Modified Atmosphere Packaging (MAP) of fresh produce has posed difficult challenges for a packaging industry accustomed to barrier packaging designed to exclude atmospheric oxygen. Fresh-cut produce has been successful in the market place, in part because of the value added to the product through its preparation and delivery in a ready-to-eat condition. However, because fresh fruits and vegetables are still living, and still require oxygen for their metabolism, barrier flexible packaging has not been appropriate in most cases. The best way to reduce respiratory metabolism and thus conserve the plants stores of carbohydrate, acids and moisture, is to reduce temperature. When excellent temperature control is in plce, MAP can be used to further reduce respiration rate, loss of moisture, production of metabolic heat, colour change, decay and sensitivity to ethylene.
By Dr Sandhya
Modified atmosphere packaging is the process by which it has become possible to optimise the atmosphere inside packaging units in such a way that the quality of the product is prolonged for an extended period of time. As the term implies, MAP involves film packaging containing a modified atmosphere. The modification process often requires reducing the amount of oxygen(O2), decreasing it from 20% to 0% in order to slow down the growth of aerobic lifeforms and the speed of oxidation The displaced oxygen can be substituted with nitrogen (N2), commonly acknowledged as an inert gas, or carbon dioxide (CO2), which can lower pH or inhibit the growth of bacteria.
In the fresh produce segment of the retail food market, MAP’s market share has risen significantly over the past few years. For MAP, barrier films and/or composite films are used, which can be rigid or soft films depending on the properties required. To ensure the gas does not escape, barrier films, with a barrier effect tuned to the specified shelf life, are needed. Alongside high-transparency, outstanding anti-fog properties are required to keep water drops from forming inside the film and causing fogging. The modified atmosphere inside the packaging creates a gas-filled space that counteracts atmospheric pressure to such an extent that even pressure-sensitive products with long shelf lives can be packaged without risk of damage – and without the addition of preservatives.
The protective atmosphere
Nitrogen serves as a supporting gas, filling the voids and ensuring pressure compensation between inside and outside. This means the merchandise rests loosely in the packaging and is protected against deformation, slices do not stick to each other and release of liquids is minimised. Carbon dioxide (CO2) dissolves in water and forms carbonic acid (H2CO3). During the packaging process a slightly acidic, preserving film forms on the surface of aqueous products, protected by CO2. This acid atmosphere inhibits the growth of most bacteria and mould fungi. The carbon dioxide evaporates as soon as the packaging is opened or the product is heated. A high oxygen concentration prevents meat surfaces from turning brown by de-oxidation under vacuum. This means the fresh colour is retained and foodstuffs – like fruit and vegetables – can ‘breathe’. In addition, this selective supply of oxygen enables a substantially longer shelf life for many products because it prevents fermentation and minimises anaerobic bacteria counts.
MAP, therefore, facilitates a longer shelf life for packaged products, reduces waste, makes products look more appetising and renders the use of preservatives superfluous. This means it can increase productivity – and open up new markets. Products like red meat, seafood, minimally processed fruits and vegetables, pasta, cheese, bakery goods, poultry, cooked and cured meats, ready meals and dried foods can be packaged utilising MAP. The three major commodity groups are fruit and vegetables, meat and meat products, and seafood. It has been estimated that 25-40 % of all fresh produce harvested will fail to reach the consumer, due to spoilage and mishandling occuring during distribution.
Effectiveness of MAP
Preserving fresh produce through packaging has been the subject of a great deal of research over the decades. Fresh-cut fruit products, both retail and for food service applications, have appeared with increasing frequency in the marketplace. In coming years, it is commonly understood that the fresh-cut fruit industry will enjoy unprecedented growth. For this reason, many leading fresh-cut salad manufacturers have targeted the development of fresh-cut fruit products as part of their long-term business plans. Fresh-cut kitchen vegetables are the largest segment of the fresh-cut produce industry. Fresh-cut salads are another important category since consumers perceive them as being healthy. A study has been conducted by Lee et al. to design a modified atmosphere package for a mixed vegetable salad, which includes 75 g of cut carrot, 55 g of cut cucumber, 20 g of sliced garlic and 50 g of whole green pepper [1]. A pouch-formed package (made of 27 mm low density polyethylene) containing a modified atmosphere of 2.0-2.1% O2 and 5.5–5.7% CO2, was beneficial for all components and provided better quality retention than other test packages. The performance of modified atmosphere packaging has also been evaluated for a mixed, prepared food i.e. Korean braised green peppers with dry anchovies [2]. The impact of MAP on product quality with different CO2 concentrations at 10°C was also studied. MAP conditions of 60% CO2/40% N2 extended the shelf life at 10°C by 130% (to 18.4 days) relative to that achieved with stretch-wrap air packaging (7.9 days). The study was based on the time taken to reach the degredation limit of an aerobic bacterial count of 105 CFU/g. It demonstrated that the relative extension of shelf life achieved with MAP was greater at lower temperatures.
Food safety qualities
The hygienic and sensory qualities of MAP have been evaluated by analysing microbial growth and atmosphere composition at the moment of packaging [3]. It has been found that the hygienic quality was comparable across all packaging systems used. However, the vegetables packaged in perforated film kept their sensory characteristics better than those packaged in air or under a modified atmosphere. The effect of a modified atmosphere of 20% CO2, 80% N2 on the microbial development and visual shelf life of a mayonnaise-based vegetable salad has also been reported [4]. The shelf life of the product was increased from 40 to 54 days at 4°C storage, 12 to 22 days at 10°C storage, and 5 to 12 days at 15°C storage. A study has also been carried out to characterise the changes that occur during refrigerated storage of six salad vegetables both individually and in a mixture [5]. Salad-cut and whole lettuce, carrot, celery, radish, green onion and endive, and a salad mixture were stored at 4.4°C in packages made from a film having low gas permeability. After two weeks of storage, organoleptic evaluation indicated that the chemical treatments were generally of no value and in some cases were even detrimental, however the modified initial headspace was beneficial.
A total of 116 commercial samples of mixed vegetable salads, packaged in plastic bags, were examined for the presence of Listeria monocytogenes during storage at 4°C [6]. The results showed that the modified atmosphere did not greatly inhibit Listeria monocytogenes. Carbon dioxide concentration, Lactobacillus casei inoculum size and the storage temperature were also varied according to a central composite design in order to assess the effects of these variables and their interactions on the growth of Aeromonas hydrophila and lactic acid bacteria in ready-to-use mixed salad vegetables packaged under modified atmosphere [7]. It has been observed that the use of these hurdles may increase the shelf life and microbiological safety of ready-to-use vegetables.
Fresh-cut products often require significantly different packaging from the complete product. For whole produce, packaging is primarily designed to avoid bruising during post-storage handling. The MAP technique consists of the enclosure of respiring produce in polymeric films, in which the gaseous environment is actively or passively altered to slow respiration, reduce moisture loss and decay and/or extend the shelf life of the products. Many of the films used in MAP do not offer, on their own, all of the properties required to contain a modified atmosphere. To provide packaging films with a wide range of physical properties, many of these individual films are combined through processes like lamination and co-extrusion. There are several groupings in MAP films. Polyethylene is most commonly used to provide a hermetic seal and also as a medium of control for characteristics like anti-fogging abilities, peelability and the ability to seal despite a degree of contamination.
A look to the future
The flexible packaging industry has become increasingly responsive to the specific atmospheric requirements of fresh produce and are now providing films specifically designed for given produce items. Films for low, medium and high respiration rate commodities are now available from many package vendors and the process of matching the oxygen transmission rate (OTR) to product is being constantly refined. This has allowed fresh-cut processors to begin providing a much greater diversity of products, including artichoke hearts, baby salad greens, sliced strawberries, stir fry mixes and many others. High respiration rate commodities such as broccoli, asparagus and mushrooms have always presented a challenge to packagers. New technologies are now allowing the manufacture of very high OTR (> 15000 cc/m2-day) films for these applications.
Although much work has already been undertaken on the benefits of MAP, more research into the influence of storage temperature and atmosphere on prepared salads and fresh cut fruit needs to be done. The use of intelligent packaging systems also needs to be evaluated. In addition, the antimicrobial effect of super atmospheric O2 in the fresh-cut produce, and the impact of antimicrobial compounds which can be incorporated into the coating need to be investigated. One area of research that is currently growing is the area of edible films for use in MAP systems.
References
1. Lee KS, Park IS & Lee DS. Modified atmosphere packaging of a mixed prepared vegetable salad dish. International Journal of Food Science & Technology 2003; 31 (1): 7-13.
2. Lee K, Kim H J, An DS, Lyu ES & Lee DS. Effectivness of modified atmosphere packaging in preserving a prepared ready-to-eat food. Packaging Technology and Science 2008; 21(7): 417-423.
3. Manzano M, Citterio B, Maifreni M, Paganessi M & Comi G. Microbial and sensory quality of vegetables for soup packaged in different atmospheres. Journal of the Science of Food and Agriculture 2006; 67(4): 521-529.
4. Buick R K & Damoglou AP. Effect of modified atmosphere packaging on the microbial development and visible shelf life of a mayonnaise-based vegetable salad. Journal of the Science of Food and Agriculture 2006; 46(3): 339-347.
5. Priepke PE, Wei L S & Nelson A I. Refigerated Storage of Prepackaged Salad Vegetables. Journal of Food Science 2006; 41(2): 379-382.
6. Garcia-Gimeno R A, Zurera-Cosano G & Amaro-Lopez M. Incidence, Survival and Growth of Listeria Monocytogenes in ready-to-use mixed vegetable salads in Spain. Journal of Food Safety 2007; 16(1): 75-86.
7. Vescovo M, Scolari, G, Orsi C, Sinigaglia M & Torriani S. Combined effects of Lactobacillus casei inoculum, modified atmosphere packaging and storage temperature in controlling Aeromonas hydrophila in ready-to-use vegetables. International Journal of Food Science & Technology 2003; 32 (5): 411-419.
The author
Dr Sandhya
Krishi Vigyan Kendra
Samrala
Punjab Agricultural University
Ludhiana-141004
India
Email: sandhya7819@gmail.com
Tel: +91 9463354523
