ANTIBACTERIAL ACTIVITY OF ESSENTIAL OILS (EOS)

The EOs being hydrophobic compounds, the primary site of action on bacteria will be cell membranes. They dissolve into the lipid portion of the cell membrane disrupting its fluidity thereby increasing the permeability [112]. This irreversible disruption causes disruption in pH gradient and leakage of divalent cations and other cellular molecules. The leakage of cellular contents beyond critical limit of the bacterial cell can lead to cell death [18, 50, 145].

Moreover, EOs depending upon the composition and outcome of interactions between individual components can act on single or multiple targets simultaneously [32, 50, 129]. Therefore, sometimes the overall activity cannot be attributed to any one constituent. Here many researchers argue on preference to whole EO over purified individual active component [31, 76].

Chemical structure of biocomponents in selected essential oils

FIGURE 2.1 Chemical structure of biocomponents in selected essential oils.

The antibacterial activity and their concerning mode of action is believed to be associated with the chemical structure of EO. The hydroxyl group in the phenolic compounds and relative positive and lipophilic character of the hydrocarbon skeleton and functional groups of EOs are important in exerting antibacterial activity of the components [141]. Therefore, EOs characterized by phenols and aldehyde (such as carvacrol, thymol, cinnamaldehyde, and eugenol) generally have high antibacterial activity on a wide range of bacteria comprising not just pathogenic but also spoilage causing [77]. Also, recent studies demonstrated that EOs have significant antimicrobial potential against antibiotic resistant bacteria (ARB) [52]. Other EOs characterized by alcohols, ketones, esters, and hydrocarbons also has vital antibacterial activities [77]. The low susceptibility is linked to obstruction offered by outer membrane for diffusion of EOs [50]. Interestingly, terpineol-4-ol was stronger on S. enteritidis and E. coli than L. monocytogenes [2]. Nevertheless, the antibacterial activity is linked to number and concentrations of low molecular components present in the EO and their interactions. The specific mechanisms and underlying associated molecular aspects are needed to be further explored.

COMBINED APPROACH BASED ON ESSENTIAL OIL (EO)

The hurdle approach is a combined application of several food preservation treatments intended to enhance the lethal effects. When lethality of combined process is higher than individual processes together, the outcome is synergy whereas lethality of combined processes is less than the sum of separate applications (Figure 2.2). The most successful combinations should be those, which results in higher lethality than separate applications. Nonetheless, in food applications, synergistic effect is preferred over additive effect [88,122]. In this sense, EOs can be combined with thermal or non-thermal processes or antimicrobials to increase the lethality of processes. The greater inactivation is presumably due to the stress exerted at multiple targets simultaneously causing damages to several functions of the cell [2, 55, 79].

The successful combinations with the resultant effectiveness should be able to reduce the effective concentrations and consequently minimize undesirable change on organoleptic acceptability of foods without compromising the level of inactivation [44]. The strategy of combining EOs with physical treatments (Heat, PEF, HHP, ultrasound, ohmic heating (OH), radiation, etc.) and antimicrobial additives (bacteriocins, EOs, organic acids, etc.) in food processing has been demonstrated in scientific studies (Table 2.1).

ESSENTIAL OILS (EOS) COMBINED WITH PHYSICAL TREATMENTS

ESSENTIAL OILS (EOS) COMBINED WITH HEAT

Thermal processing is a traditional popular industrial method applied to control foodbome diseases and spoilage. Heat resistance of target bacteria is characterized by D-value and Z-value indices that are used to check death kinetics of any bacteria and status of decimal reduction in bacterial count. D-value (decimal reduction time or decimal reduction dose) is the time or dose) required, at a given condition (e.g., temperature) or set of conditions, to achieve a log reduction, that is, to kill 90% (or 1 log) of relevant microorganisms. Z-value is the reciprocal of the slope resulting from the plot of the logarithm of the D-value versus temperature. EOs increased the bacterial sensitivity to heat, allowing reduction in the intensities of each treatment required to inactivate the same population [73]. Addition of vanillin to TSPYE (pH5) brought down the Dx of C. sctkazctkii from 14 min to less than 1 min [148]. Similarly, a five-fold reduction was reported in fruit juice with I algeriensis EO against E. coli 0157: H7 [3]. In this, EOs, and heat together proved to decrease the requirements of both quantity of EO and thermal intensity. This is a great advantage over the impact of high heat on final quality and cost of foodstuffs such as milk [23], fruit juices [24], and meat-based product [6]. In milk with vanillin 1400 ppm, Cava et al. [23] noted an average 25% reduction in time required for killing of log 4 cells L. monocytogenes. Similarly, in apple juice, 200 pl/ml of EO reduced the required temperature by 4.5°C [42].

Possible outcomes of combining different technologies of food preservation

FIGURE 2.2 Possible outcomes of combining different technologies of food preservation.

Essential Oil Tested

Combined With the Process or Other Biocomponnd

Media

Target Microorganism Tested

Log Cycles Reduction Observed

References

(+)-limonene

HHP( 175-400 MPa for 20 min)

Buffer

E. coll 0157:H7 and L. monocytogenes EGD-e

4-5

[40]

(+)-limonene.

HHP

Fruit juices

Escherichia coli 0157:H7

5

[40]

Basil

Rosemary

Chicken meat

S. Enteritidis

4

[136]

Carvacrol

Nisin

Buffer

L. mononytogenes B. cereus

3

[120]

Carvacrol (0.2 iU uiL)

Mild heat (54°C/10 min)

Apple juice

Leuconostoc sp. Saccharomyces sp.

4

[27]

Carvacrol (1% w/w)

Electrolyzed water

Shredded cabbages

Mesophilic and psyclirotropic bacteria

<1

[135]

Carvacrol (2.5-3 rnM)

HPP (250-300 MPa/20 min)

Laboratory media

L. monocytogenes

>6

[78]

Carvacrol (3 mM)

HPP (300 MPa/20 min)

Milk

L. monocytogenes

3.2

[78]

Cinnamon (1-5%

w/v)

PEF (10-30 kV/cm, 60-3000 ps)

Skun milk

Salmon ell a typhimuii urn

>1.96

[117]

Cinnamon bark oil (5 inl/100 ml)

PEF (35 kV/cm, 1700

US)

Orange, strawberry, apple, and pear juice

S. enteritidis E. coli

5-6

[105]

Essential Oil Tested

Combined With the Process or Other Biocoinpound

Media

Target Microorganisin Tested

Log Cycles Reduction Observed

References

Citral

Heat (54°C)

Phosphate buffer and apple juice

E. coli 0157 :H7

>3

[44]

Cuminum cyminum (15 pL/rnl)

L. acidophilus (0.5%)

White brined cheese

S. aureus

1

[124]

Lemon (200 pL'L)

Mild heat (54-60°C/10 min)

Apple juice

E. coli 0157 :H7

5

[42]

Lemon (200 pL'L)

PEF (25 kV and 100 KJ/ Kg) and heat (60°C)

Liquid whole egg

Salmonella saftenberg and L. mononytogenes

4

[41]

Mandarin (0.05% v/w)

y-irradiation

Green bean

L. innocua

3.3

[128]

Mandarin (0.05% v/w)

UV-C

Green bean

L. innocua

3

[128]

Mentha pulegium (0.2-0.86 pL/rnL)

Heat (54-60°C)

Apple juice

Escherichia coli 0157:H7

5

[3]

Metasequoia

glyptostroboides

(1-2%)

Nisin (62.5-500 IU/ml)

Milk

L. mononytogenes

6

[149]

Mint essential oil (0.5 and 1 (iL/mL)

HPP (100-300 MPa/3.5 min)

Yogurt

L. innocua and L. monocytogenes

>5-6

[47]

Orange, lemon, and mandarin

Heat (54°C/10 min)

5

[43]

Essential Oil Tested

Combined With the Process or Other Biocompound

Media

Target Microorganism Tested

Log Cycles Reduction Observed

References

Oregano

(0.01-0.025%v/v)

Ultrasound (26 kHz. 90 pm, 200 W, 14 mm 0, 300 sec for 5-25 min)

Lettuce

E. coli 0157:H7

>2

[101]

Oregano (0.2%)

Caprylic acid (0.5%)

Vacuum packed minced meat

L. mononytogenes, LAB.

psychrotrophic bacteria

  • 2.5
  • 1.5
  • 1.5

[68]

Oregano, lemon glass

Gamma irradiation (0.5 and 1 kGy)

UV-C (5 and 10 kJ/in2)

Cauliflower

L. mononytogenes

Escherichia coli 0157:H7

4.5

[138]

Perilla oil (1 mg/ml)

Nisin (15 pg/ml)

Pasteurized milk

L. mononytogenes S. aureus

>6

>2.9

[151]

Rosemary

Thyme

Mozzarella cheese

L. mononytogenes

1.7

[66]

Vanillin

Cinnamon and clove

Milk

L. mononytogenes E. coli 0157:H7

5

[22]

Vanillin (900-1900 ppm)

Heat (50-28°C)

Sweetened lassi

E. coli 0157:H7

4

[53]

At respective lethality, the heat sometimes generates sub-lethally injured cells and induces stress responses [114]. The repair of injuries under suitable conditions, especially in food-borne pathogens, is a food safety risk. Besides meeting the inactivation requirements of mild heat, combining with EO may control the regrowth of heat injured or adapted cells [74]. Amalaradjou et al. [6] investigated the efficacy of cooking ground beef (60 and 65°C) coupled with trans-cinnamaldehyde (0, 0.1, and 0.3%) in inactivating E. coli 0157:H7 and fate of survivors throughout one-week storage at 4 or -18°C. There was a significant reduction in D-values and no increase in cell numbers in combination (P<0.05) [6, 132].

Bacterial spore formers are well-known for poor sensitivity towards heat treatment and antimicrobials. The combination approach has also been tested against spore formers. The direct addition of EO to the heating medium was only able to show a small decrease in apparent heat resistance of spore formers and can be useful to control the germination of heat treatment surviving spores [45]. Perigo et al. [116] observed a higher loss of viability of B. cereus following mild heat treatment in carrot juice with the addition of carvacrol or thymol (0.3 mmol/L). The chilling of ground turkey with EO components was effective in controlling germination and growth of spores of Clostridium perfringens [72, 75]. Further, the overall changes in the final product during the storage should be considered.

Most of the EO and heat combined treatments were carried out by the direct addition of antimicrobials in food media. In this case, the bioavailability of EO depletion is recognized to poor solubility in aqueous media. Some researchers found that nano-emulsions may decrease the thermal resistance of pathogens in liquid foods [92]. Mate et al. [98] found that nano-emulsion of D-limonene (0.5 mM) combined with heat was better than direct addition in increasing the thermal sensitivity of L. monocytogenes. The strong lethal effect was probably based on the damages facilitated by heat on the bacterial membranes allowing EOs to penetrate the cells to reach the target sites. However, further studies are required on the mechanism of action. Overall, heat coupled with EO may be an attractive approach upon further standardization of variables such as heating temperature, EO miscibility, and intrinsic factors of foods.

 
Source
< Prev   CONTENTS   Source   Next >