Oregano and Food Preservation

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Butch Owen <butchbsi

Sun, 06 Jun 2004 19:58:38 -0700

Oregano and Food Preservation

 

 

Wageningen University Dissertation No. 2799

 

Summary

 

Title: Bactericidal action of carvacrol towards the food pathogen

Bacillus cereus. A case study of a novel approach to mild food preservation

 

Author: Annemieke Ultee

2 June 2000

 

A new trend in food preservation is the use of mild preservation

systems, instead of more severe techniques such as heating, freezing or

addition of chemical preservatives. Carvacrol, a phenolic compound

present in the essential oil fraction of oreganum and thyme, is known

for its antimicrobial activity since ancient times. This thesis

describes a study of the antimicrobial activity of carvacrol towards the

foodborne pathogen B. cereus. Carvacrol shows a dose-related inhibition

of growth of B. cereus. Concentrations of 0.75 mM and higher inhibit

growth completely at 8°C. Below 0.75 mM, carvacrol extends the lag-phase

and reduces the specific growth rate as well as the final population

density. Exposure to 0.75-3 mM carvacrol decreases the number of viable

cells of B. cereus exponentially. Spores are approximately two fold more

resistant towards carvacrol than vegetative cells.

 

The incubation and exposure temperature have a significant influence on

the sensitivity of B. cereus to carvacrol. An increase of the growth

temperature from 8°C to 30°C decreases the fluidity of the membrane of

vegetative cells and as a consequence, B. cereus becomes less sensitive

to carvacrol. The change in membrane fluidity is probably the result of

a higher percentage of lower melting lipids in the membranes at 8°C

(chemical process) as an adaptation to lower temperature. Cells need to

maintain an adequate proportion of the liquid-crystalline lipid in the

membrane, as this is the ideal physical state of the membrane. On the

other hand, an increase of the exposure temperature from 8 to 30°C,

reduces the viability again. This can be explained by an increase of the

membrane fluidity at a higher temperature as a result of melting of the

lipids (physical process). At a higher membrane fluidity, relatively

more carvacrol can dissolve in the membrane and the cells will be

exposed to relatively higher concentrations than at a lower membrane

fluidity.

 

Not only the temperature plays a role in the activity of carvacrol, also

pH is an important factor. The sensitivity of B. cereus to carvacrol is

reduced at pH 7, compared to other pH-values between pH 4.5 and 8.5.

 

Carvacrol interacts with the cytoplasmic membrane by changing its

permeability for cations such as K+ and H+. Consequently, the

dissipation of the membrane potential ( ) and pH leads to inhibition of

essential processes in the cell, such as ATP synthesis, and finally to

cell death. At carvacrol concentrations as low as 0.15 mM, is completely

dissipated, however the viable count of B. cereus is not affected.

 

Vegetative cells of B. cereus can adapt to carvacrol when the compound

is present at concentrations below the MIC-value. Compared to

non-adapted cells, lower concentrations of carvacrol are needed to

obtain the same reduction in viable count of adapted cells. Adapted

cells were found to have a lower membrane fluidity, caused by a change

in the fatty acid composition and head group composition of the

phospholipids in the cytoplasmic membrane. Adaptation to 0.4 mM

carvacrol increases the phase transition temperature of the lipid

bilayer from 20.5°C to 28.3°C. Addition of carvacrol to cell

suspensions of adapted B. cereus cells decreases Tm again to 19.5°C,

approximately the same value as was found for non-adapted cells in the

absence of carvacrol.

 

Incubation of cooked rice in the presence of different carvacrol

concentrations results in a dose-related reduction of the viable count

of B. cereus. Concentrations of 0.15 mg/g and above, reduce the viable

count, leading to full suppression of growth at 0.38 mg/g. The influence

of carvacrol on the viable count is dependent on the initial inoculum

size. Although carvacrol is an effective inhibitor of growth of B.

cereus in rice, it could affect the flavour and taste of the product at

concentrations where full suppression of growth is observed. However,

strong synergistic activity is observed when carvacrol is combined with

the biosynthetic precursor cymene or the flavour enhancer soya sauce.

This makes it possible to use lower carvacrol concentrations and

consequently a smaller influence on the sensoric properties of the rice

is expected.

 

Besides its influence on the viability of vegetative cells, carvacrol

also shows inhibition of diarrhoeal toxin production by B. cereus at

concentrations below the MIC-value. Addition of 0.06 mg/ml carvacrol to

the growth medium, inhibits the toxin to 21% of the control (no

carvacrol added). The inhibition correlates with the reduction of the

viable count of B. cereus in the presence of carvacrol. At the same

time, the total amount of cells did not change. In mushroom soup, also

an inhibition of the toxin production was observed, however, the viable

count did not change. This effect on the toxin production is most

probably caused by a lack of sufficient metabolic energy, since

carvacrol affects ATP synthesis. The cell will use its low levels of ATP

to maintain its viability, rather than using it for toxin production or

excretion. It could also be possible that the decreased toxin synthesis

in BHI was the result of the lower amount of viable cells. The

inhibition of toxin production at carvacrol concentrations which do

permit growth of B. cereus, reduces the risk of food intoxication by

this pathogen.

 

In conclusion, carvacrol may play an important role in future as a

natural antimicrobial compound.