Modified Atmosphere andVacuum Packaging to Extendthe Shelf Life of RespiringFood ProductsL.G.M. Gorris and H.W. Peppelenbos

“. . . vegetables

and fruits staymetabolicallyactive afterharvest, and thisactivity isessential forretaining theirquality.”

Summary. There has been a recent upsurgein the application of modified atmosphereand vacuum packaging to extend the shelflife of food products that rapidly deteriorate.Although these techniques are mostly usedwith nonrespiring products, both offer con-siderable prospects for respiring products aswell. Because not all commodities respire atthe same pace, packaging conditions, such asthe initial gas atmosphere applied, the typeof packaging material used, and the storagetemperature, require special consideration.Choosing the wrong packaging conditionsmay lead to loss of product quality and alsomay impair the microbiological safety nor-mally associated with fresh fruits and veg-etables. This paper stresses the importance ofthese considerations with modified atmo-sphere and vacuum packaging systems.

The postharvest quality of perish-able, respiring foods like agricul-tural and horticultural com-modities can be stabilized by al-tering the atmosphere surround-

ing a product using controlled atmosphere(CA) storage facilities. These technicallycomplex and rather expensive facilities con-tinuously adjust the composition of the at-mosphere to the optimal concentrationsprogrammed; they are mostly applied for large-scale storage. With modified atmosphere (MA)storage, there is no active control over theatmospheric composition, which starts from adefined initial gas composition but is subject tochanges due to physiological activity of theproduct and the characteristics of the physicalenvironment [e.g., composition of the gasatmosphere, temperature, relative humidity(RH)]. Current applications of CA and MAtechnologies include long-term storage of ap-

Agrotechnological Research Institute, Haagsteeg 6, P.O. Box 17,NL-6700 AA Wageningen, the Netherlands. tel: +31-8370-75000;fax:+31-8370-12260; E-mail: POSTMASTER%ATO.AGRO.NL.

HortTechnology ž July/Sept. 1992 2(3) 303

ples, pears, kiwifruits, and cabbage, temporarystorage of strawberries, cherries, and bananas(Kader et al., 1989) and shipment oflettuce.

Modified-atmosphere packaging (MAP)has proven to be a quite flexible, small-scalemeans of extending the shelf life of freshfoods (Figs. 1 and 2). With nonrespiring foodslike meat, cheese, or peanuts, modifying theatmosphere would involve replacing thenormal headspace within a packaged productwith a specific atmosphere. The atmosphereis commonly composed of N2, O2, and CO2.Although ordinary air consists mainly of thesethree gases (78% N2, 21% O2, 0.04% CO2), thebeneficial effect of MAP is obtained by ap-plying the gases in an altered ratio, which maybe different for different products (Table 1).The shelf life of nonrespiring products isprolonged because the two most importanttypes of deterioration—physicochemical de-terioration (e.g., browning of meat due to theaction of oxygen) and microbial deterioration(e.g., the growth of molds in cheese andbakery products)—are blocked or prevented.High gas barrier films or laminates are used toexclude gas exchange through the package,which would result in a less beneficial in-package gas atmosphere.

In contrast to the nonrespiring prod-ucts, fresh and freshly processed vegetablesand fruits stay metabolically active after har-vest, and this activity is essential for retainingtheir quality. To extend the shelf life of re-spiring products through MAP, a suitablepackaging system must be composed of a gasatmosphere that allows for a basic level ofmetabolism, which means that a certainamount of O2 should be available. The basiclevel of metabolism varies greatly with differentcommodities (type, maturity) and dependsalso on the degree of processing (trimming,cutting, slicing, etc.). Because the composi-tion of the gas atmosphere is the result of theinitial gas atmosphere composition, the gasexchange through the package, and the respi-

Fig. 1. Modified atmosphere packaged apples.

Fig. 2. Modified atmosphere packaged, minimally processed vegetable mixtures.

304 HortTechnology ž July/Sept. 1992 2(3)

“. . . the choice

of the packagingsystem forrespiring

ratory activity of the product, it will changeduring the course of the storage period untilan equilibrium is reached between gas ex-change and gas consumption or production.

Vacuum packaging (VP) may be regardedas a special type of MAP, since part of thenormal headspace is withdrawn, leaving an

Fruits and vegetables are characterized

altered initial atmosphere. VP of respiring

by an elaborate microflora, consisting of manydifferent types of bacteria, molds, and yeasts,

produce was first successfully practiced with a

most of which are involved in the spoilage ofproduce but are harmless to humans. Micro-

system designed by S. Burg almost a quarter

organisms that are dangerous to humans(pathogens that cause intoxifications or in-

of a century ago (Burg, 1975; Burg and Burg,

fectious diseases) normally cannot establish adangerous population density because they

1966). Although the system performed rather

have to compete with the spoilage micro-flora. However, packaging the produce

well, it was technically complex and never as

changes the microenvironment perceived bythe microorganisms and may well impair this

widely adopted as CA or MA storage. A very

safe balance. As outlined below, the choice ofthe packaging system for respiring products

simple VP system, designated as a moderate

must be carefully considered, taking intoaccount the characteristics of the commod-ity, including the microbiological safety situ-

VP system, has recently been introduced in

ation, and of the packaging system.

the Netherlands. In this system, perishablecommodities are stored at ≈400 mBar in a

Table 1. Recommended storage conditions for some(Lioutas, 1988).

HortTechnology ž July/Sept. 1992 2(3)

refrigerator. The system is currently beingtested and will be discussed later as a techni-cally simple alternative to MAP.

Modified Atmosphere Pack-aging of Respiring Products

MAP and VP slow down the metabolicactivity of a product and of the microorgan-

Nonrespiring products often are pack-

isms present, both spoilage and pathogenic,

aged with no O2, but respiring products musth a v e O2 to produce enough energy to

by limiting the O2 supply and applying an

maintain their integrity and quality throughthe process of aerobic respiration, in which

elevated level of CO2. Because the same strat-

carbohydrates are converted to CO2 and waterat the expense of O2. When the O2 level falls

egy underlies refrigerated storage, MAP and

under a certain minimum or CO2 accumulatesover a maximum, the product will have to

VP of respiring produce are usually combined

generate energy through anaerobic respiration,the end products of which (ethanol, acetalde-

with this technique. Many commodities, e.g.,

hyde) cause off-flavors and loss ofquality. FromTable 2, which lists these tolerance limits for a

avocados, mangoes, papayas, and cucumbers,

range of vegetables and fruits, it may be evidentthat the variation in the limits is substantial

are very sensitive to low-temperature injury and

among the various products.

should not be stored below ≈ 13C. Commodi-ties like apples, broccoli, and pears are notsensitive to chilling and can be stored near 0Cwithout ill effect (Zagory and Kader, 1988).

nonrespiring and respiring food products products must becarefullyconsidered. . . ”

305

“Considering thevariability in thelevels of respir-ation activity,tolerance to O2

and CO2, andchilling injuryfor individualvegetables andfruits, optimalpackagingmaterial wouldbe tailor-made.”

Table 2. Tolerance of respiring cal., 1989).

Vol (%) P

0,5 Nuts, dried fruits, an1 Broccoli, mushroom

most cut or sliced 2 Apricot, kiwifruit, ch

strawberry, pineapcorn, lettuce, celercauliflower, brusse

3 Avocado, tomato, pe

5 Citrus fruits, green p

306

Considering the variability in the levelsof respiration activity, tolerance to O2 andCO2, and chilling injury for individual veg-etables and fruits, optimal packaging materialwould be tailor-made. Some packaging-filmmanufacturers indeed claim the ability totailor-make films for any commodity by physi-cally modifying existing films or by manufac-turing films with the desired specifications.However, the logistics of food processing andpackaging companies may make it impracti-cal if they need to use many different films topackage the range ofproducts. Still, the rangeof packaging materials continues to increase.To facilitate the choice of packaging condi-tions, researchers of the Agro-technologicalResearch Inst. began compiling a rather dis-tinctive computer database that interlinksdata on product physiology, product quality,and packaging material characteristics. Dataon individual and combinations offactors willall be determined under a number ofpracticalMAP storage conditions and will not rely on,for example, in the case of the packaging ma-terials, specifications provided by manufac-turers. The database will enable one to choosethe packaging material best suited for aproduct, considering its type and degree ofmaturity and processing. It will be developedusing model products from several classes ofcommodities, classified according to key pa-rameters such as respiratory activity and toler-ances to chilling or gases (Peppelenbos, 1992).

PackagingWhen respiring products are to be pack-

aged, the initial gas atmosphere may be com-posed of pure gases or of ordinary air, whichis technically simpler and cheaper. Becausethe packaged product will respire actively, theinitial gas atmosphere will change duringstorage. Oxygen will be consumed and CO2

and H2O will be produced. Thus, the O2 level

ommodities to low O2 or high CO2 levels, determined at the o

Minimal O2 levelroduct Vol (%)

d vegetables 1 Lettus, garlic, onion, 2 Goldfruits and vegetables enderry, nectarine, peach, plum, 5 Peac

ple, most apples and pears, sweet kiwy, green beans, cabbage, bruls sproutspper, cucumber, artichoke 10 Pinea

broea, asparagus, potatoes 15 Straw

mu

will drop and the CO2 level rise. Using a high-barrier package film, O2 will be fully depleted,the product will switch to anaerobic respira-tion, and quality will be lost. Respiringproducts, therefore, need to use the recentlydeveloped types of semi-permeable or low-barrier films. Low-barrier films currentlyavailable have variable gas permeability speci-fications. As shown in Table 3 for a smallselection of films, there is about one magni-tude difference in the permeability for O2 andC O2 between individual films. Ultra-low-barrier, microperforated films are being de-veloped that have air permeabilities of ≈ 60,000liters/m 2ždayžatmosphere. These films areprobably well-suited for packaging productswith extremely high respiration rates, likeasparagus, broccoli, mushrooms, mung beansprouts, and shredded vegetables.

The equilibrium gas composition is de-termined by the gas permeabilities and respira-tory activity of the packaged product and thestorage temperature. Gas permeability specifi-cations given by film manufacturers are usuallydetermined under conditions that are remotefrom the high-humidity, refrigerated storageconditions of respiring produce. At present it isimpossible to tell from the specifications pro-vided by a packaging film manufacturer whethera specific film would provide for an in-packagegas atmosphere with tolerable Ο 2 and CO2

levels when applied in practice. Thus, the suita-bility of a film must be tested with the productunder the correct practical conditions. Figure 3depicts the result of such a test with tomatoes,using either oriented polypropylene or K-resin,a butadiene-styrene copolymer, for packaging.Taking into account the tolerance limits for O2

and CO2 (Table 2), the equilibrium O2 level istoo low, whereas the CO2 level is much toohigh with the 15-µm polypropylene film. The25-µm K-resin film gives a reasonable but notyet optimal equilibrium gas composition withO2 at ≈ 4% to 6% volume and CO2 at 4% to 5%

ptimal storage temperature of each product (Kader et

Maximal CO2 levelProduct

ceen Delicious apples, grapes, apricot, tomato,ive, artichoke, celery, sweet potato

h, plum, nectarine, orange, banana, avocado,ifruit, green pea, eggplant, cauliflower, cabbage,

ssels sprouts, radish, carrot, chicory

pple, lemon, cucumber, asparagus, green beans,ccoli, leek, onion, garlic, potatoberry, blackberry, cherry, fig, sweet corn,shroom, spinach, kale

HortTechnology ž July/Sept. 1992 2(3)

Fig. 3. Dynamics of the gascomposition for whole tomatoespackaged under air with twodifferent films and stored at 10C(data from Geeson et al., 1985).

“Fresh fruitsand vegetablesnormally have anelaborate spoilagemicroflora . . . ”

(Geeson et al., 1985). The above-mentionedcomputer database may well replace the labo-rious testing of the suitability of available typesof films every time a new fresh or freshly pro-cessed food needs to be packaged.

Many packaging materials with favor-able gas permeability characteristics have arather low permeability for water vapor. Thismay be a complicating factor, because in mostinstances the RH in a package is very high(295%). Any small fluctuation in the storagetemperature may result in condensation,which would greatly increase the prolifera-tion and spread of spoilage microorganisms.Especially in fruits, high RH causes heavylosses due to microbial decay. In-package RHmay be controlled by using packaging materialswith high water vapor permeabilities, by in-cluding sachets with water absorbers like CaC12,sorbitol, or xylitol in the package, or by usingpackaging material with suitable gas permeabilitythat has been coated with these desiccants(Barmore, 1987; Shirazi and Cameron, 1992).

Another complicating factor is the phy-tohormone ethylene, which is involved in theripening or maturation of a product. In a

Table 3. Thickness and gas permeabilities of somevarying relative humidity (data provided by man

HortTechnology ž July/Sept. 1992 2(3)

MAP system, the ethylene produced by thepackaged product may accumulate to someextent, since the permeability of most pack-aging films for ethylene is about halfwaybetween that for CO2 and O2. Alternatively,ethylene may enter a, package from the out-side when there is a prominent source there,like with ripening climacteric fruits. Manypremature fruits, green vegetables, and somecut flowers are sensitive to ethylene, whichspeeds their ripening and thus shortens theirshelf life. Inclusion of ethylene scrubbers likepotassium permanganate may counteract theeffect of ethylene, although the capacity ofsuch gas scavengers is finite.

Microbiological SafetyFresh fruits and vegetables normally have

an elaborate spoilage microflora, due to in-tensive contact with various types of microor-ganisms during growth and postharvest hand-ling. The high acidity of many fruits (pH≤4.6) limits spoilage due to acid-tolerantmolds, yeasts, and lactic-acid bacteria. Veg-etables generally have a pH around 6.0 to 7.0

semipermeable packaging films, at 20-25C andufacturers).

307

Fig. 4. Small-scalemoderate vacuumstorage system (VacuProducts b.v., Delft, theNetherlands) withmung bean sprouts. Thepiston shown is used toevacuate the containerthrough a rubber stopperby band.

“Spoilagemicroorganismsusually pose nosafety problem forconsumers, infact they maypreventpotentiallyhazardouspathogens fromgrowing todangerouslevels. . . ”

308

and lack this intrinsic protection. Microbialspoilage of undamaged, healthy products canonly be effectuated by microorganisms ableto penetrate the skin, which requires thepresence of specific enzyme systems. Withvegetables, pectinolytic types of gram-nega-tive bacteria of the genera Pseudomonas andEnterobacter are often involved in spoilage.Spoilage microorganisms usually pose nosafety problem for consumers, in fact theymay prevent potentially hazardous patho-gens from growing to dangerous levelsthrough competition for oxygen, carbohy-drates, other nutrients, and space—a mecha-nism often referred to as competitive exclu-sion. However, refrigerated MAP storagemay impair the safety balance. Refrigerationslows the growth of the spoilage flora, mostof which are mesophyles and grow best attemperatures between 20 and 40C, and re-

Table 4. Gas composition of fresh potato strips paat 5C (O’Beirne and Ballantyne, 1987).

zHigh-barrier laminate containing PVdC polyvenyle

duces their competitive capacity. Psychro-phylic pathogens like Listeria monocytogenes,Yersinia enterocolitica, enterotoxigenic E. coli,Aeromonas hydrophila, Clostridium botuli-num type E, and nonproteolytic strains of C.botulinum, which are able to grow at tem-peratures as low as 3 to 5C, may then get achance to proliferate. If inappropriate tem-peratures are used, even mesophylic patho-gens such as Salmonella typhimurium,Staphylococcus aureus, and Bacillus cereus mayrapidly develop.

Moderate VacuumPackaging

Moderate vacuum packaging is a newtype of the VP technique designed by Burg(1975; Burg and Burg, 1966). In the Burgsystem, produce is stored under an atmo-spheric pressure in the range of ≈ 10 to 100mBar at refrigerated temperatures. At thislow pressure, a constant circulation of freshair, substantially saturated with water (RH80% to 100%), is maintained. Facilities toconstantly scrub CO2 and ethylene may beincluded. In the moderate VP system, storageis at ≈ 400 mBar, and the composition of theatmosphere is not actively controlled. Theatmospheric pressure is reduced by simplyevacuating the air from the storage facility.This reduces the amount of O2 in the storageatmosphere, slows the respiration rate of thestored produce, and inhibits growth of mi-croorganisms. Any O2 consumed is con-verted to CO,. Figure 4 shows a small-scalemoderate VP system designed by Vacu Prod-ucts b.v. (Delft, the Netherlands). MAP re-quires rather sophisticated equipment and isoften used to prepare once-only packages,but the moderate VP system is technicallysimple and, when used in a rigid-containerversion, may be recycled. Investigations atour institute (Gorris and de Witte, unpub-lished) have shown that the shelf life of aperishable commodity such as mung beansprouts may be extended from 2 to 6 dayswhen stored at 400 mBar and 4 to 7C. Thesystem also prevented enzymatic browning of

ckaged under 5% O2 and 10% CO2 during storage

dene chloride.

HortTechnology ž July/Sept. 1992 2(3)

“. . . minimally

processedvegetables andfruits may safelyexpand in retailas well ascatering sectors tocomply withconsumerdemand forpremium-quality,wholesome foods.”

cut vegetables and fruits. The safety of themoderate VP system regarding survival andgrowth ofpathogenic bacteria and the type ofproduce for which the system is best suitedare currently being investigated.

Safe PackagingIn developing a MA package, the objec-

tive is to match the respiration rate of thecommodity to the gas permeability of thepackaging. Inappropriate packaging materialmay pose quality and even health problems,as can be illustrated with the data fromO’Beirne and Ballantyne (1987) on packag-ing of fresh potato strips (Table 4). A packagesuited for this product would create an equi-librium gas composition of 1% to 2% O2 and3% to 5% CO2 to prevent enzymatic brown-ing and retard senescence. In two cases, thein-package level of O2 rapidly declines to zero,which may initiate off-flavor developmentdue to anaerobic respiration. A third film didnot reduce O2 enough to inhibit enzymaticbrowning, and the fourth rendered a satisfac-tory gas composition. In the first two cases, apotential health hazard exists in that the soil-borne anaerobic pathogen Clostridiumbotulinum can grow and produce toxin at theprevailing low-oxygen situation that sup-presses the competitive action of the spoilageflora. Although only certain strains of C.botulinum thrive well at zero O2 under MAPconditions, all of the pathogens mentionedbefore have potential to survive in either ananaerobic or a low-oxygen MAP atmosphere.

In potato strips, oxygen depletion causesloss of product quality that would probablykeep consumers from eating the potentiallyhazardous foods. But such an organolepticwarning signal may not always occur, as in thecase of overwrapped mushrooms stored atambient temperatures. This condition mayproduce anaerobic conditions and supportgrowth and toxin production of C. botuli-num while the product is still of acceptablequality (Sugiyama and Yang, 1975). Clos-tridium botulinum may be effectively con-trolled by maintaining a low level of O2 (±1%to 2%) in the package, by choosing the correctpackaging material, and by adequately refrig-erating the product throughout the produc-tion-to-retail chain.

Up to now MAP and VP storage ofvegetables have enjoyed a period of compara-tive safety. However, MAP and VP may attimes inhibit the growth of the endogenousspoilage flora, thus diminishing a key signalby which consumers can tell a spoiled prod-uct. These storage methods will not eliminatepathogens but, under the right circumstances,will even select for their outgrowth, especiallyduring long-term storage and in combina-

HortTechnology ž July/Sept. 1992 2(3)

tion with temperature abuse, while the pack-aged product may still look organolepticallysound. Because the interplay of productphysiology, microbiology, and packagingconditions will eventually create the “rightconditions,” more fundamental knowledgeon these factors, individually and in combina-tion, should be gathered, and packagingconditions should be carefully considered toprevent such hazards. If these conditions arefollowed, MAP and VP of intact, minimallyprocessed vegetables and fruits may safelyexpand in retail as well as catering sectors tocomply with consumer demand for premium-quality, wholesome foods.

Literature CitedBarmore, C.R. 1987. Packaging technology forfresh and minimally processed fruits and veg-etables. J. Food Quality 10:207-217.

Burg, S. P. 1975. Hypobaric storage and transpor-tation of fresh fruits and vegetables, p. 172-188.In: N.F. Haard and D.K. Salunkhe (eds.). Posthar-vest biology and handling of fruits and vegetables.AVI, Westport, Conn.

Burg, S.P. and E.A. Burg. 1966. Fruit storage atsubatmospheric pressures. Science 153:314-315.

Geeson, J.D., K.M. Browne, K. Maddison, J. Shep-herd, and F. Guaraldi. 1985. Modified atmo-sphere packaging to extend the shelf life of toma-toes. J. Food Technol. 20:339-349.

Kader, A.A., D. Zagory, and E.D. Kerbel. 1989.Modified atmosphere packaging of fruits and veg-etables. Crit. Rev. Food Sci. Nutr. 28:1-30.

Lioutas, T.S. 1988. Challenges of controlled andmodified atmosphere packaging: a food company’sperspective. Food Technol. 42(9):78-86.

O’Beirne, D. 1990. Modified atmosphere packag-ing of fruits and vegetables. Chilled foods: Therevolution in freshness. Elsevier Applied SciencePubl. p. 183-199.

O’Beirne, D. and A. Ballantyne. 1987. Some ef-fects of modified atmosphere packaging andvacuum packaging in combination with antioxi-dants on quality and storage-life of chilled potatostrips. Intl. J. Food Sci. Technol. 22:515-523.

Peppelenbos, H. W., 1992. A systematic approach ofMAP research. Proc. Intl. Wrkshp. Modified At-mosphere Packaging, Istanbul, Turkey. (In press.)

Shirazi, A. and A.C. Cameron. 1992. Controllingrelative humidity in modified atmosphere packagesof tomato fruit. HortScience 27(4):336-339.

Sugiyama, H. and K. H. Yang. 1975. Growth po-tential of Clostridium botulinum in fresh mush-rooms packaged in semi-permeable plastic film.Applied Microbiol. 30:964-969.

Zagory, D. and A.A. Kader. 1988. Modified at-mosphere packaging of fresh produce. FoodTechnol. 42(9):70-77.