The use of EOs has great potential, even in minimally processed vegetables. In fact, herbal EOs and their constituents were studied as alternative natural disinfectants for reducing the presence of spoiling and pathogenic bacteria. Due to the strong antimicrobial activity in vitro of oregano (
Origanum vulgare) and thyme (
Thymus vulgaris) EOs and their main constituents (carvacrol and thymol), these compounds are the most suitable to be used in minimally processed vegetables
[31,54,112,119,120][30][44][58][59][60].
For example, the inclusion of oregano and thyme EOs at a concentration of 250 ppm in the washing solution of minimally processed lamb’s lettuce resulted in a product shelf life comparable to the one obtained by using chlorine (120 ppm) in the washing step
[119][59]. In fact, the use of EOs resulted in a preliminary decrease in the microbial population. In addition, the color and turgidity of the lettuce were not affected throughout the storage period while the sensorial properties were not negatively affected
[119][59].
When used in the washing solution, oregano oil (
Origanum onites) was able to inactivate
Salmonella typhimurium inoculated on iceberg lettuce. The effectiveness of oregano EO treatment (75 ppm) was comparable to that of chlorine treatment at 50 ppm
[122][61].
When oregano EO was applied to ready to eat lettuce and carrots, the initial decontamination effect was comparable to that of chlorine
[127][62]. Furthermore, oregano EO did not negatively affect the color, texture, or water activity of the samples. However, sensory acceptance of the product treated with oregano EO was only observed for carrots. Volatile antimicrobial compounds such as borneol, carvacrol, cinnamaldehyde, eugenol, menthol, thymol, and vanillin were able to inhibit the growth of
Bacillus cereus inoculated in carrot juice. Direct application of EOs in the food system has some drawbacks due to their strong odor, chemical reactivity, hydrophobicity, low solubility, and potential negative interaction with the food matrix, leading to alteration of organoleptic properties. Several technological approaches have been tested for EO delivery in food. EO encapsulation is a novel and advanced delivery system. It provides enhanced antimicrobial efficacy as well as control over the release of EO flavors into the food system
[128][63].
3. Application of Essential Oils as Meat and Dairy Preservatives
3.1. Antimicrobials as Meat Preservatives
Fresh meat and fresh meat produce are extremely perishable and prone to oxidative and microbial spoilage. These products are easily subject to oxidation, which reduces nutritional value, affecting lipids, proteins, myoglobin (pigments), texture, and flavor
[135][64]. Plant extracts are a natural source of compounds with antioxidants and antimicrobial activity that can be used as an alternative to synthetic ones. EOs and their constituents have been successfully used in meat and meat products to prevent oxidation, degradation, and microbial proliferation
[136,137,138][65][66][67]. Promising results can be achieved with EOs from the
Lamiaceae family, including oregano, thyme, sage, and rosemary, which are commonly used as flavoring agents and prevent oxidative degradation. The antioxidant activity of EOs is well documented and can be attributed to phenolic compounds including eugenol, carvacrol, and thymol, which act as hydrogen donors and scavengers of free radicals
[139,140][68][69].
Fasseas and colleagues (2007)
[141][70] measured the antioxidant activity of pork and beef meats (raw and cooked) treated with sage and oregano EOs over a 12 day storage period at 4 °C. Compared to controls without added EOs, the EO-treated meat (raw and cooked) had higher TBA (thiobarbituric acid) scores and 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activity.
Meat contaminated with spoilage microorganisms shows color changes, the development of uncharacteristic and undesirable odors and tastes, and the formation of superficial slime
[144,145][71][72]. Bacteria belonging to the genera
Pseudomonas,
Acinetobacter,
Staphylococcus,
Brochothrix,
Moraxella,
Micrococcus, and
Flavobacterium, as well as LAB and genera belonging to the
Enterobacteriaceae family, are reported to be involved in the spoilage of meat and meat derivatives
[145,146][72][73].
Cell loads of foodborne pathogen
L. monocytogenes deliberately inoculated on ham (3 log CFU/g) were decreased by 10 and 19% after supplementation of
Cinnamon cassia and oregano EOs, respectively
[151][74].
Cinnamon cassia EO was also effective in promoting the shelf life of ground lamb meat throughout refrigerated storage at 4 °C. Compared to control samples, supplementation of
Cinnamon cassia EO (0.5%) reduced from the 4th to the 16th day the lactic acid bacteria spoilage population up to 1.9 log CFU/g and
Enterobacteriaceae up to 1.1 CFU/g
[152][75]. A reduced microbial spoilage population was also observed in minced meat with rosemary, thyme, and oregano EOs at concentrations between 1 and 1.5% (
v/
w). Compared to the control group, treated samples showed reduced loads of LAB, molds, and yeast
[153][76]. Thyme EOs tested at two different concentrations (0.02 and 0.05
w/
w) inhibited the growth of coliform in chicken hamburgers
[154][77].
The use of EOs as antioxidants and preservatives for meat is dependent on the bioactivity and stability of their components. Most EO constituents lose physical stability when they interact with meat constituents
[155][78], probably due to the binding ability of fats and proteins in meat to volatiles in EOs
[155][78]. When encapsulated EOs are added to edible coatings, plastic films, or meat during marinating, their stability increases significantly. The possibility of using active packaging supplemented with natural antioxidants and antimicrobials has recently been investigated
[58,156][79][80].
The stability, antioxidant, and antimicrobial properties of rosemary EO can be enforced by encapsulation with nanogel. Compared to samples treated with free rosemary EO, rosemary EO encapsulated in chitosan-benzoic acid-based nanogels showed greater efficacy in reducing the cell load of
Salmonella typhimurium inoculated on beef chops during storage at 4 °C
[128][63]. Chitosan-based coatings also improved the performance of oregano EO in reducing lipid oxidation in dry fermented sausages in comparison to control samples after 7 months of storage
[159][81].
LABs can be added for two different purposes: as starting materials for producing fermented meat products or as a bioprotective culture used only to compete with naturally occurring microflora and produce antimicrobial peptides
[59][82].
The constant release of antimicrobial peptides from the packaging to the product surface could help to maintain the bacteriocin concentration at the optimal level to carry out their bioprotective effect
[165][83]. In various studies, films containing the bacteriocin nisin have been successfully used to preserve meat products
[59][82]. Pullulan film containing Nisin Z, which was used for the packaging of refrigerated vacuum-packed raw beef and deli ham, strongly inhibited various foodborne pathogens, including
Salmonella spp.,
S. aureus,
L. monocytogenes, and
E. coli [167][84]. Furthermore, the efficacy of this nisin/pullulan film was improved by the addition of lauric alginate
[167][84]. Reduced growth of
S. aureus on chilled sliced beef was observed when packaged with an alginate-based palmitoylated film supplemented with nisin
[168][85]. Nisin-internalized cellulose envelopes demonstrated anti-
Listeria properties on chilled vacuum-packed frankfurters (sausages)
[169,170][86][87] and vacuum-packed hot dogs wrapped with a plastic film enriched with nisin. A combination of nisin and Nisaplin
® (a commercial product from DuPont) adsorbed in cellulose-based packaging paper was shown to inhibit
S. aureus,
L. innocua, and LAB on cooked ham
[171][88].
3.2. Essential Oils as Dairy Preservatives
Plants and spices have been used in cheese production since ancient times, often linked to local traditions. Traditionally, spices and herbs or their extracts were rubbed directly on the cheese. In fact, herbs and spices can be applied as antioxidant, antimicrobial, flavoring, enriching, and functionalizing ingredients, which may improve the appearance and appeal of the product
[179][89].
Although many dairy products undergo heat treatment during their manufacture, they are perishable and can be easily contaminated by microorganisms, resulting in food spoilage, consumer health risks, and a shortened product shelf life. Furthermore, as they are rich in lipids, oxidative processes can lead to a loss of flavor, nutrients, and color, the development of off-flavors, and the accumulation of compounds that may be of concern to human health
[180][90]. For that reason, there is growing interest in the application of plants, extracts, and EOs as natural preservatives in the dairy sector as an alternative to synthetic preservatives
[181][91]. The antimicrobial activity of natural products is commonly assessed against the main pathogens and spoilage bacteria commonly found in the dairy sector, such as
L. monocytogenes,
S. aureus,
E. coli,
Salmonella spp., and
Pseudomonas spp., as well as yeasts and molds, such as
Penicillium and
Aspergillus [179][89].
Natural products have also been proven in additional types of dairy products, including milk. Jemaa et al., 2017
[190][92] showed that EO of
Thymus capitatus ameliorated pasteurization effectiveness in maintaining raw milk quality. Incorporation of EOs from basil, peppermint, and zataria in the formulation of probiotic yoghurt increased the inhibition of
E. coli and
L. monocytogenes compared to yoghurt without EOs. However, only the EOs of basil and peppermint showed good antioxidant and antiradical activity along with good sensory acceptability
[191][93]. The application of
Echinophora platyloba EO and lycopene was effective as natural preservatives for dairy products with high fat content, such as butter and cream. Pasteurized cream treated with a mixture of EO and lycopene (0.5% and 50 ppm, respectively) showed improved shelf life (both from a microbiological and chemical point of view) compared to control samples. Sensory evaluation results showed that all the samples have satisfactory overall acceptability, although the greatest sensorial features were detected in creams prepared with a combination of low concentrations of
Echinophora EO and lycopene (0.1% and 20 ppm, respectively)
[192][94]. Other authors extended the shelf life of butter using thyme and cumin EOs
[193][95], while Ozkan et al. (2007)
[194][96] explored the potential of
Satureja cilicica EO as a natural flavoring and antioxidant in the same dairy product.
Although many studies highlight the beneficial properties of herbal preservatives, other papers report that dairy fats, carbohydrates, or proteins may interact with them and reduce their functional properties. Consequently, higher amounts of EOs are necessary to attain the desired effect, with subsequent drawbacks related to sensorial impacts. To overcome this aspect, different approaches have been investigated.
Nisin is permitted as a food additive for processed cheese by the FAO/WHO Codex Committee at a concentration of 12.5 mg/kg product, while the US FDA is permitted to use up to 250 mg/kg
[203][97]. It is generally known that the effectiveness of nisin is dependent on the dairy products considered. In particular, neutral-pH dairy products made from whole milk are not suitable for nisin use
[204][98].
4. Conclusions
Essential oils and bioactive compounds can be applied as food preservatives due to their antimicrobial and antioxidant activities, which prevent spoiling processes and guarantee food safety. On the other hand, an additional strategy to increase the safety and shelf life of various types of foods, including minimally processed fruits and vegetables, vegetable beverages, meat, and dairy products, can be represented by the use of protective cultures, especially lactic acid bacteria, which are able to produce antimicrobial compounds such as bacteriocins in addition to organic acids and hydrogen peroxide. Recently, innovative new technologies and delivery strategies such as nanoencapsulation or polymer-based coatings have improved the efficacy and allowed the controlled release of natural antimicrobials. In addition, the use of EOs and bioactive plant molecules in combination with bioprotective cultures or bacteriocins can exert an additive or synergistic effect and reduce the applied concentration of EO. The combination of these antimicrobial agents represents an interesting strategy to increase the shelf life and safety of food due to their antimicrobial and antioxidant properties. The synergistic combinations of EOs and LAB metabolites such as bacteriocins or nisins allow the exploration of promising ways to overcome both the narrow range of antimicrobial action and the unpleasant sensory properties of foods.