A review on modified atmosphere packaging and preservation of fresh fruits and vegetables: Physiological basis and practical aspects—part II

PACKAGING TECHNOLOGY AND SCIENCE VOL 9 1-17 (1996)

~ ~~~

A Review on Modified Atmosphere Packaging and Preservation of Fresh Fruits and Vegetables: Physiological Basis and Practical Aspects-Part II

Laurence Leet*, Joseph Arult, Robert LenckiS and FranCois Castaignet tDepartment of Food Science and Technology, Lava1 University Quebec, Quebec, Canada, G1 K 7PA; $Department of Food Science, University of Guelph, Ontario, Canada, N1 G 2W1

Keywords: fruits and vegetables; postharvest; modified atmosphere; packaging; storage

CURRENT CA AND M A FOR FRESH PRODUCE

Currently, a few commercial storage systems are operated based on CA principles, normally controlling gas concentration, temperature and humidity. The gas compositions are created and controlled by several methods: N2 g e n e r a t i ~ n , ~ ~ O2 control,78 and C02 removal by scrubbing79 or with a C02 selective membrane.73 The hypo- baric system 80381 may also be regarded as a CA system, as the partial pressure of O2 is reduced by maintaining a low pressure. Other gases may be added, such as carbon monoxide, and systems may be added to control e th~lene .~ It is now possible to use CA in transport systems (where the major post-harvest losses can result) with the introduction of hollow fibre membranes which are compact and can handle compressed gases (e.g. the Prism@ Alpha CA System of Permea Inc., St Louis, MO). However, with CA systems, mixed load storage is impractical and unecon- omical because of variable CA requirements and the incompatibility of different produce due to ethylene action and flavour contamination. Unfortunately, most fresh produce are still stored using conventional methods, hence significant post-harvest loss is still a problem.

In contrast to CA systems, modified atmosphere (MA) technology can be more flexible in extending the CA benefits for improving shelf life to a larger number of fresh produce during distribution and storage. MA ,conditions can be realized via packaging, which is a passive system, by balancing produce respiration and gas exchange through package materials. Such systems, called modified atmosphere

*Author to whom correspondence should be addressed.

CCC 0894-3214/96/010001- 17 0 1996 by John Wiley & Sons, Ltd.

Received 28 November 1991 Revised 28 February 1992

2 L. LEE ETAL.

packaging (MAP), can be configured as bulk packaging containers, as pre-packing (unit retail packages) and even as individual produce coatings.

Bulk packaging under M A

In general, traditional bulk packaging systems for various commodities like pallet bags and paperboard containers are used for transportation, handling and even storage. The rapid development of semi-permeable polymeric materials has led to the possibility of creating modified atmospheres passively in these systems. Marcellin82 introduced silicone rubber membrane as a gas exchange device for pallet bags. The TransFresh Co. (Salinas, CA) has developed what is known as the TectrolB System for strawberries which is basically a pallet box bulk unit wrapped with a barrier plastic film.83 The gases used for the Tectrolm system (high CO2) are injected into the package, and then the bag is sealed. This type of system could hardly be regarded as an active MA or gas packaging because the permeability of the film and respiration of the produce are not factors in MA atmosphere creation and main- tenance. It is only used for short term transport and packages should be opened before the O2 level falls below the critical level which would result in anaerobic respiration.

The development of bulk MAP systems has been limited by the lack of suitable gas exchange materials with the required gas transfer properties for C 0 2 and O2 (e.g. permeability and selectivity, which is the ratio of the permeability of C 0 2 over that of 0,). More efforts are needed to eliminate these obstacles which constrain MAP applications to fresh fruits and vegetables.

A wide range of gas exchange materials to suit each particular produce is needed for MAP in bulk formats. But, a bulk MAP system should be flexible enough to handle a number of produce. This requires that the selectivity of bulk packaging should be adjustable. High gas transferability is another requirement for bulk packages as a large mass of produce is contained in each package.

In practice, passive bulk MAP systems can serve food distribution well, as they can be easily adapted to transport systems. It is also advantageous for handling mixed loads (produce with different ethylene and flavour compatibilities) in the same storage room or transport container. Another potential advantage is that bulk MAP can be returnable or disposable depending on distances involved between production and consumption points. Returnable package systems may be expensive but are ecolo- gically friendly.

P re- pac kag i ng

There has been an increase in use of plastic film packaging, such as low density polyethylene (LDPE), polyvinychloride (PVC) and polypropylene (PP), for fresh fruits and vegetables. These materials are generally transparent, provide a barrier to water vapor transmission, and are very selective in gas permeability to C 0 2 versus 02.

M 0 D I F I ED ATM 0s P H ERE PAC KAG I N G - I I 3

However, their gas transmission rates are smaller and, hence, they are often used for pre-packaging. Hardenbergs4 was the pioneer in using plastic films for fresh produce. Many of the initial applications focussed primarily on reducing moisture loss, providing protection from handling damage and improving produce appearance. For example, B e n - Y e h o s h ~ a ~ ~ ~ ~ ~ has provided strong evidence indicating that indivi- dually wrapped citrus fruit display reduced cell membrane degradation caused by water stress.

B a l l a n t ~ n e ~ ~ ’ * ~ has tested a number of plastic films for various produce, with the conclusion that there could be a risk of low 0 2 permeability of many plastic films, especially with produce having high respiration rates. The commonly used films, such as polyethylene and PVC, have gas transmission rates at a level where a large surface is needed to meet gas exchange requirements, especially with bigger packages or produce with high respiration rates. But the real challenge for pre-packaging is that most of the polymeric films provide only a narrow range of gas selectivity. Since gas selectivity requirements for different produce vary, few films can be used to develop optimal MA pre-package, and are thus applicable to only a limited number of produce .88

There are other potential problems which could jeopardize successful use of MAP for fresh fruits and vegetables. One is the effect of ethylene (produced by fresh produce) accumulation inside the package, which could act as exogenous ethylene, catalyzing senescence. In Japan, polyethylene films coated with an ethylene adsorbing agent (e.g. soapstone powder) was shown to be effective in improving MAP performance for ethylene-sensitive produce.

A problem associated with moisture condensation in MAP is severe fungal infection of fresh produce. Since polymeric materials are generally good barriers to water vapour transmission, the atmosphere inside packages of fresh produce could reach saturation humidities very quickly. Thus microbial growth is favoured. The external layers of the fruit surface and the films being cold surfaces, condensation would occur at these points. Condensation of moisture on produce favours fungal growth. Condensation on packing films could also alter the film’s permeability and lead to anaerobic conditions.

Protective coating of fresh produce

Another strategy to extend the benefits of CA consists of a protective coating on individual produce. This can be an effective alternative to plastic films, which are non- biodegradable and contribute to the problem of waste disposal. The coating acts as a semi-permeable film to gas diffusion, thereby creating a modified atmosphere inside the produce which can delay ripening. The control of deterioration of fruits as a result of coating has been attributed to internal atmosphere modification of both O2 and C02,89 and to the alleviation of water stresss4 However, there are concerns about the risk of fermentation due to excessive depletion of 02, and an increase in fungal infection. 85,90

4 L. LEE ET AL.

Edible coatings have been developed to improve the storability of fruits and vegetable^.^' The edible coating ‘Pro-Long, (a blend of sucrose esters of fatt acids and sodium carboxymentylcellulose) has been tested extensively. The suppression of ripening in response to coating was ascribed to depression of internal O2 rather than to an increase in the endogenous C02 Meheriuk and Laug4 reported that pears coated with ‘ProLong’ and ‘Nutri- Save’ were firmer, higher in titratable acidity and greener. However, coated fruits lost their ripening capacity when re-stored at 20°C. The application of ‘Nutri- Save’ (N, 0-carboxymethychitosan) to pears and apples su ressed C02 and C2H4 evolution and improved general quality attribute~9~~~’~Coating produce with chitosan delayed ripening, weight loss and controlled diseases without

However, the practical difficulty is altering the ripening capacity of fruits. in precisely manipulating the coating procedure for internal gas modification, although the anti-fungal nature of some of the coating materials, such as chitosan, is very attractive.

89,92,9y

91-99

DEVELOPMENT OF MODIFIED ATMOSPHERE PACKAGING FOR FRESH PRODUCE

Modified atmosphere packaging is a passive system based on balancing produce respiration rate and package gas transmission rate, thus creating and maintaining the required C02 and O2 levels under steady-state conditions in the package.76 The exact steady-state values of O2 and C 0 2 depend on gas exchange capacity and the respiration rate of the produce.

The respiration rate of produce after harvest is initially very high, and diminishes with storage time, reaching a steady-state level which is a function of storage temperature and atmosphere composition. In MAP, designed gas permeation fluxes through the package for C02 and 0 2 are such that the C02 produced by the produce at steady- state is transferred to the external atmosphere while the 0 2 consumed by the produce is supplied by the atmosphere. Oxygen inside the package is consumed by produce as it respires, and an approximately equal (depending on respiratory quotient, RQ) amount of C02 is produced. The reduction in O2 concentration and the increase in C02 concentration create a gradient between the package atmosphere and the extreme condition, to leave the package.

Initially, the respiration rate of the produce is much higher than the package permeation rates for C02 and 02, (i.e. respiration flux is higher than package gas exchange flux). Gas concentration gradients across the package are also small. Hence, in this initial stage, gas exchange fluxes across the package are not sufficient to compensate for respiration fluxes. This will result in accumulation of C 0 2 and depletion of 0 2 inside the package; and increase the gas concentration gradients across the package, which, in turn, will increase gas exchange rates. Eventually, the dissimilarities in respiration fluxes and gas exchange fluxes disappear, and steady- state concentrations of O2 and C02 are thus established inside the package.

MODIFIED ATMOSPHERE PACKAGING-II 5

It is obvious that to design efficient MAP and to make an intelligent selection of gas exchange materials, it is critical to obtain reliable respiration and permeability values for both C 0 2 and 02. This presents challenges, as discussed below.

Permeability of packaging films and gas exchange flux of package system

In MAP, the gas exchange requirement implies an effective gas exchange flux. The gas exchange flux of a package depends on both the permeability of the packaging film and the gas exchange surface area of that material at a given temperature and hydrostatic pressure. A packaging film should not be judged as suitable or not merely on the basis of permeability; rather, an integrated analysis is required.

The key important characteristic parameter of an MA package is its selectivity, which refers to the ratio of C 0 2 permeability to O2 permeability of the packaging material. Selectivity determines the relationship between the concentrations of C02 and O2 for a particular package. Only when package selectivity satisfies that required by a produce for O2 and C 0 2 can an optimal atmosphere of both C 0 2 and O2 be achieved. The selectivity requirement of a produce, in turn, depends on the ratio of C 0 2 production and O2 consumption of the produce (the respiratory quotient, RQ) and the expected optimal compositions of both C02 and 0 2 for that produce.

Thus 0 2 and C 0 2 fluxes across a MA package and C02/02 selectivity are the critical factors for developing a MAP. A suitable packaging material for a given produce should be able to provide a combination of both permeability and selectivity which depends on the type of produce, its mass, temperature (which affects both respiration and permeability), and expected gas composition at steady-state. One film might be able to satisfy the requirements of a variety of produce, if these produce require the same selectivity. However, it is a practical impossibility to tailor-make films suitable for each individual produce. Combination systems (usin two different gas exchange devices) could be a feasible solution to this challenge.

Selectivity is related to the required C 0 2 and O2 composition for a specific produce, and can be determined by the equation:"'

108

where S is the required selectivity, RQ is the respiratory quotient and Ap02 and ApC02 are the required gas partial pressure gradients to ambient air.

This concept is essential to screen packaging materials for a given product. Tables 1 and 2 give a summary of required gas compositions for fruits and vegetab1es.l'' The significance of this is that the mass balance calculation should be based on selecting a good package whose selectivity suits the produce. Materials currently available rarely fit the requirements for MAP of most fresh produce (Table 3).88

It should also be pointed out that the permeabilities measured by manufacturers do not simulate MAP operation. The gas exchange process in MAP involves counter diffusion of mixed gases, and the gas concentration gradient is not constant. The

L. LEE H A L . 6

Table 1. Required M A conditions and required selectivities' for modified atmosphere packaging of selected fruits

MA conditionb

Commodity Temperature %02 %CO2 Selectivity ("C)

Deciduous tree fruits Apple 0-5 Apricot 0-5 Cherry (sweet) 0-5 Fig 0-5 Kiwi 0-5 Nectarine 0-5 Peach 0-5 Pear 0-5 Persimmon 0-5 Plum and Prune 0-5 Raspberry 0-5 Strawberry 0-5

Subtropical and tropical fruits Avocado 5-1 3 Banana 12-1 5 Grapefruit 10-1 5 Lemon 10-1 5 Lime 10-1 5 Mango 10-1 5 0 I ive 8-1 2 Orange 5-1 0 Papaya 10-1 5 Pineapple 10-1 5

3.0b (2-3) 2.5 (2-3) 6.5 (3-1 0) 5 (5) 2 (2) 1.5 (1 -2) 1.5(1-2) 3' (2-3) 4 (3-5) 1.5 (1 -2) 1 oc (1 0) 6d (10)

3.5 (2-5) 3.5 (2-5) 6.5 (3-1 0) 5c (5) 5 (5) 5 (5) 3.5 (2-5) lO(10) 5 (5) 5 (5)

3.0' (1 -2) 2.5 (2-3) 1 1 (10-12) 15 (1 5) 5 (5) 5 (5) 5 (5) 1 oc (0-1 ) 6.5 (5-8) 2.5 (0-5) 6' (1 5-20) 1 5d (1 5-20)

6.5 (3-1 0) 3.5 (2-5) 7.5 (5-1 0) 10' (0-5) 7.5 (5-1 0) 5 (5) 7.5 (5-1 0) 5 (5) lO(10) lO(10)

6.0 (9-1 9) 7.4 (6-9.5) 1.3 (0.9-1.8) 1.1 (1.1) 3.8 (3.8) 3.9 (9.5-1 0) 3.9 (3.8-4) 1.8 (1 8.0-1 9.0) 2.6 (2.0-3.6) 7.8 (3.8-20.0) 1.8 (0.6-0.8) 1 (0.6-0.8)

2.7 (1.6-6.3) 5.0 (3.2-9.5) 1.9 (1 .I -3.6) 1.6 (3.2-1 6.0) 2.1 (1.6-1 6) 3.2 (3.2) 2.3 (1.6-3.8) 2.2 (2.2) 1.6 (1.6) 1.6 (1.6)

aCalculated assuming RQ is unity.

bValues in parentheses are the range of gas concentrations reported from Kader et al.5

''dother data sources are: 'Arul et al. (unpublished data): Couture (unpublished data).

methods commonly used in industry, such as the Mocon instrument,"' employ a constant gradient and involve a single gas. The dynamic method, loo which simulates the operation of a MAP, is more appropriate for evaluating gas transfer properties of different gas exchange devices, such as film membrane and perforation.

Design of modified atmosphere packaging

Many studies on MAP have concerned themselves with selecting packaging films using respiration-permeation balancing at steady-state. The method determines

M 0 D I FIE D ATM 0s PH ERE PACKAGING- I I 7

Table 2. Required M A conditions and required selectivitiesa for modified atmosphere packaging of selected vegetables

MA conditionb

Commodity Temperature %02 %C02 Selectivity ("C)

Artichokes Asparagus Beans, snap Broccoli Brussels sprouts Cabbage Cantaloupes Cauliflower Celery Corn, sweet Cucumbers Honeydews Leeks Lettuce Mushrooms Okra Onions, dry Onions, green Peppers: bell and chili Spinach Tomatoes:

mature-green partially ripe

0-5 0-5 5-1 0 0-5 0-5 0-5 3-7 0-5 0-5 0-5 8-1 2

10-1 2 0-5 0-5 0-5 0-1 2 0-5 0-5 8-1 2 0-5

12-20 8-1 2

2.5 (2-3) 5' (air) 3d (2-3) 39 (1 -2) 4c (1 -2) 4c (3-5) 4 (3-5) 2' (2-5) 5' (2-4) 3 (2-4) (3-5) (3-5) 2.2' (1 -2) 2' (2-5) sd (air) 4d (3-5) (1 -2) 2d (1 -2) 3c (3-5) (air)

5' (3-5) 3h (3-5)

4 (3-5) 10' (5-1 0)

8' (5-1 0) 6' (5-7) 6' (5-7) 12(10-15)

3c ( 0 ) 1 5 (1 0-20) ( 0 ) (0 ) 5' (3-5) 2' ( 0 ) 1 Od (1 0-1 5) 2d ( 0 ) ( 0 ) 1 Od (1 0-20) 8' (0 ) (1 0-20)

8d (5-1 0)

3'(2-5)

lO'(10) 5h (10)

4.6 (3.6-6.3) 1.6 (0.1 -0.2) 2.3 (1.8-3.8) 2.3 (1.9-4.0) 2.8 (2.7-4.0) . 2.8 (2.3-3.6) 1.4 (1.1 -1.8) 6.3 (3.2-9.5) 5.3 (> 17)

(> 16) (> 16)

1.2 (0.8-1.9) .

3.8 (3.8-6.7) 9.5 (> 16) 1.6 (< 0.1) 8.5 (> 16) (> 19) 1.9 (1 .o-2.0)

(< 0.1) 2.3 (1 6-1 8)

1.6 (1 6-1 8) 3.6 (1 5-1 8)

aCalculated assuming RQ is unity.

bValues in parentheses are the range of gas concentrations reported from Kader et Other data sources are: 'Gariepy et ~ f . , ' ~ ~ dArul et al., (unpublished data), Touture (unpublished data), fRomo-Parada et af. (unpublished data), gMakhlouf et af.,66 hWillemot (unpublished data) and 'author's data.'"

film surface area using the respiration rate of the produce at the storage temperature and the known optimal C 0 2 and 0 2 concentrations for the produce. 8 2 3 8 7 J 0 2 j 1 0 3 This simplified solution uses one optimal gas gradient and does not address the process of creating the modified atmosphere itself. In these studies, only one gas balance equation is independent and is used to determine the dimension of gas exchange. Unless a film fits a produce well, it may result in one gas being well controlled, but not both of them. Thus the MAP cannot be optimized and can be harmful, depending on whether C02 or 0 2 is used as the target gas and whether highly or less selective materials are used.lm Nevertheless,

8 L. LEE ETAL.

Table 3. Permeability and selectivity of selected polymeric materials at 4°C (thickness = lpm)

Polymers

Permeability Selectivity

KOZa Kc02a = K C 0 2 / K Q 2

Silicone rubber

Ethyl cellulose Natural rubber Polybutadiene Poly( butadiene-styrene) Polyvinychloride-vinyl acetate Polyvinylchloride (PVC-RMF-61) Polyethylene-low density (0.92 g/cm3)

Polypropylene Cellulose acetate Butyl rubber Rubber hydrochloride (plasticized Pliofilm)

Polyethylene, high density (0.96 g/cm3)

Polythylene-terephtalate (Mylar) Polyamide (Nylon) Saran

(Marcellin membrane) 1.10 x loo

1.55 x lo-’ 9.50 x 8.39 x 7.36 x lop2 2.62 x lo-’ 2.30 x lo-’ 1.20 x 1 0-’

4.66 1 0 - ~ 3.90 3.60 1.50 1 0 - ~

1.53 x 1 0-4

1.44 x 1 OP4 8.1 6 1 o - ~ 4.54 x

’Data from Exama el al. (1993) with unit of mlpm/cm2/atm.

7.13 x 10’

3.78 x lo-’ 6.72 x lo-’

6.52 x lo-’ 1.74 x lo-’ 1.40 x 1 0-‘ 8.10 x lo-’

7.73 x lo-’

1.47 x 1 0-2 3.14 x lop2 1.60 x 1 OP2 3.67

7.40 x

4.93

4.63 1 0 - ~ 3.84 x lop4

6.6

2.4 7.1 9.2 9.2 6.6 6.1 6.7

3.2 8.0 4.4 2.2

4.8

3.4 4.7

10.2

the method can be employed to estimate the permeability required for a given product and desired storage atmos here.

Some researchers 10471d: have attempted to design MAP packages on a more analytical basis of respiration rates. Using knowledge of the respiration rates of a produce at a specific temperature and in a modified atmosphere, it should be possible to calculate the permeability required to establish the desired modified atmosphere in the package. On the other hand, other researcherslo6 recognized the value of developing predictive models for the appropriate selection of polymeric films as well as following modification of the atmosphere inside the package.

have introduced a combination MAP system for fresh Droduce. Bv 107- 109 Lee et al. using two devices (silicone membrane and porous membiane) having different seleE- tives, a range of effective selectivity, 1-6, can be generated. Based on data reviewed by Kader et most fruits and vegetables require selectivity in the range 1.5-4. Combi- nation systems can suit many produce. Thus requirements for MA packages can be reduced and simplified.

M 0 D I FI ED ATM 0s P H ERE PACKAG I N G - I I 9

Table 4. Respiratory quotients of selected fresh crops in air and under M A conditions

RQ Storage conditions

Crops Air MA T"C( 50.5) co;! (f0.2) 0*(*0.2)

Brocoli 1 .o 1.1 -1.2 4 10 3 Mushroom 0.6 1.2-1.3 4 10 5 Peppers 1 .o 1 . I -1.2 13 10 5 Tomatoes

Green 1 .o 1.1 -1.2 13 10 5 Red 1 .o 1.2-1.3 13 10 5

Strawberry 1 .o 1.1 -1.2 4 10 5

Other aspects relating to the design of MAP which should be addressed are the impact of temperature variation on the development of anoxic atmospheres and on the development of the atmosphere in the package. Temperature fluctuation has an impact on respiration rate and, hence, also impacts on the package atmosphere because the permeabilities of most packaging materials are less sensitive to temperature change. An optimised MA package should be one which is able to compensate for gas composition changes due to respiration changes. Temperature can also aggra- vate condensation problems because of heat-mass exchange as it relates to transpiration and vapour phase changes during temperature fluctuation.

Temperature variation occurs during handling and distribution of fruits and vegetables. However, the impact depends on the magnitude and duration of the fluctuation. A small package is more vulnerable to the impact of temperature variation because of its lower mass load and smaller void volume. A bulk package may be less sensitive to temperature changes since the heat transfer process to heat or cool a bulk package can be very slow. Packaging materials and pack design can lead to packages with different overall heat transfer coefficients. Although, MA packaged produce will show less sensitivity to temperature variations in terms of changes in the respiration rate than air-stored produce, MA packaged produce could suffer from other consequences. Since the permeabilities of most packaging materials are less sensitive to temperature changes than respiration rate changes of the produce if temperature varies, MAP will lead to anoxic conditions (C02 accumulation and 0 2 depletion) during temperature variation. Depending on the sensitivity of the produce to high C02 and low O2 conditions, physiological damage to the produce can occur, leading to loss of quality and shelf-life.

Another serious consequence of temperature variation on MAP performance is the effect on condensation and the associated infection problem, more than its effect on changes in gas composition. Changes in package, package air and packaged produce temperatures will have a significant effect on the water vapour pressure in the package atmosphere. A greater vapour deficit would be expected when package temperature increases. Thus produce transpiration will increase which would result in more water

10 L. LEE €7 AL.

being pumped out of the fruit and deposited inside the package, since most packaging materials are barriers to water vapour. Condensation would occur mainly on package interior walls and fruit surfaces during a cooling period, where heat exchange takes place on colder areas. Since a temperature increase as well as high humidity favour microbial growth temperature variation can promote infection, leading to heavy losses. Lee et al.'" have developed a method to reduce condensation and improve quality by using an absorbent. This works well even with tomato packages (unpublished) exposed to temperature fluctuations.

MAP of climacteric fruits need not be different from packaging of non-climacteric fruits. Climacteric fruits are characterized by an ethylene overshoot as well as an overshoot in respiration. As discussed earlier, MA lowers respiration rate and ethylene production. In addition, MA lowers respiration and ethylene overshoots. When MAP is designed with the preclimacteric respiration rate, one should expect a higher than expected level of COZ and a lower level of 0 2 during the overshoot. Since respiration overshoot is somewhat suppressed by MA and the gas gradient across the package is transiently higher, levels of C 0 2 and O2 may not be affected to any great extent. Second, since packaging materials are more permeable to ethylene than to C02 and 0 2 , accumulation of ethylene in the package would also be minimal. Addi- tional precautions can be taken for produce, such as banana and papaya, which produce high levels of ethylene, by incorporating ethylene oxidizers or scrubbers either in the packaging material or inside packages.

Measurements and modelling of respiration rate under M A conditions

The respiration rate of produce is affected by the composition of the ambient atmosphere. Once produce is packaged, the package atmosphere gradually changes from the initial ambient air to a modified atmosphere of lowered 0 2 and elevated COZ which reaches a steady-state value with storage time. Thus, the respiration rate of packaged produce is a function of storage time and package air Composition. Hence, dynamic methods to measure respiration as well as modelling are appropriate. Weichmanns6 made a rough estimation of steady-stated respiration rate under MA. He observed that the respiration rate under MA was approximately 70% of the rate in ambient air. Unfortunately, there has been no further progress in developing methods and models for the respiration process under MA conditions. It is often hypothesized that the respiration rate under CA conditions is the same as under MA conditions although in the latter, gas concentration is gradually modified until steady-state is reached.

Two methods are commonly used to measure the respiration rate and these have been discussed by Lee:Io3 (1) a constant flush method where the produce is continuously ventilated with a gas

(2) a closed system method, where the initial ambient air is gradually modified by mixture of known concentration;

respiration of the produce.

M 0 D I FI ED ATM 0s PH ERE PACKAG I NG-I I 11

The constant flush method is simple and most data in the literature was generated by this method. However, this type of data is difficult to verify for MAP applications, since the MA process is not regarded as the same as CA storage. With the closed system, at any given time only a single combination of C 0 2 and O2 levels in the package atmosphere can be obtained, such that [C02] + [02] = 21% and other combinations of [CO,] + [02] may not be possible. Secondly, the process of CA modification is rapid (typically 12 h before the O2 level falls below the critical level which induces anaerobic respiration). This is not what typically happens in MA packages. Atmosphere modification is rather slow, and it takes a few days before a steady-state is reached. Furthermore, there is a lag of produce response to MA which is affected b factors such as the mass of the produce, the package void volume and temperature.

Several regression models have been developed to estimate respiration rate under suggested that there MA from data collected by the gas flush method.

is a need to describe the transient feature in respiration measurement and modelling. He believed that use of a closed system could generate a transient gas composition change. This suggestion was experimentally verified by Cameron et al., lo' who characterized this phenomenon using a first-order equation. As discussed earlier, a closed system can only be useful when the desired gas compositions meet [CO,] + [ 0 2 ] = 21%. However, in practice such situations are not numerous.

Gariepy et al.' l 4 measured respiration rate under MA using a semi-permeable membrane system. This system resembles a MA package, however, being similar to the closed system method, this procedure also determines the respiration rate only under certain combinations of CO2 and 0 2 . With this method, combinations of CO, and O2 are such that ([C02] x 6) + [02] = 21%, where 6 is the selectivity of a silicone membrane, assuming an RQ of unity. The gas composition produced in this method could be different from what is required by the particular produce and thus it still has limitations. Lee"' developed a respiration model based on enzyme kinetics (Michael-Menton model) where the effects of C02 and 0 2 on respiration rate can be evaluated. However, the gas concentration for this model should be the internal gas concentration in the produce tissue, which is not easily measured and controlled in practice. Lee"' has also developed a dynamic method by elaborating on the combination system. The unique advantage is that the required combination of C 0 2 and 0 2 can be generated by combining the use of two gas exchange devices.

Knowledge about the respiratory quotient (RQ), is another important aspect in respiration modelling. The RQ is the ratio of C02 production to O2 consumption of fresh produce. Unless the RQ is unity, C 0 2 production and O2 consumption are not equal, and this should be taken into consideration for MAP design. Most fruits and vegetables have an RQ of about one in ambient air, with the exception of mushrooms (RQ about 0.6 in air). However, produce metabolism under MA is typically different from that in air. As discussed earlier, the effects of low O2 and raised COa are not on the same basis. There is a possibility that substances other than hexose are used as substrate, which will have a dominant impact on RQ, and that a possibility of partial anaerobic respiration is involved, which can only be completely eliminated if the O2 level is higher than 10% and the C02 level lower than 5%.8 For these reasons, the RQ of fresh produce under MA can hardly be regarded as the same as

117

112,113 ~ ~ ~ 1 0 3

12 L. LEE ET AL.

in normal air storage. The significance of RQ can be addressed in three aspects related to MAP development: package selection, MAP design and understanding produce metabolism in adapting to the modified atmosphere.

One of the critical parameters for selecting packaging materials is the required selectivity. This is determined based on the produce respiration gas exchange charac- teristics and optimal atmosphere composition, for both C 0 2 and 02. 107J08 For MAP design, if RQ is not unity, as in air storage, it will be difficult to justify whether to use the 0 2 consumption rate or the C 0 2 production rate as the parameter for calculation, especially where a single gas exchange device is used. Furthermore, the effects of the RQ can contribute to changes in total pressure inside a MAP which can also affect gas exchange. Thirdly, knowing how the produce adapts to the modified atmosphere has significances for a better understanding of the physiological basis of MA and for providing information which can be used to improve quality retention of produce stored under MA. For example, removing the by-products of partial anaerobic respiration (ethanol and acetaldehyde) from the package atmosphere, can signifi- cantly improve the freshness of MA-stored broccoli. ' l o

Raghavan et a1.'I6 reported some RQ values for several produce (e.g. cabbage, celery and carrot) stored in the Marcellin system with an RQ in a value range below unity. However, calculation and analysis of the storage data of the 10 produces investigated by Marcellin82 and our collected data indicate that RQs were more than unity. Our data'" (Table 4) for broccoli, tomato, strawberry and mushroom confirmed Marcellin's storage test. We believe that partial anaerobic respiration is MA operation-oriented, but it is not a dominant factor compared with the true anaerobic dominant respiration process. In the MA/CA literature, respiration data is mostly reported on a C02 basis. Nevertheless, it is clear that RQ is an important aspect for successful MAP development because the selectivity of a package material is related to RQ,Ioo and should be well addressed in future investigations.

Modelling of the MAP system

Modelling of MAP can serve two purposes: package design, and prediction or simulation of gas composition in the MA package during storage. Karel and Jurin'I7 proposed a simple model based on a mass balance for package design. It has been extensively used since then for calculating required gas exchange surface. Marcellin82 developed mathematical equations for MAP and pointed out the necessity of considering the exchange of three gases, i.e. 02, C 0 2 and N2. Henig and Gilbert'I2 developed first-order differential equations and solved them using numerical solutions to optimize the package parameters. However, they assumed that the 0 2 concentration in the package has only a ne ligible effect on CO2 production and vice versa. Subsequently, Hayakawa et a d 4 integrated into this model the effects of O2 and C 0 2 on respiration rate. Lee'" developed a model for designing a combination MAP system. The model can be used to determine the selectivity required for a particular product and the dimensions of two gas exchange devices.

M 0 D I FI ED A T M 0s PH E R E PACKAG I N G-I I 13

Modelling for simulation purposes is on a mass balance basis. The gases considered in MAP modelling are mainly C02 and 02, with the exception of flexible packages and MAP initially flushed with a gas mixture whose N2 content is not that of the atmosphere."' Since respiration is a key factor in regulating mass transfer and mass balance, the way of incorporating respiration rate in predicting gas composition dynamics mainly contributed to the diversity in different models, such as using a constant respiration rate or as a respiration function of gas composition and time. The model proposed by Deily and Rizvi"' assumed an O2 concentration between 5 and 21% and a C02 concentration between 0 and 25% did not affect respiration. The model was improved by integrating the mass balance of gases C 0 2 and 0,) and by using the respiration rate as a function of storage time." This model was reported as being able to predict precisely the time required to reach steady- state.

There are other models which are worth mentioning.'20-122 All these models, in principle, consist of two simultaneous first-order differential equations for O2 and C 0 2 , and a regression equation to take into account the effect of O2 and C02 on respiration rate. However, the model of Mannaperuma et makes a rigorous analysis of gas diffusion in the plant tissues and also introduces a convective term. Although they did not explain the mechanism of convective gas transfer, presumably they introduced this term to account for additional gas transfer which may take place due to differences in the total pressure across the packaging material.

Although better estimations can be achieved with these models, they still have limitations. First, the mass balance of the gas components are assumed to be independent for each gas species and usually do not include N2 flux. Second, it is almost always assumed that the RQ is unity, which is not what is observed in MA packages. Certain parameters, such as package pressure and volume, are considered constant. These assumptions are contrary to experimental observations as well as theoretical considerations. Marcellin82 observed a volume reduction in flexible MA packages. Thus, it is fair to say that these models do not completely portray MA packaging: gas species do not permeate independently, respiration rate is not constant the RQ is not always unity and package pressure and volume are not constant. Leeioo developed a model by integrating two functions: the total hydrostatic pressure in MAP and a respiration function of time which is an exponential equation predicting respiration starting from the initial ambient rate to a steady-state rate for the expected gas compositions verified with collected data. The total model was found satisfactory by validation, in that it can reasonably predict the steady-state gas compositions and the time required to reach the steady-state, as well as the total pressure change for MAP made of rigid material.

CHALLENGES FOR M A P

MAP has attractive potential as an alternative to low temperature storage for preser- ving fresh fruits and vegetables, particularly many tropical fruits and vegetables which possess a high respiration rate and are sensitive to chilling injury. However,

14 L. LEE ET AL.

there are still practical and theoretical difficulties limiting the commercial use of modified atmosphere packaging for fresh produce.

Practical aspects

Bulk packaging. CA/MA benefits can be well exploited with bulk systems of MAP where the modified atmosphere can be created and maintained in an independent package container. Such a system can facilitate mixed load transportation and handling, which is not economically viable with a CA system. Developing new bulk package design and reducing packaging cost are required. Unless logistically and economically feasible, MAP technology will not be appreciated by industry. Bulk systems should also be more environmental friendly, being disposable and re-usable and promoting waste reduction.

Respiration. Produce respiration behaviour under MA is not well understood; more effective and practical methods are needed for measuring respiration rate under MA conditions. In addition, more respiration rate data for both COz production and 0 2 consumption need to be collected under specific MA conditions.

Condensation and temperature variation. These are potential hazards in employing MAP for fresh produce. Little attention has been devoted to this aspect. Since most packaging material is a barrier to water vapour transmission, MAP of fresh produce will operate with a saturated package atmosphere which is favourable to microbial infection and, more seriously, offset the designed optimal gas compositions. Temperature variation can even induce anoxic atmospheres, which can damage produce tissue, with shelf life being reduced instead of prolonged. Additional measures should be incorporated with MAP.

Quality aspects. More investigations are required on the problems of off-flavour and loss of freshness of MA packaged produce, which is one of the main obstacles limiting application of the MAP technique to fresh produce. Understanding the mechanism and compounds which are responsible for off-flavour can help to generate innovative solutions to the problem. Unless MAP can keep fresh produce ‘fresh’, its full potential is unlikely to be realised.

Theoretical aspects

Further investigation of biochemical and physiological aspects of fresh produce under MA (namely respiratory metabolism, ethylene action, transpiration, physiological disorders) which produce tolerance to critical gas compositions and their depen- dence on temperature is required. Perhaps, post-harvest pathological aspects incorporated in the produce defence compartment need to be investigated. Fungal infection is important with MA packaged produce, particularly for tropical and

MODIFIED ATMOSPHERE PACKAGING-It 15

subtropical produce which require moderate storage temperature to prevent chill injury.

The greatest difficulty for packaging manufacturers is the standardization criteria for material fabrication, as well as for package selection. Package materials with suitable combinations of selectivity and permeability should be developed based on current packing practice and regulations. More investigations are also needed on optimal gas composition, capable of compensating for the impact of temperature variation on the package atmosphere. Packaging materials and methods which are able to remove undesired gases, such as ethylene and acetaldehyde, from the package atmosphere and control of infection are also needed at a competitive price.

A systematic investigation on the use of MA packages with optimal modified atmospheres for storage and handling fresh produce is needed, to develop comprehensive theory for analysis, material selection, design and performance evaluation of MAP for fresh produce. Such an investigation needs efforts by researchers from teams of a multi-disciplinary nature, including plant pathologists, material engineers, and packaging specialists.

Comprehensive modelling of a MA package is needed, using a dynamic respiration model. Other factors to be considered in modelling should include the impact of package volume, package atmosphere moisture dynamics, the impact of temperature fluctuation on gas composition and moisture dynamics, microbial safety etc.

Investigation to predict shelf life and storability of fresh produce under MA is required. Such predictions should be related to the storage condition and changes in conditions likely to be encountered during handling and distribution of fresh produce from packing house to supermarket.

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Documents

A review on modified atmosphere packaging and preservation of fresh fruits and vegetables: Physiological basis and practical aspects—part II