seminar sbg 1

7/30/2019 Seminar SBG 1

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What is plasma ?

Plasma is ionized gas that consists of a large number of

different species such as electrons, positive and negative ions,

free radicals, and gas atoms, molecules in the ground or

excited state and photons. It is considered to be the forth state

of matter in the world


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Thermal and nonthermal plasmas

If Te Tion then we have athermal plasma

If Te Tion then plasma isnonthermal or nonequilibriumor cold plasma

Thermal and nonthermal

plasma


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Cold atmospheric gas plasma

technology

Cold atmospheric gas plasma (CAP) treatmentis of particular interest for the

decontamination of food surfaces since it doesnot require extreme process conditions

compared to classical preservation methods

such as heat treatments, which have a negativeimpact on vegetable tissues


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Food-borne human illnesses resulting from

contaminated fresh produce have been

reported in several Countries .

Most reporting countries identified leafy greensand

berries as the main vectors, and either Salmonell

Escherichia coliO157:H7 or norovirus as thetar et atho ens.


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Salmonella was identified as the most frequent cause of food-

borne outbreaks .

A wide range of fresh fruit and vegetable products have been

implicated in Salmonella infections in recent years, such as

lettuce, tomatoes, pepper and basil


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The objective of the present work was

to study the effect of :-

on the inactivation of Salmonella entericaserovar Typhimurium (S. Typhimurium)by Nitrogen CAP.

growth phase.

growth temperature.

influence of the surface topography

chemical treatment regime

.


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further aim was to evaluate the usefulnessof CAP treatment for the inactivation of

S. Typhimurium inoculated on the surface

of lettuce and strawberry, as well as onpotato tissue.


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Bacterial culture and sample

inoculationThe bacterium used in this study was S. Typhimurium

Cultures were grown aerobically in Luria Bertani broth (LB)

(Difco) at 20 C to either :_

stationary phase

mid-exponentialphase

late-exponentialphase

(28 h)

(10 h)

(14 h)


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In order to study the effect of the growth temperature,

cells were also incubated at 25, 37 and 45 C for 28 h

(Fig. 1).


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Dilutions of harvested cells were prepared

in :

1) peptone salt dilution fluid (PSDF)

2) 30 mL aliquots were deposited onto 0.2

mm pore size 25 mm diameter Whatman

polycarbonate membrane

placed singly on LB agar plates (LBA,

Oxoid)


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lettuce and strawberry surfaces and potatotissue, with no detectable initial levels of S.Typhimurium cells, were used to study the

efficacy of CAP treatment for theinactivation of this microorganism on realfood matrices.

Fresh produce transportedimmediately to the laboratory, where theywere kept in a refrigerator overnight at 4 Cbefore use

Electron micrographshowing Salmonella in thepores of a lettuce leaf


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Food discs were obtained by using a sterile 25

mm - diameter cork borer and also deposited on

LBA plates.

Aliquots (30 mL) of the diluted bacterial culture

were carefully pipetted onto the centre of the

food samples and spread.

Membrane filters and food samples were then

allowed to dry for up to 45 min in a laminarflow cabinet before plasma treatment.


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Plasma inactivation procedure

Plasma treatments were carried out in a commerciallyavailable nitrogen plasma jet.

nitrogen plasma jet


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electrical potential by theafterglow of a jet ofexcited state nitrogen

produced by a highvoltage dischargetypically sampled at a rateof 1 kHz

. The electrode is located25 mm above the gasoutlet.

A grid near the electrode,held at the same potential,

is used to remove space

charge


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The plasma system also allows the grid to bepositively and negatively biased using an

external voltage source, thereby determining thechemical treatment regime

redox reactions are favored as a result of theequilibrium state or at zero bias

reduction process is favoredover oxidationPositive bias

oxidation process is favoredover reduction.Negative bias


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Under the experimental conditions

assayed the:-

temperature of the samples never

exceeded 35 C.

atmospheric pressure and nitrogen

throughput of 12 standard litres perminute, at approximately 1 W output

power.


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Inoculated membrane filters andfood discs were treated with

plasma for various time periods.

As a control, bacteria wereexposed to nitrogen (discharge

turned off) according to thesame time series.

It was found that in all cases cellviability remained constantthroughout the nitrogen treatmentperiod


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Following plasma exposure, eachmembrane filter and vegetable disc was

carefully removed from the agar withsterile forceps and placed into a stomacherbag containing 10 mL of PSDF.

Plasma treated cells were recovered fromthe membrane filters through agitationusing a stomacher at medium speed for 1

min.


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Diluted aliquots werespread on Plate Count

Agar (PCA) plates to

allow enumeration ofsurviving bacteria.


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Viable cells from

treated foods were

determined on

Xylose Lysine

Deoxycholate agar

(XLD agar,Oxoid) to select

for Salmonella.


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showed that XLD yielded the same rate of

Salmonella recovery as the non-selectivemedia PCA.

The PCA and XLD plates were incubated

at 25 C for 48 h and 37 C for 24 h,

respectively, before enumeration.


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Scanning electron microscopy (SEM)

we evaluated the physical structure and

microbial distribution of S. Typhimurium on

the surfaces of the target foods using Scanning

Electron Microscopy (SEM).

The microstructure of the food surfaces and its

effect on the distribution of S. Typhimurium

suggest a possible reason for the food-specific

variability of CAP inactivation

.


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D-values, expressed as the mean value ofthree independent experiments and

standard deviation, were useful for the

purpose of comparing microbial CAP

resistance.

determine significant differences between

D-values at p < 0.05


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Experimental parameters can influence the

inactivation efficiency of Salmonella byCAP treatment these include the:

Microbial loadType of substrateon which bacteria

are deposited

Processparameters such

as gascomposition,

relative humidity,flow rate, input

power and type ofdischarge ,Temp.,

pH etc.


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Cold plasma inactivation curves and D-values of S. Typhimurium cells

harvested at different growth phases. D-values (mean of three different

experiments standard deviation) with same superscript are not statistically

different> 0.05 .


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The effect of the growth temperature the D-values

(min) obtained were for the cells grown at 20, 25,

37 and 45 C


grown at different temperatures. D-values (mean of three different


different(p > 0.05).

h l b i d d h diff


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the D-values obtained under the different

chemical treatment regimes


treated under different chemical regimen. D-values (mean of three different


different(p > 0.05).

The inactivation effect of CAP treatment on lettuce strawberry


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The inactivation effect of CAP treatment on lettuce,strawberry,

and potato tissue superficially contaminated with Salmonella,

compared to the inactivation on membrane filters

Cold plasma inactivation curves and maximum log reduction-values of S.

Typhimurium cells treated on different substrates.

Th i fl f th f t h


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The influence of the surface topography on

the efficacy of CAP treatment was observed using

SEM. shows images of the microstructure of the

inoculated filter, lettuce andstrawberry surfaces and potato tissue.

Micrograph A shows the

appearance of the surface ofan inoculated membrane filter,

where Salmonella cells are

spread evenly on the smooth

surface.


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Micrographs B, C and D

show inoculated lettuce,

strawberry and potato disks.

The micrographs show that

the convoluted surface

features of these tissues result

in sequestration of the

bacterial cells.


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In conclusion,

emerging technology, CAP, has the potential todecontaminate food produce and therefore to

replace or augment traditional preservation

techniques in the future.

The efficiency of inactivation by CAP s related

to food surface structures, perhaps suggestingcombined decontamination approaches may

achieve even greater levels of decontamination.


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A further point is that the plasma device used in this study

was a laboratory model and food treatment on a commercialscale would require scale-up devices that should

significantly reduce the treatment times and increase

inactivation levels

It should also be noted that for the application of this

technique to the food industry, further studies are needed to

confirm that no harmful by-products are generated by CAPtreatment.


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seminar sbg 1

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