15.1 Introduction
Modified Atmospheric Packaging (MAP) is a precise description of this shelf-
life extension technique (Bennett 1995). In the UK, MAP mainly involves the
use of three gases – carbon dioxide, nitrogen and oxygen although other gases
are used elsewhere. Products are packed in various combinations of these three
gases depending on the physical and chemical properties of the food.
15.1.1 MAP and food preservation, food spoilage and shelf-life
Over time, food spoilage inevitably sets in and the rate at which it occurs
depends on the physical structure and properties of the food itself, the type of
microorganisms present and the environment the food is kept in. By carefully
matching individual modified atmospheres to specific food products, adopting
appropriate manufacturing, handling and packaging methods and observing
recommended storage and display conditions, a retailer can successfully extend
the shelf-life of most foodstuffs. Fine tuning this process can result in substantial
benefits. Selecting the correct mixture of gases for the modified atmosphere is
determined by looking at a combination of shelf-life and visual appearance. For
the longest shelf-life red meat uses 100% carbon dioxide but the meat would not
have the bright red colour desired by consumers. The redness of meat, an
essential part of the consumer’s decision to buy, can be maintained longer by
using a MAP gas mixture between 60% and 80% oxygen. Once it has been
accepted that it can, in certain cases, make economic sense to sacrifice some
shelf-life to ensure visual appearance, then it has been established which mixture
produces the best result for each product. The effect of the individual gases on
15
Integrating MAP with new germicidal
techniques
J. Lucas, University of Liverpool, UK
Table 15.1 MAP gas mixtures for food items
Food item Retail gas mix Storage temp.
o
C Shelf-life days
O
2
CO
2
N
2
MAP gas In air
Raw red meat 70 30 1 to + 2 5–8 days 2–4 days
Raw offal 80 20 1 to + 2 4–8 days 2–6 days
Raw poultry and game 30 70 1 to + 2 10–21 days 4–7 days
Raw fish and seafood 30 40 30 1 to + 2 4–6 days 2–3 days
Cooked, cured and processed meat products 30 70 0 to + 3 3–7 weeks 1–3 weeks
Cooked, cured and processed fish and seafood 30 70 0 to + 3 7–21 days 5–10 days
products
Cooked, cured and processed poultry and game 30 70 0 to + 3 7–21 days 5–10 days
bird products
Ready meals 30 70 0 to + 3 5–10 days 2–5 days
Fresh pasta products 50 50 0 to + 5 3–4 weeks 1–2 weeks
Bakery products 50 50 0 to + 5 4–12 weeks 4–14 days
Hard cheese 100 0 to + 5 2–12 weeks 1–4 weeks
Soft cheese 30 70 0 to + 5 2–12 weeks 1–4 weeks
Dried food products 100 Ambient 1–2 years 4–8 months
Cooked and dressed vegetable products 30 70 0 to + 3 7–21 days 3–14 days
Liquid food and beverage products 100 0 to + 3 2–3 weeks 1 week
Carbonated soft drinks 100 0 to + 3 1 year 6 months
both food and microorganisms will now be outlined. Table 15.1 gives summary
advice on recommended gas mixtures, storage temperatures and achievable
shelf-lives for 16 different foodstuffs.
There are sound commercial reasons why MA packed foods are in such
demand in the UK. These are:
? extension of shelf-life by 50% to 500%
? minimisation of waste – restocking and ordering can become more flexible
? quality, presentation and visual appeal – all improved
? reduction of need for artificial preservatives
? increased distribution distances of products
? semi-centralised production is possible.
15.1.2 New germicidal techniques
No matter how effectively modified atmosphere technology is applied to food,
no product can remain on the supermarket shelf indefinitely. For each food there
is a recommended gas mixture, storage temperature and achievable shelf-life as
given in Table 15.1. At the end of the shelf-life, a summary of the main sources
of food spoilage and poisoning which have occurred under the MAP process is
given in Table 15.2. In all cases the principal spoilage mechanism is microbial
and the main microorganisms responsible for food poisoning for that particular
product have been identified.
Over time, food spoilage inevitably sets in but the rate at which it occurs can
be slowed down by combining germicidal and MAP techniques. Both UV and
Table 15.2 Sources of food spoilage and poisoning
Food item Principal spoilage
mechanisms
Some food poisoning hazards
Raw red meat Colour change
(red to brown).
Microbial.
Clostridium species, Salmonella species,
S. aureus, Bacillus species, Listeria
monocytogenes, E. coli.
Raw poultry and
game
Microbial. Clostridium species, Salmonella species,
S. aureus, Listeria monocytogenes,
Campylobacter species.
Raw fish and seafood Oxidative rancidity.
Microbial.
Clostridium botulinum (non-proteolytic E,
B and F) Vibris parahaemolyticus.
Ready meals Microbial. Clostridium species, Salmonella species,
S. aureus, Bacillus species, Listeria
monocytogenes, Yersinia enterocolitica
Bakery products Microbial, staling.
Physical separation.
Moisture migration.
S. aureus, Bacillus species,
Cheese Microbial, oxidative
rancidity.
Physical separation
Clostridium species, Salmonella species,
S. aureus, Bacillus species, Listeria
monocytogenes, E. coli.
314 Novel food packaging techniques
ozone are able to kill microorganisms therefore the combining of UV and ozone
with modified atmospheric packaging (MAP) results in a safer product and an
extended shelf-life. Compact germicidal systems can be incorporated within the
MAP packaging process, resulting in a sustainable increase in shelf-life.
The survival (S) of microorganisms when exposed to either UV or ozone is
represented by two rates of decay (Wekhof 2000) as follows
S C exp kD for D < D
o
S C exp mD for D < D
o
This relationship is illustrated in Fig. 15.1. 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 15.2 shows a comparison of the
dosages (D
o
) required for UV, ozone and chlorine required to achieve a 99.9%
kill level when compared with the dosage for 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 – 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.
15.2 Ultraviolet radiation
Ultraviolet (UV) radiation is a form of energy that can be absorbed by and can
bring about structural changes of systems (Koller 1965). The exposure of
microbiological systems to UV radiation, within the wavelength range defined
by Fig. 15.3, can dissociate the DNA, which are vital to metabolic and
Fig. 15.1 Fraction of living microorganisms (S).
Integrating MAP with new germicidal techniques 315
Fig. 15.2 Mortalities of bacteria and pathogens in sterilisation of water.
Fig. 15.3 Ultraviolet radiation spectrum.
316 Novel food packaging techniques
reproductive functions and thus inactivate the microorganisms. The most
common source for producing light within a germicidal region is the low
pressure mercury vapour lamp. At room temperature approximately 73% of the
output radiation produces 254nm UV radiation, 19% produces 185nm UV
radiation and 8% is output as a series of wavelengths 313, 365, 405, 436 and
546nm.
This is shown in Fig. 15.4. It operates with the same principle as a fluorescent
lamp but without the phosphor coating. A voltage applied across the lamp
generates an electric field E within the lamp which ionises the mercury vapour
to produce UV light emission. The bulb is made of type 219 quartz which
excludes light below 220nm. When operating at a temperature of 40oC this lamp
emits 92% of its radiation at 254nm wavelength. The characteristics of this
family of lamps are given in Table 15.3. They operate using a.c. (50Hz) mains
power and produce an output of no more than 25W per metre lamp length.
Microwaves are high frequency electromagnetic waves generated by
magnetrons, which can be stored in a resonance cavity made of metal or
dielectric material (Wilson 1992). The principle is illustrated in Fig. 15.5 in
which microwaves are launched into the lamp via a coupled metal cavity
resonator. The electric field (E) ionises the mercury vapour in the lamp to
produce the UV emission. The microwave frequency is 2.46GHz and is the same
as that used in a microwave oven. This allows low cost magnetrons to be used
(Kraszewski 1967). The lamps differ significantly from conventional UV lamps
because they have no warm-up time, do not deteriorate with age, have adaptable
shapes and can be used in pulsed mode. There is also the possibility of
producing ozone and UV from the same lamp to produce a synergistic effect.
Fig. 15.4 Conventional low-pressure mercury lamp.
Table 15.3 Conventional UV lamp characteristics
Lamp and Lamp wattage Lamp current UV output UV output @
arc length W mA W 1000mm,
(mm) W/cm
2
212, 131 10 425 2.9 24
287, 206 14 425 3.9 35
436, 356 23 425 7.0 69
793, 711 37 425 12.8 131
Integrating MAP with new germicidal techniques 317
Two different lamp designs are shown in Fig. 15.6. The lamps are energised
from one end and operate in free space to emit both 185nm and 254nm by using
214 quartz glass or can emit only 254nm by using 219 quartz glass (Al-
Shamma’a et al. 2001). Because the microwaves produce a transverse electric
field compared with the longitudinal electric field of the conventional lamp, the
microwave lamp is able to emit UV of an order of magnitude higher in intensity,
e.g., at least 250W/m.
UV light can be detected by silicon photodiodes having enhanced responses
in the 190 to 400nm wavelength range. The 5.8mm
2
detector area is housed in a
metal can package whilst the 33.6 and 100mm
2
devices are housed in ceramic
packages (RS Components 1998). All packages incorporate a quartz window for
enhanced spectral response. The device is illustrated in Fig. 15.7 with all
Fig. 15.5 The microwave UV lamp.
The UV flat lamp The UV cylindrical lamp
Fig. 15.6 Microwave UV lamp shapes.
318 Novel food packaging techniques
dimensions being given in mm. It operates with a voltage of 5V and a maximum
current of 10mA. The electrical characteristics are given in Table 15.4 and the
responsitivity in Fig. 15.8. The device produces a current output which is linear
with input UV power.
UV light is able to kill microorganisms by using wavelengths within the
germicidal region. The 254nm wavelength emitted from a mercury discharge is
ideal for this action. The kill rate is usually represented by a logarithmic value of
Fig. 15.7 The UV detector diode (mm units).
Table 15.4 Diode characteristics
Active area Responsivity amp/watt (typical) Peak
mm
2
mm @ 190nm @ 245nm @ 340nm responsivity
(typical)
5.8 2.4 2.4 0.12 0.14 0.19 950nm
33.6 5.8 5.8 0.12 0.14 0.19 950nm
Fig. 15.8 Typical spectrum response and typical quantum efficiency curves.
Integrating MAP with new germicidal techniques 319
the kill rate with 90% being 1, 99% being 2, 99.9% being 3. Table 15.5 gives the
3 log kill rate for a wide range of microorganisms. The UV light power is given
in microwatts per cm
2
and a typical value would be 6000 W/cm
2
for bacteria.
Higher kill rates can be obtained by increasing the UV light dosage (intensity
time) but there is usually a limit attained for the kill rate.
Table 15.5 Ultraviolet energy levels in microwatt-seconds per square centimetre at
wavelength of 254nm required for 99.9% destruction of microorganisms
Bacteria Mould spores
Agrobacterium tumefaciens 8500 Mucor ramosissimus (white 35200
Bacillus anthraci 8700 gray)
Bacillus megaterium (vegetative) 2500 Penicillum expensum 22000
Bacillus subtilis (vegetative) 11000 Penicillum roqueforti (green) 26400
Clostridium tetani 22000
Corynebacterium diphtheriae 6500 Algae
Escherichia coli 7000
Legionella bozemanii 3500
Legionella dumoffii 5500 Chlorella vulgaris 22000
Legionella gormonii 4900
Legionella micdadei 3100
Legionella longbeachae 2900
Legionella pneumophila 3800 Viruses
(Legionaires disease)
Leptospira interrogans 6000
(Infectious Jaundice)
Mycobacterium tuberculosis 10000 Bacteriophage (e. coli) 6600
Neisseria cattarhalis 8500 Hepatitis virus 8000
Protius vulgaris 6600 Influenza virus 6600
Pseudomonas aeruginosa 3900 Poliovirus 21000
(laboratory strain)
Pseudomonas aeruginosa 10500 Rotavirus 24000
(environmental strain)
Rhodospirilium rubrum 6200 Yeast
Salmonella enteritidis 7600
Salmonella paratyphi 6100 Baker’s yeast 8800
(Enteric fever)
Salmonella typhimurium 15200 Brewer’s yeast 6600
Salmonella typhosa 6000 Common yeast cake 13200
(typhoid fever)
Sarcini lutea 26400 Saccharomyces var. ellipsoideus 13200
Serratia marcescens 6200 Saccharomyces sp 17600
Shigella dysenteriae (Dysentery) 4200
Shigella flexneri (Dysentery) 3400 Cysts
Shigella sonnei 7000
Staphylococcus opidermidis 5800 Cysts normally cannot be killed with UV,
Staphylococcus aureus 7000 but are removed with a sub-micron filter
Staphylococcus faecalis 10000 such as the EPCB filter by PURA
Staphylococcus hemolyticus 5500
Staphylococcus lactis 8000
Viridans streptococci 3800 Cysts include Giardia, Llambila and
Vibrio cholerae (Cholera) 6500 Chryptosporidiun
320 Novel food packaging techniques
15.3 Ozone
Ozone is toxic and concentrations in excess of 5ppm are required to produce a
significant microbiocidal effect in a short exposure time consistent with modern
high-speed production lines. Ozone is a compound in which three atoms of
oxygen are combined to form the molecule O
3
. It is a strong, naturally occurring
oxidising and disinfecting agent. The weak bond holding ozone’s third oxygen
atom causes the molecule to be unstable. Because of this instability an
oxidisation reaction occurs upon any collision between an ozone molecule and
microorganisms (bacteria, viruses and cysts). Bacteria cells and viruses are split
apart or inactivated through oxidisation of their DNA chains.
O
3
X O
2
XO
ozone microorganism oxygen oxide
Ozone has a half life of 4 to 12 hours in air depending on the temperature and
humidity of the ambient air. The half life in water ranges between seconds and
hours depending on the temperature, pH and water quality.
Two commercial methods are used for generating ozone namely corona
discharge and ultraviolet radiation. The corona discharge (CD) system is
produced by passing air through a high voltage electric field which is close to
the ignition voltage required for electrical breakdown. Typical operating
conditions range from 5000 volts for high frequency voltages of 1000Hz to
16000 volts for low frequency voltages of 50Hz (mains frequency). Air
(containing approximately 21% oxygen) or concentrated oxygen (up to 95%
pure oxygen) dried to a minimum of 60oC dew point passes through the corona
which contains free electrons (e) which causes the oxygen (O
2
) bond to split
allowing two O atoms to collide with other O
2
molecules to create ozone
O
2
e 2O e
O
2
O O
3
The ozone/gas mixture discharged from the CD ozone generator normally
contains 1% to 3% when using dry air and 3% to 10% when using high purity
oxygen.
As indicated in Fig. 15.9, the production of ozone with un-dried air ( 10oC)
is less than half of that at the dew point of 60oC. The figure alone shows the
increase in the production of nitrogen oxides increases exponentially above
40oC dew point. The nitrogen oxides dissolve in water creating nitric acid,
which is corrosive to the CD system construction materials causing increased
maintenance. Moisture can be removed by passing the air through molecular
sieves, activated alumina, silica gel, membranes or by a combination of
refrigeration and desiccation. Oxygen is concentrated in air by passing ambient
air through molecular sieve material which absorbs moisture and nitrogen when
pressurised to 2 bar. The production rates for commercial units are indicated in
Table 15.6.
Integrating MAP with new germicidal techniques 321
Ozone is produced by irradiating ambient air with UV having wavelengths
below 200nm. Longer wavelengths, around 250nm, are more efficient at
destroying ozone rather than producing it. The energy of the UV splits some of
the O
2
molecules into two O atoms which collide with other O
2
molecules to
produce ozone (O
3
). The system is shown in Fig. 15.10 for which air is flowed
through a larger cylinder placed around the UV lamp. Because UV light sources
are not monochromatic, both long and short wavelengths are generated therefore
in UV systems ozone is simultaneously produced and destroyed. The
concentration of ozone from the UV generator depends on the UV energy
output of the lamp used, the enclosure surrounding the lamp, the temperature,
humidity and oxygen content of the air and the volume of air flowing through the
generator. Figure 15.11 shows the increase in production rate g/kWhr as the gas
flow increases when using the microwave UV lamp. For a flow rate of 160lpm the
production rate is 13gm/kWRr for a 45.9W lamp when using 214 quartz. By
using refined quartz it is possible to transmit wavelengths as low as 160nm and
for these conditions the ozone production rate is higher than 40g/kWhr.
Fig. 15.9 Corona discharge ozone production.
Table 15.6 Ozone production rates for commercial ozone units
Ozone production Oxygen feed Power consumption Efficiency
g/h gas flow rate lpm with compressor W g/kWhr
16 4.5 835 19
30 9.0 1415 21
45 13.5 1930 23
60 18.0 2430 25
322 Novel food packaging techniques
Sensors for ozone employ electrochemical sensors. These sensors operate
continually and require minimum maintenance. The rated ambient temperatures
are between 25oC to + 50oC. The ranges can vary between 0 and 10ppm with a
sensitivity of 0.1ppm up to a range between 0 and 100ppm. The output signal
can be transmitted on a 4–20 mA current loop to remote displays or data logger.
The device normally operates from mains power but hand-held sets operating
from 12V d.c. batteries are available. The warm-up period is a few minutes and
the response to ozone changes only takes a few seconds. Figure 15.12 shows the
compact sensor system produced by ATI (Manchester UK).
Fig. 15.10 Generation of ozone using 186nm UV.
Fig. 15.11 Ozone results – compressed air.
Integrating MAP with new germicidal techniques 323
Some results for ozone in water have been given in Fig. 15.2. The dosage of
ozone for E coli bacteria is 0.5mg/litre of water with a kill rate of 99.9% being
obtained. The results for the kill rates of ozone with contaminated air is given in
Table 15.8.
The kill rate as a function of time is shown in Figs 15.13 and 15.14 for the
E. coli and S. aureus bacteria. There is a two decay process with different rate
Table 15.7 Comparison of ozone generation by corona discharge versus ultraviolet
radiation
Parameter Ultraviolet radiation Corona discharge
Maximum ozone
production rate
13g/kWh using185nm bulbs with air
40g/kWhr using 160nm bulbs
>25g/kWh from dry
concentrated oxygen
air
Concentration of
ozone in output gas
per kW
~0.29% by weight of air ~1.6%
by weight of oxygen
Need to dry feed gas Desirable if needed for consistent ozone
output in a given application, but not
critical for equipment longevity
(moisture with UV generators does not
produce nitric acid as moisture does with
corona discharge generators)
Critical for optimum
equipment life and
decreased equipment
maintenance
Capital costs Relatively low Relatively low
Operating costs
(electrical energy)
High High
Fig. 15.12 ATI ozone sensor 1–100ppm.
324 Novel food packaging techniques
constants which occur simultaneously namely the rapid die-off of the
individual bacterial cells and the slow death of resistant or protectively
clumped bacteria. The response to ozone is as if two different species were
present and the total effect of ozonation is simply the addition of the separate
effects.
15.4 Integration with MAP
These are a range of machines such as horizontal form-fill-seal machines,
thermoform-fill-seal machines, vacuum chambers and snorkels. Horizontal
form-fill-seal (HFFS) or so-called flow pack machines are capable of making
flexible pillow-pack pouches from only one reel of film. Horizontal form-fill-
seal machines can also overwrap a pre-filled tray of product. Form-fill-seal
Table 15.8 Bacteria kill rates and ozone dosages in contaminated air
Organism Survival % Ozone ppm Time sec Dosage
(ppmxsec)
S. salivarius 2 0.6 600 360
S. epidermis 0.6 0.6 240 144
E. coli 0.056 300 15 4500
0.007 631 15 9460
S. aureus 0.004 300 15 4500
0.003 1500 15 22500
Fig. 15.13 Death curves for E. coli in ozonated air. (Ozone concentrations during this
series of experiments varied from 300 to 1500ppm.)
Integrating MAP with new germicidal techniques 325
machines can be of two types – horizontal and vertical (VFFS) as illustrated in
Fig. 15.15. VFFS machines are suitable for gravity-fed loose products such as
coffee, snack products, grated cheese, salads, etc.
Thermoform-fill-seal machines produce packages consisting of a thermoformed
semi-rigid tray which is hermetically sealed to a flexible lidding material.
Rollstock film (typically PVC/PE) is automatically conveyed into a thermoforming
section where a vacuum or compressed air is used to draw the film into dies, giving
the trays their desired shape. The product is then manually or automatically loaded
into the trays before evacuation, back-flushing with the desired MA gas mixture,
and heat sealing with lidding material. The hermetically sealed packages are then
finally separated by cross-cutting and longitudinal cutting units. The Multivac
Rollstock machine is illustrated in Fig. 5.16.
Fig. 15.14 Death curves for S. aureus in ozonated air. (Ozone concentrations during this
series of experiments varied from 300 to 1500ppm.)
Fig. 15.15 Horizontal form-fill-seal machine.
326 Novel food packaging techniques
The choice of films for MAP is largely determined by their gas and water
vapour transmission rates. Materials such as polyester (PET), nylon,
polyvinylidene chloride (PVdC) and ethylene vinyl alcohol copolymer (EVOH)
provide good gas barriers but in many cases poor water vapour barriers.
Polythene, polypropylene and ethylene vinyl acetate have gas transmission rates
which are too high to maintain a chosen gas mixture or vacuum for long enough
to provide an adequate shelf-life for most products. However, they are good
barriers to water vapour and hence prevent products drying out or dry products
becoming moist. Oxygen transmission rates of films for most applications lie in
the range 10–125cm
3
/m
2
.day.atm.
15.4.1 Installation of the UV/ozone systems
The UV/ozone lamp shown in Fig. 15.5 has been incorporated into the Rollstock
Machine shown in Fig. 15.16. The food is exposed to the ozone or UV before the
lidding material is applied to the packaging, as shown in Fig. 15.17. The lamp is
able to produce both UV and ozone either separately or as a combined output as
illustrated in Fig. 15.18a, b, c. The UV germicidal effect is produced by the
254nm mercury line which is transmitted by both 214 and 219 quartz glass. The
ozone is produced by the 185nm mercury line which is only transmitted by the
214 quartz glass as shown in Fig. 15.19. Hence the lamp may be arranged to give
UV (Fig.15. 18a), ozone (Fig. 15.18b) or UV and ozone (Fig. 15.18c). Pyrex
glass prevents the transmission of both 185nm and 254nm UV radiation.
The UV/ozone lamp has been mounted onto the Rollstock machine shown in
Fig. 15.16. The arrangement is shown in Fig. 15.20a and b with the indication of
the best location for the system. A gantry type arrangement to position the lamp
Fig. 15.16 Multivac Rollstock machine.
Integrating MAP with new germicidal techniques 327
above the food trays is shown in Fig. 15.21a and b. This allows the lamp and
reflector clearly to illuminate the food trays with UV, ozone or its combination.
The solid construction of the device is shown in Fig. 15.22a and the absence of UV
radiation close to the operating area is seen in Fig. 15.22b. The screening also
maintains a trap for the ozone. The bacteria kill rate for the S. aureus
microorganism is given in Fig. 15.23 and shows the killing actions of ozone and
UV/ozone jointly producing 4 log performance.
Fig. 15.17 Lidding material.
Fig. 15.18 UV/ozone germicidal arrangement. Type 214 (1mm thickness) Type 219
(1mm thickness).
328 Novel food packaging techniques
Fig. 15.19 Transmission of UV in quartz glass.
Fig. 15.20a Installation location.
Integrating MAP with new germicidal techniques 329
Fig. 15.20b Overview of mounting arrangement.
Fig. 15.21b Microwave cavity with UV lamp fixing bracket.
Fig. 15.21a Frame mounting arrangement.
330 Novel food packaging techniques
Fig. 15.22b UV screening of UV radiation from operating personnel.
Fig. 15.22a Gantry mounting of the UV lamp.
Integrating MAP with new germicidal techniques 331
15.5 Future trends
In order to obtain higher kill rates it is necessary to break up the microorganism
clumps into single microorganisms. This is possible by using large doses of UV
or ozone (Wekhof 2001). Figures 15.24 and 15.25 illustrate enhanced energy (J/
cm
2
) levels required when using white light. Such energy can be generated only
by using a xenon flashlamp which produces a wide radiation spectrum from
Fig. 15.23 Data for S. aureus and Ps aeruginosa after treatment with UV + ozone (44–
67ppm).
Fig. 15.24 Microbiological effects of pure white light on E. coli (petri dish test).
332 Novel food packaging techniques
1100nm to 200nm. The amount of germicidal radiation within the range 230 to
280nm is about 5% compared with 73% for the low-pressure lamp. The rate of
kill for the single microorganism is therefore about 15 times less efficient for the
flashlamp because of the reduced percentage of germicidal radiation. However,
the breaking up of the clumps of microorganisms is mainly a thermal effect and
can be achieved by all the intense emitted radiation and hence the observed
faster kill rate for higher intensities.
The deactivation rate for E. coli, B. subtilis and S. aureus is given in Fig.
15.26. Filtering out the UV germicidal radiation from the spectrum produces a
dramatic reduction in the obtained kill values. The kill rate with 254nm UV
radiation for E. coli is 6mJ/cm
2
for a 3 log kill rate when using a conventional
UV lamp and this has to be compared with 500mJ/cm
2
using a xenon flashlight.
Likewise the kill rate for B. subtilis with 254nmUV radiation is 11mJ/cm
2
compared with 250mJ/cm
2
. However, a large overall power has the ability to
improve the overall kill rate by up to 4 logs of addition kill levels by using total
radiation effects as shown in Fig. 15.25.
The advantage of the microwave system, shown in Fig. 15.5, is its ability to
produce pulsed UV light at high pulse powers. Figure 15.27 shows the same
average power of 6mJ/cm
2
being produced in 100 s pulses with repetition time
of 70 s. The advantage of using UV over white light is that it is more readily
absorbed by the substrate and hence less pulse power is required to break up the
clumps of microorganisms. In addition the kill rate of the single microorganisms
resulting from the breakup of clumps is also enhanced when compared with
white light. Theory suggests that the surface temperature must rise to over 100oC
during the pulse duration in order to detach the surface bacteria.
Fig. 15.25 Pure white light effect on B. subtilis spores (petri dish test).
Integrating MAP with new germicidal techniques 333
Fig. 15.27 Pulsed UV lamp waveform.
Fig. 15.26 Comparison of bacteria deactivation with a flashlamp for a full spectrum and
for the UV filtered spectra. 1. E. Coli at 8 flashes of 12J/cm
2
of full spectra and 10 flashes
each of 12J/cm
2
with UV filtering. 2. B. subtilis (vegetative form) at 1 flash (4 to 12) j/
cm
2
of a full spectra and 15 flashes each of (8 to 10) j/cm
2
with the UV filtering. 3. B.
subtilis (spores) at 1 flash of 8/cm
2
of a full spectra and 10 flashes of same energy with
UVC filtering. 4. S. aureus at 1 flash of 2J/cm
2
of a full spectra and with 5 flashes at 4J/
cm
2
each with UV filtering.
334 Novel food packaging techniques
Equation 15.1 shows the surface temperature (T) as a function of time (t) after
the surface has been irradiated by a plane wave light source of power P (W/m
2
)
Pt T
Dt
p
15:1
with D K= :
where K thermal conductivity W/(mk)
specific heat kJ/(kgk)
density (kg/dm
3
)
The diffusion distance (r) is r
Dt
p
: 15:2
For a specimen plate of thickness r then
Pr KT: 15:3
For aluminium K 209, 0.904, 2.7,
whilst for glass K 0.81, 0.84, 2.5
In order to reach a surface temperature of 100oC then for a glass substrate Pr
81 and t 2.59r
2
. If r 10mm then P 8100W/m
2
(i.e. 800nmW/cm
2
) and
t 259 s, alternatively if r 1mm then P 8W/cm
2
and t 2.59 s. Thus a
series of 8kW/m
2
pulses of 259 s duration will boil off the bacteria from the
surface for destruction by the UV light.
The food industry is keen to adopt and exploit techniques that improve the
safety and/or extend the shelf-life of food products without the use of
preservatives. There is currently considerable interest in the use of UV and
ozone particularly with the prospect of chlorine washing being discontinued for
organic produce. One of the drawbacks of conventional UV lamps is that they do
not work in shadowed areas. However, one of the advantages of microwave UV
lamps is that their shape and size can be adapted to suit the product being
irradiated. Some products such as sliced meat present a flat surface which lends
itself readily to UV treatment. Other products such as bread have a porous
crumb structure which is less easily sterilised by UV light but could be treated
with ozone. Unfortunately ozone is toxic with maximum exposure levels of
0.2ppm. Doses in excess of this (2–5ppm minimum) are required to produce
significant microbiocidal effects in a short exposure time consistent with modern
high-speed production lines. Hence safety aspects for operating personnel need
to be carefully considered.
Combining UV and ozone could provide sufficient sterilisation which when
combined with MAP, results in a safer product and/or extended shelf-life. Some
products containing fatty acids can unfortunately be oxidised by ozone leading
to off flavours. It may be possible to counteract the oxidation during the
sterilisation phase by use of an appropriate MAP gas system. Combined UV/
ozone systems can provide more options for food (and packaging) sterilisation.
They provide the option of ‘flash’ sterilisation. The challenge will be to
determine the optimum frequency, intensity or waveform for the greatest
biocidal effect. There is also the option of producing ozone and UV to produce a
Integrating MAP with new germicidal techniques 335
synergistic effect. This will also be combined with MAP gases. The combined
UV/ozone system (Lucas and Al-Shamma’a 2001) has the following attributes.
? To kill bacterial growth and moulds by using recently invented, compact
systems for producing UV radiation and ozone by microwaves.
? To enhance the shelf-life of food products by integrating the germicidal
system into modified atmosphere packaging (MAP) machines (e.g., Multivac
Chamber and Rollstock Machines).
? To destroy microbes in water washing systems for fruit and vegetable
produce.
? To be applicable within a factory environment for a wide range of food
products.
15.6 References
AL-SHAMMA’A, A.I., PANDITHAS, I. and LUCAS, J., (2001), ‘Low Pressure
Microwave Plasma UV Lamp for Water Purification and Ozone
Production’, J. Phys D: Appl. Phys. Special Issue, Vol 34, No. 14,
2775–81.
AL-SHAMMA’A, A.I., PANDITHAS, I., LUCAS J. and STUART R.A., (2001), ‘Microwave
Plasma UV Lamp for Water Purification’, Accepted for Publication in the
IEEE Transaction on Plasma Science, Ref: S016.
BENNETT, B. (1995), The freshline Guide to Modified Atmospheric Packaging,
Crew, Air Products.
KOLLER, L. R. (1965) Ultraviolet Radiation, John Wiley and Sons, Inc., N.Y.,
Second Edition.
KRASZEWSKI, A. (1967) Microwave Gas Discharge Devices, Ilife Books Ltd.,
Second Edition.
LUCAS J. and AL-SHAMMA’A, A.I., (2001), Germicidal Modified Atmospheric
Packaging, DEFRA, Food Link News, Newsletter for the Food Link
programme, No. 36, 9.
RS COMPONENTS DATASHEET, Photodiodes, issued July 1998 (order code 298-
4562).
WEKHOF, A. (2000), ‘Disinfection with flash lamps’, PDA Journal of
Pharmaceutical Science and Technology, 54 (3), 264–76.
WEKHOF, A. (2001), ‘Pulsed UV to Sterilise Packaging and to preserve
foodstuffs’, AGIR Conference on New Sterilisation Technologies, Talence,
1–6.
WILSON F.A. (1992) An Introduction to Microwaves, Bernard Babani (publishing)
Ltd. ISBN 0 85934 257 3.
336 Novel food packaging techniques