15 Integrating MAP with new germicidal techniques J. Lucas, University of liverpool, UK 15.1 Introduction Modified Atmospheric Packaging(MAP)is a precise description of this shelf- ife 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 )ver 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 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
15.1 Introduction Modified Atmospheric Packaging (MAP) is a precise description of this shelflife 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.°C Shelf-life days CO N2 MAP gas In air Raw red meat 70 30 I to 5-8 days 24 days Raw offal I to 4-8 days 2-6 days Raw poultry and game I to 10-21 days 4-7 days Raw fish and seafood 0000000 2-3 days Cooked, cured and processed meat products 0to+3 Cooked, cured and processed fish and seafood 0to+3 5-10 days Cooked, cured and processed poultry and game 0to+3 7-21 days 5-10 days bird products Ready meals 0to+3 Fresh pasta products 00000 5-10 days 2-5 days 3-4 weeks 1-2 week Bakery products 0to+5 4-12 weeks 4-14 days Hard cheese 0to+5 2-12 weeks 1-4 weeks Soft cheese 0to+5 2-12 weeks 1-4 weeks Dried food products 1-2 4-8 months Cooked and dressed vegetable products 00 0to+3 7-21 days 3-14 days Liquid food and beverage products 0to+3 2-3 weeks Carbonated soft drinks 100 0to+3 I year 6 months
Table 15.1 MAP gas mixtures for food items Food item Retail gas mix Storage temp. oC Shelf-life days O2 CO2 N2 MAP gas In air Raw red meat 70 30 ÿ1 to + 2 5–8 days 2–4 days Raw offal 80 20 ÿ1 to + 2 4–8 days 2–6 days Raw poultry and game 30 70 ÿ1 to + 2 10–21 days 4–7 days Raw fish and seafood 30 40 30 ÿ1 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
314 Novel food packaging techniques both food and microorganisms will now be outlined. Table 15. 1 gives summary ended gas mixtures, storage temperatures and achievable shelf-lives for 16 different foodstuffs There are sound commercial reasons why MA packed foods are in such emand in the 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 semi-centralised production is possibe ucts increased distribution distances of prod 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 proc 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 Some food poisoning hazards mechanism Raw red meat Colour change red to brown) S. aureus, Bacillus species, Listerpecies, monocyte poultry and Microbial Clostridium species, Salmonella species, Campylobacter species Raw fish and seafood Oxidative rancidity. Clostridium botulinum(non-proteolytic E, B and F)vibris parahaemolyticus Ready mea Microbial Clostridium species, Salmonella spec Listeria monocytogenes, Yer enterocolitica S. aureus, Bacillus species, Moisture Cheese Microbial, oxidative Clostridium species, Salmonella spec ancid S. aureus, Bacillus species, Listeria Physical separation monocytogenes, E coli
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
Integrating MAP with new germicidal techniques 315 Log s-Log, C 0 LOg,s-Log(C)-3 Fig. 15. 1 Fraction of living microorganisms (S) 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 epresented by two rates of decay(Wekhof 2000)as follows S=C exp(-kD) for D< Do S=C exp(-mD)for D< Do This relationship is illustrated in Fig. 15.1. The dosage D is the product of the UV or ozone intensity and duration(n)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. Figure 15.2 shows a comparison of the dosages(D)required for UV, ozone and chlorine required to achieve a 99.9% kill level when compared with the dosage for Escherichia coli(. 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 ce 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
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 ÿkD for D < Do S C exp ÿmD for D < Do 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 ÿ 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 (Do) 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 micronsized 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
16 Novel food packaging techniques ∽>c ≌学> O O 2 Q-e∽w uy cl O, cl 2 Fig. 15.2 Mortalities of bacteria and pathogens in sterilisation of water 1849200 280300 400nm Ozone forming range Gcmicidc Ra Fig. 15.3 Ultraviolet radiation spectrum
Fig. 15.2 Mortalities of bacteria and pathogens in sterilisation of water. Fig. 15.3 Ultraviolet radiation spectrum. 316 Novel food packaging techniques
Integrating MAP with new germicidal techniques 317 Filament Filament se face to base face length Fig. 15.4 Conventional low-pressure mercury lamp eproductive 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 ac.(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 Table 15.3 Conventional UV lamp characteristics Lamp and Lamp wattage Lamp current UV output UV output arc length 1000mm uW/cm- 212.131 0 425 2.9 24 287,206 425 3.9 436.356 425 793.711 37 425 12.8
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/cm2 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
318 Novel food packaging techniques MPUVL To sensor power supply UV radialion 0-200245GHIL pp) power microwave DVM Fig. 15.5 The microwave UV lamp Two different lamp designs are shown in Fig. 15.6. The lamps are energise om 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- Shammaa 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 detector area is housed in a metal can package whilst the 33.6 and 100mm* 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 The UV flat lamp The UV cylindrical lamp Fig. 15.6 Microwave UV lamp shapes
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 (AlShamma’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 5.8mm2 detector area is housed in a metal can package whilst the 33.6 and 100mm2 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
Integrating MAP with new germicidal techniques 319 Active are 0.45p Metal can package Cathode -Anode Fig. 15.7 The Uv detector diode(mm units) 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 Table 15.4 de characteristics Active ity amp/watt(typical) @245mm@340nm responsivity 2.4×2.4 950nm 33.6 58×5.8 0.14 950nm 006 Wavelength. nanome Fig. 15.8 Typical spectrum response and typical quantum efficiency curves
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 mm2 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
320 Novel food packaging techniques Table 15.5 Ultraviolet energy levels in microwatt-seconds per square centimetre at wavelength of 254nm required for 99.9% destruction of microorganisms Mould grobacterium tumefaciens 8500 Mucor ramosissimus(white 35200 Bacillus anthraci Bacillus subtilis(vegetative) 1 1000 Penicillum roqueforti(green) 26400 6500 gae Escherichia col 7000 3500 gionella dumoffii 5500 Chlorella vulgaris 22000 Legionella gormon 4900 Legionella mcdade 3100 Legionella longbeachae Legionella 3800 Viruses Legionaires disease Leptospira interrogans 6000 (Infectious Jaundice Mycobacterium tuberculosis 6600 Neisseria cattarhalis virus 8000 Proteus vulgaris vIrus Pseudomonas aeruginosa poliovirus 21000 aboratory strain) Pseudomonas aeruginosa 10500 Rotavirus 24000 (environmental strain) Rhodospirillum rubrum 6200 Yeast Salmonella enteritidis Salmonella paratyphi 6100 Baker's yeast (Enteric fever) Salmonella typhimurium 15200 Brewers yeast Salmonella typhosa (typhoid fever) 26400 Saccharomyces var ellipsoideus 13200 Serratia marcescens 17600 Shigella dysenteriae(Dysentery) 4200 Shigella flexneri (Dysentery) sonnel Staphylococcus epidermidis Staphylococcus aureus with a sub-micron filter taphylococcus faecalis 10000 the EPCB filter by PURA Staphylococcus hemolyticus Staphylococcus lactis Viridans streptococci 3800 Cysts include Giardia, Llambila and Vibrio cholerae(Cholera 6500 Chryptosporidiun 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" and a typical value would be 6000 u W/cm- for bacteria Higher kill rates can be obtained by increasing the UV light dosage(intensity x time)but there is usually a limit attained for the kill rate
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 cm2 and a typical value would be 6000 W/cm2 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
Integrating MAP with new germicidal techniques 321 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 O3. 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 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 hich contains free electrons(e)which causes the oxygen(O2)bond to split allowing two o atoms to collide with other o, molecules to create ozone O2+e=20+e O2+O=0 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 purit 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 -60C. The figure alone shows the increase in the production of nitrogen oxides increases exponentially above -40C 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
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 O3. 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. O3 X O2 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 ÿ60ºC dew point passes through the corona which contains free electrons (e) which causes the oxygen (O2) bond to split allowing two O atoms to collide with other O2 molecules to create ozone O2 e 2O e O2 O O3 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 (ÿ10ºC) is less than half of that at the dew point of ÿ60ºC. The figure alone shows the increase in the production of nitrogen oxides increases exponentially above ÿ40ºC 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