Definition: Food-packaging method in which the proportionsof carbon dioxide, nitrogen, and oxygen in asealed container are different from those in the normal(ambient) air to enhance the foods shelf lifeThe shelf-life of perishable foods as meat, poultry, fish, fruitsand vegetables and bakery products is limited in the presenceof normal air by two principal factors -the chemical effect of atmospheric oxygen and the growth ofaerobic spoilage micro organisms.
These factors either individually or in association with oneanother bring about changes in odour, flavour, colour andtexture leading to an overall deterioration in quality. Chilledstorage will slowdown these undesirable changes but will notnecessarily extend the shelf -life sufficiently for retaildistribution and display purposes.The modified atmosphere concept for packaged goodsconsists of modifying the atmosphere surrounding a foodproduct by vacuum, gas flushing or controlled permeability ofthe pack thus controlling the biochemical, enzymatic andmicrobial actions so as to avoid or decrease the maindegradations that might occur.
This allows the preservation of the fresh state of the foodproduct without the temperature or chemical treatments usedby competitive preservation techniques, such as canning,freezing, dehydration and other processes.MAP is the replacement of air in a pack with a single gas ormixture of gases; the proportion of each component is fixedwhen the mixture is introduced. No further control is exertedover the initial composition, and the gas composition is likelyto change with time owing to the diffusion of gases into andout of the product, the permeation of gases in to and out of thepack, and the effects of product and microbial metabolism.
The normal composition of air is 21% oxygen, 78% nitrogen and lessthan 0.1% carbon dioxide. Modification of the atmosphere withinthe package by reducing the oxygen content while increasing thelevels of carbon dioxide and/or nitrogen has been shown tosignificantly extend the shelf-life of perishable foods at chilltemperatures.Advantages of MAP: Increased shelf-life allowing less frequent loading of retail displayshelves; Reduction in retail waste Hygienic stackable pack, sealed and free from product drip andodour; Little or no need for chemical preservatives; Increased distribution area and reduced transport costs due to lessfrequent deliveries; Centralized packaging and portion control; )Reduction in production and storage costs due to better utilizationof labour, space and equipment.
Disadvantages of MAP: Capital cost of gas packaging machinery Cost of gases and packaging materials Cost of analytical equipments to ensure that correct gasmixtures are being used Cost of quality assurance systems to prevent thedistribution of leakers, etc. increased pack volume which will adversely affecttransport costs and retail display space potential growth of food-borne pathogens due totemperature abuse by retailers and consumers; benefits of MAP are lost once the pack is opened orleaks.
Gases Used in MAPThe basic concept of the MAP of fresh foods is the replacementof the air surrounding the food in the package with a mixtureof atmospheric gases different in proportion from that of air.Oxygen (O2)Food deteriorates due to physical, chemical andmicrobiological factors. Oxygen is probably the mostimportant gas in this context being used metabolically by bothaerobic spoilage of micro organisms and plant tissues andtaking part in some enzymic reactions in food including thecompounds such as vitamins and flavours.
For these reasons, in modified atmosphere packaging, oxygenis either excluded or the levels set as low as possible. Theexceptions occur where oxygen is needed for fruit andvegetable respiration, colour retention as in the case of redmeat or to avoid anaerobic conditions in white fish. In MAP,oxygen levels are normally set as low as possible to reduceoxidative deterioration of foods. Oxygen will generallystimulate the growth of aerobic bacteria and can inhibit thegrowth of strictly anaerobic bacteria, although there is a verywide variation in the sensitivity of anaerobes to oxygen. Oneof the major functions of O2 in MAP meats is to maintainmyoglobin in its oxygenated form, oxymyoglobin. This is theform responsible for the bright red colour, which mostconsumers associate with fresh red meat.
Carbon dioxide is both water and lipid soluble and although it isnot a bactericide or fungicide, carbon dioxide has bacteriostaticand fungistatic properties. The overall effect on micro organismsis an extension of the lag phase of growth and a decrease in thegrowth rate during the logarithmic growth phase. However, theformer effect is greater and therefore as bacteria move from thelag to log phase of growth the inhibitory effects are reduced.Thus, the earlier the product is gas packaged the more effectiveCO2 will be . This bacteriostatic effect is influenced by theconcentration of CO2, the partial pressure of CO2,volume ofheadspace gas, the type of micro organism, the age and load ofthe initial bacterial population, the microbial growth phase, thegrowth medium used, the storage temperature, acidity, wateractivity, and the type of the product being packaged .
Yeasts which produce carbon dioxide during growth arestimulated by high levels of carbon dioxide and thus for someproducts where they are potentially a major cause of spoilage,MAP may not be an advisable option. Also the food-associated pathogens Clostridium perfringens and Clostridiumbotulinum are not affected by the presence of carbon dioxideand their growth is encouraged by anaerobic conditions. Ingeneral carbon dioxide is most effective in foods where thenormal spoilage organisms consist of aerobic, gram-negativepsychrotropic bacteria .
For maximum antimicrobial effect, the storage temperature of MAPproduct should be kept as low as possible, because the solubility ofCO2 decreases dramatically with increasing temperature. Thus,improper temperature control will usually eliminate the beneficialeffects of elevated CO2. The absorption of CO2 is highly dependenton the moisture and fat content of the product. If product absorbsexcess CO2, the total volume inside the package will be reduced,giving a vacuum package look known as “pack collapse”. ExcessCO2 absorption in combination with “pack collapse” can alsoreduce water holding capacity of meats, resulting in unsightly drip.Some dairy products (e.g. cream) are very sensitive to CO2concentrations and will be tainted if packed in MAP with high CO2levels. Fruits and vegetables can suffer physiological damage due tohigh CO2 levels.
Nitrogen (N2):Nitrogen is an inert tasteless gas, which displayslittle or no antimicrobial activity on its own. Because of itslow solubility in water and fat, the presence of N2 in a MAPfood can prevent pack collapse that can occur when highconcentrations of CO2 are used. In addition, N2, by displacingO2 in the pack, can delay oxidative rancidity and also inhibitthe growth of aerobic micro organisms. In foods such as nuts,removing oxygen to <%1 by nitrogen flushing helps preventoxidative rancidity of fats. Nitrogen can also indirectlyinfluence the micro organisms in perishable foods by retardingthe growth of aerobic spoilage organisms. The second role ofnitrogen in MAP is to act as a filler gas and keeps flexiblepackages from developing a vacuum.
Exactly what combination of gases is used depends on manyfactors, such as the type of the product, packaging materialsand storage temperature. The packaging system selected musthave sufficient headspace to provide enough gas to interactwith the entire product. The headspace must contain areservoir of CO2 to compensate for gas absorbed by theproduct and lost across the packaging material . The longerthe required shelf-life then the larger the headspace should be.Carbon monoxide (CO)This has been found to be very effective in maintaining the redcolour in fresh meat due to the formation ofcarboxymyoglobin.
It has not been used commercially for this purpose however sincecarbon monoxide, a highly toxic gas is not approved by theregulatory authorities owing to the possible health hazard topackaging machine operatives. Its use has, however, beensanctioned in the United States to prevent browning in packedlettuce. Carbon monoxide has little inhibitory effect on microorganisms.Other GasesThe potential of various other gases such as chlorine, etyhlene oxide,nitrogen dioxide, ozone propylene oxide and sulphur dioxide formodified atmosphere packaging have been investigatedexperimentally but their commercial use for packaging foods isunlikely to meet with approval from the regulatory authorities.
Gas Mixtures: There are three types of gas mixtures used in modifiedatmosphere packaging Inert blanketing (N2) Semi-reactive blanketing (CO2/N2 or O2/CO2/N2) Fully-reactive blanketing (CO2 or CO2/O2)Packaging MaterialsThere are six main characteristics to consider when selectingpackaging material for MAP foods:1) Resistance to puncture2) Sealing reliability3) Antifogging properties4) Carbon dioxide permeability5) Oxygen permeability6) Water transmission rate
Although an increasing choice of packaging materials isavailable to the MAP industry, most packs are still constructedfrom four basic polymers: polyvinyl chloride (PVC),polyethyleneterephthalate (PET), polyproylene (PP) andpolyethylene (PE).Machine Systems for MAPThe first element for optimum gas packaging is appropriateequipment. There are two different techniques to replace theair:1) Gas flushing2) Compensated vacuum.
The gas flush technique is normally accomplished on a formfill-seal machine. The replacement of air inside a package isperformed by a continuous gas stream. This gas stream dilutesthe air in the atmosphere surrounding the food product. Thepackage is then sealed. Since the replacement of air inside thepackage is accomplished by dilution, there is a limit on theefficiency of this unit. Typical residual oxygen levels in gasflushed packs are 2-5% O2.Therefore, if the food item to bepackaged is very oxygen sensitive, the gas flush technique isnormally not suitable. So when considering a packagingsystem it is important to consider the oxygen sensitivity of thefood product. The great advantage of the gas flush techniqueis the speed of the machine. Since the action is continuous, theproduct rate can be very high.
The compensated vacuum technique removes the air inside bypulling a vacuum on the atmosphere inside the package andthen breaking the vacuum with the desired gas mixtures. Sincethe replacement of the air is accomplished in a two-stepprocess, the speed of operation of the equipment is slowerthan the gas flush technique. However, since the air isremoved by vacuum and not simply diluted, the efficiency ofthe unit with respect to residual air levels is better. Therefore,if the food product is extremely sensitive to oxygen, acompensated vacuum machine must be used.
Methods of Creating Modified Atmosphere ConditionsModified atmospheres can be created either passively by thecommodity or intentionally via active packaging Passivemodified atmosphere : Modified atmospheres can passivelyevolve within a hermetically sealed package as a consequenceof a commodity’s respiration, i.e. O2consumption and CO2evolution. If commodity’s respiration characteristics areproperly matched to film permeability values, then abeneficial modified atmosphere can be passively createdwithin a package. If a film of correct intermediarypermeability is chosen, then a desirable equilibrium modifiedatmosphere is established when the rates of O2 andCO2transmission through the package equal a product’srespiration rate.
Active packaging: By pulling a slight vacuum and replacingthe package atmosphere with a desired mixture of CO2, O2and N2, a beneficial equilibrium atmosphere may beestablished more quickly than a passively generatedequilibrium atmosphere. Another active packaging techniqueis the use of O2, CO2 or ethylene scavengers/emitters. Suchscavengers/emitters are capable of establishing a rapidequilibrium atmosphere within hermetically sealed producepackages.
Application of MAP to Fruit and VegetablesThere are many advantages of MAP fruit and vegetables, butthe most obvious one must be the extension of shelf-life. Bydecreasing the amount of available oxygen to the produce, therespiration rate and the rate of all metabolic processes arecorrespondingly decreased. This results in delayed ripeningand senescence, which may be seen as chlorophyll retention,delayed softening and the prevention of discoloration. Theextension of shelf -life is most noticeable with preparedproducts; this, combined with ease of use for the consumer,makes a MAP pack an attractive form of product presentation.Additionally, MAP packs reduce the quantity of water vapourlost from the produce.
Although fresh fruit and vegetables have been removed fromthe parent plant and from their normal nutrient supplies, theywill continue to respire. Under normal aerobic conditions, therate of respiration of a product may be determined by eitheroxygen uptake rate or carbon dioxide production rate. A highrespiration rate is usually associated with a short shelf -life.When the rate of packaging film transmission of oxygen andcarbon dioxide equals the rate of respiration of the product, anequilibrium concentration of both gases is established. Theequilibrium values attained depend on:a) The respiration rate of the productb) Fill weight of productc) The film surface area which is available for gas exchange
The respiration rate of the product is influenced by:a) Storage temperatureb) Produce varietyc) Growing area and conditionsd) Injury to the produceChill temperatures are generally chosen for fresh producebecause fruit and vegetables respire slowly at lowtemperatures, as do many micro organisms, which are likelyto spoil the product. Some fruits and vegetables, most notablyones from tropical or sub-tropical areas, are susceptible tochilling injury.