Mary Finn, B.Sc., Department of Industrial Microbiology, UCD, Belfield
Introduction
The ability of Modified Atmosphere Packaging (MAP) to extend the shelf life of foods has been recognised for many years. MAP may be defined as the packaging of a perishable product in an atmosphere, which has been modified so that its composition is other than that of air 12. The impetus behind these products came mainly from an increased consumer demand for fresh and chilled products, although other factors, such as a consumer desire for preservative-free products, the growth of centralised packaging and portion control, and a decline in the growth of canned and frozen foods, certainly played a role 11. The potential advantages and disadvantages of MAP have been reviewed by Farber (1991) in Table 1.
Potential Advantages and Disadvantages of MAP
ADVANTAGES
- Potential shelf life increases of 50 to 400%
- Reduced economic loss
- Products can be distributed over longer distances and with fewer deliveries, leading to decreased distribution costs
- Provides a high quality product
DISADVANTAGES
- Visible added cost
- Temperature control necessary
- Different gas formulations for each product type
- Special equipment and training required
- Product safety to be established
*Adapted from Farber (1991)
MAP technology is largely used for minimally processed fruits and vegetables14&16 including fresh, "ready-to-use" vegetables. The disadvantage of pre-cut produce is that its storage life may be greatly reduced as compared to the intact vegetable3. MAP combined with low temperature storage is a common method to improve the storage stability of "ready-to-use" vegetables. These consist of fresh, washed, peeled, sliced, shredded or grated vegetables, sold within 7-8 days of preparation, after storage at low temperatures (below 100C)7. In recent years a rapid expansion in the sale of prepacked fresh vegetables in the United States and in Europe has been observed6.
Gases Used In MAP
The three main gases used commercially in MAP are oxygen, nitrogen and carbon dioxide. The gases and their concentrations should be tailored for each individual product. The required combinations of temperature, oxygen and carbon dioxide levels vary with vegetable type, variety, origin and season2. Carbon dioxide is important because of its biostatic activity against many spoilage organisms that grow at refrigeration temperatures10. Oxygen inhibits the growth of anaerobic pathogens, but in many cases does not directly extend shelf life13. Nitrogen is used as a filler gas to prevent pack collapse, which may occur in high C02 containing atmospheres11.
Modified atmospheres may be produced naturally by respiration (Passive MA) and by the application of gas flushing techniques (Equilibrium MA) 24. A sliced or grated vegetable is still alive and it continues to respire- it creates an MA within the pack with a reduced level of oxygen and an increased level of carbon dioxide6. For respiring produce, the permeability characteristics of the film determine to a large degree, the equilibrium gas concentration achieved in the package13. The actual equilibrium MA attained within a package will also depend on factors such as the prepared form of the vegetable studied, the rate of respiration at storage temperature, the pack volume and fill-weight, and the surface areas for gas exchange2.
General Effects of MA on Micro-organisms
Although MA can change the general microbial profile of foods5, these effects have not been well studied. C02 has an inhibitory effect on many common spoilage organisms. Coyne (1933) conducted experiments with several spoilage organisms and showed that C02 increases the lag phase of the growth curve. Gram negative bacteria are generally more sensitive to C02 than Gram positive bacteria. In MA's the inhibition of Gram-negative rod-shaped spoilage organisms such as Pseudomonas coincides with the growth of Gram-positive, lactic acid-producing organisms such as Lactobacillus. Carbon dioxide at sufficient concentrations can inhibit the growth of spoilage moulds12.
Nature of Micro-organisms in MAP Vegetables
Mesophilic bacteria counts on plate count agar or equivalent media range from 103 to 109 colony-forming units (CFU) g-1. The quality of the vegetables is often acceptable, despite such high counts. Counts of lactic acid bacteria enumerated on MRS 9 reach 109CFU g-1 in some cases, but are usually lower for a given sample than those of mesophilic bacteria. Yeasts and moulds are usually less numerous than mesophilic or lactic acid bacteria17. Pseudomonas spp. appear to be common organisms on minimally processed vegetables. Nguyen-The and Prunier (1989) suggested that a relationship between the deterioration of "ready-to-use" salads and the growth of Pseudomonas spp. particularly Pseudomonas marginalis exists, following 10 days storage at 100C.
Safety Concerns - Foodborne Pathogens
The great vulnerability of MAP foods from a safety standpoint is that with MA's containing moderate to high levels of carbon dioxide, the aerobic spoilage organisms which usually warn consumers of spoilage are inhibited, while the growth of pathogens may be allowed or even stimulated. Thus contaminated foods that lack off odours or other signs of decomposition may be unwittingly consumed11. Another important factor is the emergence of a group of "new" foodborne pathogens which are capable of growth at 500C in foods. Bacteria fitting this criterion include Clostridium botulinum type E, Yersinia enterocolitica, enterotoxigenic Escherichia coli, Listeria monocytogenes and Aeromonas hydrophila 20. L. monocytogenes has been isolated from cabbage, cucumbers, potatoes and radishes. Beuchat and Brackett (1990) reported that L. monocytogenes is capable of growing on lettuce subjected to commonly used packaging procedures used in the food industry. Sizmur and Walker (1988) reported isolating L. monocytogenes from 4 of 60 samples of prepacked salads
There is little published work on the effects of MAP on pathogenic microorganisms in raw or minimally processed (eg. cut and washed) produce. The most frequently conducted study to assess safety hazards associated with MAP foods is an inoculation study. Typically in this type of study, a product is inoculated with a pathogen, packaged in a MA, stored, often under some degree of temperature abuse, and the development of the pathogen followed over time. Kallander et al. (1991) examined the fate of L. monocytogenes inoculated onto shredded cabbage stored at 50C and 250C under air and under MA (70% C02). The MA was ineffective in controlling growth at 50C. At 250C, spoilage was rapid and L. monocytogenes counts declined under both atmospheres. Omary et al. (1993) reported an increase in populations of Listeria innocua in MAP shredded cabbage, after 21 days storage at 110C.
The fate of other pathogenic microorganisms inoculated onto MAP vegetables has been investigated. Abdul-Raouf et al. (1993) investigated the effect of MAP on the survival and growth of E. coli 01 57:H7 inoculated onto shredded lettuce, sliced cucumber and shredded carrot. They reported that packaging under an atmosphere containing 3% oxygen and 97% nitrogen had no apparent effect on populations of E. coli 01 57:H7. Solomon et al. (1990) studied the ability of Cl. botulinum spores Type A and B to grow in shredded cabbage at room temperature packaged under an MA. Only type A grew and produced toxin in cabbage. This study followed an outbreak of botulism in Florida (1987) which was attributed to consumption of coleslaw made from packaged shredded cabbage mixed with coleslaw dressing. Satchell et al. (1990) studied the survival of Shigella sonnei in shredded cabbage. They reported that S. sonnei can survive and even proliferate in shredded cabbage packaged and stored in a vacuum or MA as well as aerobic conditions, thereby posing a potential hazard to the consumer.
Conclusions
The complexity of MAP dictates that a team approach should be adapted in order to successfully implement the technology. Key experts from food science, microbiology, packaging, engineering, marketing, sales, postharvest physiology, transportation and operations will have to be involved from the beginning. However, some aspects of MAP need further study. These include the micro-biological safety of refrigerated, "ready-to-eat" foods with extended shelf life, product safety during temperature abuse, gas flush system failure and loss of packaging integrity. Mathematical modelling of microbial growth/survival and the development of specific HACCP plans will also afford extra assurance of the safety of MAP foods.
References
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8. Coyne, (1933) Cited by Farber (1991)
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