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Experimental Investigations of Microwave
Plasma UV Lamp for Food Applications
Montserrat Ortoneda*, Sinead O’Keeffe, Jeff D. Cullen,
Ahmed I. Al-Shamma’a and David A. Phipps
1
RF and Microwave Group, General Engineering Research Institute,
Liverpool John Moores University, Liverpool, L3 3AF, UK
*
[email protected]
The food industry is keen to have new techniques that improve the safety and/or shelf life of food
products without the use of preservatives. There is considerable interest in developing UV light and
ozone (O3) treatments to enhance shelf life. A microwave radiation device that is a novel source of
germicidal UV and O3 suitable for the food industry has been developed, which offers speed, cost
and energy benefits over existing sources. With this system comes the need to monitor a number of
conditions, primarily UV intensity and ozone gas concentrations. The effectiveness of intense UV
exposure for short periods of time was assessed on different microorganisms. Culture plates were
exposed to a range of doses of UV-C light, and the reduction in numbers of surviving microorganisms was recorded. The results on the biocidal capacity of the microwave generated UV light are
presented.
Submission Date: 17 January 2008
Acceptance Date: 1 August 2008
Publication Date: 4 November 2008
INTRODUCTION
Each year the food distribution chain has an
increased demand for fresh products such as
minimally processed vegetables [Ragaert et al.,
2004]. The economical losses related to highly
perishable products such as fresh processed
lettuce are substantial because this type of product
is likely to lose quality during processing and
distribution. Food producers and supermarkets
are always looking for improved production
methods allowing better preservation of these
products and longer shelf lives [Corbo et al.,
2005; Yahia et al., 1988]. At the same time, these
treatments need to preserve, or even enhance the
sensorial and nutritional qualities of the fresh
product. Aspects such as general appearance,
texture, off-odours, aroma, browning, colour and
Keywords: Microwave plasma UV lamp, optical fibre,
bacteria, food packaging
International Microwave Power Institute
taste are important for consumers, and therefore
affect the sales of the products [Allende et al.,
2004; Ragaert et al., 2004].
One of the main factors influencing the
lifespan of fresh products is the microbial content
of the product. Fresh fruits and vegetables
normally carry non-pathogenic, epiphytic
microflora. Although it does not represent a
threat for human health, it can contribute to the
shortening of the product’s lifespan [Corbo et
al., 2005]. That is the reason why these products
are routinely decontaminated using different
protocols. On the other hand, these products
can become contaminated with pathogens,
occasionally. This contamination can occur
during production on the farm (dirty irrigation
water or use of raw manure), and also at any
stage of product handling from harvest to point
sale (washing, cutting, slicing, packaging)
[James, 2006].
Minimally processed foods such as packaged
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salads present specific challenges because cutting
and slicing removes the natural protective barriers
of the intact plant, thus accelerating the decay
of the product [Allende et al., 2006]. Typical
decontamination methods such as washing
with hypochlorite or commercial surfactant
formulations, have a limited success with only
a 1 to 2-log reduction (90 – 99 % killing).
Other methods such as washing with hydrogen
peroxide reduce the microbial contamination by
3-logs (99.9 %) [Sapers et al., 2000]. Traditional
detergents are known to be partially effective
in removing pathogens, however each type of
disinfectant varies both in efficiency and in
allowable maximum concentration [Beuchat,
1998]. The use of a nonselective treatment for
the destruction of pathogens on the surface
of fresh fruits and vegetables that leaves no
residue would be desirable. Ozone is one of
several new sanitizing agents recently used in
post harvest management systems [Xu, 1999]. It
decomposes spontaneously to non toxic products
and it can be used in combination with other
disinfection treatments to reduce the presence of
pathogenic and non pathogenic microorganisms
in fresh products [Selma et al., 2007]. Modified
atmosphere packaging (MAP) is another method
that improves the environment of the food whilst
preventing potential bacterial growth. Fruit,
vegetables, salads, poultry, fish and red meat are
just some of the food products that can benefit
from a modified atmosphere. However, there is a
major concern about the safety of foods packaged
using MAP because it suppresses the growth of
spoilage organisms, but it seems to have no
effect on slow growing pathogenic bacteria
such as Yersinia enterocolitica, Aeromonas
spp. and Listeria monocytogenes. In this case,
removing the spoilage microorganisms would
favour the growth of the pathogenic ones when
present in the product [Allende et al., 2006].
One alternative process is the use of germicidal
ultraviolet light at a wavelength of 200–280 nm
(UV-C), alone or combined with other treatments
such as ozone, mild thermal treatements or MAP.
Ultraviolet light is a non chemical disinfectant
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system that uses an extremely rapid physical
light energy of a specific wavelength to destroy
microorganisms.
The microwave plasma UV lamp (MPUVL)
developed in a research project supported by
the EU, is able to produce both UV light at the
germicidal region (254 nm) and at the Ozone
forming region (185 nm), simultaneously. The
system has proven to have higher efficiency
than both a conventional UV lamp and the low
pressure electrical DC discharge lamp [Cullen
et al., 2006].
Pathogens associated with fresh produce
Epidemiologic evidence shows a relationship
between some foodborne pathogens and specific
food products. Numerous pathogens have
been isolated from a variety of fresh fruits and
vegetables. For example, Aeromonas hydrophila,
A. caviae, Campylobacter jejuni, Clostridium
botulinum, C. perfringens, Cryptosporidium
parvum, E. coli O157:H7, Giardia lamblia,
Listeria monocytogenes, Salmonella spp.,
Shigella spp., Staphylococcus aureus and
Yersinia enterocolitica have been found in lettuce
and mixed salads and have been associated
with outbreaks in different countries [Archer
and Young, 1988]. These microorganisms will
affect people with a weak immune system, such
as children and elderly people, and those with
underlying diseases [McCabe-Sellers and Beattie,
2004]. The diseases caused by these pathogens
range from self limited diarrhoea, abdominal
pain and fever, to more serious diseases such as
severe diarrhoea, kidney failure (caused by E.
coli O157:H7), respiratory and musculoskeletal
paralysis (caused by Clostridium) or meningitis,
infection of the nervous system and miscarriages
(caused by Listeria) [Archer and Young, 1988;
Tauxe, 1997].
Use of UV light in food technology
UV light has been used to sterilize a wide range
of materials and surfaces for years [Andersen
Journal of Microwave Power & Electromagnetic Energy ONLINE
Vol. 42, No. 4, 2008
et al., 2006]. In the food industry it is used in
a broad range of antimicrobial applications
including disinfection of water, air, food
preparation surfaces and containers [Wang et
al., 2005]. The germicidal properties of UV
irradiation are situated in the UV-C region of the
spectrum (200-280 nm), and more specifically
around 254 nm. This wavelength is absorbed by
the nucleic acids (DNA and RNA) present in all
living organisms and provokes the formation of
pyrimidine dimers and other nucleic acid lesions.
These lesions in the DNA and RNA inhibit
replication and transcription and prevent the
microorganism from multiplying [Hinjen et al.,
2006]. UV light is effective against a wide range
of microorganisms, including pathogens that do
not respond to chlorine treatments in drinking
water such as oocysts of Cryptosporidium and
Giardia [Hingen et al., 2006].
Treatment with ultraviolet energy offers
several advantages to available washing and
sanitizing methods, as it does not leave a
residue, have legal restrictions and does not
require extensive safety equipment to utilize
[Yousef and Marth, 1988]. The use of UVC light
could be useful as a treatment step in HACCP
protocols because it is effective at reducing
microbial numbers on the surface of fresh fruits
and vegetables [Yaun et al.,2004 , Allende et al.,
2004, Allende et al., 2006, Gomez-Lopez et al.,
2005] and can be applied in a continuous mode
or in pulses to the food samples. Reducing the
number of bacteria and other microorganisms is
especially significant in the case of minimally
processed products, where UV together with
refrigeration and several modifications in
packaging, including modified atmospheres, can
play a very important role in increasing lifespan
and decreasing the risk of food poisoning.
Although high doses of UV-C radiation are
harmful to plant tissues, low doses can help
reduce the number of microorganisms that would
cause spoilage and shorten the lifespan of the
food product or could represent a health risk
[Krishnamurti et al., 2004]. At the same time,
this type of light induces a series of changes in
International Microwave Power Institute
the plant tissues as a defence mechanism against
this type of radiation that can delay senescence
in several types of fruits and vegetables [Erkan
et al., 2001, Marquenie et al., 2003].
Germicidal kinetics
The survival (S) of microorganisms when exposed to either UV or ozone is represented by
two rates of decay, as shown in equations 1 and
2 [Koller, 1965]. This relationship is illustrated
in Figure 1.
S=C exp(-KD) for D <D0
S=C exp(-mD) for D<D0
(1)
(2)
The dosage D is the product of the UV or ozone
intensity and duration (t) of exposure. There is
an initial rapid rate of kill (k) to a level (1-C)
and this is followed by a much slower kill rate
(m). The value of C is of the order of 10-3. Figure 2 shows a comparison of the dosages (Do)
required for UV, ozone and chlorine required to
achieve a 99.9% kill level when compared with
the dosage for the Escherichia Coli (E coli) in
water. They show comparative responses with
a range of microorganisms. The most likely
explanation for the tailing off of the survival
curves is the clumping effect suggested by
various investigators, which is the tendency of
micron-sized particles to clump together naturally. The clumping of bacteria cells protects a
small percentage of bacteria and causes them to
behave as if they had much higher resistance to
both UV and ozone.
EXPERIMENTAL PROCEDURES
Experimental setup of the Microwave Plasma
UV Lamp
The microwave power is produced by a
magnetron source operating at 2.45 GHz. Figure
3 shows the main MPUVL components. The
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Figure 1. Survival curve of microorganisms in response to UV radiation.
Figure 2. Comparative mortalities of bacteria and other pathogens in response to UV radiation,
ozone and chlorine treatments in water sterilisation.
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Journal of Microwave Power & Electromagnetic Energy ONLINE
Vol. 42, No. 4, 2008
Figure 3. Microwave plasma Ultraviolet Light system setup.
custom made lamps utilise a high purity GE214
quartzTM glass [Cullen et al., 2006)] that allows
254 nm and 185 nm UV light to pass through.
Inside the lamp, there is a low pressure mixture
of Argon gas combined with Mercury vapour
produced by double-distilled liquid Mercury
drops (considered safer than the extremely
toxic and volatile fumes). The lamp itself does
not contain any electrodes, as in the case of the
commercial lamps. The cavity was connected
to the magnetron with a low loss coaxial
RG214 cable with N-Type connectors. The
microwaves are generated by a magnetron 200
power supply. This is a variable supply which
is capable of outputs up to a maximum of 200
Watts CW or 300 W (peak power) pulsed with
a 4 kHz frequency repetition rate. The unit is
also equipped with an incident power meter and
reflected power meter. The microwave power
is coupled to the lamp via a tunable resonant
cavity using the fundamental TE010 mode. The
E-field propagated into the cavity resonator
is a radial mode and propagates through the
quartz tube via a surface wave [Al-Shamma’a
et al., 2001]. In the MPUVL, the electric field
component is transverse with standing wave
present compared with the longitudinal electric
field of the commercial lamp; this leads the lamp
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to emit UV of an order of magnitude higher in
intensity with power per unit length of at least
250W/m [Kraszewski, 1967]. The high rate of
power per unit length allows for the setting up of
the lamp for the investigation of various cases of
bacteria inactivation. In addition, the resonant cavity
of the MPUVL can be designed to allow one or
more lamps to be used and in any shape. A twin
MPUVL system arrangement can be seen in
Figure 4. The lamp arrangement does not induce
any severe losses to the microwave coupling, and
the power is divided equally among the lamps of
which the thermal burden of the lamps reduced.
An optical detector system, based on a transimpedance amplifier, was developed to monitor
the UV intensity at 254 nm. A UV enhanced
photodiode was used with a narrowband optical
filter that transmits light at 254 nm. A similar
system can be developed to monitor the optical
signal at 185 nm [O’Keeffe et al., 2007].
Figure 5 shows the microwave UV lamp
system set-up for bacteria-killing experiments. The
lamp has been positioned horizontally based upon
the testing requirements. One end of the lamp is
energised by microwave power and the other end
has a computer controlled stepping motor for a
lamp shutter, which controls the UV exposure
time against various types of bacteria.
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Design of a parabolic reflector
The samples that have to be irradiated would
normally be situated underneath the lamp in a
fixed position or moving over a conveyor belt.
With the initial design of the lamp, most of the
UV radiation emitted by the lamp was wasted,
since only the portion directed to the area below
the lamp had an effect on the samples. To
maximize the efficiency of the lamp, a parabolic
reflector was designed. The intensity of UV
radiation on the surface under the lamp (60 mm
from the lamp) was measured with and without
the parabolic reflector.
Figure 6 shows the intensity map for the
lamp with no cover. The lamp shows the highest intensity directly under the lamp and nearest
the microwave cavity (15-16 W/m2). Along the
length of the lamp it decreases slightly where
the Primarc lamp tapers in the middle.
The intensity of the UV light decreases rapidly as the sensor moves away from under the
lamp. At 10 mm from the lamp, the intensity is
approx. 10-12 W/m2. At 20 mm from the lamp,
the intensity is approx. 4-6 W/m2. Beyond 30
mm the intensity is negligible.
Figure 7 shows the results of the intensity
map for the Primarc lamp with a parabolic cover
in place. The parabolic cover was designed to
be 12.5 mm from the lamp centre, therefore it
was placed 3.5 mm from the outside of the lamp.
Given the design of the lamp holder, this is the
closest the parabolic cover can be placed to the
lamp.
It is clear that the parabolic cover significantly improves the intensity of the UV light and
the overall efficiency of the lamp. The UV light
directly under the lamp is 16-20 W/m2 when the
reflector is in place. As the sensor is moved away
from under the lamp the UV intensity remains
high. Where previously, without a cover, at 30
mm there was negligible UV intensity, this is
now increased to 10-14 W/m2. The intensity remains high until 90 mm either side of the lamp.
The cover ends at 100 mm, thus the UV intensity outside this range is minimal. The intensity
42-4-18
Figure 4. The twin MPUVL system
arrangement.
Figure 5. The MPUVL set-up.
increases slightly at 70-80 mm, which may be
due to the uneven surface of the parabolic cover
used.
Microorganisms used
The study of pathogens in a laboratory is
important for the understanding of their biology
and specific characteristics. It is unsafe, though,
to work with virulent strains and very strict
safety measures need to be observed. Due to
this, surrogate organisms are sometimes used
instead of the pathogenic strains. Surrogate
strains are tools in assessing fresh fruit and
vegetable safety and the effectiveness of
Journal of Microwave Power & Electromagnetic Energy ONLINE
Vol. 42, No. 4, 2008
Figure 6. Intensity map (J/m2) for the Primarc Microwave Plasma UV Lamp at 60 mm without
the parabolic reflector.
Figure 7. Intensity map (J/m2) for the Primarc Microwave Plasma UV Lamp at 60 mm with the
parabolic reflector.
International Microwave Power Institute
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microbial control measures because they mimic
growth and survival patterns of a pathogen
and can help in studying what occurs with a
pathogen during handling and storage. They are
important tools used when studying the effects
and responses to specific processing treatments,
such as the effectiveness of a produce wash or
other decontamination processes. In this study
we have used the strain E. coli ATCC 25922.
This is a non pathogenic strain that is considered
to be a surrogate of the enterotoxigenic strain
of E. coli O157:H7 [Duffy et al., 2000]. It has
the same UV sensitivity/resistance as three
pathogenic strains of E. coli O157:H7 and it
was used to evaluate the response to UV-C light
emitted by a Microwave Plasma Ultraviolet
Lamp (MPUVL) developed previously in our
lab [Cullen et al., 2006]. Other organisms used
in this study are: Pseudomonas aeruginosa
NCTC 10662, Bacillus cereus (vegetative cells
and spores) and Staphylococcus aureus NCNTC
6571. Overnight liquid cultures were adjusted to
different concentrations (106-108 cfu/ml) and 0.1
ml was plated on to Nutrient Agar plates. The
plates were then exposed to the UV light for a
certain time and immediately placed in the 37ºC
incubator. Surviving colonies were counted the
next day.
RESULTS AND DISCUSSION
The maximum intensity of the UV radiation
emitted by the lamp is 10 W/m2 in the area
below the lamp. This intensity is lower in the
areas not directly below the lamp. To correlate
the intensity of ultraviolet radiation emitted by
the MPUV lamp with its biocidal power, an
experiment was designed in which the plates
containing the E. coli cells were placed in five
different locations in the area illuminated by the
lamp (three were placed directly underneath the
lamp, covering the whole length of the lamp, one
was placed 20 cm in front and another 20 cm at
the back, forming a cross shape) and exposed
for 5 seconds to the UV light. Two sets of
plates were exposed to UV light: the first set of
42-4-20
plates (low inoculum) contained approximately
2x105 cfu/plate. These plates did not develop
any colonies, meaning that all the cells were
killed by the UV radiation. The second set of
plates, containing a high inoculum of 2x107
cfu/plate, showed a significant reduction in the
number of colonies, though this reduction was
not homogenous for all the plates. In the plates
situated at the back and in the centre row (left,
centre and right), there was a 6 log reduction.
Although the results for these three plates were
in the same range, the reduction in survival was
slightly smaller in the plate situated at the right.
The plate situated at the front only had a 5 log
reduction, showing that the intensity of the light
is not equal in all the areas, which agrees with
the distribution of intensities observed for the
lamp. This shows that in order to get accurate
and highly reproducible results, the position of
the samples has to be maintained constant.
The germicidal effect of the UV-C light
emitted by the microwave powered lamp was
assessed on 4 different microorganisms. Culture
plates containing from 105 to 107 cfu (colony
forming units) of the bacteria were exposed to a
range of doses of UV-C light. The culture plates
were placed in the area directly underneath
the lamp, at 10 cm distance. Plates were then
exposed to the UV light for different amounts
of time depending on the microorganism. For
example, E. coli and P. aeruginosa plates were
exposed for 2 to 8 seconds. Both organisms
are gram-negative and have similar membrane
structure. S. aureus was also exposed to UV light
for 2 to 8 seconds. In the case of B. cereus the
exposure times were longer because it is known
that gram positive bacteria, and more specially
the members of the genus Bacillus, are highly
resistant to UV light.
Procaryotic cells divide by a process called
binary fission (invagination of the plasma
membrane and the laying down of new cell wall
to produce two separate daughter cells). Some
bacteria, though, are able to produce endospores
(spores) when there are no nutrients available.
These are highly heat-resistant, dehydrated
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Vol. 42, No. 4, 2008
Figure 8. Growth of E. coli on Nutrient Agar plates after exposure to different doses of UV light.
resting cells formed intracellularly in members
of the genera Bacillus and Clostridium. Spores
are not metabolically active and can remain in
that state for long periods of time. To distinguish
the active cells from the inactive spores, cells are
also called vegetative cells. For our experiment,
spores were obtained from 2-3 week old plates,
and the abundance of vegetative cells and
spores was measured using a microscope. Plates
containing >95% spores were scraped and the
spores suspended in a buffered solution. Spores
and vegetative cells were treated for 10 to 60
seconds.
After exposure to UV-C light, the plates were
incubated overnight at 37ºC. Surviving bacteria
developed a colony, which were then counted.
An example of this can be seen in Figure 8. The
plates situated on the top row were inoculated
with 105 E. coli bacteria, and exposed for different
amounts of time to UV light. After incubation,
surviving bacteria developed colonies, which
can be seen in Figure 8. The longer the plates
were exposed to UV light, the lower the amount
of bacteria that were able to survive.
Figure 9 shows the reduction in the numbers
for the 4 different microorganisms tested.
Experiments were performed in triplicate, and
the average values are shown. The exposure
times for the plates varied from 2 to 8 seconds
for E. coli, P. aeruginosa and S. aureus, and
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from 10 to 60 for B. cereus. A reduction of up to
6 Log in the number of living microorganisms
was observed for E. coli and P. aeruginosa in
less than 2 sec exposure, which corresponds to
less than 20 J/m2. S. aureus was slightly more
resistant, and needed between 20 and 50 J/m2 to
obtain the same Log reduction. Vegetative cells
of Bacillus were more resistant to UV radiation,
and an exposure to 600 J/m2 was needed to
produce a 6 log reduction in the number of
surviving microorganisms in the test plates.
When spores of B. cereus were used to perform
the tests, they proced to be highly resistant to
damage by UV light, and very high doses of
radiation (600 J/m2) were needed to observe
a reduction of even 2 Log in the number of
surviving microorganisms.
Due to aggregation of bacteria, which occurs
naturally due to their very small size, some of the
bacteria that happen to be in those clumps may
survive the exposure to UV-C light. This is why
there is a “trailing” effect observed, and after a
high kill rate observed for the first seconds, this
kill rate decreases and it is difficult to find plates
with 0 colonies, mainly when the inoculums is
high. However, the 6 log reduction observed
for three of the studies species after only 2- 5
seconds of exposure, which equals a 99.9999
% killing, should be sufficient for most food
products to achieve a long lifespan without any
42-4-21
Figure 9. Effect of UV radiation on different bacterial species. The diagrams show the number of
surviving bacteria after exposure to different doses of UV light (grey and white bars), compared
to the initial number of bacteria (dark bars). Error bars represent standard deviation of the
mean. Experiments were performed at least in triplicate.
detectable spoilage effects. Microorganisms like
Bacillus cereus are more resistant to UV light,
and their spores especially might respond better
to other type of treatments, alone or combined
with UV light.
CONCLUSIONS AND FUTURE WORK
The MPUVL used in these experiments has proven
to be an alternative to the commercial ultraviolet
irradiation systems. It can significantly reduce
the microbial contamination in surfaces. The
next steps are to evaluate the biocidal capacity
of the lamp with real samples, such as various
lettuce types, and other vegetables as well.
The effect of the reduction of microbiological
contamination and its relationship to an increase
of shelf life will also be evaluated using different
packaging techniques and storage conditions.
ACKNOWLEDGMENTS
The authors would like to thank the EU for their
kind fund toward the Micro-GMAP Project
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contract MTKD-CT-2006-042609.
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