. These properties were compared to those of Thymus vulgaris EO, used as a reference ingredient.
Antioxidant and radical-scavenging properties were tested by means of 1,1-diphenyl-2-picrylhydrazyl (DPPH) assay, ? -carotene 120 bleaching test and luminol-photochemiluminescence (PCL) assay. In the DPPH assay, C. odorata, C. citratus, R. officinalis and C. longa showed major effectiveness, with a radical inhibition ranging from 59. 6 ± 0. 42–64. 3 ± 0. 45%. In the ? -carotene bleaching test, C. odorata (75. 5 ± 0. 53%), R. officinalis (81. 1 ± 0. 57%) and C. longa (72. 4 ± 0. 51%) gave the best inhibition results. Similar results were obtained for the same EOs in the PCL assay (Gianni et al. 2005).
Total antioxidant activity of water-soluble components in raw spinach was in the order of BI ? BM ? BPG ; BP, whereas free radical-scavenging activity was in the order of BI ; BPG ; BM ; BP (Amin et al. 2006). Kartal and co workers (2007) examined the in vitro antioxidant properties of the EO and various extracts prepared from the herbal parts of Ferula orientalis A. (Apiaceae). The highest 2,2-diphenyl-l-picrylhydrazyl (DPPH) radical-scavenging activity was found in the polar extract, e. g. methanol–water (1:1), obtained from non-deodorised materials with IC50 values at 99. 1 ? g/ml. In the ? carotene/linoleic acid assay, the deodorised acetone extract exhibited stronger activity than the polar one.
The relative antioxidant activities of the extracts ranged from 10. 1% to 76. 1%, respectively. Extraction with methanol–water (1:1) mixture was concluded to be the most appropriate method in terms of higher extract yield, as well as effectiveness, observed in both assays. Although the EO showed antioxidative potential, it was not as strong as that of positive control (BHT). Bektas et al. (2005) compared the antioxidant potentials of two Thymus species on the basis of the chemical compositions of EOs obtained by hydrodistillation.
Using 2,2-diphenyl-1-picrylhydrazyl (DPPH), the free radical scavenging activity of the EO of T. sipyleus subsp. sipyleus var. rosulans was superior to var. sipyleus oil (IC50=220 ± 0. 5 and 2670 ± 0. 5 ? g/ml, respectively). In the case of ? -carotene/linoleic acid assays, oxidation of linoleic acid was effectively inhibited by T. sipyleus subsp. sipyleus var. rosulans (92. 0%), while the var. sipyleus oil had no activity. In the latter case, the linoleic acid inhibition rate of var. rosulans oil is close to the synthetic antioxidant BHT (96. 0%).
It was reported that oxidative stress is associated with the pathogenesis of Alzheimer’s disease (AD) and cellular characteristics of this disease are either causes or effects of oxidative stress (Vina et al. 2004). These evidences clearly show that oxidative stress, an early event in AD, may play a key pathogenic role in the disease (Zhu et al. 2004). Interestingly, intake of polyphenols through diets rich in fruits, vegetables and beverages such as red wine was stated to reduce incidence of certain age-related neurological disorders including macular degeneration and dementia (Commenges et al. 000; Bastianetto and Quirion 2002). Herbs and spices have been used for many centuries to improve the sensory characteristics and to extend the shelflife of foods.
As a result, considerable research has been carried out on the assessment of the antioxidant activity of Food biopreservation. Iraj Rasooli many herbs, spices and their extracts when added in a variety of foods and food model systems. Mate (Ilex paraguariensis) leaves contain many bioactive compounds, such as phenolic acids, which seem to be responsible for the antioxidant activity of green mate infusions, both in vivo and in vitro (Filip et al. 000; Schinella et al. 2000; Bracesco et al. 2003; Markowicz-Bastos et al. 2006).
The antioxidative effect of dietary Oregano EO and ? -tocopheryl acetate supplementation on susceptibility of chicken breast and thigh muscle meat to lipid oxidation during frozen storage at ? 20°C for 9 months was examined. Dietary oregano EO supplementation at the level of 100 mg/kg feed was significantly (P ? 0. 05) more effective in reducing lipid oxidation compared with the level of 50 mg oregano EO k/g feed and control, but less effective (P ? 0. 05) compared with ? -tocopheryl acetate supplementation (Botsoglou et al. 003).
Oregano, a characteristic spice of the Mediterranean cuisine obtained by drying leaves and flowers of Origanum vulgare subsp. hirtum plants, is well known for its antioxidative activity (Economou et al. 1991). Carvacrol and thymol, the two major phenols that constitute about 78-82% of the EO, are principally responsible for this activity (Adam et al. 1998; Yanishlieva 1999). The antioxidant effect of two plant EOs (sage and rosemary EOs) and one synthetic antioxidant (BHT) on refrigerated stored liver pate (4°C/90 days) was evaluated. Pates with no added antioxidants were used as controls.
Plant EOs inhibited oxidative deterioration of liver pates to a higher extent than BHT did (Estevez et al. 2007). Oxidative reactions in foodstuffs are enhanced after cooking and refrigerated storage through the increase of their oxidative instability due to the degradation of natural antioxidants and the release of free fatty acids and iron from the haeme molecule (Kristensen and Purslow 2001; Estevez and Cava 2004). Sage (Salvia officinalis) and rosemary (Rosmarinus officianalis) are popular Labiatae herbs with a verified potent antioxidant activity (Dorman et al. 2003).
The antioxidant activity of sage and rosemary EOs is mainly related to two phenolic diterpenes: carnosic acid and carnosol which are considered two effective free-radical scavengers (Dorman et al. 2003; Ibanez et al. 2003). The antioxidant activity of these molecules has been compared to that from other recognized antioxidant substances, and Richheimer et al. (1999) indicated that the antioxidant potential of the carnosic acid was approximately seven times higher than that of BHT and BHA.
Bektas and co workers (2007a) studied in vitro antioxidant activity of the EO of Clinopodium vulgare by using 2,2-diphenyl-1-picrylhydrazyl (DPPH) and ? carotene-linoleic acid assays. In the first case, IC50 value of the C. vulgare EO was determined as 63. 0 ± 2. 71 ? g/ml. IC50 value of thymol and ? -terpinene, the major compounds of the oil, was determined as 161 ± 1. 3 ? g/ml and 122 ± 2. 5 ? g/ml, respectively, whereas p-cymene did not show antioxidant activity. In ? -carotene-linoleic acid system, C. vulgare EO exhibited 52. 3 ± 1. 19% inhibition against linoleum acid oxidation. Bektas et al. (2007b) screened the methanolic extracts of Salvia verticillata subsp. verticillata and S. verticillata subsp.
Amasiaca for their possible antioxidant activity by two complementary test systems, namely DPPH free radical-scavenging and ? -carotene/linoleic acid systems. In the first case, S. verticillata subsp. verticillata was superior to the subsp. amasiaca with an IC50 value of 14. 5 ± 1. 21 ? g m/g. In the ? -carotene/linoleic acid test system, inhibition capacity of S. verticillata subsp. verticillata was 74. 4 ± 1. 29%. Activity of rosmarinic acid was also screened for better establishing the relationship between rosmarinic acid level and antioxidant activity for the plant extracts. S. verticillata subsp. erticillata had the highest rosmarinic acid level with a value of 28. 7 ± 0. 89 ? g m/g. There was a strong correlation between the rosmarinic acid level and antioxidant activity potential.
Honey has been reported to contain a variety of phenolics and represents a good source of antioxidants, which makes it a good food antioxidant additive and increases its usability potential in ethnomedicine (Al-Mamary et al. 121 2002; Aljadi and Kamaruddin 2004; Beretta et al. 2005; Kucuk et al. 2007). Several methods have been developed, in recent years, to evaluate the antioxidant capacity of biological samples (Rice-Evans et al. 997; Schlesier et al. 2002).
The total phenolic content of natural samples, such as plants and honey, reflects, to some extent, the total antioxidant capacity of the sample (Beretta et al. 2005). The most widely used antioxidant methods involve the generation of oxidant species, generally radicals, and their concentration is monitored as the present antioxidants scavenge them. Radical formation and the following scavenging are applied in 2,2-diphenyl-1-picrylhydrazyl (DPPH)- and superoxide radical-scavenging activity measurements (Gulcin et al. 2003).
In radical-scavenging activity, the higher extract concentration required to scavenge the radicals means the lower antioxidant capacity. Ferric-reducing/antioxidant power (FRAP) is another widely used antioxidant activity measurement method, which has been used for the assessment of antioxidant and reducing power of many different samples, including honey (Aljadi and Kamaruddin 2004) and plant exudates (Gulcin et al. 2003). ESSENTIAL OILS IN FOOD Food composition and structure have a significant effect on the dynamic and final outcomes of an interaction.
Naturally present ingredients can favor or inhibit the interactive cultures. Food composition can be manipulated to achieve the desired effect. In food products, the EOs have been used in bakery (Nielsen and Rios 2000), cheese (Vazquez et al. 2001), meat (Quintavalla and Vicini 2002) and fruit (Lanciotti et al. 2004), among others. The advantage of EOs is their bioactivity in the vapor phase, a characteristic that makes them useful as possible fumigants for stored commodity protection. Antimicrobial packaging is a form of active packaging that could extend the shelf-life of product and provides microbial safety for consumers.
It acts to reduce, inhibit, or retard the growth of pathogen microorganisms in packed foods and packaging material. In order to control undesirable microorganisms on food surfaces: volatile and non-volatile antimicrobial agents can be incorporated into polymers or coating or adsorbing antimicrobial onto polymer surfaces can be applied (Appendini and Hotchkiss 2002).
The coating can serve as a carrier for antimicrobial compounds and/or antioxidants compounds in order to maintain high concentrations of preservatives on the food surfaces (Siragusa et al. 999; Oussallah et al. 2004). Although a small number of food preservatives containing EOs is commercially available, until the early 1990s very few studies of the activity of EOs in foods had been published (Board and Gould 1991). Since then a fair number of trials have been carried out with EOs in foods. There are reports of studies using diluted foods or food slurries (Pol et al. 2001; Smith-Palmer et al. 2001) and studies using dried herbs or spices or their extracts (Tassou et al. 1996; Hao et al. 1998a, 1998b).
It has generally been found that a greater concentration of EO is needed to achieve the equivalent in-vitro effect in foods (Smid and Gorris 1999). The ratio has been recorded to be approximately twofold in semi-skimmed milk (Karatzas et al. 2001), 10-fold in pork liver sausage (Pandit and Shelef 1994), 50-fold in soup (Ultee and Smid 2001) and 25- to 100-fold in soft cheese (Mendoza-Yepes et al. 1997). An exception to this phenolmenon is Aeromonas hydrophila; no greater proportion of EO was needed to inhibit this species on cooked pork and on lettuce in comparison to tests in vitro (Stecchini et al. 993; Wan et al. 1998).
Several studies have recorded the effect of foodstuffs on microbial resistance to EOs but none appears to have quantified it or to have explained the mechanism, although suggestions have been made as to the possible causes. The greater availability of nutrients in foods compared to laboratory media may enable bacteria to repair damaged cells faster (Gill et al. 2002). Generally, the susceptibility of bacteria to the antimicrobial effect of EOs also appears to increase with a decrease in the pH of the Food 1(2), 111-136 ©2007 Global Science Books ood, the storage temperature and the amount of oxygen within the packaging (Skandamis and Nychas 2000; Tsigarida et al. 2000).
At low pH the hydrophobicity of an EO increases, enabling it to more easily dissolve in the lipids of the cell membrane of target bacteria (Juven et al. 1994). The physical structure of a food may limit the antibacterial activity of EO. A study of the relative performance of oregano oil against S. typhimurium in broth and in gelatine gel revealed that the gel matrix dramatically reduced the inhibitory effect of the oil.
This was presumed to be due to the limitation of diffusion by the structure of the gel matrix (Skandamis et al. 2000). MICs for a particular EO on a particular bacterial isolate have been shown to be generally slightly lower in broth than in agar (Hammer et al. 1999). Research into the growth characteristics of L. monocytogenes and Yersinia enterocolitica in oil-in-water emulsions has shown that, depending on the mean droplet size of the emulsion, the bacteria can grow in films, in colonies or as planktonic cells (Brocklehurst et al. 1995).
It is known that colonial growth restricts diffusion of oxygen (Wimpenny and Lewis 1977) and cells situated within a colony may be shielded to a certain extent by the outer cells from substrates in the emulsion. If the oil droplets in a food emulsion are of the appropriate size, it could be possible for bacteria growing within colonies to be protected from the action of EOs in this way. Meat products A high fat content appears to markedly reduce the action of EOs in meat products. It is generally supposed that the high levels of fat and/or protein in foodstuffs protect the bacteria from the action of the EO in some way.
For example, if the EO dissolves in the lipid phase of the food there will be relatively less available to act on bacteria present in the aqueous phase (Mejlholm and Dalgaard 2002). Another suggestion is that the lower water content of food compared to laboratory media may hamper the progress of antibacterial agents to the target site in the bacterial cell (Smith-Palmer et al. 2001). Mint oil in the high fat products exhibited little antibacterial effect against L. monocytogenes and S. enteritidis, whereas in low fat food the same EO was much more effective (Tassou et al. 995).
Immobilising cilantro EO in a gelatine gel, however, improved the antibacterial activity against L. monocytogenes in ham (Gill et al. 2002). One study found that encapsulated rosemary oil was much more effective than standard rosemary EO against L. monocytogenes in pork liver sausage, although whether the effect was due to the encapsulation or the greater percentage level used was not further elucidated (Pandit and Shelef 1994). Certain oils stand out as better antibacterials than others for meat applications.
Eugenol and coriander, clove, oregano and thyme oils were found to be effective at levels of 5-20 ? l/g in inhibiting L. monocytogenes, A. hydrophila and autochthonous spoilage flora in meat products, sometimes causing a marked initial reduction in the number of recoverable cells (Tsigarida et al. 2000; Skandamis and Nychas 2001) whilst mustard, cilantro, mint and sage oils were less effective or ineffective (Gill et al. 2002; Lemay et al. 2002).
In fish, just as in meat products, a high fat content appears to reduce the effectiveness of antibacterial EOs. For example, oregano oil at 0. 5 ? /g is more effective against the spoilage organism Photobacterium phosphoreum on cod fillets than on salmon, which is a fatty fish (Mejlholm and Dalgaard 2002). Oregano oil is more effective in/on fish than mint oil, even in fatty fish dishes; this was confirmed in two experiments with fish roe salad using the two EOs at the same concentration (5-20 ? l/g) (Koutsoumanis et al. 1999; Tassou et al. 1996). The spreading of EO on the surface of whole fish or using EO in a coating for shrimps appears effective in inhibiting the respective natural spoilage flora (Ouattara et al. 2001; Harpaz et al. 2003).
The activity of oregano EO against Clostridium botuli122 num spores has been studied in a vacuum packed and pasteurised minced (ground) pork product. Concentrations of up to 0. 4 ? l/g oregano EO were found not to significantly influence the number of spores or to delay growth. However, in the presence of low levels of sodium nitrite which delayed growth of bacteria and swelling of cans when applied alone, the same concentration of oregano EO enhanced the delay. The delay of growth was dependent on the number of inoculated spores; at 300 spores/g the reduction was greater than at 3000 spores/g (Ismaiel and Pierson 1990).
Active packagings with the packaging materials delivering antimicrobials, can play an important role in satisfying current requirements because inhibitors are more effective when delivered in this manner. When AIT was used as an antimicrobial agent in active packaging of rye bread, it was found that 1 ? l AIT completely inhibited the growth of A. flavus, Penicillium commune, Penicillium corylophilum, Penicillium discolor, Penicillium polonicum, Penicillium roqueforti and Endomyces fibulige (Nielsen and Rios 2000). Smith-Palmer et al. (2001) found hydrophobic plant EOs were more effective against L. onocytogenes in low fat (16%) that in high fat (30%) cheeses. Hasegawa et al. (1999) reported that AIT was more effective against V. parahaemolyticus in high fat (20. 8%) than in low fat (0. 4%) tuna tissue.
The potential for intrinsic fat levels in food to moderate the antimicrobial activity of EOs is clear, and results from these two studies showed that interference can be expected at fat levels in food of ;16%. Allyl isothiocyanate (AIT), a major antimicrobial component in mustard and horseradish oil, has been used in a number of foods against a variety of rganisms. It has been found to be generally more effective against Gram-negative bacteria.
In a study, Hasegawa et al. (1999) found AIT more effective in fatty (20. 8%) than lean (0. 4%) tuna meat suspension against 4 strains of V. parahaemolyticus. After 24 h of incubation, AIT at 152. 6 ? g /ml was able to inhibit only one strain in the lean suspension, but it reduced all strains below 10 cfu /ml in the fatty suspension. At 101. 7 ? g /ml, AIT inhibited 3 of the strains to the same level in the fatty suspension.
The higher activity of AIT in fatty tuna meat flesh may be related to the high level of unsaturated fat. The main fatty acids of tuna flesh are cis-vaccenic, palmitic and docosahexaenoic acid, which may stabilize AIT in tuna tissue suspensions. AIT possesses strong antimicrobial activity against E. coli O157:H7 as well as V. parahaemolyticus. Nadarajah et al. (2002) killed 3. 6 log cfu/g E. coli O157:H7 in ground beef with AIT (200– 300 ppm) after 21 d at 4°C. The antimicrobial effectiveness of AIT against E. coli O157:H7 varied with storage temperature and inoculation level.
There was very little inhibitory effect on the natural microflora. In subsequent work, Muthukumarasamy et al. (2003) examined the effectiveness of AIT at 1300 ppm in ground beef stored at 4°C under nitrogen with Lactobacillus reuteri against E. coli O157:H7. As an ingredient, AIT by itself eliminated 3 log10 cfu/g E. coli O157:H7 within 15 d and reduced 6 log10 cfu/g by 47 log cfu/g during 25 d storage. AIT did not interact synergistically with Lb. reuteri against E. coli O157:H7.
When AIT was used in acidified chicken meat (0. % w/w), it failed to exert a significant effect on the growth of Brochothrix thermosphacta, but it was able to delay growth of some LAB and aerobic mesophilic bacteria for at least 2 days (Lemay et al. 2002). In another similar study, when AIT was evaluated for its effectiveness in precooked roast beef against pathogenic bacteria (E. coli O157:H7, L. monocytogenes, S. typhimurium, and S. aureus) and spoilage bacteria (Serratia grimesii and Lb. sakei), it was found that pathogenic bacteria were inhibited by AIT at a concentration in the head space of 20 ? l/l. E. coli O157:H7, S. ureus and S. typhimurium were most sensitive.
Food biopreservation. Iraj Rasooli Dairy products A reaction between carvacrol, a phenolic component of various EOs, and proteins has been put forward as a limiting factor in the antibacterial activity against Bacillus cereus in milk (Pol et al. 2001). Protein content has also been put forward as a factor inhibiting the action of clove oil on Salmonella enteritidis in diluted low-fat cheese (SmithPalmer et al. 2001).
Carbohydrates in foods do not appear to protect bacteria from the action of EOs as much as fat and protein do (Shelef et al. 984). A high water and/or salt level facilitates the action of EOs (Skandamis and Nychas 2000). Mint oil at 5-20 ? l/g is effective against S. enteritisdis in low fat yoghurt and cucumber salad (Tassou et al. 1995). Mint oil inhibits the growth of yoghurt starter culture species at 0. 05-5 ? l/g but cinnamon, cardamom and clove oils are much more effective (Bayoumi 1992).
SmithPalmer et al. (1998) reported that the oils of clove, cinnamon and thyme were effective against L. monocytogenes and S. enteritidis in tryptone soya broth (TSB). The oils had MICs of 0. 4%, 0. 075% and 0. 03%, respectively, against L. monocytogenes. Similarly, concentrations of 0. 075%, 0. 1%, and 0. 04% were required to inhibit the growth of S. enteritidis in TSB. On the other hand, when Smith-Palmer et al. (2001) evaluated clove, cinnamon, thyme and bay oil for their activity against L. monocytogenes and S. enteritidis in both low (16%) and high fat (30%) cheese it was observed that the oils of clove and cinnamon were highly effective against L. monocytogenes.
However, a 1% concentration of the oils was required to inhibit L. onocytogenes and reduce its number to rosemary ; mustard ; cilantro/sage. An approximate general ranking of the EO components is as follows (in order of decreasing antibacterial activity): eugenol ; carvacrol/cinnamic acid ; basil methyl chavicol ; cinnamaldehyde ; citral/geraniol.
