Non-thermal Processing withPlasma Technologies

Brendan A. Niemira

Food Safety and Intervention Technologies Research UnitU.S. Department of Agriculture, Agricultural Research Service

Eastern Regional Research Ctr.600 E. Mermaid Ln, Wyndmoor, PA, USA

Cold plasma: this isn’t it

B.A. Niemira.

Non-thermal plasma• What is a plasma?

– Fourth state of matter– Equivalent to a highly energetic form of ionized gas

B.A. Niemira

SOLID

ENERGY

LIQUID

ENERGY

GAS

ENERGY

PLASMA

• Why is it sometimes called “cold” plasma?– For food processing, intended to operate at

conventional room temperatures

Non-thermal plasma• Inputs to the system

– energy (electricity, microwaves, etc.)– carrier gas: air, a pure gas (He, O2, N2, etc.) or a

defined gas mixture• Output

– self-quenching plasma– resolves to UV light and ozone– chemical residues are expected to be minimal to

non-existent• New technology for food processing

– adaptation from existing applications– regulatory status

B.A. Niemira

Non-thermal plasma

B.A. Niemira

OxygenNitrogen Carbon dioxide

Injected volatiles

Ozone

UV light

NOx

ElementalOxygen

Freeradicals

e-

Nanoparticles

Making cold plasma: gas and pressure

pd = pressure*distance between parallel plates

One atm., 760 torr

Making cold plasma: gas and pressure

Cost of feed gas

Ease

of i

oniz

atio

n

Higher voltagerequired;

equipment = $$$

$ $$$$$$$$

Lower volatagerequired;

equipment = $

He, Ne, ArH2N2, Air

Non-thermal plasma: technologies• Remote treatment and enclosed

chambers• Contact with electrodes, corona

discharges• Direct applications• In-package treatments

B.A. Niemira

OAUGDP (Kayes, M.M. et al., 2007. Foodborne Path Dis 4(1). DOI:

10.1089/fpd.2006.62)

Enclosed plasma treatment chambers

Microwave pumped plasma, enclosed chamber (Amidi, M., et al.

2007. Food Science Australia)

Enclosed plasma treatment chambers

Dielectric barrier discharge, applied to E. coli on almonds (Deng, S. R. et al. 2007. J. Food Sci.

72(3):M62-M66.)

Electrode contact plasma treatment

1.00E+00

1.00E+01

1.00E+02

1.00E+03

1.00E+04

1.00E+05

1.00E+06

1.00E+07

0 5 10 15 20 25 30 35

Treatment time (second)

Surv

ival

cou

nts

air nitrigen co2 argon

-4.5

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

00 10 20 30 40

Lo

g(N

/No

)

Time (s)

16 kV

20 kV

25 kV

Direct application of plasma, open air

(Niemira and Sites. 2008. J Food Prot.)

USDA-ARScold plasma research subjects

Feed gas: 99.5% He, 0.05% O2

Direct application of plasma, carrier gas

Perni, S. et al. 2008.JFP, 71(2):302–308

Honeydew

Mango

In-package treatments: ozone generation

E. coli O157:H7 inactivation on spinach (Klockow, P.A., K. Keener. 2009. LWT)

“Electrodes were placed above and below the bag, oriented on top of each other to allow for maximum ozone production. Electrodes rested on top of each other with the bag in between having an approximate gap distance of 3-3.5 mm [1/8 inch]. The system was then activated

for a 5-min treatment.”

“Treated samples showed varying levels of discoloration”

In-package treatments: ozone generation• PlasmaLabel. (Schwabedissen, A. et al. 2007.

Contrib. Plasma Phys. 47, 551-558 )• Electrically conductive labels on

inside surface– Rigid container, clamshell, bag, etc.

• Cold plasma generated by induction• 4 log cfu reduction of B. subtillis on

agar, 10’ treatment.– Ozone concentration inside the

package to 2000 ppm• Sensory impact?• Optimization

– shape of the applied electrodes– method of application (screen-

printed, applied, bonded, etc)

(+)

(-)

Plasma treatment of liquids• Air plasma microjet in a quasi-steady gas cavity

– reduces pH to 3.0-4.5 after 10’.– NO3

- & NO2- increases to 37 mg · L−1 and 21 mg · L−1

after 20’– Suspended Staphylococcus aureus inactivated by pH 4.5.– Mode of action: perhydroxyl radical (HOO•) reaction

with cell membranes (Liu et al, 2010, Plasma Processes and Polymers 7(3-4):231-236)

• Thin film application• Continuously renewed liquid surface• Co-injected spray into plasma discharge

– Can yield H2, H2O2 or NOx, depending on plasma feed gas (Burlica et al., 2010. Ind. Eng. Chem. Res., 49(14):6342–49)

B.A. Niemira

Commercial Equipment

Ingersoll-Rand

PlasmaTreat

Enercon Industries

Non-thermal plasma: conclusions• Many different ways to make plasma• How well it works is determined by:

– Type of plasma– Nature of power delivered– Feed gas composition

• What are you trying to achieve?• What product are you trying to treat?• What kind of packaging are you using?

B.A. Niemira

Brendan.Niemira@ars.usda.govwww.tinyurl.com/Niemira

B.A. Niemira

Non-thermal plasma: technologiesA. remote exposure

reactor (Gadri et al., 2000)

B. plasma pencil (Laroussi and Lu, 2005)

C. plasma needle (Sladek and Stoeffels, 2005)

D. gliding arc (Niemira et al., 2005)

E. microwave plasma tube (Lee et al., 2005)

F. dielectric barrier discharge (Deng et al., 2005)

G. resistive barrier discharge (Laroussi et al., 2003)

NTP Technology Class

I. Remote treatment II. Direct treatment III. Electrode contact

Nature of NTP applied Decaying plasma (afterglow) - longer lived chemical species

Active plasma - short and long-lived species

Active plasma - all chemical species, including shortest lived and ion bombardment

NTP density and energy Moderate density - target remote from electrodes. However, a larger volume of NTP can be generated using multiple electrodes

Higher density - target in the direct path of a flow of active NTP

Highest density - target within NTP generation field

Spacing of target from NTP-generating electrode

Approx. 5 - 20 cm; arcing (filamentous discharge) unlikely to contact target at any power setting

approx. 1 - 5 cm; arcing can occur at higher power settings, can contact target

approx. ≤ 1 cm; arcing can occur between electrodes and target at higher power settings

Electrical conduction through target

No Not under normal operation, but possible during arcing

Yes, if target is used as an electrode OR if target between mounted electrodes is electrically conductive

Suitability for irregular surfaces

High - remote nature of NTP generation means maximum flexibility of application of NTP afterglow stream

Moderately high - NTP is conveyed to target in a directional manner, requiring either rotation of target or multiple NTP emitters

Moderately low - close spacing is required to maintain NTP uniformity. However, electrodes can be shaped to fit a defined, consistent surface

Examples of technologies Remote exposure reactor, plasma pencil

Gliding arc; plasma needle; microwave-induced plasma tube

Parallel plate reactor; needle-plate reactor; resistive barrier discharge; dielectric barrier discharge