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Temperature Switchable Membranes for Packaging Fresh Produce

Published: November 01, 2000, By Raymond Clarke, Landec Corp. Menlo Park, CA

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Application: Temperature switchable membranes using a side chain crystallizable polymer have use in packaging fresh produce to create and maintain the specific atmospheres required to preserve fresh fruits and vegetables even with moderate temperature abuse.

Fresh fruits and vegetables when harvested consume oxygen and emit carbon dioxide. When the fruits and vegetables are in a sealed package, the atmosphere will reach equilibrium levels of oxygen and carbon dioxide depending on the weight of the produce, its respiration rate, and the permeability of the package. A specific beneficial atmosphere exists for each fruit and vegetable that will help preserve the quality and freshness of the produce with good temperature control. If a break occurs in temperature control, the oxygen consumption may increase beyond the capacity that the package film can supply. The package will then become anaerobic, and the produce will spoil.

Challenges in packaging fresh produce are the following:

• Different produce materials need different beneficial atmospheres
• Oxygen consumption requirements vary
• Oxygen consumption of produce increases with increasing temperature

When packaging fresh produce, a need therefore exists to provide different package permeabilities and different selectivities to oxygen and carbon dioxide to create select atmospheres. The package atmosphere must also be maintained as temperature changes.

Membrane Technology
Membrane technology is a technique to provide different package permeabilities to create specific oxygen and carbon dioxide levels in a package. The technology also maintains this optimum atmosphere within limits even as temperature is changing. The highly permeable membrane technology is over a hole in the wall of the product and essentially controls the flow of gases into and from the package.

Making the membrane involves coating a porous substrate with a proprietary side chain crystallizable (SCC) polymer. Altering the properties of the polymer provides specific oxygen permeabilities, specific carbon dioxide to oxygen permeability ratios, and changes in permeability with temperature.

SCC Polymers
SCC polymers are those in which the side chain crystallizes independently from the main chain. Examples of such polymers are siloxanes or acrylic polymers in which the side chain has eight or more carbon atoms. Varying the chain length of the side chain can change the melting point of the polymer. Preparation of the acrylic polymers occurs in solution using conventional free radical initiators. Using appropriate comonomers allows production of a polymer with melting point values of 0øC-68øC.

SCC polymers are unique due to their sharp melting transition and the ease of producing melting points in a specific temperature range. An SCC polymer in a solid or crystalline state becomes a molten fluid when heated to the switch temperature. This change of state is useful in product applications requiring a drastic change in permeability when going from below to above the crystalline melting point.

Table I

Table I shows that a membrane coated with an SCC polymer has very high permeability compared with polyethylene film. The oxygen and carbon dioxide permeability of the membrane is more than 1000 times more permeable than with a 2-mil polyethylene film while still maintaining the same carbon dioxide/oxygen permeability ratio. This allows packaging of highly respiring produce or large quantities of produce up to 1000 lb without compromising the mechanical integrity of the package. An example of the ability to package highly respiring produce is a bag of green bananas weighting 40 lb.

Produce will increase its respiration rate with temperature. If a package does not have adequate permeability, the oxygen in the package will be consumed with concomitant high levels of carbon dioxide. This will cause off odors and off flavors. Using SCC polymers as the coating on a membrane will provide a rapid increase in permeability as the temperature increases.

Conclusion
Membranes coated with SCC polymers are currently in successful use in the domestic market for broccoli, cauliflower, vegetable medley, and asparagus.