View source for Chemistry:Modified atmosphere

[[File:Test Modified Atmosphere.jpg|right|thumb|Testing the atmosphere in a [[Engineering:Plastic bag|plastic bag]] of carrots]]
'''Modified atmosphere packaging''' ('''MAP''') is the practice of modifying the composition of the internal atmosphere of a [[Social:Packaging|package]] (commonly food packages, drugs, etc.) in order to improve the [[Finance:Shelf life|shelf life]].<ref>{{Citation 
  | last =Ogg
  | first =M
  | title = Modified Atmosphere Packaging Adds More Value To Value-Added
  | publisher =Produce Business
| date =April 2020
  | url =https://www.producebusiness.com/modified-atmosphere-packaging-adds-more-value-to-value-added/
  | access-date =20 August 2020
}}</ref><ref name=":0">{{Cite book|title=Principles and applications of modified atmosphere packaging of foods|last=Parry|first=R. T.|publisher=Springer US|year=1993|isbn=9781461358923|location=Boston, MA|oclc=840284063}}</ref> The need for this technology for food arises from the short shelf life of food products such as meat, fish, poultry, and dairy in the presence of [[Chemistry:Oxygen|oxygen]]. In food, oxygen is readily available for lipid oxidation reactions. Oxygen also helps maintain high [[Biology:Cellular respiration|respiration]] rates of fresh produce, which contribute to shortened shelf life.<ref>{{Cite book|title=Frying of food : oxidation, nutrient and non-nutrient antioxidants, biologically active compounds and high temperatures|last=Boskou, D., Elmadfa, I.|publisher=CRC Press|year=2011|isbn=9781439806821|edition=2nd|location=Boca Raton|oclc=466361000}}</ref> From a microbiological aspect, oxygen encourages the growth of [[Biology:Cellular respiration#Aerobic respiration|aerobic]] [[Medicine:Decomposition|spoilage]] [[Biology:Microorganism|microorganisms]].<ref name=":0" /> Therefore, the reduction of oxygen and its replacement with other gases can reduce or delay oxidation reactions and microbiological spoilage. [[Physics:Oxygen scavenger|Oxygen scavenger]]s may also be used to reduce [[Biology:Food browning|browning]] due to lipid oxidation by halting the auto-oxidative chemical process. Besides, MAP changes the gaseous atmosphere by incorporating different compositions of gases.

The modification process generally lowers the amount of oxygen (O<sub>2</sub>) in the [[Engineering:Ullage|headspace]] of the package. Oxygen can be replaced with [[Chemistry:Nitrogen|nitrogen]] (N<sub>2</sub>), a comparatively inert gas, or [[Chemistry:Carbon dioxide|carbon dioxide]] (CO<sub>2</sub>).<ref name=":0" />

A stable atmosphere of gases inside the packaging can be achieved using active techniques, such as gas flushing and compensated vacuum, or passively by designing “breathable” films.

== History ==
The first recorded beneficial effects of using modified atmosphere date back to 1821. [[Biography:Jacques Étienne Bérard|Jacques Étienne Bérard]], a professor at the School of Pharmacy in Montpellier, France, reported delayed ripening of fruit and increased shelf life in low-oxygen storage conditions.<ref name=":6">{{Cite journal|last=Kirtil, E and Oztop, M.H.|date=2016|title=Controlled and modified atmosphere packaging|journal=Reference Module in Food Science|doi=10.1016/B978-0-08-100596-5.03376-X|isbn=9780081005965}}</ref> [[Earth:Controlled atmosphere|Controlled atmosphere]] storage (CAS) was used from the 1930s when ships transporting fresh apples and pears had high levels of CO<sub>2</sub> in their holding rooms in order to increase the shelf life of the product.<ref name=":1">{{Cite book|title=Principles and applications of modified atmosphere packaging of foods|last=Blakistone|first=B.A.|publisher=Blackie Academic & Professional|year=1998|isbn=978-0751403602|edition=2nd|location=London|pages=1–38}}</ref> In the 1970s MA packages reached the stores when bacon and fish were sold in retail packs in Mexico. Since then development has been continuous and interest in MAP has grown due to consumer demand.

== Theory ==
Atmosphere within the package can be modified passively or actively.<ref name=":5">{{Cite book|title=Food Packaging Principles and Practice (2nd ed)|last=Robertson|first=G.L.|publisher=CRC Press|year=2006|isbn=978-0-8493-3775-8|location=Florida|pages=313–330}}</ref> In passive MAP, the high concentration of CO<sub>2</sub> and low O<sub>2</sub> levels in the package is achieved over time as a result of respiration of the product and gas transmission rates of the packaging film. This method is commonly used for fresh respiring fruits and vegetables. Reducing O<sub>2</sub> and increasing CO<sub>2</sub> slows down respiration rate, conserves stored energy, and therefore extended shelf life.<ref name=":52">{{Cite book|title=Modified atmosphere packaging for fresh-cut fruits and vegetables|last=Brody, A.L., Zhuang, H.|first=Han, J.H|publisher=Blackwell Publishing Ltd|year=2011|isbn=978-0-8138-1274-8|location=West Sussex, UK|pages=57–67}}</ref> On the other hand, active MA involves the use of active systems such as O<sub>2</sub> and CO<sub>2</sub> scavengers or emitters, moisture absorbers, [[Chemistry:Ethylene|ethylene]] scavengers, [[Chemistry:Ethanol|ethanol]] emitters and gas flushing in the packaging film or container to modify the atmosphere within the package.<ref name=":52" /> 

The mixture of gases selected for a MA package depends on the type of product, the packaging materials and the storage temperature. The atmosphere in an MA package consists mainly of adjusted amounts of N<sub>2</sub>, O<sub>2</sub>, and CO<sub>2.</sub><ref name=":5" /><ref name=":3">{{Cite book|title=Food processing technology: principles and practice (4th ed)|last=Fellows, P.J|publisher=Woodhead Publishing|year=2017|isbn=978-0-08-101907-8|location=Duxford, UK|pages=992–1001}}</ref> Reduction of O<sub>2</sub>  promotes delay in deteriorative reactions in foods such as [[Chemistry:Lipid peroxidation|lipid oxidation]], [[Biology:Food browning|browning reactions]] and growth of spoilage organisms.<ref name=":1" /><ref name=":5" /> Low O<sub>2</sub> levels of 3-5% are used to slow down respiration rate in fruits and vegetables.<ref name=":5" /> In the case of red meat, however, high levels of O<sub>2</sub> (~80%) are used to reduce oxidation of [[Biology:Myoglobin|myoglobin]] and maintain an attractive bright red color of the meat.<ref name=":622">{{Cite journal|last=Djenane, D.|first=Roncales, P.|date=2018|title=Carbon monoxide in meat and fish packaging: advantages and limits|journal=Foods|volume=7|issue=2|pages=12|doi=10.3390/foods7020012|pmid=29360803|pmc=5848116|doi-access=free}}</ref> Meat color enhancement is not required for pork, poultry and cooked meats; therefore, a higher concentration of CO<sub>2</sub> is used to extend the shelf life.<ref name=":3" /> Levels higher than 10% of CO<sub>2</sub> are phytotoxic for fruit and vegetables, so CO<sub>2</sub> is maintained below this level.

N<sub>2</sub> is mostly used as a filler gas to prevent pack collapse.<ref name=":1" /><ref name=":3" /> In addition, it is also used to prevent oxidative rancidity in packaged products such as snack foods by displacing atmospheric air, especially oxygen, therefore extending shelf life.<ref name=":1" /><ref name=":3" /> The use of noble gases such as [[Chemistry:Helium|helium]] (He), [[Chemistry:Argon|argon]] (Ar) and [[Chemistry:Xenon|xenon]] (Xe) to replace N<sub>2</sub> as the balancing gas in MAP can also be used to preserve and extend the shelf life of fresh and minimally processed fruits and vegetables. Their beneficial effects are due to their higher [[Chemistry:Solubility|solubility]] and diffusivity in water, making them more effective in displacing O<sub>2</sub> from cellular sites and enzymatic O<sub>2</sub> receptors.<ref name=":4">{{Cite journal|last=Ghidelli, C|first=Perez-Gago, M.B|date=2018|title=Recent advances in modified atmosphere packaging and edible coatings to maintain quality of fresh-cut fruits and vegetables.|journal=Critical Reviews in Food Science and Nutrition|volume=58|issue=4|pages=662–679|doi=10.1080/10408398.2016.1211087|pmid=27469103|s2cid=205692928|hdl=20.500.11939/6137|hdl-access=free}}</ref> 

There has been a debate regarding the use of [[Chemistry:Carbon monoxide|carbon monoxide]] (CO) in the packaging of red meat due to its possible toxic effect on packaging workers.<ref name=":622" /> Its use results in a more stable red color of carboxymyoglobin in meat, which leads to another concern that it can mask evidence of spoilage in the product.<ref name=":1" /><ref name=":622" />

== Effect on microorganisms ==
Low O<sub>2</sub> and high CO<sub>2</sub> concentrations in packages are effective in limiting the growth of Gram negative bacteria, [[Biology:Mold|mold]]s and aerobic microorganisms, such as ''[[Biology:Pseudomonas|Pseudomonas]]'' spp. High O<sub>2</sub> combined with high CO<sub>2</sub> could have bacteriostatic and bactericidal effects by suppression of [[Biology:Aerobic organism|aerobes]] by high CO<sub>2</sub> and [[Biology:Anaerobic organism|anaerobes]] by high O<sub>2</sub>.<ref name=":4" /> CO<sub>2</sub> has the ability to penetrate bacterial membrane and affect intracellular [[Chemistry:PH|pH]]. Therefore, lag phase and [[Physics:Generation time|generation time]] of spoilage microorganisms are increased resulting in shelf life extension of [[Physics:Refrigeration|refrigerated]] foods.<ref name=":622" /> Since the growth of spoilage microorganisms are suppressed by MAP, the ability of the [[Biology:Pathogen|pathogen]]s to grow is potentially increased. Microorganisms that can survive under low oxygen environment such as ''[[Biology:Campylobacter jejuni|Campylobacter jejuni]]'', ''[[Biology:Clostridium botulinum|Clostridium botulinum]]'', ''[[Biology:Escherichia coli|E. coli]]'', ''[[Biology:Salmonella|Salmonella]]'', ''[[Biology:Listeria|Listeria]]'' and ''[[Biology:Aeromonas hydrophila|Aeromonas hydrophila]]'' are of major concern for MA packaged products.<ref name=":52" /> Products may appear [[Biology:Organoleptic|organoleptically]] acceptable due to the delayed growth of the spoilage microorganisms but might contain harmful pathogens.<ref name=":52" /> This risk can be minimized by use of additional hurdles such as temperature control (maintain temperature below 3 degrees C), lowering [[Physics:Water activity|water activity]] (less than 0.92), reducing pH (below 4.5) or addition of [[Chemistry:Preservative|preservative]]s such as [[Chemistry:Nitrite|nitrite]] to delay metabolic activity and growth of pathogens.<ref name=":3" />

== Packaging materials ==
Flexible films are commonly used for products such as fresh produce, meats, fish and bread seeing as they provide suitable permeability for gases and [[Physics:Water vapor|water vapor]] to reach the desired atmosphere. Pre-formed trays are formed and sent to the food packaging facility where they are filled. The package headspace then undergoes modification and sealing. Pre-formed trays are usually more flexible and allow for a broader range of sizes as opposed to [[Engineering:Thermoforming|thermoformed]] packaging materials as different tray sizes and colors can be handled without the risk of damaging the package.<ref name=":2">{{Cite book|title=Food and Beverage Packaging Technology|last1=Mullan|first1=Michael|last2=McDowell|first2=Derek|date=2011-03-17|publisher=Wiley-Blackwell|isbn=9781444392180|location=Oxford, UK|pages=263–294|doi=10.1002/9781444392180.ch10}}</ref> Thermoformed packaging however is received in the food packaging facility as a roll of sheets. Each sheet is subjected to heat and pressure, and is formed at the packaging station. Following the forming, the package is filled with the product, and then sealed.<ref>{{Cite journal|last=Schmidt|first=F|date=2003-12-20|title=Modelling of infrared heating of thermoplastic sheet used in thermoforming process|journal=Journal of Materials Processing Technology|volume=143-144|pages=225–231|doi=10.1016/s0924-0136(03)00291-7|s2cid=136832404|url=https://hal.archives-ouvertes.fr/hal-01709512/document}}</ref> The advantages that thermoformed packaging materials have over pre-formed trays are mainly cost-related: thermoformed packaging uses 30% to 50% less material, and they are transported as rolls of material. This will amount in significant reduction of manufacturing and transportation costs.<ref name=":2" />

When selecting packaging films for MAP of fruits and vegetables the main characteristics to consider are gas permeability, water vapor transmission rate, mechanical properties, transparency, type of package and sealing reliability.<ref name=":5" /> Traditionally used packaging films like LDPE (low-density polyethylene), [[Chemistry:Polyvinyl chloride|PVC]] (polyvinyl chloride), EVA (ethylene-vinyl acetate) and OPP (oriented [[Chemistry:Polypropylene|polypropylene]]) are not permeable enough for highly respiring products like fresh-cut produce, mushrooms and broccoli. As fruits and vegetables are respiring products, there is a need to transmit gases through the film. Films designed with these properties are called permeable films. Other films, called barrier films, are designed to prevent the exchange of gases and are mainly used with non-respiring products like meat and fish.

MAP films developed to control the humidity level as well as the gas composition in the sealed package are beneficial for the prolonged storage of fresh fruits, vegetables and herbs that are sensitive to moisture. These films are commonly referred to as modified atmosphere/modified humidity packaging (MA/MH) films.

== Equipment ==
In using form-fill-seal packaging machines, the main function is to place the product in a flexible pouch suitable for the desired characteristics of the final product. These pouches can either be pre-formed or thermoformed. The food is introduced into the pouch, the composition of the headspace atmosphere is changed within the package; it is then heat sealed.<ref name=":2" /> These types of machines are typically called pillow-wrap, which horizontally or vertically form, fill and seal the product.<ref name=":1" /> Form-fill-seal packaging machines are usually used for large scale operations.

In contrast, chamber machines are used for batch processes. A filled pre-formed wrap is filled with the product and introduced into a cavity. The cavity is closed and vacuum is then pulled on the chamber and the modified atmosphere is inserted as desired. Sealing of the package is done through heated sealing bars, and the product is then removed. This batch process is labor-intensive and thus requires a longer period of time; however, it is relatively cheaper than packaging machines which are automated.<ref name=":2" />

Additionally, snorkel machines are used to modify the atmosphere within a package after the food has been filled. The product is placed in the packaging material and positioned into the machine without the need of a chamber. A nozzle, which is the snorkel, is then inserted into the packaging material. It pulls a vacuum and then flushes the modified atmosphere into the package. The nozzle is removed and the package is heat sealed. This method is suitable for bulk and large operations.<ref name=":2" />

== Products ==
Many products such as red meat, seafood, minimally processed fruits and vegetables, salads, pasta, cheese, bakery goods, poultry, cooked and cured meats, ready meals and dried foods are packaged under MA.<ref name=":6" /> A summary of optimal gas mixtures for MA products is shown in the following table.

'''Modified Atmosphere Packaging for different food products and optimal gas mixtures'''<ref name=":0" />
{| class="wikitable"
|-
! Product !! Oxygen (%) !! Carbon Dioxide (%) !! Nitrogen (%)
|-
| Red Meat || 80 - 85 || 15 || -
|-
| Poultry || - || 25 || 75
|-
| Fish || - || 60 || 40
|-
| Cheeses || - || 100 || -
|-
| Bread || - || 70 || 30
|-
| Fresh Pasta || - || - || 100
|-
| Fruits and Vegetables || 3 - 5 || 3 - 5 || 85 - 95
|-
|}

=== Grains ===
Modified atmosphere may be used to store grains. 

{{CO2}} prevents insects and, depending on concentration, mold and oxidation from damaging the grain. Grain stored in this way can remain edible for approximately five years.<ref>{{Cite web|url=http://entomology.k-state.edu/doc/finished-chapters/s156-ch-16-contr-or-mod-atmospheres--mar2.pdf|title=Controlled or Modified Atmospheres|last1=Navarro|first1=Shlomo|last2=Timlick|first2=Blaine|date=March 2012|website=k-state.edu|access-date=17 March 2018|last3=Demianyk|first3=Colin|last4=White|first4=Noel}}</ref> One method is placing a block of [[Chemistry:Dry ice|dry ice]] in the bottom and filling the can with the grain. Another method is purging the container from the bottom by gaseous carbon dioxide from a cylinder or bulk supply vessel. 

[[Chemistry:Nitrogen|Nitrogen]] gas ({{chem2|N2}}) at concentrations of 98% or higher is also used effectively to kill insects in the grain through [[Medicine:Hypoxia (medical)|hypoxia]].<ref>Annis, P.C. and Dowsett, H.A. 1993. Low oxygen disinfestation of grain: exposure periods needed for high mortality. Proc. International Conference on Controlled Atmosphere and Fumigation. Winnipeg, June 1992, Caspit Press, Jerusalem, pp. 71–83.</ref> However, carbon dioxide has an advantage in this respect, as it kills organisms through hypercarbia and hypoxia (depending on concentration), but it requires concentrations of roughly over 35%.<ref>Annis, P.C. and Morton, R. 1997. The acute mortality effects of carbon dioxide on various life stages of Sitophilus oryzae. J. Stored Prod.Res. 33. 115–124</ref> This makes carbon dioxide preferable for fumigation in situations where a hermetic seal cannot be maintained.

Air-tight storage of grains (sometimes called hermetic storage) relies on the respiration of grain, insects, and fungi that can modify the enclosed atmosphere sufficiently to control insect pests. This is a method of great antiquity,<ref>Various authors, Session 1: Natural Air-Tight Storage In: Shejbal, J., ed., Controlled Atmosphere Storage of Grains, Elsevier: Amsterdam, 1–33</ref> as well as having modern equivalents. The success of the method relies on having the correct mix of sealing, grain moisture, and temperature.<ref>Annis P.C. and Banks H.J. 1993. Is hermetic storage of grains feasible in modern agricultural systems? In "Pest control and sustainable agriculture" Eds S.A. Corey, D.J. Dall and W.M. Milne. CSIRO, Australia. 479–482</ref>

A patented process uses [[Physics:Fuel cell|fuel cell]]s to exhaust and automatically maintain the exhaustion of [[Chemistry:Oxygen|oxygen]] in a shipping container, containing, for example, fresh fish.<ref name=ADN51813>{{cite news|title=Laine Welch: Fuel cell technology boosts long-distance fish shipping |url=http://www.adn.com/2013/05/18/2907670/laine-welch-fuel-cell-technology.html |access-date=19 May 2013 |newspaper=Anchorage Daily News |date=18 May 2013 |author=Laine Welch |url-status=dead |archive-url=https://web.archive.org/web/20130609190326/http://www.adn.com/2013/05/18/2907670/laine-welch-fuel-cell-technology.html |archive-date=9 June 2013}}</ref>

== See also ==
* Active packaging
* [[Physics:Cold chain|Cold chain]]
* Modified atmosphere/modified humidity packaging
* [[Physics:Permeation|Permeation]]
* [[Finance:Shelf life|Shelf life]]

== Citations ==
<references />

== References ==
* Church, I.J. & Parsons, A.L.: (1995) Modified Atmosphere Packaging Technology: A Review, Journal Science Food Agriculture, 67, 143-152
* Day, B.P.F.: (1996) A perspective of modified atmosphere packaging of fresh produce In Western Europe, Food Science and Technology Today, 4,215-221
* European Food Information Council (EFIC: (2001) [http://ec.europa.eu/food/fs/sc/scf/out112_en.pdf Opinion of the Scientific Committee on Food on the use of carbon monoxide as component of packaging gases in modified atmosphere packaging for fresh meat].
* Parry, R. T.: (1993) Principles and applications of MAP of foods, Blackie Academic & Professional, England, 1-132
* Phillips, C.A.: (1996) Review: Modified Atmosphere Packaging and its effects on the microbial quality and safety of produce, International Journal of Food Science and Tech, 31, 463-479
* Robertson, G. L., "Food Packaging: Principles and Practice", 3rd edition, 2013, {{ISBN|978-1-4398-6241-4}}
* Zagory, D. & Kader, A.A.: (1988) Modified atmosphere packaging of fresh produce, Food Technology., 42(9), 70-77

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[[Category:Food technology]]
[[Category:Industrial gases]]

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