Strategy and Software Application of Fresh Produce Package Design to Attain Optimal Modified Atmosphere

Modified atmosphere packaging of fresh produce relies on the attainment of desired gas concentration inside the package resulting from product respiration and package’s gas transfer. Systematic package design method to achieve the target modified atmosphere was developed and constructed as software in terms of selecting the most appropriate film, microperforations, and/or CO 2 scavenger. It incorporates modeling and/or database construction on the produce respiration, gas transfer across the plastic film and microperforation, and CO 2 absorption by the scavenger. The optimization algorithm first selects the packaging film and/or microperforations to have the target O 2 concentration in response to the respiration and then tunes the CO 2 concentration by CO 2 absorber when it goes above its tolerance limit. The optimization method tested for green pepper, strawberry, and king oyster mushroom packages was shown to be effective to design the package and the results obtained were consistent with literature work and experimental atmosphere.


Introduction
Modified atmosphere packaging (MAP) of fresh produce is based on attaining the modified atmosphere (MA) resulting from contributions of produce respiration and plastic film gas permeation.For effective preservation of the fresh produce, desired or optimal modified atmosphere of reduced O 2 and elevated CO 2 concentrations specific for the commodity should be created and maintained.Control variables to attain the desired atmosphere in MAP design are type of packaging film, its area and thickness, presence and number of microperforations, and so forth, for the given pack size.Systematic way of MAP design has been implemented and is available in Internet web-based software for convenient and versatile design application to a wide variety of commodities and conditions [1][2][3].The design tools have been built based on the databases of fresh produce respiration and plastic film gas permeability.While the currently available software can present the optimized package design consisting of plastic film condition (type and thickness) and perforation conditions if required, there is still a need for incorporating other options such as active packaging attachment [4].
Inclusion of carbon dioxide scavenger can help to ease the package optimization and widen the available MAP design window by attaining the desired MA without invading the injurious CO 2 level [5].Therefore, augmentation of MAP design scheme with CO 2 scavenging has been tried in this study.Specific objectives of this study are to build and test a comprehensive fresh produce MAP design platform applicable to a wide variety of commodities and storage conditions.

Model of Design Package.
As the designable package model, modified atmosphere package of gas-permeable plastic tray or bag was conceptualized to be potentially appended with perforation or CO 2 absorber (Figure 1).It was assumed that the package stays or is stored at constant temperature and respiration rate of fresh produce depends on the atmosphere surrounding the produce.Parameters of uncompetitive respiration model (1) describing the produce respiration rate [6] have been compiled as a database for many commodities and products, which is ready for being called from the design program.Thus, the respiration rate is given as: where Produce respiration and plastic film gas permeability at temperature other than the listed one can be supplied to the design program application by using Arrhenius equation [7,8]: where  , and  ,  are the respective respiration rates at temperatures  (K) and   (K);  , and  ,  are the respective gas permeabilities (mol mm m −2 h −1 Pa −1 ) at temperatures  (K) and   (K);   means the activation energy (J mol −1 ) corresponding to temperature dependence of respiration or gas permeation;  is universal gas constant (8.314J K −1 mol −1 ); the subscript  refers to O 2 or CO 2 gas.

Mass Balances of Gases on the Produce Package.
As a basis for designing fresh produce MAP, mass balance equations on the package with CO 2 absorber incorporated have been set up: where The above equations (3)-( 5) have been used as the basis for design of MAP to attain the desired MA close to the optimal one.All the derivatives equal to zero ( O 2 / = 0,  CO 2 / = 0,  N 2 / = 0) can be used to deal with steady state, and negation of the first terms on right side of (3)-( 5) amounts to the nonperforated packaging conditions.The solution of simultaneous differential equations ( 3)-( 5) can give the history of gas moles in the package, which can be converted using the Ideal Gas Law to partial pressures of O 2 , CO 2 , and N 2 ( O 2 ,  CO 2 , and  N 2 ) or to volumetric percentages under normal atmosphere package atmosphere.

Design Scheme to Attain the Desired MA at Steady State.
The design application starts with receiving the input information on commodity, temperature, expected shelf life, package's physical condition (type, produce weight, dimension, permeable surface area, and preferred microperforation), and film and type of CO 2 absorption sachet.Selection of commodity dictates optimal MA, CO 2 tolerance limit, and respiration parameters (Figure 2).nonperforated package film to provide the optimal MA are calculated from the steady state assumption of (3) and ( 4) with  = 0 and   = 0: where If this  O 2 , can be covered by available plastic films (usually lower than the most permeable one such as linear low density polyethylene (LLDPE)), then the plastic film having the O 2 permeability closest to  O 2 , is selected from the list in the gas permeability database.Unless this condition is not satisfied, we can conclude that perforation is needed for achieving the high gas transfer, which will be handled later separately as another conditional case of design.Now, the selected film is tailored in the thickness () by using its O 2 permeability ( O 2 , ): While the optimum O 2 concentration ([O 2 ]  ) is ensured by the selection of the available film and its thickness (), the resultant CO 2 concentration ([CO 2 ]  ) for this film condition (with CO 2 permeability of  CO 2 , ) is newly calculated: If this CO 2 concentration is above the CO 2 tolerance limit, there is a potential risk of physiological injury due to high CO 2 concentration.Thus, CO 2 absorption is needed to lower the CO 2 concentration down to the optimal level and its amount in moles ( CO 2 , ) can be assumed to be equal to the difference between respiratory CO 2 production and CO 2 permeation toward outside during the shelf life of   (h): Because the CO 2 absorption of the scavenger is achieved through permeation process of the sachet film, the surface area of sachet (  , m 2 ) can be inferred from the steady state CO 2 balance on the sachet: From the steady state arrangement of (4) with foregoing relationship, the resultant CO 2 concentration ([CO 2 ]  ) for the package with CO 2 scavenger can be reached: If  O 2 , calculated by (8) is not enough to be provided by available permeable plastic films, the required number () of basic perforations in diameter  is obtained from the steady state case of (3): where O 2 permeability of basic plastic film is adopted as With this number of perforations, the steady state O 2 and CO 2 concentrations ([O 2 ]  and [CO 2 ]  , resp.) of basic film package (with CO 2 permeability of  CO 2 , ) are stated anew from the steady state cases of ( 3) and ( 4) with   = 0 (no scavenger): If the CO 2 concentration from ( 16) is above the CO 2 tolerance limit, CO 2 scavenger is required to lower the CO 2 concentration down to the optimal level ([CO 2 ]  ) and the scavenger amount can be obtained as respiratory CO 2 production minus outward CO 2 transfer through the perforation and film layer during the shelf life of   (h): Similar to the nonperforated case, surface area of CO 2 scavenger can be obtained by the same equation (12) given above and the resulting CO 2 concentration for the perforated package with the scavenger can be derived from the steady state condition of (4):

Estimation of Package Atmospheric Change under Designed Package Condition.
Once all the package variables to give the desired MA close to optimal MA were decided based on the steady state analysis, the package atmospheric change as function of time can be estimated by the solution of simultaneous differential equations ( 3)-( 5) as mentioned before.Runge-Kutta method with short time step was used for the solution in this study.For the simplified and stable solution, the condition of constant volume and normal atmospheric pressure was enforced at every time step of evolution with assuming simultaneous deflation of package atmosphere or infiltration of ambient air.Simplification for the solution stability of the mass balance equations on the perforated fresh package has been examined and discussed by Kwon et al. [11].Usually nitrogen permeability of polymeric package film is much lower than that of oxygen or carbon dioxide and its partial pressure differential across the film is lower in the passive MA package, resulting in little change of nitrogen concentration, which tells limited influence of the N 2 permeability on the package atmosphere.While there is sufficient information on O 2 and CO 2 permeabilities, there is not enough information on N 2 permeability and thus N 2 permeability was assumed as one-fifth of O 2 permeability valid for common plastic films [12,13].All the design scheme and atmosphere estimation were set up with a link to respiration and plastic permeability databases in an application of smart phone as a name, ProduceMAP.Comprehensive algorithm for the package optimization can be summarized as a flowchart in Figure 2.

Application and Validation of the Developed Design Method by Using Case Studies
Three commodities with different MA requirements were subjected to the developed design optimization program providing optimal conditions for their packages.The desired MA, respiration, and package dimension of the products supplied to the program as input information are given in Table 1.

Green Pepper Package.
Green pepper is known to have low respiration activity and optimal MA of low O 2 and low CO 2 concentrations [14,15] (Table 1).Thus, small permeable package unit has been reported by Lee et al. [9] to achieve the optimal MA by intact permeable film like low density polyethylene (LDPE).It also has moderately low tolerance to CO 2 with its limit of 8.0%.Submission of the same package unit of 110 g to ProduceMAP could provide the optimized solution giving an equilibrated MA close to the target (4% O 2 and 5% CO 2 concentrations) (Table 2 and Figure 3).The optimized bag package of 0.042 mm thick LDPE film was estimated to have equilibrated MA of 4.0% O 2 and 5.1% CO 2 concentrations, which was closer to the target than that of a reported experimental package of 0.025 mm thick LDPE (5.1% O 2 and 3.3% CO 2 concentrations) [9].The systematic optimization could select the film thickness better to create the optimal MA than the commonly used trial-and-error approach.The equilibrated O 2 and CO 2 concentrations with solution of differential equations ( 3)-( 5) in Figure 3 (4.6 and 5.2%, resp.) are a little different from those from ( 6) and (10) (4.0 and 5.1%, resp.).This difference would have come from assuming the respiration as constant in the optimal MA and taking into consideration only O 2 and CO 2 balances, respectively, in steady state equations ( 6) and ( 10) without  1 and 2.
considering the nitrogen balance.However, this little difference less than 1% may be tolerated in practices of fresh produce MAP design.It is noted that the optimized package design for green pepper could be obtained without using perforation or CO 2 absorption.Relatively green pepper's low respiration rate and location of optimal MA window at low O 2 and low CO 2 concentrations allowed the LDPE film having ( CO 2 / O 2 ) ratio of ≈3.3 to fit to the package of the target MA [16,17].

Strawberry Package.
Strawberry can tolerate high CO 2 concentration and be benefited from storage under an MA of low O 2 and high CO 2 concentrations [15], which can be offered by microperforated package [3,10,18].A package unit tried by Sousa-Gallagher and Mahajan [3] was optimized for the target of 8% O 2 and 18% CO 2 with CO 2 tolerance limit of 20% by ProduceMAP program.The output of the design was a tray consisting of permeable oriented polypropylene (OPP) with two perforations of 0.25 mm resulting in an equilibrated MA of 9.3% O 2 and 17.5% CO 2 concentrations (Table 2).The solution of differential equations (3)-( 5) for the optimal design resulted in the similar equilibrated MA (8.6 and 16.6% for O 2 and CO 2 concentrations, resp., in Figure 4).This optimized package design of two perforations is the same condition obtained by Sousa-Gallagher and Mahajan [3] but gave a little difference in the equilibrated MA (4.8 and 19.8% of O 2 and CO 2 concentrations, resp., in the latter).Because the produce respiration and package gas permeability data used by them [3] may be different from ours, direct comparison of current outcome to the source is impossible.But the optimal MA conditions are found to be attained for strawberry with high respiration activity by microperforations in common plastic film.Presence of microperforations in permeable plastic package plays roles to increase gas transfer across the film layer and reduce the CO 2 /O 2 permeability ratio near to 1, which helps to design MAP for commodities Respiration parameters for green pepper and strawberry are from literature sources [9,10], and those for mushroom are determined by the closed system experiment.

Package of King Oyster Mushroom.
Mushroom is of high respiration activity and highly perishable [14].The respiration parameters of (1) for the optimization were determined by closed system method [19,20], and high   values represent high respiration rate of this commodity (Table 1).Optimal MA condition suggested for quality preservation varies with literatures or sources.In this study of MAP design, MA of 1% O 2 and 10% CO 2 , reported to be beneficial, was used as a target for the optimization [21,22] (Table 1).The upper limit of CO 2 concentration range 15% was adopted as the tolerance limit in the optimization.The outcome of design optimization showed the inclusion of 17 microperforations of 100 m on 0.03 mm thick OPP film and CO 2 absorber of 9.5 g Ca(OH) 2 to attain the desired MA (Table 2).Different conditions of CO 2 sachet film could be selected to give the same resultant  1 and 2.  1 and 2. Open and close symbols are experimental gas concentration data obtained in two separate trials with LLDPE CO 2 sachet film of 0.020 and 0.013 mm, respectively.equilibrium package atmosphere and two film thicknesses were given in Table 2 for later experimental testing.Apparently the high respiration activity of respiration seemed to need high number of microperforations to meet the O 2 concentration of 1%, but the presence of microperforations was not enough to maintain CO 2 concentration below 15%.Thus, the package design required CO 2 absorber to let the CO 2 concentration stay exactly at the optimal level of 10% according to (17) (Table 2).The simulation outcome of differential equations (3)-(5) in Figure 5 is nearly the same with package atmosphere from the equilibrated relationships in Table 2.
As a further way to validate the design method, the optimized package of mushroom was prepared and measured in its atmosphere during storage at 10 ∘ C. O 2 , CO 2 , and N 2 concentrations of the package were measured for one milliliter of gas samples taken through a silicon-sampling port by using a gas-tight syringe.Varian Model 3800 Gas Chromatography (Varian Inc., Palo Alto, CA, USA) equipped with an Alltech CTR I Column (Alltech Associates Inc., Deerfield, IL, USA) and a thermal conductivity detector was used for the gas analysis.The experimental package atmospheres tried with two different CO 2 sachet systems were in very close agreement with the estimated ones (Figure 5), verifying the validity of the MAP optimization method developed in this study.

Conclusions
MAP design optimization methodology capable of attaining the desired MA was developed to select the most appropriate film, microperforations, and/or CO 2 scavenger.The optimization algorithm first selects the film and/or microperforations to have the target O 2 concentration and then tunes the CO 2 concentration by CO 2 absorber when it goes above its tolerance limit.The optimization method tested for three different commodities was shown to be effective to design the package and the results obtained were consistent with literature work and experimental atmosphere.

Figure 1 :
Figure 1: Fresh produce package structure to be optimized.

Figure 2 :
Figure 2: Flowchart for optimizing fresh produce MAP to give the target MA.

Figure 3 :
Figure 3: Optimal MA profile for green pepper package with requirements and design conditions in Tables1 and 2.

Figure 4 :
Figure 4: Optimal MA profile for strawberry package with requirements and design conditions in Tables1 and 2.

Figure 5 :
Figure 5: Optimal MA profile for king oyster mushroom package with requirements and design conditions in Tables1 and 2. Open and close symbols are experimental gas concentration data obtained in two separate trials with LLDPE CO 2 sachet film of 0.020 and 0.013 mm, respectively.
O 2 ,  CO 2 , and  N 2 are the respective mole numbers in the container of O 2 , CO 2 , and N 2 gases at time  (h);  O 2 ,  CO 2 , and  N 2 are the respective diffusivities of O 2 , CO 2 , and N 2 gases in air (m 2 h −1 );  O 2 ,  CO 2 , and  N 2 are the respective partial pressures of O 2 , CO 2 , and N 2 in the package and   is normal atmospheric pressure (1.013 × 10 5 Pa);  is the number of perforations of diameter  (m) and cross-sectional area  (m 2 ) on the package;  is a correction term for gas diffusion resistance in the perforations (1.1);  and  are the thickness (mm) and surface area (m 2 ) of the plastic layer, respectively;  O 2 ,  CO 2 , and  N 2 are the respective gas permeabilities of the plastic layer against O 2 , CO 2 , and N 2 (mol mm m −2 h −1 Pa −1 );  is the produce weight (kg);  CO 2 , is the CO 2 permeability of the CO 2 absorber sachet (mol mm m −2 h −1 Pa −1 );   and Then the required permeance values (Φ O 2 as  O 2 / and Φ CO 2 as  CO 2 /) of [O 2 ]  and [CO 2 ]  are optimal O 2 and CO 2 concentrations (atm or decimal), respectively.It is noted that  O 2 in (6) and  CO 2 in (7) are for the optimal MA.Now it is examined whether commercially available films of usual thickness (  , mm) can provide the required O 2 permeance by calculating the correspondent O 2 permeability,  O 2 , (mol mm m −2 h −1 Pa −1 ):

Table 1 :
Commodity properties and package size for optimized MAP.

Table 2 :
Output from optimized MAP design for the package sizes in Table1.