Oxygen, ethylene and other scavengers L. Vermeiren, L. heirlings, F. Devlieghere and J. Debevere, Ghent University, Belgium 3.1 Introduction The best known and most widely used active packaging technologies for foods today are those engineered to remove undesirable substances from the headspace of a package through absorption, adsorption or scavenging. To achieve this goal a physical or chemical absorbent or adsorbent is incorporated in the packaging material or added to the package by means of a sachet. In most publications, the term absorption is used loosely to describe any system that removes a substance from the headspace. However, there is a clear difference between absorption and adsorption. Adsorption is a two-dimensional phenomenon while absorption is three-dimensional. According to Mortimer(1993),absorption Involv es a substance being take the bulk of involves a substance being taken onto a surface. Both, absorption and adsorption are physical phenomena while scavenging implies a chemical reaction(Brody al, 2001). This chapter focuses mainly on oxygen and ethylene scavenging and finally also discusses carbon dioxide absorbers and odour removers 3.2 Oxygen scavenging technology 3.2.1 Introduction In many cases, food deterioration is caused by the presence of oxygen, as oxygen is responsible for oxidation of food constituents and proliferation of moulds, aerobic bacteria and insects. Modified atmosphere packaging(MAP) and vacuum packaging have been widely adopted to exclude oxygen from the headspace. However, these physical methods of oxygen elimination do not
3.1 Introduction The best known and most widely used active packaging technologies for foods today are those engineered to remove undesirable substances from the headspace of a package through absorption, adsorption or scavenging. To achieve this goal a physical or chemical absorbent or adsorbent is incorporated in the packaging material or added to the package by means of a sachet. In most publications, the term ‘absorption’ is used loosely to describe any system that removes a substance from the headspace. However, there is a clear difference between absorption and adsorption. Adsorption is a two-dimensional phenomenon while absorption is three-dimensional. According to Mortimer (1993), absorption involves a substance being taken into the bulk of a phase while adsorption involves a substance being taken onto a surface. Both, absorption and adsorption are physical phenomena while scavenging implies a chemical reaction (Brody et al., 2001). This chapter focuses mainly on oxygen and ethylene scavenging and finally also discusses carbon dioxide absorbers and odour removers. 3.2 Oxygen scavenging technology 3.2.1 Introduction In many cases, food deterioration is caused by the presence of oxygen, as oxygen is responsible for oxidation of food constituents and proliferation of moulds, aerobic bacteria and insects. Modified atmosphere packaging (MAP) and vacuum packaging have been widely adopted to exclude oxygen from the headspace. However, these physical methods of oxygen elimination do not 3 Oxygen, ethylene and other scavengers L. Vermeiren, L. Heirlings, F. Devlieghere and J. Debevere, Ghent University, Belgium
Oxygen, ethylene and other scavengers 23 al ways remove the oxygen completely. Some oxygen (0. 1-2%) generally remains in the package and even more when the food is porous. Moreover, the oxygen that permeates through the packaging film during storage cannot be removed by these techniques. In the presence of such amounts of oxygen, many of the oxidation reactions and mould proliferation still proceed. Oxygen scavengers are able to reduce the oxygen concentration to less than 0.01% and can maintain those levels(Rooney, 1995; Hurme and Ahvenainen, 1998, Vermeiren et al, 1999). An oxygen scavenger is a substance that scavenges oxygen chemically or enzymatically and therefore, protects the packaged food completely against deterioration and quality changes due to oxygen 3.2.2 Role of oxygen scavengers Preventing oxidation Oxygen scavengers effectively prevent oxidative damage in a wide range of food constituents such as(i)oils and fats to prevent rancidity, (ii) both plant and muscle pigments and flavours to prevent discolouration(e.g. meat) and loss of aste and (iii)nutritive elements, e.g., vitamins to prevent loss of the nutritional alue. Berenzon and Saguy(1998)investigated the effect of oxygen scavengers on the shelf-life extension of crackers packaged in hermetically sealed tin cans which were stored at 15, 25 and 35C for up to 52 weeks. Oxygen scavengers reduced the hexanol concentration significantly. Peroxide values were markedly reduced by the presence of oxygen scavengers. In the presence of oxygen scavengers, the lag period before the peroxides started to build up was prolonged to, respectively, 17 and 10 weeks at 25 and 35C. Sensory evaluations showed that in the presence of oxygen scavengers and independently of storage temperature, no oxidative rancid odours were observed for up to 44 weeks Preventing insect damage Oxygen scavengers are effective for killing insects and worms or their eggs growing in cereals such as rice, wheat and soybeans. Fumigation treatments using gases such as bromides and methyl disulfide kill insects but their residues can remain in the food. Additionally, insects in the egg or pupal stages can be resistant against fumigation treatments. Oxygen scavengers are very effective against insects because they remove the oxygen the insects need to survive Prevention of proliferation of moulds and strictly aerobic bacteria Ox ging is effective in preventing growth of moulds and aerobic bacteria. Mould spoilage is an important microbial problem limiting the shelf- life of high and intermediate moisture products. Losses due to mould spoilage are a serious economic concern in the bakery industry. Some moulds, such as Aspergillus flavus and Aspergillus parasiticus, can also produce highly toxic substances called mycotoxins. In gas packaging aerobic growth can still occur depending on the residual oxygen level in the package headspace. It has been demonstrated that moulds can proliferate in headspaces with oxygen
always remove the oxygen completely. Some oxygen (0.1–2%) generally remains in the package and even more when the food is porous. Moreover, the oxygen that permeates through the packaging film during storage cannot be removed by these techniques. In the presence of such amounts of oxygen, many of the oxidation reactions and mould proliferation still proceed. Oxygen scavengers are able to reduce the oxygen concentration to less than 0.01% and can maintain those levels (Rooney, 1995; Hurme and Ahvenainen, 1998; Vermeiren et al., 1999). An oxygen scavenger is a substance that scavenges oxygen chemically or enzymatically and therefore, protects the packaged food completely against deterioration and quality changes due to oxygen. 3.2.2 Role of oxygen scavengers Preventing oxidation Oxygen scavengers effectively prevent oxidative damage in a wide range of food constituents such as (i) oils and fats to prevent rancidity, (ii) both plant and muscle pigments and flavours to prevent discolouration (e.g. meat) and loss of taste and (iii) nutritive elements, e.g., vitamins to prevent loss of the nutritional value. Berenzon and Saguy (1998) investigated the effect of oxygen scavengers on the shelf-life extension of crackers packaged in hermetically sealed tin cans which were stored at 15, 25 and 35ºC for up to 52 weeks. Oxygen scavengers reduced the hexanol concentration significantly. Peroxide values were markedly reduced by the presence of oxygen scavengers. In the presence of oxygen scavengers, the lag period before the peroxides started to build up was prolonged to, respectively, 17 and 10 weeks at 25 and 35ºC. Sensory evaluations showed that in the presence of oxygen scavengers and independently of storage temperature, no oxidative rancid odours were observed for up to 44 weeks. Preventing insect damage Oxygen scavengers are effective for killing insects and worms or their eggs growing in cereals such as rice, wheat and soybeans. Fumigation treatments using gases such as bromides and methyl disulfide kill insects but their residues can remain in the food. Additionally, insects in the egg or pupal stages can be resistant against fumigation treatments. Oxygen scavengers are very effective against insects because they remove the oxygen the insects need to survive. Prevention of proliferation of moulds and strictly aerobic bacteria Oxygen scavenging is effective in preventing growth of moulds and aerobic bacteria. Mould spoilage is an important microbial problem limiting the shelflife of high and intermediate moisture products. Losses due to mould spoilage are a serious economic concern in the bakery industry. Some moulds, such as Aspergillus flavus and Aspergillus parasiticus, can also produce highly toxic substances called mycotoxins. In gas packaging aerobic growth can still occur depending on the residual oxygen level in the package headspace. It has been demonstrated that moulds can proliferate in headspaces with oxygen Oxygen, ethylene and other scavengers 23
24 Novel food packaging techniques concentrations as low as 1-2%(Smith, 1996). Oxygen levels of 0. 1% or lower are required to prevent the growth and mycotoxin production of many moulds (Rooney, 1995). The effects of modified atmosphere packaging involving oxygen scavengers, age temperature and packaging film barrier characteristics on the growth of and aflatoxin production by Aspergillus parasiticus in packaged peanuts was investigated(Ellis et al, 1994). A slight mould growth was visible in air-packaged peanuts using a high gas barrier film (Oxygen Transmission Rate (OTR) of 3-6 cc. m. day at 23C and dry conditions)while extensive growth was observed in peanuts packaged under similar air conditions using a low gas barrier film(OTR of 4000 cc m-day-) When an oxygen scavenger(Ageless type S)was incorporated, mould growth was inhibited in peanuts packaged in a high gas barrier film and was reduced when a low barrier film was used. Aflatoxin B1 production was inhibited in peanuts packaged in a high barrier film with an oxygen scavenger, while a limited amount of aflatoxin less than the regulatory level of 20 ng g was detected in absorbent packaged peanuts using a low barrier film. This study showed that oxygen scavengers are effective for controlling the growth of and aflatoxin production by Aspergillus parasiticus. However, the effectiveness of the scavengers will be dependent on the gas barrier properties of the packaging Smith et al.(1986) showed that oxygen scavengers are three times more effective than gas packaging for increasing the mould-free shelf-life of crusty rolls. In gas packaged (40% N2/60% CO2)crusty rolls with Ageless the headspace oxygen never increased beyond 0.05% and the product remained mould-free for over 60 days at ambient storage temperature. A similar mould- free shelf-life was obtained in air and N2 packaged crusty rolls with Ageless The mould-free shelf-life of white bread packaged in a polypropylene film could be extended from 4-5 days at room temperature to 45 days by using an Ageless sachet. Pizza crust, which moulds in 2-3 days at 30oC was mould-free for over 10 days using an appropriate O2 scavenger(Nakamura and Hoshino, 1983) It is well known that an oxygen-free atmosphere at a water activity greater than 0.92 can favour the growth of many microbial pathogens including Clostridium botulinum (Labuza and Breene, 1989). Clostridium botulinum mainly grows under anaerobic conditions but can also have a limited growth under low O2 conditions. The use of oxygen scavengers could be dangerous if the temperature is not kept close to 0oC. Daifas et al.(1999)investigated the growth and toxin production by Clostridium botulinum in English-style crumpets, using an Ageless FX200 oxygen scavenger at room temperature All inoculated crumpets were toxic within 4 to 6 days and were organoleptically acceptable at the time of toxigenesis. Counts of C. botulinum increased to approximately 105 CFUIg at the time of toxin production. This study confirms that C. botulinum could pose a public health hazard in high aw - high pH crumpets using an oxygen scavenger when stored at non-chilled conditions Lyver et al.(1998)have done challenge studies on raw surimi nuggets, which were inoculated with 10" spores/g of Clostridium botulinum type E spores. All
concentrations as low as 1–2% (Smith, 1996). Oxygen levels of 0.1% or lower are required to prevent the growth and mycotoxin production of many moulds (Rooney, 1995). The effects of modified atmosphere packaging involving oxygen scavengers, storage temperature and packaging film barrier characteristics on the growth of and aflatoxin production by Aspergillus parasiticus in packaged peanuts was investigated (Ellis et al., 1994). A slight mould growth was visible in air-packaged peanuts using a high gas barrier film (Oxygen Transmission Rate (OTR) of 3–6 cc. mÿ2 . dayÿ1 at 23ºC and dry conditions) while extensive growth was observed in peanuts packaged under similar air conditions using a low gas barrier film (OTR of 4000 cc mÿ2 dayÿ1 ). When an oxygen scavenger (AgelessÕ type S) was incorporated, mould growth was inhibited in peanuts packaged in a high gas barrier film and was reduced when a low barrier film was used. Aflatoxin B1 production was inhibited in peanuts packaged in a high barrier film with an oxygen scavenger, while a limited amount of aflatoxin less than the regulatory level of 20 ng. gÿ1 was detected in absorbent packaged peanuts using a low barrier film. This study showed that oxygen scavengers are effective for controlling the growth of and aflatoxin production by Aspergillus parasiticus. However, the effectiveness of the scavengers will be dependent on the gas barrier properties of the packaging film. Smith et al. (1986) showed that oxygen scavengers are three times more effective than gas packaging for increasing the mould-free shelf-life of crusty rolls. In gas packaged (40% N2/60% CO2) crusty rolls with Ageless Õ the headspace oxygen never increased beyond 0.05% and the product remained mould-free for over 60 days at ambient storage temperature. A similar mouldfree shelf-life was obtained in air and N2 packaged crusty rolls with AgelessÕ. The mould-free shelf-life of white bread packaged in a polypropylene film could be extended from 4–5 days at room temperature to 45 days by using an AgelessÕ sachet. Pizza crust, which moulds in 2–3 days at 30ºC was mould-free for over 10 days using an appropriate O2 scavenger (Nakamura and Hoshino, 1983). It is well known that an oxygen-free atmosphere at a water activity greater than 0.92 can favour the growth of many microbial pathogens including Clostridium botulinum (Labuza and Breene, 1989). Clostridium botulinum mainly grows under anaerobic conditions but can also have a limited growth under low O2 conditions. The use of oxygen scavengers could be dangerous if the temperature is not kept close to 0ºC. Daifas et al. (1999) investigated the growth and toxin production by Clostridium botulinum in English-style crumpets, using an AgelessÕ FX200 oxygen scavenger at room temperature. All inoculated crumpets were toxic within 4 to 6 days and were organoleptically acceptable at the time of toxigenesis. Counts of C. botulinum increased to approximately 105 CFU/g at the time of toxin production. This study confirms that C. botulinum could pose a public health hazard in high aw – high pH crumpets using an oxygen scavenger when stored at non-chilled conditions. Lyver et al. (1998) have done challenge studies on raw surimi nuggets, which were inoculated with 104 spores/g of Clostridium botulinum type E spores. All 24 Novel food packaging techniques
Oxygen, ethylene and other scavengers 25 Table 3.1 Effects of oxygen scavengers on foods(Abe, 1994; Smith et al, 1990) Effect Typical application Fresh taste and aroma arious food items, coffee, tea lusts, cheese, processed Nuts, fried foods, processed meat, whole er produc duration Processed meat, green noodle, herbs, tea, ← Insect damage Beans, grain, herbs, spices Maintaining nutritional value All kinds of foods products were packaged in air and air with an Ageless Ss oxygen absorber and stored at 4, 12 and 25C. Toxin was not detected in any raw product throughout stor 8 days). The absence of toxigenesis was attributed to the low pH (4.1 4.3)due mainly to the growth of lactic acid bacteria. Whiting and naftulil (1992) showed that controlling the pH and NaCl concentration of the food product is an important factor in controlling growth of C. botulinum under low oxygen concentrations. When oxygen absorbers are used, challenge studies should be done to investigate if C. botulinum is able to grow. An overview of the effects of oxygen scavengers and their most important food applications is shown in Table 3.1 3.3 Selecting the right type of oxygen scavenger Oxygen scavengers must satisfy several requirements: they must 1. be harmless to the human body. Though the oxygen scavengers themselves are neither food nor food additives, they are placed together with food in a package, and there is therefore the possibility of accidental intake 2. absorb oxygen at an appropriate rate. If the reaction is too fast, there will be a loss of oxygen absorption capacity during introduction into the package. If it is too slow, the food will not be adequately protected from oxygen 3. not produce toxic substances or unfavourable gas or odour. performance 5. absorb a large amount of oxygen 6. be economically priced(Nakamura and Hoshino, 1983: Abe, 1994; Rooney An appropriate oxygen scavenger is chosen depending on the O2-level in the headspace, how much oxygen is trapped in the food initially and the amount of
products were packaged in air and air with an AgelessÕ SS oxygen absorber and stored at 4, 12 and 25ºC. Toxin was not detected in any raw product throughout storage (28 days). The absence of toxigenesis was attributed to the low pH (4.1– 4.3) due mainly to the growth of lactic acid bacteria. Whiting and Naftulin (1992) showed that controlling the pH and NaCl concentration of the food product is an important factor in controlling growth of C. botulinum under low oxygen concentrations. When oxygen absorbers are used, challenge studies should be done to investigate if C. botulinum is able to grow. An overview of the effects of oxygen scavengers and their most important food applications is shown in Table 3.1. 3.3 Selecting the right type of oxygen scavenger Oxygen scavengers must satisfy several requirements: they must 1. be harmless to the human body. Though the oxygen scavengers themselves are neither food nor food additives, they are placed together with food in a package, and there is therefore the possibility of accidental intake by consumers. 2. absorb oxygen at an appropriate rate. If the reaction is too fast, there will be a loss of oxygen absorption capacity during introduction into the package. If it is too slow, the food will not be adequately protected from oxygen damage. 3. not produce toxic substances or unfavourable gas or odour. 4. be compact in size and are expected to show a constant quality and performance. 5. absorb a large amount of oxygen. 6. be economically priced (Nakamura and Hoshino, 1983; Abe, 1994; Rooney, 1995). An appropriate oxygen scavenger is chosen depending on the O2-level in the headspace, how much oxygen is trapped in the food initially and the amount of Table 3.1 Effects of oxygen scavengers on foods (Abe, 1994; Smith et al., 1990) Effect Typical application Fresh taste and aroma Various food items, coffee, tea $ Mould growth Bakery products, cheese, processed seafood, pasta $ Rancidity Nuts, fried foods, processed meat, whole milk powder product $ Discolouration Processed meat, green noodle, herbs, tea, dried vegetables $ Insect damage Beans, grain, herbs, spices Maintaining nutritional value All kinds of foods Oxygen, ethylene and other scavengers 25
26 Novel food packaging oxygen that will be transported from the surrounding air into the package during storage. The nature of the food (e. g. size, shape, weight), water activity and desired shelf-life are also important factors influencing the choice of oxygen absorbents For an oxygen scavenger(sachet) to be effective, some conditions have to be fulfilled(Nakamura and Hoshino, 1983 Abe, 1994, Smith, 1996). First of all packaging containers or films with a high oxygen barrier must be used, otherwise the scavenger will rapidly become saturated and lose its ability to trap O2. Films with an oxygen permeability not exceeding 20 ml/md atm are recommended for packages in which an oxygen scavenger will be used. Examples of barrier layers used with oxygen scavengers are VOH(ethylene vinyl alcohol) and PVDC (polyvinylidene chloride)(Nakamura and Hoshino, 1983; Rooney, 1995). If films with high O2 permeabilities are used( 100 ml/m2. d atm), the O2 concentration will reach zero within a week but after some days, it will return to ambient air level because the absorbent is saturated. If high-barrier films(e.g. <10 ml/m=d atm)are used, the headspace O2 will be reduced to 100 ppm within 1-2 days and remain at this level for the duration of the storage period provided that package integrity is maintained(Rooney, 1995). Secondly, for flexible packaging heat sealing should be complete so that no air invades the package through the sealed part. A rapid, inexpensive and efficient method of monitoring package integrity and ensuring low residual headspace oxygen throughout the storage period is through the incorporation of a redox indicator, e.g. Ageless Eye. Ageless Eye is a tablet which indicates the presence of oxygen by a colour change. When placed inside the package, the colour changes from blue to pink when the O2 concentration approaches zero. If the indicator reverts to its blue colour, this is an indication of poor packaging integrity (Smith et al, 1990; Nakamura and Hoshino, 1983, Rooney, 1995). Finally, an oxygen scavenger of the appropriate type and size must be selected. The appropriate size of the scavenger can be calculated using the following formulae(Roussel, 1999; ATCO technical information, 2002). The volume of oxygen present at the time of packaging(A)can be calculated using the formula A=(V-P)×[O2/100 V= volume of the finished pack determined by submersion in water and expressed in ml P= weight of the finished pack in g: Jo2= initial O2 concentration in package(=21% if air) In addition, it is ary to calculate the volume of oxygen likely to permeate through the packaging during the shelf-life of the product(B). Thi quantity in ml may be calculated as follows B=SxP×D
oxygen that will be transported from the surrounding air into the package during storage. The nature of the food (e.g. size, shape, weight), water activity and desired shelf-life are also important factors influencing the choice of oxygen absorbents. For an oxygen scavenger (sachet) to be effective, some conditions have to be fulfilled (Nakamura and Hoshino, 1983; Abe, 1994; Smith, 1996). First of all, packaging containers or films with a high oxygen barrier must be used, otherwise the scavenger will rapidly become saturated and lose its ability to trap O2. Films with an oxygen permeability not exceeding 20 ml/m2 .d.atm are recommended for packages in which an oxygen scavenger will be used. Examples of barrier layers used with oxygen scavengers are EVOH (ethylene vinyl alcohol) and PVDC (polyvinylidene chloride) (Nakamura and Hoshino, 1983; Rooney, 1995). If films with high O2 permeabilities are used (> 100 ml/m2 .d.atm), the O2 concentration will reach zero within a week but after some days, it will return to ambient air level because the absorbent is saturated. If high-barrier films (e.g. < 10 ml/m2 .d.atm) are used, the headspace O2 will be reduced to 100 ppm within 1–2 days and remain at this level for the duration of the storage period provided that package integrity is maintained (Rooney, 1995). Secondly, for flexible packaging heat sealing should be complete so that no air invades the package through the sealed part. A rapid, inexpensive and efficient method of monitoring package integrity and ensuring low residual headspace oxygen throughout the storage period is through the incorporation of a redox indicator, e.g. AgelessÕ EyeÕ. AgelessÕ EyeÕ is a tablet which indicates the presence of oxygen by a colour change. When placed inside the package, the colour changes from blue to pink when the O2 concentration approaches zero. If the indicator reverts to its blue colour, this is an indication of poor packaging integrity (Smith et al., 1990; Nakamura and Hoshino, 1983; Rooney, 1995). Finally, an oxygen scavenger of the appropriate type and size must be selected. The appropriate size of the scavenger can be calculated using the following formulae (Roussel, 1999; ATCOÕ technical information, 2002). The volume of oxygen present at the time of packaging (A) can be calculated using the formula: A V ÿ P O2=100 V volume of the finished pack determined by submersion in water and expressed in ml; P weight of the finished pack in g; O2 initial O2 concentration in package ( 21% if air). In addition, it is necessary to calculate the volume of oxygen likely to permeate through the packaging during the shelf-life of the product (B). This quantity in ml may be calculated as follows: B S P D 26 Novel food packaging techniques