22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Plasma sterilization of dispersed material
G. Paskalov
Applied Quantum Energies LLC, Naples, FL, U.S.A.
Abstract: Our interest is especially focused to organic food powder treatment. It is shown
that RF plasma technology is capable to de-contaminate and sterilize powder without
heating. The disinfection and microstructural evolution of the samples are studied for
different plasma conditions and processing time. Low pressure plasma systems at two
different configurations: drum and shelf, are presented.
Keywords: rf plasma, vacuum, powder, sterilization, non-thermal treatment
Introduction
RF plasma is efficient tool to process powdered
material [1]. Current methods of decontamination must
weigh the level of microbial reduction with the amount of
acceptable product degradation. Some current methods
affect detriment upon the substrate.
For instance,
substances with heat-labile active ingredients are prone to
degradation when exposed to high heat; the active
element in dry and steam is heat treatments. Oxidizing
agents, such as, ethylene oxide or sodium hypochlorite
can either be reactive toward or be absorbed into the
processed material. Cold, low pressure RF plasma
(CLPP) was modeled for decontamination of powdered
botanicals, such as: hydrilla, stinging nettle leaf, organic
wheat grass powder and saw palmetto. Chia seeds were
tested as a coarse-material out-group.
1. Experimental Set Up
A typical RF Plasma system is shown on Fig.1. The
CLPP device includes: RF generator; matching network;
plasma reactor; gas supply and vacuum unit.
RF Generator
P = 0.1 to 10 kW
f = 13.56 MHz
Matching Network
RF Plasma ( Air )
Internal RF electrode
External electrode
(Drum)
Processing Material
Fig. 1. RF Plasma System - Drum configuration.
RF source include RF generator (3 kW at 13.56 MHz),
matching network and modulator. Modulation frequency
could be varying from 10 Hz to 10 kHz. The vacuum
chamber is equipped with thermocouple and 2 pressure
P-II-7-25
gauges. Drum is connected to the motor (variable speed
from 0 to 4 rpm) by rotary vacuum feed-through. Also
drum could be easily remove and replace by 7 shelves
connected to the matching network in parallel. The
plasma gas system supplies the required gases or gas
mixtures. Argon, Nitrogen, Oxygen and pure Air have
been used as a plasma gases. The flow rates were
controlled by MFC and varied from 50 to 500 sccm.
Intensity of plasma radiation in absence and presence of
powder was measured by using portable spectrometer
(model EPP2000Cs StellarNet Inc.). Exhaust gas was
measures and analyzed by industrial combustion gas &
analyzer (model E4400, E-Instrument Inc.).
2. Process Description
CLPP can decontaminate without modifying
compounds liable to heat or oxidation [2]. During CLPP
treatment, decontamination is achieved as a result of
direct contact with ions of high kinetic energy, electrons
and UV radiation. In this study, plasma decontamination
of different nutritional supplements was carried out in a
controlled atmosphere for two different plasma reactor
configurations: stationary shelf and rotary drum.
Variables include: power; plasma gas composition;
pressure and processing time. Treated materials were
assessed for Total Coliform, Escherichia coli,
Staphylococcus aureus, Pseudomonas aeruginosa, yeast
and mold. A comparative analysis of air, pure oxygen
and pure nitrogen plasma gas upon an oxidation-prone
substrate was also done. All processed materials were in
the temperature range between 20 to 60 °C immediately
after venting of the chamber.
3. Results and Discussion
For most samples, a 3 to 5 log reduction in bacterial
colonies was observed within 15 minutes of processing.
Efficiency of inactivation was variable for different types
of bacteria and also depends of the powder size
distribution and plasma gas mixture. Some experimental
data is shown below in Table 1.
Samples 030211-1,2,3 are processed in Oxygen plasma.
Samples 100311-P2,3-A are processed in air plasma.
Plasma power was 1 000 Watts. A comparison analysis
1
of survival curves and plasma optical emission spectral data
Table 1. Plasma processing of Hydrilla powder (particle size 100 to 600 µm).
Sample ID
Aerobic Plate Count
C1- Initial
1 200 000
030211-2
285 000
030211-1
212
030211-3
28 000
C2-Initial
8 800 000
100311-P2-A 10 000
100311-P3-A 10 000
Note: All counts are in cfu/g.
E-coli and Coli form
Neg
Neg
Neg
Neg
9000
<1 000
<1 000
Pseudomonas
Neg
Neg
Neg
Neg
Neg
Neg
Neg
Staphylococcus
Neg
Neg
Neg
Neg
Neg
Neg
Neg
Mold
100
100
0
0
1 000
0
0
50.0
45.0
40.0
Current, Amps
35.0
air, 500 mTorr
30.0
air, 800 mTorr
25.0
air, 1000 mTorr
O2, 500 mTorr
20.0
O2, 800 mTorr
15.0
O2, 1000 mTorr
10.0
5.0
Voltage
0.0
0
100
200
300
400
4.0
plasma resistance, Ohm
3.5
3.0
2.5
100mTorr;air
800mTorr;air
2.0
1000 mTorr;air
1.5
800mtorr;O2
1000 mTorr;O2
1.0
0.5
0.0
100
300
500
Power,W
800
1000
Fig. 3. Plasma Parameters
demonstrate that Nitrogen/Oxygen gas mixture play a
significant role in inactivation of bacteria, yeast and mold.
Some critical plasma parameters are shown on Fig. 3.
When using a mixture of nitrogen and oxygen gas to
create plasma, radicals of both species are produced.
Often, nitrogen and oxygen radicals combine to produce
NOx species. Within the NOx moiety, nitric oxide acts as
an antimicrobial agent. Preservation of ascorbic acid
using pure nitrogen was observed, with only 2.5%
decomposition to dehydroascorbic acid after 15 minutes
of treatment.
treatment of food powder is a promising technology due
to that processed material does not content toxic residuals
and the temperature range can be kept to an acceptable
level. Experimental data allow us to underline few
interesting and unusual effect during RF plasma
processing of organic materials. Plasma modulation is a
powerful tool to increase the efficiency of the process.
The next step of plasma development is pairing it with
other decontamination process such as RF pulse electric
field treatment.
Synergetic effects might be more
significant than either technology alone.
Conclusion
Processing parameters, such as power, processing time
and temperature, are unique to the product and batch in
order to inactivate microorganisms without affecting the
material property.
Using non-thermal plasma for
References
[1] G. Paskalov. Application of Modulated RF-Plasma
for Powder Treatment.
14th International
Symposium on Plasma Chemistry, August 2-6,
1999, Prague Czech Republic, p. 2265
2
P-II-7-25
[2]
G. Paskalov. Plasma powder sterilization apparatus
and method. Patent USA 8,771,595 (2014)
P-II-7-25
3