Antimicrobial Agents of Plant Origin

In recent years, antimicrobial properties of essential oils (EOs) have been documented, and interest in these EOs persists. A wide range of natural antimicrobial agents derived from essential oils has attracted much attention in the food and packaging industries as a replacement for synthetic ones to protecting food products from microbial contamination. The essential oils are aromatic oil liquids obtained mostly from plant material. They exhibit antiviral, antibacterial, antimycotic, antitoxigenic, antiparasitic and insecticidal properties, and their use is allowed in European Union (EU) in food (as flavoring, either extracted from plant material or synthetically manufactured), perfumes and pharmaceuticals (Burt, 2004). Several essential oils were used as antimicrobial agents to prolong the shelf life of dairy products.

The potential antimicrobial effectiveness of rosemary essential oil (REO) on Escherichia coli inoculated on Coalho cheese was evaluated. The E. coli strain EC16 was used to inoculate two samples of commercial cheese, and then one sample was added with 20% (v/v) REO, and another sample was used without the addition of REO as a control. Results showed that during seven days of refrigeration, the rosemary essential oil has an inhibitory activity against the E. coli strain EC16 in vitro and in vivo (Ribeiro et al., 2013). The antimicrobial effectiveness of basil oil and their principal constituents (linalool or methylchavicol) was evaluated. Tsiraki and Savvaidis (2013) assessed the effect of basil essential oil on the quality characteristics of whey cheese “Anthotyros.” The experiment was performed by adding the basil EO to the cheese samples to a final concentration of 0.4% (v/w). The sensory evaluation and microbiological data showed that the combined use of modified atmosphere packaging (MAP) and vacuum packaging (VP) with added basil EO extended the shelf life of fresh Anthotyros by approximately 10 to 12 days for samples packaged in MAP and 6 days for those in VP as compared to aerobic packaging. The principal constituents of basil oil (linalool or methylchavicol) were mixed with low-density polyethylene (LDPE) pellets to manufacturing antimicrobial films (AM). Suppakul et al. (2008) tested the inhibitory effect against microbial growth on cheddar cheese wrapped with the antimicrobial films and stored at 4°C. In addition, cheese samples inoculated with E. coli or Listeria innocua were wrapped with the AM films, stored at refrigerated (4°C) or at abuse (12°C) temperatures. In abuse temperature conditions, methylchavicol-LDPE based film exhibited a higher efficacy of inhibition than that of linalool-LDPE based film. An antimicrobial starch-based film was realized by incorporating linalool, carvacrol, and thymol in the coating of the film. Then, the inhibitory effect against S. aureus and A. niger of the antimicrobial agents was tested on the surface of cheddar cheese (Kuorwel et al., 2011; 2012). Current studies suggest that linalool, carvacrol, or thymol, could be successfully coated onto starch-based films to produce packaging materials that exhibit activity against S. aureus and A. niger. The inhibitory effect of the essential oils depended on the concentration of the active agent in the film coating and was found to be in the order of thymol>carvacrol>linalool.

Ramadan et al. (2014) studied the antimicrobial effectiveness of black cumin seed oil against some pathogenic bacteria (S. aureus, E. coli, L. monocytogenes and S. enteritidis) inoculated in soft cheese during cold storage. For the cheese supplemented with essential oil (0.1% or 0.2%, v/w), the counts of the inoculated pathogens were significantly reduced by about 1.3 log10 and 1.5 log10 CFU g-1 after 21 days of storage.

A study was conducted by Belewu et al. (2012) to compare the effect of different levels of eucalyptus oil (EO) and lemongrass oil (LO) on the shelf life of fresh West African soft cheese. The experiment was performed by adding the essential oils at different concentrations (75% EO + 25% LO and 50% EO + 50% LO) to the cheese samples; moreover, cheese kept in the whey was used as control. The results showed that the eucalyptus oil 75% plus 25% lemongrass had a positive impact on the nutritional, sensory, and microbial values; whey did not significantly enhance the nutritional, sensory, and microbial qualities of the soft cheese. The antibacterial activity of oregano and thyme essential oils added at doses of 0.1 or 0.2 and 0.1 mL/100 g, respectively, to feta cheese inoculated with E. coli O157:H7 or L. monocytogenes and stored under modified atmosphere packaging was investigated. In feta cheese with oregano oil at the dose of 0.1 ml/100 g, the E. coli O157:H7 or L. monocytogenes, strains survived up to 22 or 18 days of storage, respectively, whereas at the dose of 0.2 mL/100 g up to 16 or 14 days of storage, respectively. The antibacterial activity of thyme oil at 0.1 mL/100 g against either E. coli O157:H7 or L. monocytogenes was similar to that of oregano oil at 0.1 mL/100 g in feta cheese (Govaris et al., 2011).

The efficiency of plant essential oils as natural food preservatives in Fior di latte cheese was assessed by Gammariello et al. (2008). The Fior di latte cheese samples were packaged in polypropylene tubs using either brine or active brine. The use of natural compounds such as lemon, sage, and thyme in Fior di latte cheese inhibited the growth of microorganisms involved in spoilage. Conte et al. (2007) evaluated the effectiveness of three different concentrations (500, 1000, 1500 ppm) of lemon extract on the microbial quality decay kinetics during storage of mozzarella cheese. The active agent was added to salt solutions with and without adding sodium alginate to obtain, respectively, an active solution and an active gel. The results confirmed that the investigated substance might exert an inhibitory effect on the growth of spoilage microorganisms such as coli- forms and Pseudomonas spp. without affecting the functional dairy microbiota and the sensorial characteristics of the product. The shelf life of Minas frescal cheese made with oregano essential oil was estimated. Different concentrations of oregano essential oil (0.0075% and 0.015%) were used in the samples, and the sample without active agents was used as control. The researchers found that the count of Staphylococcus coagulase positive, aerobic mesophilic, psychrotrophic, coliforms, and Salmonella were within the limits established by applicable Brazilian legislation until to 28 days. Moreover, the sensorial analysis of all samples did not show significant differences for the attributes assessed (Santos et al., 2014). An alginate-based edible coating loaded with essential oils of rosemary and oregano was applied on ricotta cheese in order to evaluate the physical, chemical, microbiological, and sensory characteristics. The authors found that there was no difference between the samples in relation to sensory analysis, mass loss, texture, and color (Tavares et al., 2014). Also, Asensio et al. (2014) evaluated the antimicrobial activity of Argentinean oregano essential oils on flavored ricotta cheese. The results showed that ricotta flavored with oregano essential oil improves their physicochemical characteristics and lessens its microbiological count, especially ricotta supplemented with oregano essential oil obtained from variety Cordobes.

Thyme and oregano essential oils were used against S. aureus both in vitro and on fresh cheese. In vitro test, the antimicrobial power against the S. aureus of the EOs of thyme and oregano has been proven; whereas, in vivo the results showed that the concentration of S. aureus remained almost unchanged for all types of cheeses produced (Amatiste et al., 2014). The effects of some plant-derived essential oils (cinnamon, cumin, mint, dill, and caraway essential oils) on the shelf life of Labneh traditional fermented milk were tested (Thabet et al., 2014; Wafaa et al., 2013). The results suggested that the cinnamon oil at 0.3% extended the shelf life for up to 24 day at 6±1°C with acceptable taste and flavor, and without any microbial spoilage. The dill and caraway essential oils enhanced the cheese quality and the organoleptic properties and also extended the shelf life to 28 days.

Also, Otaibi and Demerdash (2008) estimated improvement of the quality and shelf life of concentrated yogurt (Labneh) by the addition of some essential oils. In particular, thyme, marjoram, and sage were added at concentrations of 0.2, 0.5, and 1.0 ppm. Results indicated that the Labneh containing 0.2 ppm thyme, marjoram, or sage oils was the most acceptable after the control. An amount of 0.2 ppm thyme, marjoram, or sage oils increased the shelf life of Labneh for up to 21days. Mohamed et al. (2013) found that among seven screened essential oils, caraway as well as dill seeds oil had the highest antibacterial effect. The two selected essential oils were incorporated in cheese yogurt manufactured from milk, which was previously contaminated by five strains of pathogens (L. monocytogenes, S. aureus, Bacillus cereus used as Gram-positive bacteria and E. coli and S. typhimurium used as Gram-negative). The results highlighted that the caraway and dill seeds oils had the same ability to inhibit the growth of all pathogens and were the most inhibiting to L. monocytogenes and S. typhimurium. The physicochemical and microbiological qualities of yogurt samples treated with anise volatile oil and its oleoresin at varying concentrations (0.1 to 1.0 g/L) were assessed up to 20 days at 4±1°C. The results revealed that incorporation of essential oil and oleoresin of anise at 1.0 g/L concentration was moderately effective in controlling the growth of spoilage microorganisms and also suggested that it had no adverse effect on the physicochemical properties of yogurt (Singh et al., 2011).

Alsawaf and Alnaemi (2011) carried out a study on antibacterial effect of Nigella sativa (seed and oil) on the quality of soft white cheese. Nigella sativa seed (1% and 3%) and oil (0.3% and 1%) were tested on some food poisoning and pathogenic bacteria as well as on the total bacterial count. A decrease in the total bacterial count of S. aureus, Brucella melitensis, and E. coli count in cheese samples treated with N. sativa seed and oil was observed. Moreover, N. sativa oil was more effective as an antibacterial agent than seed. Dried spearmint and its essential oils were used as preservatives in white cheese. The authors found that the oil showed significant reduction in the total bacterial count, proteolytic and psychrotrophic bacteria count, as compared with control (Foda et al., 2009). An innovative Karish cheese was made by adding curcumin (0.3% w/v) to obtain a new dairy product named Karishcum. The behavior of pathogenic bacteria in artificially contaminated cheese revealed that the addition of aqueous curcumin extract achieved a reduction of bacterial counts, about one log cycle of S. typhimurium, two log cycles of Pseudomonas aeruginosa and E. coli, respectively. S. aureus, B. cereus and L. monocytogenes vanished at the end of the storage period (Hosny et al., 2011). Moreover, Metwalli (2011) achieved a Karish cheese by mixing propolis (5%, 10%), garlic, and ginger (0.5%), respectively, essential oils, in order to improve the quality of the cheese. Total bacterial count, yeast, and mold decreased in all treated samples with respect to the control cheese; lipolytic bacteria counts were affected by propolis more than essential oils. The use of propolis and essential oils increased the shelf life of Kareish cheese for 30 days while imparting good flavor.

Essential oils can be also used as antimicrobial volatile compounds incorporated in sachets. Pires et al. (2009) and Han et al. (2014) studied the antimicrobial effectiveness of sachets incorporating allyl-isothiocyanate (AIT), rosemary, and thyme oils, respectively, on the quality of mozzarella cheese. The sachet incorporated with AIT was tested against growth of yeasts and molds, Staphylococcus spp. and psychrotrophic bacteria on sliced mozzarella cheese. The authors found that yeasts and molds were the most sensitive to the antimicrobial effects; while, psychrotrophic bacteria species were the most resistant to the antimicrobial action. The efficacy of the volatiles of the oils (rosemary and thyme) embedded in a sachet containing microcellular foam starch was tested to reduce bacterial growth of L. monocytogenes inoculated in shredded mozzarella cheese. The results showed that rosemary and thyme oil volatiles released from the sachet restricted the growth of L. monocytogenes, resulting in a 2.5 log10 CFU/g reduction on day 9 at 10°C compared to untreated samples.

 
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