Safety and aroma appealing of fresh-cut fruits and vegetables could be improved using a right combination of flavors among the antimicrobial essential oils (EOs) and the target fresh-cut produce.
Premise I. Microbial and aroma attributes limiting safety and sensory appealing of fresh-cut produce
The fresh-cut industry has undergone a rapid growth and has been one of the most successful businesses within the food processing industry. This industrial growth has been attributed to the increase in the healthy alternative in restaurants, supermarkets, and schools (Sanford and others 2008). Among the wide variety of fresh-cut vegetables on the market, the most common are vegetable mixtures served as salads. Carrot, tomato, broccoli, cauliflower, cabbage, and other vegetables also have established markets and are used as individual product (Rico and others 2007). However, there are important quality considerations as well as safety concerns involved in the fresh-cut fruit and vegetable production (Artés and others 2007).
Fruit decay and loss of quality of fresh-cut fruits and vegetables are enhanced by processing and storage (Busta and others 2003). Minimal processing that includes peeling, coring, chopping, slicing, dicing, or shredding cause cell injury, releasing its liquid content at the sites of wounding (Figure 1). Subcellular compartmentalization is disrupted at the cut surfaces, favoring contact of substrates and enzymes, normally separated, initiating deteriorative reactions that normally are absent in the whole produce (Toivonen and Brummell 2008). Microbiological, enzymatic, and physicochemical reactions simultaneously take place, causing negative quality changes due to the fact that fresh-cut fruits are complex and more active systems than whole fruit (Artés and others 2007). Therefore, fresh-cut products tissues deteriorate more rapidly and their physiology significantly differs from that of intact fruit and vegetables. Among the quality attributes that limit mostly the shelf life of fresh-cut produce are microbial growth and sensorial decay.
Microbial growth can seriously limit the safety and shelf life of fresh-cut fruits and vegetables. The high content of organic acids and sugars present in the fruit tissue can be available after peeling and cutting and be a good source of nutrients for bacteria, yeast, and mold growth (Brecht 2006; Ayala-Zavala and others 2008a). Natural microbial flora may have about 104 to 105 colony forming units per gram (CFU/g), but can increase rapidly under conditions mentioned previously (Busta and others 2003). The commonly encountered microflora of fruits and vegetables are Pseudomonas spp., E. herbicola, E. agglomerans, lactic acid bacteria, molds, and yeasts (Busta and others 2003). Among the deteriorative microflora, fungi are the most important microorganisms causing wastage of fresh-cut fruit, where the relatively acid conditions tend to suppress bacterial growth (Frazier and Westhoff 1993). However, bacterial infections are more common in vegetables due to their high pH and are important agents of both, spoilage and foodborne diseases. Epidemiological surveys demonstrate that there is a potential risk for a wide range of raw fruits and vegetables to become contaminated with microorganisms, including human pathogens. Outbreaks of gastrointestinal illness have been reported for intact and fresh-cut produce; for example, in the last years, several problems have raised on vegetables such as spinach, green onions, lettuce, and melons (Whipps and others 2008).
The variety of organisms associated with outbreaks linked to fresh and fresh-cut produce encompasses bacteria, viruses, and parasites. Most of the reported outbreaks have been associated with bacterial contamination, particularly members of the family Enterobacteriaceae. The viruses involved in outbreaks have a human reservoir (for example, Norwalk-like and Hepatitis A) and can be associated with intact products grown in contact with the soil and/or water (Busta and others 2003). Outbreaks linked to protozoa (for example, Cryptosporidium, Cyclospora, Giardia) have been associated more with fruits than with vegetables (Busta and others 2003). Protozoa and viruses are most often associated with contaminated water or food handlers, and can be transmitted to the final fresh-cut produce during cultivation, harvest, and minimal processing, compromising consumer health (Garrett and others 2003). With this in mind, microbiological risk is one of the major factors affecting safety of fresh-cut fruit and vegetables; however, these produce can suffer from other deteriorative processes that affect also sensorial quality.
Sensorial appealing of fresh-cut fruits and vegetables by consumers is a major factor in the purchase decision (Toivonen and Brummell 2008). Among the most important sensory attributes of fresh-cut produce, flavor and aroma are included, which are often the major indicators of shelf life from the point of view of the consumers (Beaulieu and Lea 2003). However, it has to be highlighted that after minimal processing and storage, deterioration of fresh-cut fruit occurs, compromising also their sensorial appealing (Martinez and others 2008). Analyzing sensorial changes of fresh-cut produce, the 2 primary mechanisms of flavor loss are metabolic and diffusional (Figure 2) (Forney 2008). Metabolic changes in flavor are the result of the synthesis or catabolism of either flavor compounds or compounds responsible for off-flavors. These metabolic processes are dependent on product physiology, which is influenced by maturity and a variety of environmental, handling, and processing factors (Beaulieu and Lancaster 2007). However, such deleterious reactions can be diminished with several preserving technologies, with the objective to accomplish consumer demands of a high-quality produce.
Chemical synthetic additives can reduce decay rate, but consumers are concerned about chemical residues in the product, which could affect their health and cause environmental pollution (Roller and Lusengo 1997; Ayala-Zavala and others 2008b). Therefore, alternative methods for controlling fresh-cut fruit decay are required. One of the major emerging technologies for reducing quality loss and safety assurance of fresh-cut fruits and vegetables is the application of natural additives (Lanciotti and others 2004).
Nowadays, the interest in natural antimicrobial compounds to preserve fresh-cut produce quality has increased, and numerous studies on the antimicrobial activity of a wide range of natural compounds have been reported (Burt 2004; Ayala-Zavala and others 2008a). There has been an increasing concern of consumers about foods free or with lower levels of synthetic chemical preservatives, because these could be toxic for humans and environment (Wang 2006). Concomitantly, consumers also demanded foods with long shelf life and absence of pathogens. The food industry has paid attention to this perception and has been working in the removal of synthetic preservatives and adoption of natural alternatives to reach safety and quality of fresh-cut produce. This resulted in a continuous search for effective natural antimicrobial compounds that preserve fresh-cut produce quality without detriment of its sensorial attributes.
Premise II. Antimicrobial and aromatic potential of EOs
An EO is a concentrated, hydrophobic liquid containing a mixture of antimicrobial volatile aroma compounds commonly derived from plant tissues (Ha and others 2008). EOs are “essential” in the common sense that they carry a distinctive scent of the source plant and are generally extracted by steam distillation and other processes like maceration, cold pressing, or solvent extraction. They are used in perfumes, cosmetics, bath products, scenting incense, household cleaning products, as well as in the flavoring food and drink area (Berger 2007). The aromatic constituents of known EOs are mostly constituted of short hydrocarbon chains, complemented with oxygen, nitrogen, and sulfur atoms attached at various points of the chain (Braca and others 2008). Such mixture of different scented molecules with highly reactive atoms gives different functional properties to EOs, which could be considered for many food applications.
Several EOs such as oils of garlic, cinnamon, thyme, oregano, clove, basil, coriander, citrus peel, laurel, ginger, rosemary, and peppermint, among others, have been studied as antimicrobial natural products against both bacteria and molds (Table 1) (Edris and Farrag 2003; Novak and others 2003; Arcila-Lozano and others 2004; Jugl-Chizzola and others 2006; Wang 2006; Mukherjee and Datta 2007; Ravi and others 2007; Senhaji and others 2007; Zeller and Rychlik 2007). The inherent aroma and antimicrobial activity of EOs are related commonly with the chemical configuration of the components, the proportions in which they are present and to interactions between them, affecting their bioactive properties (Fisher and Phillips 2008). Some studies have concluded that whole EOs have a greater antibacterial activity than their major constituents separately (Marino and others 1999, 2001; Chang and others 2008), which suggests that the components at lower concentration are critical to the activity and the entire mixture must have a synergistic effect. Considering that EOs are formed by several constituents is difficult to attribute the antimicrobial efficacy to only one compound, in this context, the mode of action of a given compound could be different compared to others, being reported several targets in the microbial cell (Figure 3). It seems that they may cause deterioration of cell wall, damage to cytoplasmic membrane, damage to membrane proteins, leakage of cell contents, coagulation of cytoplasm, depletion of proton motive active sites, inactivation of essential enzymes, and disturbance of genetic material functionality (Burt 2004; Ayala-Zavala and others 2008a; Gutierrez and others 2008).
|Essential oil||Major volatile constituents||Antimicrobial effect against||Aroma notes||References|
|Garlic root (A. sativum)||Methyl disulfide, allyl sulfide, allyl disulfide, allyl trisulfide, trimethylene trisulfide, allyl tetrasulfide||B. cereus, E. coli, Shigella spp., Vibrio parahaemolyticusi Y. enterolitica, S. enterica serovars Enteritidis, Infantis, Typhimurium, B. subtilus, E. faecalis, S. Faecalis, A. Alternata||Pungent, spice||(Ross and others 2001; Ayala-Zavala and others 2008c)|
|Cinnamon leaf (C. zeylanicum)||Cinnamaldehyde, eugenol, copaene, β-caryophyllene||E. coli, Ps. aeruginosa, E. faecalis, S. aureus, S. epidermidis, methicillin-resistant S. aureus, K. pneumoniae, Salmonella sp., V. parahemolyticus, A. alternate, A. flavus, A. niger, P. corylophilum||Sweet, wood, spice||(Chang and others 2001; Guynot and others 2003; Ayala-Zavala and others 2008c)|
|Thyme (T. vulgaris)||Thymol, p-cymene, γ-terpinene, linalool||B. cereus, C. botulinum, E. faecalis, E. coli, S. aureus, L. monocytogenes, A. flavus, A. niger, P. corylophilum, E. faecalis, E. coli, K. pneumoniae, P. aeruginosa, S. aureus, Salmonella sp.||Spice, citrus, wood||(Hammer and others 1999; Guynot and others 2003; Lee and others 2005)|
|Oregano (O. vulgare)||Sabinyl monoterpenes, terpinen-4-ol, γ-terpinene, carvacrol, thymol||B. cereus, B. subtilis, C. botulinum, E, faecalis, E. coli, S. aureus, A. niger, L. monocytogenes, E. faecalis, E. coli, K. pneumoniae, P. aeruginosa, S. aureus, Salmonella sp.||Spice, herb||(Charai and others 1996; Hammer and others 1999; Elgayyar and others 2001; Burt 2004)|
|Clove (E. aromaticum)||Eugenol, eugenyl acetate, caryophyllene||B. brevis, B. subtilis, C. botulinum, E. faecalis, Candida sp., A. flavus, A. niger, P. Corylophilum, A. flavus, A. niger, P. Corylophilum, E. faecalis, E. coli, K. pneumoniae, P. aeruginosa, S. aureus, Salmonella sp., E. coli, L. monocytogenes||Sweet, spice, wood||(Akgül and Kivanç 1988; Hammer and others 1999; Guynot and others 2003; Burt 2004)|
|Basil (O. basilicum)||Linalool, methylchalvicol, eugenol, methyl eugenol, methyl cinnamate, 1,8-cineole, caryophyllene||B. brevis, E. coli, A. Flavus, A. niger, P. Corylophilum, E. faecalis, E. coli, K. pneumoniae, P. aeruginosa, S. aureus, L. monocytogenes, L. Plantarum||Fresh, sweet, herb, spice||(Hammer and others 1999; Elgayyar and others 2001; Guynot and others 2003; Opalchenova and Obreshkova 2003)|
|Coriander (C. sativum)||2(E)-decanal, 2(E)dodecenal, linalool||E. coli, L. monocytogenes, L. Plantarum, S. aureus||Sweet, flower, spice, citrus||(Elgayyar and others 2001)|
|Citrus peel (Citrus sp.)||Limonene, linalool, citral||A. niger, A. flavus, P. verrucosum, P. chrysogenum||Sweet, citrus||(Viuda-Martos and others 2007, 2008)|
|Laurel (L. nobilis)||1,8-cineole, α-terpinyl acetate, linalool, methyl eugenol||S. aureus, B. cereus, M. luteus, E. faecalis||Fresh, herb, spice||(Demo and de las Mercedes Oliva 2009)|
|Ginger (Z. officinale)||β-sesquiphellandrene, zingiberene||A. flavus, A. niger, P. Corylophilum, E. faecalis, E. coli, K. pneumoniae, P. aeruginosa, S. aureus||Pungent, spice||(Hammer and others 1999; Guynot and others 2003)|
|Rosemary (R. officinalis)||Borneol, verbenone, camphor, a-pinene, 1,8-cineole||A. flavus, A. niger, P. corylophilum, E. faecalis, E. coli, K. pneumoniae, P. aeruginosa, S. aureus, L. monocytogenes, L. Plantarum, Salmonella sp., B. cereus, S. aureus||Fresh, herb, resinous||(Hammer and others 1999; Elgayyar and others 2001; Guynot and others 2003; Burt 2004)|
|Peppermint (M. piperita)||Menthol, menthone, menthyl acetate, menthofurane||B. brevis, S. aureus, V. Choleraei E. faecalis, E. coli, K. pneumoniae, P. aeruginosa, A. flavus, A. niger, P. corylophilum||Fresh, herb||(Hammer and others 1999; Guynot and others 2003)|
There are several volatile compounds present in the EOs that possess antimicrobial activity and flavoring properties. Some of the most representative and studied are terpenoids, organic sulfur compounds, aldehydes, and alcohols, among others (Berger 2007).
Among the studied compounds, terpenoids are used extensively for their aromatic qualities and significant contribution to the scent of coriander, cinnamon, oregano, rosemary, cloves, thyme, and citrus oils, among others (Berger 2007). Well-known terpenoids include citral, menthol, camphor, citral, geraniol, eugenol, menthol, and cinnamaldehyde, that have also been found to inhibit the in vitro growth of bacteria and fungi, and form the characteristic odor classes of the different EOs (Chaumont and Léger 1992). The α-terpinene inhibited several bacterial species and its odor perception is lemon like (Baranauskiene and others 2005). Cinnamaldehyde is known to inhibit the growth of E. coli O157:H7 and S. typhimurium and it is perceived as a sweet cinnamon–honey odor (Aggarwal and others 2002; Zhou and others 2007). Carvone presents a warm herb-like character and suppress the specific growth rate of E. coli, Streptococcus thermophilus, and L. lactis, suggesting that it acts by disturbing the metabolic energy status of cells (Aggarwal and others 2002). Different studies have reported the possible mode of action of specific compounds, but considering the wide variety of these compounds, more basic research is needed.
Organic compounds containing either nitrogen or sulfur are commonly found in garlic, onion, and leek oils, which have demonstrated antimicrobial activity against a wide variety of microorganisms. Organosulfur compounds from garlic such as diallyl mono-, di-, and trisulfide have been reported to have antimicrobial activity against fungi and bacteria, presenting a characteristic smell to sulfur spices (Chung and others 2007). For example, diallyl sulfide and dimethyl trisulfide are volatile constituents of garlic oil that presented antimicrobial activity against E. aerogenes, E. coli, Salmonella sp., L. monocytogenes, and Y. enterocolytica, are perceived as sulfur-, onion-, garlic-, and/or cabbage-like odorants (Benkeblia 2004). Recent study reported that the main compounds detected in garlic oil were allyl disulfide, allyl trisulfide, and allyl tetrasulfide (Ayala-Zavala and others 2008c).
EO and/or their components have been registered by the legislation of different regions as flavorings in foodstuffs (Ayala-Zavala and others 2008a; Fisher and Phillips 2008). The flavorings registered are considered to present generally no risk to human health of the consumer and include amongst others carvacrol, carvone, cinnamaldehyde, citral, p-cymene, eugenol, limonene, menthol, and thymol (CFSAN/FDA 2006). Estragole and methyl eugenol were deleted from the list in 2001 because they were found to be genotoxic (Zeller and Rychlik 2007). The European Union registered flavorings that also appear on the “Everything Added to Food in the US” (EAFUS) list, which means that the U.S. Food and Drug Administration (FDA) has classified the substances as generally recognized as safe (GRAS) or as approved food additives.
Premise III. Connecting aroma of essential oils and fresh-cut produce: sensory descriptors and the traditional art of seasoning
For the application of EOs as fresh-cut fruits and vegetables preservatives, the sensorial impact should be considered (Ayala-Zavala and others 2008a). Several fresh-cut fruits and vegetables that generally are associated with herbs, spices, or seasonings would be the least affected by this phenomenon; however, new combination of flavors can be considered based in consumer studies (Baranauskiene and others 2006). One of the most important keys to success in the food industry, including the fresh-cut fruit and vegetable market, is delivering products that please the palate of consumers (Kader 2008). Even the most healthful foods are not routinely accepted and regularly consumed if they have poor sensory properties according to the consumer point of view (Park and others 2005). In this context, the application of the right combination of a given EO aroma could increase consumer acceptability of the treated fresh-cut produce.
The common relation of flavor profiles normally used in the global seasoning and flavoring mixtures could be helpful to support a given combination among the antimicrobial EO and the fresh-cut fruit (Rozin and Rozin 1981). The aroma profile of a fresh-cut fruit could be merged with an EOs using their aroma notes as connectors. As can be observed in Table 2 aroma notes of fresh-cut vegetables and fruits are different among them. Most vegetables present herb and spice notes, and fruits present sweet and fresh aroma notes. In this context, aroma notes of fresh-cut produce and EOs could be combined, for example, oregano, coriander, garlic, thyme, laurel, rosemary, and ginger oils, present a spice, herb aroma that could be combined with the aroma notes of fresh-cut vegetables. On the other hand, cinnamon, clove, citrus, and peppermint oils present sweet and fresh aroma notes that could be combined with sweet and tropical fresh-cut fruits. The idea of adding an extra flavor to the EO-treated product could be perceived within the philosophy of seasoning (Rozin and Rozin 1981).
|Fresh-cut product||Odor class||Consumed as||Aroma notes at consumption||Possible EO to apply|
|Broccoli||Vegetable||Florets, dressed salad mixture||Spice, fresh, herb||Oregano, coriander, garlic, thyme, laurel, rosemary, ginger|
|Carrot||Vegetable||Alone, dressed salad mixtures||Fresh, herb||Oregano, coriander, thyme, laurel, rosemary|
|Potato||Vegetable||Fry, dressed with ketchup||Spice, herb||Oregano, coriander, garlic, thyme, laurel, rosemary, ginger|
|Cucumber||Vegetable||Alone, dressed salad mixtures||Spice, fresh, herb||Oregano, coriander, garlic, thyme, laurel, rosemary, ginger|
|Onion||Root spices||Sandwich, dressed salad mixtures, salsas||Spice, herb, pungent||Oregano, coriander, garlic, thyme, laurel, rosemary, ginger|
|Tomato||Vegetable, berry||Sandwich, pizza, salad mixtures, salsa||Spice, herb, citrus, pungent||Oregano, coriander, garlic, thyme, basil, laurel, rosemary, ginger|
|Apple||Pome||Alone, fruit salad||Sweet, fresh||Cinnamon, clove, citrus, peppermint|
|Pear||Pome||Alone, fruit salad||Sweet, fresh||Cinnamon, clove, citrus, peppermint|
|Peach||Pome||Alone, fruit salad||Sweet, fresh||Cinnamon, clove, citrus, peppermint|
|Watermelon||Pome||Alone, fruit salad||Sweet, fresh||Cinnamon, clove, citrus, peppermint|
|Strawberry||Berry||Alone, fruit salad||Sweet, fresh||Cinnamon, clove, basil, citrus, peppermint|
|Grapes||Berry||Alone, fruit salad||Sweet, fresh||Cinnamon, clove, basil, citrus, peppermint|
|Lemon||Citrus||Fruit or vegetable salad||Fresh, sweet, citrus||Cinnamon, clove, basil, citrus, peppermint|
|Mandarin||Citrus||Alone, fruit salad||Fresh, sweet, citrus||Cinnamon, clove, basil, citrus, peppermint|
|Orange||Citrus||Alone, fruit salad||Fresh, sweet, citrus||Cinnamon, clove, basil, citrus, peppermint|
|grapefruit||Citrus, tropical||Alone, fruit salad||Fresh, sweet, tropical||Cinnamon, clove, basil, citrus, peppermint|
|Pineapple||Tropical||Alone, fruit salad||Sweet, fresh, tropical||Cinnamon, clove, basil, citrus, peppermint|
|Mango||Tropical||Alone, fruit salad||Sweet, fresh, tropical||Cinnamon, clove, basil, citrus, peppermint|
|Papaya||Tropical||Alone, fruit salad||Fresh, sweet, tropical||Cinnamon, clove, basil, citrus, peppermint|
The traditional art seasoning for fresh-cut vegetables consist generally with the addition of fresh, herb, and pungent odors of herbs and spices, with this in mind, the use of EOs as flavorings could be considered for this purpose. Fresh-cut broccoli, cabbage, potato, cucumber, carrot, onion, and tomato are generally expended as ready-to-eat dressed salads, or as ingredients for other dishes (Pollack 2001), where they could be mixed with aromatic spices like oregano, coriander, garlic, thyme, laurel, rosemary, and/or ginger. Herein, EOs of these spices could be used to add an acceptable extra flavor and additionally preserve safety of the treated vegetables.
Among the different EOs studied, many studies have been focused on the oil extracted from garlic, which possess the aroma and odor notes of its source plant (Ayala-Zavala and others 2008b). Garlic (A. sativum) is an important crop for culinary purposes; its pungent flavor adds a special taste to food. In some traditional Chinese, Egyptian, Mediterranean, and Mexican dishes, garlic flavor is mixed with several vegetables produce as a seasoning in salad dressing, vinaigrettes, and sauces, providing a special taste and smell to the dish (Sherman and Billing 1999). Oregano aroma notes are present in several Mexican and Mediterranean dishes, and used as salad dressings in vegetable salads, among others. Coriander aroma notes are indispensable in Thai soup like the Tom Yum type—with coconut milk, lemongrass, ginger, and chili-peppers; also, in Mexican dishes, coriander is normally used in fresh-cut vegetable sauce (fresh-cut tomato, onion, and jalapenos) (Sherman and Billing 1999). As can be observed in these examples several fresh-cut vegetables are consumed with aroma notes also found in EOs. Therefore, other application forms could be implemented and performed with similar purposes, increasing its use as natural additives and/or preservatives.
Fresh-cut fruit like apple, pear, peach, watermelon, strawberry, grapes, lemon, mandarin, orange, grapefruit, pineapple, mango, and papaya, present a sweet, fresh, flavor with characteristic pome, citrus, berry, and tropical aroma notes, depending on the produce (Kader 2008). Fresh-cut fruit is expended alone or as fruit salads, where final flavor continues to be sweet and fresh; in this case, sweet and/or fresh flavors like cinnamon, clove, basil, citrus, and peppermint could fairly be added. These combinations could increase value and acceptability whilst the fresh-cut produce is naturally preserved.
Cinnamon (C. zeylanicum) aroma notes can be obtained from leaves, bark, flowers, fruits, wood, and root that always contain EOs of various compositions. The main component of cinnamon bark oil is the aromatic cinnamaldehyde (about 60%). However, the oil from the leaves of the cinnamon bush has eugenol as the main component (about 80%) (Ayala-Zavala and others 2008c). Cinnamon aroma notes as seasoning are used in Mexican, Middle Eastern, and other ethnic dishes, curries, vegetables, tea, beverages, and of course, desserts (Sherman and Billing 1999). The agreeably sweet woody aroma is quite delicate and intense and the taste is fragrant and warm (Farrell 1998). In American cooking, cinnamon notes are often paired with apples and used in other fruit and cereal dishes. Cloves add sweet spicy notes depth to applesauce, muffins, cakes, and other sweets. Basil (O. basilicum) EO aroma vary according to the given specie plant source (anis-like, cinnamon-like, camphor-like, lemon-like, among others) (Gahagan 2008). Major constituents of basil EO are linalool and methylchavicol (estragole) together with eugenol, methyleugenol, methyl cinnamate, 1,8-cineole, and caryophyllene, among others (Gahagan 2008). Basil aroma notes are widely used in Italian cuisine and are often paired with tomatoes, but they also blend well with garlic, thyme, and oregano notes (Gahagan 2008).
Lemon (C. limon) EO possesses the characteristic lemon flavor notes due to the aldehyde citral combined with smaller amounts of linear aliphatic aldehydes (C7 to C13). As with most citrus oils, (+)-limonene is by far the major component (approximately 65%) (Gahagan 2008). These compounds contribute to the typical aroma of distilled lime oil (Gahagan 2008). Mandarin (C. reticulata) oil is made by cold pressing the peels of mandarin. It has a deep, sweet, orange-like character, and is used in liqueurs, fragrances, and cosmetics. Peppermint (M. piperita) EO is obtained by steam distillation of the flowering herb. It has almost colorless to pale greenish-yellow liquid with a characteristic peppermint odor. The main component of peppermint oil is (−)-menthol (approximately 50%) followed by (−)-menthone (approximately 20%) and (−)-menthyl acetate (approximately 10%) (Gahagan 2008). These flavors could be easily incorporated to fresh-cut fruits, combining the sweet and fresh characters of the EOs and cut produce. The addition of a precise EO to a specific fruit has to consider the characteristics of both systems to develop an acceptable combination for consumers.
Premise IV. State of the art: successful and unsuccessful combinations between EOs and fresh-cut produce treatment
Following the natural food preservation trends, research has been focused on the effect of EOs on microbial inhibition of fresh and fresh-cut fruit, despite the global effect, which concerns sensorial consumer acceptability. In this section, the state of the art of the use of EOs and other aromatic volatile compounds to preserve fresh and fresh-cut produce quality is described. EOs stand out as an alternative to chemical preservatives and their use in fresh-cut produce could meet the demand of consumers for fresh natural like products (Burt 2004).
The chemical reactivity of natural antimicrobial agents with fresh fruits and vegetables and package matrix, could significantly affect the sensorial properties of produce. EOs have been proved to be effective antimicrobials; however, their aromatic volatile constituents can be sorbed by the food product and alter its sensory characteristics. In addition, reactions with lipids, proteins, carbohydrates, and other additives, may result in an overall decrease in the activity of the antimicrobial compound (Ayala-Zavala and others 2008a). The ability of most antimicrobials, including EOs to inhibit microorganisms can be overcome on extended storage (Ayala-Zavala and others 2008a). Depending on the time and storage temperature, these antimicrobials can be volatilized or become inactive (Tripathi and Dubey 2003; Ayala-Zavala and others 2008a). Normally, direct application of antimicrobials to food must be done at high concentrations to achieve good antimicrobial activity against target microorganisms on extended storage of fresh-cut fruit and vegetables (Ayala-Zavala and others 2008a). Obviously, antimicrobial compounds that possess high antimicrobial activity but negatively affect flavor and odor acceptability, would be unacceptable and not considered for these purposes.
Appropriate or compatible use of natural antimicrobial agents would involve using these compounds to add positive sensory characteristics, additionally to the improvement of food safety and/or shelf life extension of fresh-cut fruit and vegetables (Tripathi and Dubey 2003; Ayala-Zavala and others 2008a). Choosing the right combination in aromas between the antimicrobial EO and the fresh-cut produce, the quality involving safety and flavor can be improved.
Recently, the efficacy of alginate and gellan edible coatings has been combined with the antimicrobial effect of plant EOs (lemongrass, oregano oil, and vanillin) to prolong shelf life of fresh-cut apples. They observed a 4 log CFU/g reduction in the inoculated population of Listeria innocua in fresh-cut apple when lemongrass or oregano oils were incorporated into an apple puree–alginate edible coating (Rojas-Graü and others 2007). Previously it has been indicated that the addition of cinnamon, clove, or lemongrass oils at 0.7% (v/v) or their active compounds at 0.5% (v/v) into an alginate-based coating increased their antimicrobial effect reducing the population of E. coli O157:H7 by more than 4 log CFU/g and extending the microbiological shelf life of Fuji apples for at least 30 d (Raybaudi-Massilia and others 2008). Despite the good results achieved so far with the incorporation of EOs into edible films and coatings, the major drawback is their strong flavor, which could affect negatively the original taste of foods.
Rojas-Graü and others (2007) evaluated the sensory quality of coated fresh-cut apples containing plant EOs. They studied the effect of lemongrass oil, oregano oil, and vanillin incorporated into apple puree–alginate edible coatings on sensorial quality of fresh-cut “Fuji” apples. Sensory evaluation indicated that coated fresh-cut apples containing vanillin (0.3% w/w) were the most acceptable in terms of flavor quality after 2 wk of storage. However, coated apple pieces containing oregano exhibited the lowest overall preference. It was reported that, despite the low concentration of oregano oil (0.1% w/w) used, some consumers detected a residual aromatic herbal taste, which diminished the overall preference of fresh-cut apples. Raybaudi-Massilia and others (2008) evaluated the effect of alginate coating as carrier of EOs (cinnamon, palmarosa, and lemongrass) to improve safety and sensorial quality and increase the shelf life of fresh-cut melon. According to their results, the incorporation of 0.3% palmarosa oil into the coating appears to be promising, since it was well accepted by consumers. The consumer test demonstrated that the combination of oregano EO aroma notes and the sweet fruity character of fresh-cut apples were not compatible.
The inclusion of hexanal in combination with 2-(E)- hexenal in the surrounding atmosphere of fresh sliced apples results in a significant extension of shelf life even when a spoilage yeast such as P. subpelliculosa was inoculated at levels of 103 CFU/g and abuse storage temperatures were used (Lanciotti and others 2004). Hexanal, 2-(E)-hexenal, and hexyl acetate presented a significant inhibitory effect against pathogenic microorganisms, such as E. coli, S. enteritidis, and L. monocytogenes inoculated in fresh-cut apples, in addition to their activity on shelf life, in terms of color retention and spoilage microflora (Lanciotti and others 2004). In fact, the results of nonstructured sensorial analysis revealed that such molecules were able to enhance the sensorial properties of sliced apples.
The antimicrobial and sensorial efficacy of EOs (oregano and thyme) for preserving fresh-cut lettuce and carrots has been reported (Gutierrez and others 2009). Initial decontamination effects achieved using EOs that were comparable to that observed with chlorine and a solution containing oregano has a significantly lower initial level than carrots treated with water. The sensory panel found that EO treatments were acceptable for carrots throughout storage, while lettuce washed with the EO solutions were rejected for overall appreciation by day 7. Contemplating that on the 1st day, the vegetable aroma perceived from samples treated with oregano + thyme was significantly less intense than that of oregano or chlorine. Although, the researchers did not present these conclusions, it seems that over dose of oregano EO was the major cause of acceptability loss.
EOs as natural sanitizing agents were sprayed on Swiss chard leaves produced by organic methods. Samples were stored at 0 and 5 °C and 97% to 98% relative humidity (Ponce and others 2004). Microbial populations and sensory attributes were monitored during storage. No significant differences were observed between treated and control samples stored at 0 °C. However, samples treated with the EO of eucalyptus, tea tree, and clove and stored at 5 °C, presented microbial counts significantly lower than those of control samples. Although some of the EOs appeared to reduce microbial counts compared with controls, they were not effective in extending the shelf life of the Swiss chard leaves from a sensory point of view.
Guillen and others (2007) studied grapes wrapped with 2 distinct films, with different permeability, with or without the addition of a mixture of eugenol, thymol, and carvacrol. Microbial counts (molds, yeasts, and mesophilic aerobics) were drastically decreased, and consequently diminished berry decay. Although slight odor was detected after opening the packages, typical flavors of these active compounds were not detected by trained panelists after tasting the berries. Thus, with this safe and simple technology, the overall quality (sensory and safety) of grapes could be improved significantly (Guillen and others 2007).
In conclusion, the present study suggests that the right combination among aroma notes and antimicrobial activity must be contemplated and evaluated to optimize the use of EOs as natural additives for fresh-cut produce, fulfilling the requirements of consumers of natural preserved and flavorful, healthy, and convenient fruits and vegetables. We have to considered that new appeals and tastes are always welcome for modern consumers especially when the source are of natural origin.