Jan. 19, 2022
Dec. 21, 2021
There is ample research on Modified Atmosphere Packaging (MAP) specification for storing and transporting a wide range of fresh and perishable food items like vegetables, fruits, meat, fish, and dairy products. Passive MAP is useful for fresh produce, while active packaging is more relevant for non-respiring animal products. This packaging technology is becoming more important, as it can reduce food loss, preserve quality, and extend shelf-life in global food supply chains.
About 30% of food is lost in the supply chain. Packaging is one of the main reasons for rejection by suppliers and supermarkets, leading to 14% food loss, along with improper processing, logistics, and storage.
Modified Atmosphere Packaging (MAP) uses modified gas mixtures to protect and maintain the quality of perishable food and extend its shelf life.
At a time when most products are moved in global supply chains, food is transported and stored for long periods. Perishable food has to remain fresh and also retain its sensory attributes and quality to meet consumer demands.
A package should ideally fulfill many requirements, mainly the following:
MAP also eliminates the need for the addition of chemicals for food preservation. This makes food healthier and less polluting. It also makes MAP suitable for packing organic food.
The space in packages is not a vacuum, but they are filled with different mixtures of carbon dioxide (CO2), oxygen (O2), and nitrogen (N2). Air has 78% of N2, 21% O2, and 0.04% CO2. The proportions of these gases in MAP will depend on the products they have to protect. Hence, every product has specific Modified Atmosphere Packaging suited to its individual needs.
Some of the following factors need to be considered while designing MAP for a particular product:
There are two ways to achieve MAP:
MAP applications aim to reduce respiration and microbial spoilage. External factors like temperature, relative humidity, and storage time that influence the biotic reactions will influence the gas mixture choice for MAP.
MAP used for respiring fresh produce and non-respiring meat, seafood, and dairy products are designed under varying considerations. Table 1 shows the MAP requirements for each food type.
Table 1: "Recommended storage conditions for some respiring and nonrespiring food products (Lioutas, 1988). (Credits: Gorris and Peppelenbos 1992, Hort Technology.DOI: 10.21273/HORTTECH.2.3.303
MAP for fresh produce helps to preserve freshness, form, firmness, color, and aroma, while also preventing diseases.
However, of all the food categories, designing MAP for fresh produce is the most challenging, especially passively. Vegetables and fruits are still alive, whether they are whole or sold as cut produce; therefore, more factors need to be considered for fresh produce than for animal products. The following biological processes influence MAP in fresh produce:
Respiration: Fresh produce continues to respire even after harvest. They break down carbohydrates, lipids, and proteins using oxygen to produce energy. Post-harvest respiration leads to biomass, freshness, and firmness loss as reserves are used up. Moreover, the products of respiration, such as pectolytic enzyme, will change aroma and taste, while ethanol and acetaldehyde produce off-flavors and reduce quality.
Respiration rate is greater in cut vegetables than intact fruits. Due to the influence of external factors, there can be a 10% difference in respiration in different batches of the same product. Every 10oC increase in temperature increases respiration rate by a factor of 3-4. Moreover, the timing of harvest and maturity can also influence respiration rates, for example, by up to 60% in apricots harvested early or late in the season.
Ethylene: Fruits and vegetables produce ethylene, a natural hormone for ripening in climacteric fruits, while several non-climacteric fruits are ethylene sensitive. Ethylene will change color, firmness, aroma, and quality; higher phytohormone levels lead to senescence. The heat produced by respiration will also increase ethylene formation. Storage temperature will also change the ethylene production rate.
Physical damage: Rough handling that bruises fresh produce sets off a chain reaction that leads to tissue softening, browning, and decay. Breaks in skins can also aggravate microbial activity.
Diseases: If the storage conditions are not ideal, fresh produce can also get infected by bacteria, fungus, viruses, etc. The infection could have started preharvest or post-harvest. The result can be greater ethylene production, loss of water, texture, and color. Considerable research is focused on pretreatments before packing to control a disease outbreak. Heat treatment is being tried in combination with MAP to reduce diseases.
MAP could still be ineffective against microbes that can survive in cold temperatures like Aeromonas hydrophila and Yersinia enterocolitica or in anaerobic conditions like Salmonella and E. coli. Many microbes, such as anaerobic Clostridium botulinum, can produce harmful toxins even in low temperatures before visible deterioration happens.
Therefore, lowering O2 will encourage anaerobic microbes that were already present. Using passive MAP is better in these cases than active MAP. Thus, maintaining food safety in MAP is a major industry concern.
Table 2: "Tolerance of respiring commodities to low O2 or high CO2 levels, determined at the optimal storage temperature of each product (Kader et al., 1989). (Credits: Gorris and Peppelenbos 1992, Hort Technology.DOI: 10.21273/HORTTECH.2.3.303)
Gas concentrations can be altered in fresh produce to varying effect:
It is important to use the correct packaging material and gas mixture, otherwise, there will be a loss in quality and shelf-life.
Passive MAP is usually used for fresh produce, where the rate of respiration will reduce O2 and increase CO2 to the desired concentrations. If passive MAP is used, semipermeable packing materials are necessary to allow diffusion of atmospheric O2 into the package to prevent anaerobic conditions from developing. This is necessary for products with high respiration rates like broccoli, mushrooms, sprouts, etc.
Mixing vegetables will also influence MAP, as gas mixtures and respiration rates for different products will vary.
Recommended storing temperatures and relative humidity must be maintained for each vegetable or fruit to achieve the correct MAP conditions, as shown in Table 1.
Animal products like meat, fish, and dairy also benefit from MAP.
MAP can extend the shelf-life of animal products, as these are also susceptible to physiochemical and microbial deterioration. MAP is the ideal technology to store and transport animal products without chemical additives. For meat products, the following factors are important:
Composition: Different meats such as beef, pork, or chicken will have varying percentages of fat, proteins, moisture content, etc. Even different parts of an animal have varying content and will need different MAP. See Table 3. High moisture content and non-protein content in meat and fish lead to microbial spoilage.
Table 3: “Suitable gas mixtures for storing meat in MAP at temperatures between 1-4oC,”
Schmid et al. (2016). (Image credits: Austin Food Sci. 2016; 1(1): 1005, https://austinpublishinggroup.com/food-sciences/fulltext/afs-v1-id1005.php)
External conditions: As is the case with respiring products, temperature plays a vital role in determining the quality of products and their shelf-life.
The amount of O2 in contact with the products is another major factor, as it can lead to browning and discoloration of meat, making the product less attractive to consumers. The red color in meat is due to ferrous blood proteins like myoglobin. High temperatures and light intensity increase oxidation, where the initially purple deoxymyoglobin is changed to red oxymyoglobin, which on further oxidation changes to brown metmyoglobin. Autooxidation of lipids will interact with discoloration synergetically to produce rancidity.
The main purpose of MAP is to retain purple deoxymyoglobin by using less O2 or by using a vacuum. MAP for meat products usually uses a combination of high and low-barrier packaging. Before displaying the meat products, retailers will remove the high barrier film to allow some O2 in, which produces the fresh red meat color.
Prime cuts or ground meat not meant to be sold immediately are stored in modified atmosphere packages with less than 0.1% O2. Shelf-ready products could have MAP with a high O2 and CO2 content. To prevent oxidation deterioration, they will also contain carbon monoxide (CO) that binds to myoglobin and prevents it from being oxidized.
Microbial spoilage: The disadvantage of CO is that it can persist after cooking and it also masks microbial spoilage symptoms and is, thus, not optimal for food safety. It has been found that 0.4% of CO and 99.6% of CO2 allowed Listeria spp to grow undetected in pork.
On the other hand, O2 will encourage fast-growing microbes like Pseudomonas, Moraxella, and Psychrobacter.
In poultry, Pseudomonas and Achromobacter spp cause spoilage. These Gram-negative bacteria can be controlled by increasing CO2 levels beyond 20%, along with low temperature and good hygiene. If CO2 levels increase beyond 25%, it causes browning of the meat.
Microbial spoilage causes the decomposition of muscle protein and the production of slime and off-odors. Usually, high CO2 is maintained to control most microbes. However, some CO2 tolerant lactic acid bacteria produce off aroma and flavors and have become the main reason for spoilage in MAP meat products. Hence, 10-40% and 60-90% of CO2, going up to 100%, are being used.
Very high levels of CO2 will cause package collapse, as CO2 can seep out of the package or be absorbed by the meat product. To prevent this, the product-to-gas ratio should be one to two and gas should be replenished, so the headspace left is important and has to be closely monitored. A product-to-gas ratio of 1:0.5 resulted in the loss of quality in beef.
Table 4: MAP conditions and reported storage life of seafood. ICEP. [Source: Davis H.K., Fish in Principles & Packaging of MAPackaging of Food,
For fish, MAP has to prevent microbial growth, physical damage, and dehydration. As can be seen in Table 4, there is a great variation in the MAP gas mixture used for seafood, depending on species, batches, treatments, and methods. The processing and handling of fish after the catch and before packaging is one of the most crucial factors.
The effectiveness of MAP in extending shelf-life becomes apparent when longer storage times are needed. The shelf-life can be increased two or three-fold with MAP storage. Using higher levels of initial CO2 is the key for increasing shelf life.
The following are important factors to consider when designing MAP for seafood:
Microbial spoilage: Seafoods are susceptible to microbial degradation that can spoil their flavor, aroma, and appearance. Seafood-specific-spoilage organisms will grow if there is high moisture content and temperature. Microbes produce hydrogen sulfide, ammonia, acetic acid, biogenic amines, and hypoxanthine to spoil aroma, and Trimethylamine (TMA) produces the fishy odor. MAP can prevent the development of off-odors by limiting microbial growth of Pseudomonas spp., Photobacterium phosphoperum, Shewanella putrefaciens, Vibrionaceae, and Enterobacteriacea by using high levels of CO2 and less O2. This will also control lipid oxidation.
Physical damage: Fish are very delicate and have to be protected from physical damage to retain their appearance and quality.
Packaging material is especially important in the case of seafood as it provides several uniquely applicable benefits:
Table 5: “Original gas mixture and changes of N2, O2, and CO2 levels during storage. (Adopted from Khoshgozaran et al., 2012).” (Credits: Ščetar et al. Mljekarstvo, 69(1), 3–20. https://doi.org/10.15567/mljekarstvo.2019.0101)
The use of MAP for dairy products is still relatively new and considered a novel solution. However, MAP can solve the problems of microbial deterioration, oxidative rancidity, and pollutants and contaminants in dairy products like dry milk powder and cheese.
Rancidity: Since dairy products are rich in fat, milk powder is susceptible to rancidity. When milk is sprayed during drying, the air is absorbed so that the milk powder has about 1-5% of oxygen. The permissible level of O2 is less than 1%, so the absorbed O2 has to be removed.
To overcome this problem, the air is removed from milk powder packages and refilled with an active mixture of N2 or an N2-CO2 mixture. O2 scavenging methods are also used as a MAP solution.
Microbial spoilage: Cheeses suffer from mold and yeast infection and were earlier packed in a vacuum. Nowadays, MAP using a high concentration of CO2 mixed with N2 is increasingly used to prevent microbial spoilage. CO2 gets absorbed by the moisture content in cheese, so N2 is added to prevent package tension.
The choice of gas mixtures depends on cheese type, the production process, and packaging materials, etc.; see Table 5. MAP is especially useful in extending the shelf-life of high moisture cheese, for example:
Plastic Pollution: Milk, milk powder, yogurt, butter, and cream are contaminated by 21 to 43 ng g-1 of bisphenol A (BPA), a compound used in plastics. It is carcinogenic and tends to bioaccumulate. This occurs because dairy products interact with plastic packaging. Moreover, this interaction can reduce the probiotic attributes of dairy products.
To get around this problem, scientists are researching the effectiveness of several materials to package cheese. Some materials have been successfully tested with MAP for cheese:
More attempts can help to eliminate the proliferate plastic use in MAP to reduce its health effects and reduce plastic packaging pollution for the environment.
Figure 1: About 14% of food is lost between farm and shelf for various reasons, Wood 2020. (Image credits: https://www.weforum.org/agenda/2020/02/supermarket-food-waste-surplus-germany/)
Since agriculture uses 30% of land and 70% of freshwater resources, food loss is a massive waste of natural resources, at a time when 11% of people are suffering from hunger.
To avoid food loss and waste, FAO advocates using innovative packaging solutions and sees Modified Atmosphere Packaging as one of the sustainable and innovative solutions to food waste today. The international organization also encourages the use of appropriate technology to address this problem. Small, portable, handheld tools, such as the F-920 Check It! Gas Analyzer, manufactured by Felix Instruments - Applied Food Science, which has been made for measuring gas composition in the headspace is ideal. The tool gives rapid and accurate gas analysis and is easy to use in the food supply chain.
MAP is not going to solve all the food loss and waste problems—even countries where new packaging methods are ubiquitous, food waste still often occurs in households—but it can be one of the solutions.
Science Writer, CID Bio-Science
Ph.D. Ecology and Environmental Science, B.Sc Agriculture
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