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