The combined action of carvacrol and high hydrostatic pressure on Listeria monocytogenes Scott A

Combined application of origanum vulgare l. essential oil and acetic acid for controlling the growth of staphylococcus aureus in foods

This study evaluated the occurrence of an enhancing inhibitory effect of the combined application of Origanum vulgare L. essential oil and acetic acid against Staphylococcus aureus by the determination of Fractional Inhibitory Concentration (FIC) index and kill-time assay in nutrient broth, meat broth and in a food model (meat pieces). Acetic acid showed MIC and MFC of 0.6 and 1.25 μL.mL^-1, respectively. For O. vulgare essential oil MIC and MBC were 1.25 and 2.5 μL.mL^-1, respectively. FIC indexes of the mixture of essential oil and acetic acid at MIC x ½ were ≤ 1.0, showing an additive effect. No synergy was found at kill-time study. Anti-staphylococcal effect of the antimicrobials alone or in mixture (MIC x ½) was lower in meat than in nutrient and meat broths. The effective combination of essential oils and organic acids could appear as an attractive alternative for the food industry, as the doses to inhibit the microbial growth in foods can be lowered.

High pressures in combination with antimicrobials to reduce Escherichia coli O157:H7 and Salmonella Agona in apple juice and orange juice

The effect of high pressure processing in conjunction with the chemical antimicrobials, dimethyl dicarbonate (DMDC), hydrogen peroxide, cinnamic acid, potassium sorbate, and sodium benzoate (NaB) on E. coli O157:H7 strain E009 and Salmonella enterica serovar Agona was investigated in apple juice and orange juice, respectively. Juices were inoculated with approximately 10(6) CFU/ml and subjected to pressures of 550 MPa (E. coli O157:H7 samples) and 400 MPa (Salmonella Agona samples) for 2 min at 6 degrees C (initial temperature). Populations of each pathogen were determined before pressurization, immediately after pressurization, and after samples had been held after treatment for 24 h at 4 degrees C. The most effective treatment for E. coli O157:H7, as determined by plating immediately after pressurization, was 125 ppm of DMDC, which caused a >4.98-log reduction. Other treatments that were significantly different from the sample with no added antimicrobial were 62.5 ppm of DMDC, 300 ppm of hydrogen peroxide, and 500 ppm of NaB, which produced 4.97-, 5.79-, and 3.91-log total reductions, respectively. After 24 h at 4 degrees C, E. coli O157:H7 was undetectable in all treatment groups (and controls). In samples inoculated with Salmonella, the most effective treatment was 62.5 ppm of DMDC, which produced a 5.96-log decrease immediately after pressure treatment. The results for 1,000 ppm of NaB, which produced a 3.26-log decrease, also were significantly different from those for the sample containing no antimicrobials. After 24 h at 4 degrees C, all samples with added antimicrobials had near or more than a 5-log total reduction of Salmonella Agona.

Synergistic effect of thymol and carvacrol combined with chelators and organic acids against Salmonella Typhimurium

To identify synergistic combinations of different food additives, the antimicrobial effects of thymol and carvacrol against Salmonella Typhimurium were assessed alone and in combination with various other preservatives including EDTA, acetic acid, lactic acid, and citric acid. Overall, growth of Salmonella Typhimurium was significantly inhibited in Mueller-Hinton broth containing thymol, carvacrol, EDTA, acetic acid, lactic acid, or citric acid at concentrations of 400 mg/liter, 400 microl/liter, 300 mg/liter, 0.2% (vol/vol), 0.2% (vol/vol), and 0.2% (wt/vol), respectively. The combination of different antimicrobials such as thymol or carvacrol with EDTA, thymol or carvacrol with acetic acid, and thymol or carvacrol with citric acid all resulted in significantly reduced populations of Salmonella Typhimurium. In samples treated with combinations, these antimicrobials had synergistic effects compared with samples treated with thymol, carvacrol, EDTA, acetic acid, or citric acid alone. However, the combined use of lactic acid with thymol or carvacrol did not produce a synergistic effect against Salmonella Typhimurium. Thus, some chelators or organic acids can be used as food preservatives in combination with thymol and carvacrol to reduce the concentrations needed to produce an adequate antimicrobial effect.

In vitro assessment of antimicrobial activity of carvacrol, thymol and cinnamaldehyde towards Salmonella serotype Typhimurium DT104: effects of pig diets and emulsification in hydrocolloids

AIMS

To determine the effect of pig diets in vitro on the antimicrobial activity of carvacrol, thymol and cinnamaldehyde, and to identify an emulsifier/stabilizer that can stabilize the essential oil (EO) components in aqueous solution and retain their antimicrobial activity in the presence of the diets.

METHODS AND RESULTS

Emulsification of essential oil components with hydrocolloid solution was achieved by blending with a Polytron. Antimicrobial activity was measured through in vitro assays to determine the inhibition of bacterial growth by measuring the optical density at 600 nm or plating on nutrition agar after incubation of the mixtures of an EO component with the culture of Salmonella serotype Typhimurium DT104 in the presence or absence of pig diets. The results generated through the in vitro assays indicated that pig diets were able to abolish the antimicrobial activity of EOs. Xanthan, fenugreek and yellow mustard gums were the best in forming stable emulsions of five different EO components among ten different plant polysaccharides and surfactants examined. Emulsification of all the EO components in the fenugreek gum solution did not alter their antimicrobial activity. However, the antimicrobial activity of geraniol was significantly reduced when emulsified with other polysaccharides and surfactants. Both fenugreek and xanthan gum solutions were unable to protect the antimicrobial activity of carvacrol and thymol when mixed with the diets. Although cinnamaldehyde required no emulsification, but a high concentration (equivalent to at least three times of minimum bactericidal concentration for cinnamon oil) to inhibit Salmonella growth significantly in the presence of the diets, emulsification in fenugreek gum appeared to be essential for cinnamaldehyde solution to retain its antimicrobial activity during storage.

CONCLUSIONS

The diets for newly weaned pigs were a significant factor limiting the antimicrobial activity of EOs and their components. Cinnamaldehyde required a high concentration to retain its antimicrobial activity in the diets, in addition to its requirement for emulsification to stabilize its activity during the storage.

SIGNIFICANCE AND IMPACT OF THE STUDY

The assay with the diets used in this study for measuring the antimicrobial activity can be used in vitro for rapid and effective screening of potential antimicrobials for swine production. This study has identified polysaccharides that are able to stabilize EO component solutions. It has also identified cinnamaldehyde for further in vivo studies that may have potential in future application in controlling Salmonella and possibly other enteric pathogens in swine production.

Use of phenolic compounds for sensitizing Listeria monocytogenes to high-pressure processing

Three Listeria monocytogenes strains (Scott A, OSY-8578, and OSY-328) that differ considerably in barotolerance were grown to stationary phase and suspended individually in phosphate buffer (pH 7.0). Twelve phenolic compounds, including commercially used food additives, were screened for the ability to sensitize L. monocytogenes to high-pressure processing (HPP). Each L. monocytogenes strain was exposed to each of the 12 phenolic compounds (100 ppm each) for 60 min; this was followed by a pressure treatment at 400 MPa for 5 min. Six phenolic compounds increased the efficacy of HPP against L. monocytogenes but tert-butylhydroquinone (TBHQ) was the most effective. The additives alone at 100 ppm were not lethal for L. monocytogenes. Subsequently, the three L. monocytogenes strains were exposed to TBHQ before or after pressure treatments at 400 or 500 MPa for 5 min. When TBHQ was added after the pressure treatment, the combined treatment was more lethal than was pressure alone. However, the lethality attributable to TBHQ was greater when the additive was applied before rather than after pressure treatment. The inactivation kinetics of the L. monocytogenes strains at 300, 500, and 700 MPa, in the presence or absence of TBHQ, was investigated. All survivor plots showed non-linear inactivation kinetics, but tailing behavior was most pronounced when HPP was used alone. Combinations of TBHQ and HPP eliminated tailing behavior when survivors were monitored by direct plating or an enrichment procedure. Pressure and phenolic additives are apparently a potent bactericidal combination against L. monocytogenes.

Inactivation of barotolerant Listeria monocytogenes in sausage by combination of high-pressure processing and food-grade additives

Food-grade additives were used to enhance the efficacy of high-pressure processing (HPP) against barotolerant Listeria monocytogenes. Three strains of L. monocytogenes (Scott A, OSY-8578, and OSY-328) were compared for their sensitivity to HPP, nisin, tert-butylhydroquinone (TBHQ), and their combination. Inactivation of these strains was evaluated in 0.2 M sodium phosphate buffer (pH 7.0) and commercially sterile sausage. A cell suspension of L. monocytogenes in buffer (10(9) CFU/ml) was treated with TBHQ at 100 ppm, nisin at 100 IU/ml, HPP at 400 MPa for 5 min, and combinations of these treatments. Populations of strains Scott A, OSY-8578, and OSY-328 decreased 3.9, 2.7, and 1.3 log with HPP alone and 6.4, 5.2, and 1.9 log with the HPP-TBHQ combination, respectively. Commercially sterile sausage was inoculated with the three L. monocytogenes strains (10(6) to 10(7) CFU/g) and treated with selected combinations of TBHQ (100 to 300 ppm), nisin (100 and 200 ppm), and HPP (600 MPa, 28 degrees C, 5 min). Samples were enriched to detect the viability of the pathogen after the treatments. Most of the samples treated with nisin, TBHQ, or their combination were positive for L. monocytogenes. HPP alone resulted in a modest decrease in the number of positive samples. L. monocytogenes was not detected in any of the inoculated commercial sausage samples after treatment with HPP-TBHQ or HPP-TBHQ-nisin combinations. These results suggest that addition of TBHQ or TBHQ plus nisin to sausage followed by in-package pressurization is a promising method for producing Listeria-free ready-to-eat products.

Inactivation of Colletotrichum gloeosporioides spores by high hydrostatic pressure combined with citral or lemongrass essential oil

Anthracnose, caused by the fungus Colletotrichum gloeosporioides, is the main post-harvest disease of the papaya. Inactivation of the spores of C. gloeosporioides in saline solution by the use of high hydrostatic pressure, citral oil and lemongrass oil, alone and in combination, was studied. C. gloeosporioides spores were efficiently inhibited after a pressure treatment of 350 MPa for 30 min. When C. gloeosporioides was treated with 0.75 mg ml(-1) of citral or lemongrass oil, the pressure needed to achieve the same spore inhibition was 150 MPa. This work suggests the use of high hydrostatic pressure and plant essential oils as an alternative control for fruit diseases.

Essential oils: their antibacterial properties and potential applications in foods--a review

In vitro studies have demonstrated antibacterial activity of essential oils (EOs) against Listeria monocytogenes, Salmonella typhimurium, Escherichia coli O157:H7, Shigella dysenteria, Bacillus cereus and Staphylococcus aureus at levels between 0.2 and 10 microl ml(-1). Gram-negative organisms are slightly less susceptible than gram-positive bacteria. A number of EO components has been identified as effective antibacterials, e.g. carvacrol, thymol, eugenol, perillaldehyde, cinnamaldehyde and cinnamic acid, having minimum inhibitory concentrations (MICs) of 0.05-5 microl ml(-1) in vitro. A higher concentration is needed to achieve the same effect in foods. Studies with fresh meat, meat products, fish, milk, dairy products, vegetables, fruit and cooked rice have shown that the concentration needed to achieve a significant antibacterial effect is around 0.5-20 microl g(-1) in foods and about 0.1-10 microl ml(-1) in solutions for washing fruit and vegetables. EOs comprise a large number of components and it is likely that their mode of action involves several targets in the bacterial cell. The hydrophobicity of EOs enables them to partition in the lipids of the cell membrane and mitochondria, rendering them permeable and leading to leakage of cell contents. Physical conditions that improve the action of EOs are low pH, low temperature and low oxygen levels. Synergism has been observed between carvacrol and its precursor p-cymene and between cinnamaldehyde and eugenol. Synergy between EO components and mild preservation methods has also been observed. Some EO components are legally registered flavourings in the EU and the USA. Undesirable organoleptic effects can be limited by careful selection of EOs according to the type of food.

Characterization of a Listeria monocytogenes Scott A isolate with high tolerance towards high hydrostatic pressure

An isolate of L. monocytogenes Scott A that is tolerant to high hydrostatic pressure (HHP), named AK01, was isolated upon a single pressurization treatment of 400 MPa for 20 min and was further characterized. The survival of exponential- and stationary-phase cells of AK01 in ACES [N-(2-acetamido)-2-aminoethanesulfonic acid] buffer was at least 2 log units higher than that of the wild type over a broad range of pressures (150 to 500 MPa), while both strains showed higher HHP tolerance (piezotolerance) in the stationary than in the exponential phase of growth. In semiskim milk, exponential-phase cells of both strains showed lower reductions upon pressurization than in buffer, but again, AK01 was more piezotolerant than the wild type. The piezotolerance of AK01 was retained for at least 40 generations in rich medium, suggesting a stable phenotype. Interestingly, cells of AK01 lacked flagella, were elongated, and showed slightly lower maximum specific growth rates than the wild type at 8, 22, and 30 degrees C. Moreover, the piezotolerant strain AK01 showed increased resistance to heat, acid, and H(2)O(2) compared with the wild type. The difference in HHP tolerance between the piezotolerant strain and the wild-type strain could not be attributed to differences in membrane fluidity, since strain AK01 and the wild type had identical in situ lipid melting curves as determined by Fourier transform infrared spectroscopy. The demonstrated occurrence of a piezotolerant isolate of L. monocytogenes underscores the need to further investigate the mechanisms underlying HHP resistance of food-borne microorganisms, which in turn will contribute to the appropriate design of safe, accurate, and feasible HHP treatments.