Lynneric Potter of Campden BRI explains the principles behind modified atmosphere packaging (MAP) and discusses the problems caused by the shortages in carbon dioxide supply for food and beverage packaging in the summer months of 2018.
Modified atmosphere packaging (MAP) is used as a means of preservation and is well known in the food industry as a method to extend the shelf-life of a range of different food products. It relates to the removal of atmospheric air, which is then replaced with alternative gases that are optimal to the product in order to provide the longest shelf-life whilst maintaining quality and safety. Changing the atmosphere in the headspace of a package can reduce biochemical changes, retard microbial growth and maintain organoleptic qualities whilst reducing the need for additives.
Modified atmospheres
A modified atmosphere can be produced in a number of ways. Passive modification is more commonly associated with fruits and vegetables by matching the respiration rate with the permeability of the film to create a favourable atmosphere. Gas packing can be used in combination with a vacuum (compensated vacuum) or just by gas flushing. Compensated vacuum firstly applies a vacuum to the pack to draw out the air and then the desired gas mixture is flushed into the pack.
Gas packing relies on a continuous stream of gas being injected into the pack to replace the air. The disadvantage of gas packing is the residual oxygen levels can be higher and therefore unsuitable for oxygen sensitive products. Compensated vacuum can be slower as it is a two-stage process, but the residual oxygen levels are usually lower.
The first recorded scientific study with modified atmospheres was conducted in 1821 by Jacques Etienne Berard, who found that the ripening of fruits could be delayed by removing oxygen. Advances with controlled atmosphere storage continued, initially with a number of studies on fresh fruit contained within a gas tight building relying on the respiration of the produce to generate a passive atmosphere.
Controlled atmosphere storage refers to bulk storage where the atmosphere is controlled and monitored. The first controlled atmosphere store was built in 1929 and a decade later over 200 units were in use[1]. Controlled atmosphere storage progressed further for fruit and vegetables, enabling transportation by sea, making the export of a range of different products across the world possible. Controlled atmospheric storage is still used for this purpose and also for storing produce so it is available throughout the year.
Following Berard’s work, studies continued, mainly with meat and fish. These focused predominantly on the effects of carbon dioxide with results showing a dramatic increase in shelf-life compared to air. MAP made its debut in the retail industry in the 1970s, initially with meats and fish, and since then the market has rapidly grown to cover the majority of food groups.
The interaction between the gas and the food is critical and needs to be understood to select the correct gas mixture. Commonly a mixture of one, two or three gases, usually nitrogen, carbon dioxide or oxygen, are used. However there has been a lot of research looking at the potential benefits of other gases including carbon monoxide, argon and nitrous oxide. The gases used play an integral role in the finished pack.
Oxygen is usually removed from the pack, particularly with dried foods, ready cooked products and those with high fat contents to reduce oxidation and inhibit aerobic microbial growth. However, for some products the presence of oxygen can be beneficial. With red meat, oxygen helps develop the red colour and is used in high concentrations (70-80%) within the pack. In the presence of high levels of oxygen, purple myoglobin (in the meat) may be oxygenated to the red pigment oxymyoglobin, while at low oxygen levels, it can be oxidised to metmyoglobin, giving an undesirable brown colour.
Fresh produce requires oxygen to continue respiring. It also produces carbon dioxide as a by-product. During the modified atmosphere packing of fresh produce, the level of oxygen is usually reduced from that of atmospheric air (21%) to approximately 5% depending on the product. Reducing the oxygen slows the respiration of the produce, which can be high, as the cells try to repair themselves following the preparation processes of washing and cutting. This delays fermentation which can occur as the atmosphere becomes anaerobic.
Oxygen is often included with white fish as it can have a bleaching effect on the fish and also reduces drip.
Nitrogen is an inert gas and is often used to displace oxygen, particularly with products that are prone to oxidation. These products are often packed in 100% nitrogen. Nitrogen is also used as a filler gas to prevent pack collapse and offers some cushioning to the product.
Raw and cooked meats, fish, ready meals, combination products and bakery goods will all commonly be packed with a proportion of carbon dioxide.
Role of carbon dioxide
Image may be NSFW. Clik here to view.Carbon dioxide is normally included as an antimicrobial agent. It is often suggested that a minimum of 20% is required for it to have an effect, but there is little scientific data to back this. Carbon dioxide can be used in combination with oxygen with red meat, white fish and fresh produce but it is often used in combination with nitrogen to form an anaerobic environment to inhibit aerobic microorganisms.
Raw and cooked meats, fish, ready meals, combination products and bakery goods will all usually be packed with a proportion of carbon dioxide. Carbon dioxide is more commonly used at levels between 25-40% apart from in bakery goods and some cheeses, where carbon dioxide can be used up to 100%.
Bakery goods can be packed in high levels of carbon dioxide to increase the mould-free shelf life. Studies have shown that with higher concentrations of carbon dioxide the mould free shelf-life (product specific) can be increased from days to weeks and months[2]. However, MAP will not prevent the inevitable staling of bakery goods although it may slow the rate.
Bulk packs of meat and poultry can also be packed in 100% carbon dioxide prior to being packed for retail.
Carbon dioxide is highly soluble in moisture and fats. Solubility of carbon dioxide in the water phase of the product produces carbonic acid, which can increase the acidity of the product resulting in a small drop in pH. Studies have shown that intrinsic and extrinsic factors influence the solubility of carbon dioxide. These may include pH, water activity, fat type and content, gas to product ratio and temperature. The solubility of carbon dioxide increases as the temperature decreases, similarly the antimicrobial activity of carbon dioxide is higher at lower temperatures[3]. The change in pH may be beneficial with some products, but not if it causes an undesirable taint.
The concentration of dissolved carbon dioxide in the water phase of the food is thought to help with the inhibition of microorganisms. Gram negative microorganisms, such as Psuedomonas, are more sensitive to carbon dioxide compared to Gram positive organisms. Lactic acid bacteria are the least sensitive to carbon dioxide[4] and growth can be stimulated by the presence of carbon dioxide[3].
Due to solubility, high levels of carbon dioxide can cause pack collapse, therefore nitrogen is often included to prevent this. High levels of carbon dioxide can also cause damage to plant and muscle tissues, discoloration and excessive drip.
One aspect of MAP, which is often overlooked, is the gas to product ratio. This can often be confused with the composition of gas rather than the volume of gas added to the pack. It is particularly important with carbon dioxide, which is absorbed into the product to get enough gas into the pack for it to have an effect.
Depending on the product, it is normally recommended that a 2:1 gas to product ratio is used. However, this can result in a large seemingly empty headspace and manufacturers can be criticised for using too much material.
Carbon dioxide shortage
Carbon dioxide is usually recovered as a by-product from the production of hydrogen or ammonia used in the manufacture of fertilisers. It is captured and then sold on to different industries. The summer of 2018 saw a shortage of carbon dioxide, which affected several industries, bringing to light the importance and benefits of the gas.
Several ammonia plants were shut down for maintenance and, although it is not unusual for plants to close in the summer, a higher that usual number of plants closed at the same time. The food industry was not given sufficient time to react and prepare for the shortage with only 1-2 weeks’ notice being provided by the gas suppliers that stocks of carbon dioxide would be short or delayed.
Since the carbon dioxide shortage occurred during the football World Cup and an extremely hot summer, the media was largely focused on the shortage of beer and carbonated drinks. However, behind the scenes, food manufacturers were seriously affected too. Well-established gas mixes providing extended shelf life to a range of products including ready meals, bakery goods, meat and poultry were in jeopardy. With imminent short supplies of carbon dioxide, industry was faced with packing its products with reduced levels of carbon dioxide or in atmospheric air with no carbon dioxide. Campden BRI was inundated with requests from many of its members for advice on alternative gas mixes and the effect on the quality and shelf life if carbon dioxide levels were reduced. Changing a gas mix – even reducing the range of gases by 5-10% – requires shelf-life to be reassessed, but the short timeframe gave little opportunity for this.
Other areas of the food industry that use carbon dioxide were also affected. It is used to produce dry ice, which may be used to transport frozen goods, for example for online orders. Carbon dioxide can also be used for the slaughter of animals, leading to concerns that the shortage could result in animal welfare issues and a shortage of meat.
Outside the food industry, carbon dioxide is used in many applications, including as an alternative to water in fire extinguishers, refrigeration systems, water treatment and production of some pharmaceuticals.
In light of the carbon dioxide shortage, many food manufacturers have been reviewing their current gas mixes. Studies have been carried out to look at how reducing carbon dioxide within the mix impacts on their assigned shelf-life in case this situation should arise again so that they are better prepared. An industry meeting, held at Campden BRI, confirmed that there is very little literature on the effects of carbon dioxide and pathogens.
Most studies have concentrated on spoilage and establishing shelf-life. As a result, Campden BRI is working with industry to develop a research club to study the effects of different levels of carbon dioxide on the growth and control of selected pathogens.
With imminent short supplies of carbon dioxide, industry was faced with packing its products with reduced levels of carbon dioxide or in atmospheric air with no carbon dioxide.
Plastics challenge
Modified atmosphere packing will continue to be used as a means of preservation within the food industry. For ‘clean’ products that require a longer shelf life, modified atmosphere is an ideal solution, however concern over the excessive use of plastics in packaging is something we need to consider for the future.
The food industry has faced significant criticism for its use of plastics and this is driving the development of alternative materials. Although there are many options to explore, there are specific food groups for which this will be more technically challenging.
Products packed in a modified atmosphere require the materials to have specific barrier properties and maintain seal integrity for the duration of the shelf-life of the product or the modified atmosphere will be lost. This not only relates to chilled foods but many ambient products. Many snack products, such as crisps, which have been targeted by anti-plastic groups, rely on nitrogen flushing to provide a longer shelf life and to maintain the quality of the snack, slowing rancidity, which can result in off flavours. If the food industry is going to continue to supply and develop the vast range of products consumers are currently used to, then new materials must provide the same properties to ensure the quality and safety of the product is maintained.
Lynneric Potter, Food Packaging Technical Lead
Department of Food Manufacturing Technologies, Campden BRI, Station Road, Chipping Campden, Gloucestershire, GL55 6LD
Lynneric has worked at Campden BRI since 1999, where her main activities involve consultancy and testing of packaging materials to ensure they are fit for purpose. This includes new and novel materials, as well as traditional packaging materials such as plastic, glass, metals and paper board. Main areas of packaging work include, strength and integrity testing, food and packaging interactions, shelf life studies and a lot of troubleshooting.
2. Seiler, D.A.L. 1998. Bakery products. In: Principles and Applications of Modified Atmosphere Packaging of Foods. Edited by Blakistone, B.A. 2nd Edition. Blackie A&P
3. Mullan, M. and McDowell, D. 2011. Modified atmosphere packaging. In: Food Packaging Technology. Edited by Coles, R. and Kirwin, M.J. 2nd Edition. Blackwell Publishing
4. Devlieghere, F. and Debevere, J. 2003. MAP, product safety and nutritional quality. In: Novel Food Packaging Techniques. Edited by Ahvenainen, R. Woodhead Publishing Ltd
