Temperature-Compensating Films for Modified Atmosphere

Chapter 16

Temperature-Compensating Films for Modified Atmosphere Packaging of Fresh Produce Ray F. Stewart , Judy M . Mohr , Elizabeth A. Budd , Loc X. Phan , and Joseph A r u l

Downloaded by NORTH CAROLINA STATE UNIV on October 29, 2012 | http://pubs.acs.org Publication Date: March 5, 1993 | doi: 10.1021/bk-1993-0520.ch016

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Landec Corporation, 3603 Haven Avenue, Menlo Park, C A 94025 Department of Food Science, Laval University, University City, Quebec G 1 K 7P4, Canada 2

Temperature responsive gas permeable films based on side chain crystallizable polymers have been developed that can match or exceed the increasing respiration rates of fresh produce subjected to temperature fluctuations. The development of the films is discussed and experimental results of test produce packages are compared with computer simulations. There is potential to use this film to compensate for temperature fluctuations in the cold chain of storage and distribution. Fruits and vegetables are perishable produce which actively metabolize during the postharvest phase. The use of low temperature is probably the most important means of extending the storage life of postharvest produce. Modifying the gas atmosphere inside an enclosure containing the produce can also decrease the respiration rate and thus further extend the storage life. Altered gas atmospheres can be passively created and maintained by flexible film packaging, a process known as Modified Atmosphere Packaging (MAP). In a sealed package containing a produce with a permeable film, a modified atmosphere is created naturally as a result of dynamic molecular balance between the respiratory process (O uptake and C O external production) and the exchange of gases with the atmosphere through the film. Eventually, steady-state concentrations of O , C O and the third gas N are established passively at a given temperature when the rates of O consumption and C O production of the produce equal the rates of permeation of these gases through the package. The magnitude of the C O increase and O decrease at steady-state is dependent on the gas flux through the film and its C O / O selectivity. In order to obtain the maximum benefit from MAP, the steady-state gas concentrations should correspond to the storage optima for a given crop. On the other hand, when CO accumulates above the tolerance limit of the crop, it can cause injury to the crops. Lowering O levels below the critical levels 2

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0097-6156/93/0520-Ο232$06.00/0 © 1993 American Chemical Society

In Polymeric Delivery Systems; El-Nokaly, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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Temperature-Compensating Films for Packaging

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may result i n anaerobic conditions, with the development of off-flavors and undesirable texture changes. A t extremely low 0 levels, toxin p r o d u c t i o n by anaerobic pathogenic organisms can also occur (1,2,3). A number of researchers have developed design methodologies to a i d the construction of M A packages suitable for a specific crop assuming that the packages w o u l d be constantly maintained at the optimal storage temperature of the produce (4,5,6,7,8, Exama, A, Laval University, personal communication, 1992). T h e practical application and the c o m m e r c i a l impact of M A P has thus far been quite limited, i n spite of its beneficial affects o n the quality a n d the longevity of fresh crops. This is because some of the serious practical limitations of M A P to ensure the safety of the produce r e m a i n unresolved. T h e primary hurdle relates to the low gas flux and/or improper selectivity of the commercially available films to create and m a i n t a i n o p t i m a l M A for many crops w i t h a few exceptions such as apple and green pepper (Exama, Α., Laval University, personal communication, 1992). T h e second p r o b l e m arises f r o m the condensation of water o n the packaged produce w h i c h favors fungal growth, as w e l l as o n the films which could add resistance to gases through the film. E v e n if a films could be developed to meet the required package permeabilities at the o p t i m u m storage temperature, as w e l l as prevent the condensation of water vapor prevented, M A P should still prove safe against the temperature fluctuations encountered i n the cold chain of storage a n d distribution. T h e temperature may fluctuate from the expected storage temperature up to ambient temperatures at w h i c h produce packages are often displayed. This has two implications: (1) creation of anoxic atmospheres inside the package at the higher temperature; (2) aggravation of the condensation of the saturated vapor. B o t h respiration of produce and the f i l m permeability are dependent o n temperature. W h i l e the values of respiration rate coefficient ( R value) of most crops are generally between 2.0 to 3.0, the permeability coefficient (P ) of most films ranges between 1.0 and 2.0 for the same temperature range (Exama, Α., Laval University, personal communication, 1992). T h i s disparity could lead to excessive accumulation of C 0 and/or depletion of 0 at higher temperatures, a situation which must be avoided. This p r o b l e m can be avoided by using films possessing appropriate P values matching the R value of the produce. W i t h regard to this possibility, it w o u l d seem to be a daunting task to develop a f i l m that w o u l d possess the required 0 and C 0 permeabilities, i n addition to appropriate P values. T h e second possibility could involve a temperature sensitive safety value - a value of a membrane - that w o u l d o p e n at elevated temperatures and reclose when the desired temperature is re established (Exama, A, Laval University, personal communication, 1992).


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W o r k conducted at L a n d e c C o r p o r a t i o n i n the area of Intelimer (a registered trademark of L a n d e c Corporation) temperature responsive polymers has led to the development of semipermeable membranes that exhibit large changes i n gas permeation i n response to small temperature changes. Intelimers belong to a family of materials known as Side-Chain-Crystallizable ( S C C ) polymers w h i c h exhibit abrupt thermal transitions associated w i t h the


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side chain rather than the polymer backbone. This unique feature allows membrane materials to be designed that exhibit tailored permeation properties over a range of temperatures useful i n food packaging. T h e general structure of Intelimer polymer is as follow: R I -(CH -CH)-

Where:

R= H, C H Z = oxygen, carbonyl, ester, amide x = 11 to 21 3

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I (Z)

I (CH )x 2

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CH

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T h e change i n the permeation property mentioned above is associated with the melting point (Tm) of the side chains. T h a t is, at temperatures below the T m the polymer is not very permeable, but at temperatures above the T m , the permeability increases dramatically. T h e melt temperature of the side chains can be varied by systematically changing their chain lengths. The melting point of the polymer changes directly with respect to the change i n the chain lengths. Specific compositions and uses of this class of polymer is the subject of issued and pending patents (U.S. Patent N o . 4,830,855; 5,129,180 and 5,129,349) assigned to L a n d e c C o r p o r a t i o n . A development effort was made at L a n d e c C o r p o r a t i o n i n collaboration with L a v a l University to determine the feasibility of using these materials for compensation strategy against temperature abuse for M A P . Experimental Procedures and Results Polymer Preparation. Side chain crystallizable polymers exhibiting melting transitions i n the range of 4°C to 20°C were prepared by radical polymerization of n-alkyl acrylates having average side chain lengths of 12 to 14 carbon atoms. T h e polymers were purified by precipitation into cold ethanol and isolated. R e p o r t e d thermal transitions were measured v i a D S C at 10°C/min. Film Preparation and Permeation Measurement. Intelimer films were prepared as 0.001" thick films and laminated to a highly permeable microporous polyolefin for permeation testing. Pure gas 0 and C 0 permeability values were measured o n an automated system constructed i n our laboratories. A l l films were tested i n quadruplicate, first for 0 permeation as a function of temperature and followed by C 0 permeability measurements. T h e P values of each f i l m for 0 and C 0 were calculated by comparing the permeabilities over a 10°C range below and above the side chain transition. T h e C 0 / 0 selectivities were calculated above and below the transition temperature. 2

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A summary of the permeability behavior for a selected series of Intelimer films is given i n T a b l e I. It was found that the permeability c o u l d be varied over a range of approximately 3,000 to 21,000 cc · m i l / m · day · atm. F i l m s w i t h permeabilities i n the upper end of this range offer intriguing possibilities for the packaging of rapidly respiring produce such as mushrooms and broccoli. T h e gas selectivity of the films ranged f r o m approximately 3 to 7, comparable with existing films. 2


T a b l e I. Permeation Characteristics for Intelimer F i l m s Film

Transition

CQ

C 0 / 0 Selectivity below Tt above T t 2

Pio Values 0 CO; ι

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T t is an abbreviation for transition temperature Units: cc · mil/m · day · atm 2

Evaluation of Film Suitability for Produce Packaging. T h e ideal packaging f i l m w o u l d have a P value that closely matches the R value of the produce to be packaged so that the gas concentrations i n the package w o u l d r e m a i n i n balance even under variable temperature conditions. B y comparing the selectivities and P values of the films w i t h reported values of o p t i m u m storage conditions and R values for various produce, one can estimate the suitability of these films for use i n specific packaging applications. Figure 1 shows the effect of temperature on the respiration rate of cabbage, the carbon dioxide permeability of one of the films tested and comparative data for a f i l m used commercially, Cryovac SSD-310 (Cryovac). T h e Cryovac f i l m is a multi-ply, co-extruded polyolefin film, often used i n food packaging. Based on the permeation rate data for these films it w o u l d appear that the Intelimer f i l m w o u l d be an excellent fit with cabbage. 1 0

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Computer Modeling of Food Packages. Cabbage was selected for initial evaluation and testing because it is an important commodity, w o u l d benefit f r o m M A P , is not chilling sensitive, and is available year r o u n d . T h e r e are reliable data i n the literature o n the respiration response of cabbage to different temperatures and atmospheres.



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Figure 1. T h e Effect of Temperature o n the R e s p i r a t i o n R a t e of Shredded Cabbage o n ( A ) the C o m m e r c i a l F i l m Cryovac S S D - 3 1 0 and o n (B) the Intelimer F i l m L L 4 7 - 1 7 3 .


16. STEWART E T AL.

A n iterative computer m o d e l developed for purposes of package simulation by a team at the University of C a l i f o r n i a , D a v i s was used. T h e m o d e l takes into account the produce respiration properties as a function of temperature and atmosphere conditions, f i l m permeability and headspace i n the package, and calculates the 0 and C 0 concentrations w i t h i n the package as a function of time until e q u i l i b r i u m is established. Simulations were preformed for three different films i n order to compare their performance as packaging materials for shredded cabbage. T w o films were temperature responsive Intelimer films and a third was a c o m m e r c i a l Cryovac film, SSD-310. Simulations were done at 2.5, 5, 7.5, 10 and 20°C, a range of temperatures typically encountered i n the distribution chain for fresh produce. Results of the simulations for the Intelimer f i l m L L 4 7 - 1 7 7 and for the Cryovac f i l m are presented i n Figures 2 and 3. T h e c o m m e r c i a l f i l m is suitable at temperatures up to approximately 10°C, but at 20°C anaerobic conditions result after a short p e r i o d of time. A n identical package constructed with the temperature responsive Intelimer film, i n contrast, maintains suitable conditions at a l l temperatures tested clearly showing an ability to respond to significant temperature changes. 2


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C o n s t r u c t i o n a n d E v a l u a t i o n of Prototype Packages. T w o types of tests were performed o n packages that were prepared utilizing the Intelimer f i l m and the Cryovac f i l m : an isothermal test and a temperature step experiment. Isothermal Experiment. In the isothermal test, four identical packages were constructed: two utilizing Cryovac and two utilizing the Intelimer film. E a c h package had 324 c m of f i l m surface area. T h e amount of shredded cabbage, W to be loaded into the packages was determined from the following equation: 2

Po xA_ 2

RRxW

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W h e r e P 0 is the oxygen permeability of the package f i l m , A is the f i l m surface area, L is the film thickness, R R is the respiration rate of the cabbage, and the quantity ( 0 - 0 ) is the difference between atmospheric oxygen concentration and the oxygen concentration inside the package. In this manner each package was balanced for the relative permeability of the films at the design temperature of 10°C. Comparative tests were then conducted isothermally at 10°C and 20°C and the gas concentrations inside the packages were monitored for up to 200 hours. T h e gas concentrations inside the packages versus time are shown i n Figures 4 and 5. A s expected, when maintained at either 10°C or 20°C b o t h packages show an initial rapid drop i n 0 and an increase i n C 0 . T h i s response is expected because freshly cut cabbage respires rapidly. A s the 2

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F i g u r e 2. C o m p u t e r Simulated Changes at T h r e e Temperatures i n a Package Constructed with Cryovac F i l m SSD-310 and F i l l e d with Shredded Cabbage.



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Time (Hours)

Figure 3. C o m p u t e r Simulated Changes at T h r e e Temperatures i n a Package Constructed with the Intelimer F i l m L L 4 7 - 1 7 7 and F i l l e d with Shredded Cabbage.


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F i g u r e 4. Oxygen and C a r b o n D i o x i d e C o m p o s i t i o n Inside Packages Constructed from Cryovac F i l m SSD-310 and F i l l e d w i t h Shredded Cabbage and Stored at 10°C and 20°C.



16. STEWART E T A L


Figure 5. Oxygen and C a r b o n D i o x i d e C o m p o s i t i o n Inside Packages Constructed from the Intelimer F i l m L L 4 7 - 1 7 7 and F i l l e d w i t h Shredded Cabbage and stored at 10°C and 20°C.


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cabbage recovers from the cutting injury and the atmosphere is modified, respiration rates decrease and the packages begin to reach e q u i l i b r i u m conditions. In the case of the Cryovac f i l m package, it can be seen that at 10°C the package w o u l d equilibrate at about 8 % 0 and 1 0 % C 0 . A t 20°C however, a very different result is obtained. T h e 0 concentration drops to 1% while the C 0 concentration rises initially to 2 0 % and then continues to increase throughout the experiment. These conditions are not healthy for cabbage and clearly demonstrate the practical p r o b l e m of designing a package w h i c h can both generate a suitable modified atmosphere at the low design temperature and prevent rapid generation of anaerobic conditions at higher temperatures. In the case of the Intelimer f i l m package maintained at 10°C, the system equilibrates at about 1.5% 0 and 5 % C 0 w h i c h is a suitable environment. W h e n maintained at 20°C the package again equilibrates w i t h a suitable atmosphere not much different from that obtained at the lower temperature. It should be pointed out that these packages do not represent optimized systems and the ideal f i l m for cabbage w o u l d exhibit a C 0 / 0 selectivity of about 3 whereas the tested f i l m exhibit a selectivity of about 67. 2

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Temperature Step Experiment. A more realistic test of the performance of the temperature sensitive food package is a temperature step experiment. Test packages were prepared as before, loaded with the appropriate amount of shredded cabbage and placed for 3 days at 3°C. A s expected, the gas composition inside the packages is similar, i n both cases the 0 drops to approximately 1.5% and the C 0 levels rise to about 1 3 % . W h e n the packages are moved to 20°C the C 0 level i n the Cryovac f i l m package continues to increase even faster up to 20 % while the C 0 level i n the Intelimer f i l m package drops to a n e q u i l i b r i u m value of about 6 % w h i c h is maintained even after the temperature is again lowered to 3°C. W h i l e this is an encouraging result it should be noted that during the initial 3°C period of these experiments true e q u i l i b r i u m conditions were not yet attained. Experiments probing additional iterations of f i l m surface area or cabbage loading levels w o u l d be needed to find the specific conditions that give comparable e q u i l i b r i u m gas concentrations at the target temperature of 3°C. T h e experimental results obtained i n conjunction w i t h the computer simulations clearly indicate that Intelimer films can be used to design temperature compensating modified atmosphere produce packages. 2

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Conclusions W o r k conducted to date shows that Intelimer membranes may be u t i l i z e d to prepare temperature compensating food packages. A variety of gas selectivities, P values and permeabilities can be designed into these films to meet the specific needs of various commodity and high value crops. Because 1 0


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actively respiring produce is a dynamic material that changes w i t h time a n d conditions, computer simulations can serve as a valuable a i d i n the design o f a package construction optimized for specific produce subjected to a variety of probable time-temperature conditions. Temperature abuse compensating food packages m a y play a n important role i n the c o m m e r c i a l growth o f m o d i f i e d atmosphere produce packaging.


Acknowledgments T h i s material is based u p o n w o r k supported i n part by the U . S . D e p a r t m e n t of A g r i c u l t u r e under G r a n t 91-33610-5932. T h e authors w o u l d l i k e to acknowledge helpful discussions with D r . D e v o n Zagory o f Z a g o r y & Associates. Literature Cited

1. Kader, Α.; Zagory, D. and Kerbel, E. Critical Reviews in Food Sci. 1989, 28, pp 1. 2. Sugiyama, H . and Yang, Κ. H. Applied Microbiology, 1975, 30(30), pp 964. 3. Aylsworth, J. Fruit Grower, 1989, 9, pp 10. 4. Cameron, A. C.; Boylanpett, W. and Lee, J., J. Food Sci., 1989, 54(6), pp 1413. 5. Massignan, L.; in Computerized Selection of Plastic Films for the Packaging of Fresh Fruits and Vegetables; in Third Subproject: Conservation and Processing of Foods. A Research Report 1982-1986, National Research Council of Italy, Milano, 1987. 6. Zagory, D.; Mannapperuma, J. D.; Kader, A. A. and Singh, R. P. In Use of a Computer Model in the Design of Modified Atmosphere Packages for Fresh Fruits and Vegetables; in Proceedings of the 5th International Controlled Atmosphere Research Conference; Feuman, John F. (Ed.); Wenatchee, Washington, 1989, Vol. 1; pp 479. 7. Deily, R. F. and Rizvi, S. S. H. J. Food Process Eng. 1981, 5, pp 23. 8. Henig, Y. S. and Gilbert, S. G. J. Food Sci. 1975, 40, pp 1033. RECEIVED October 1, 1992


Temperature-Compensating Films for Modified Atmosphere

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