In packaged and stored foods, lipid oxidation occurs in the presence of oxygen reducing the quality and life of the product. According to research done in the year 2004, an oxygen level of 4-5 ppm is just enough to drastically reduce the shelf life of products like canned meats and vegetables to a period of less than one year.
Quality Losses Due To Oxidation Process in Packaging
Continued research on ways of reducing oxygen contact with the foods has lead to evolution of various packaging methods of heat processed foods other than the metallic cans. Glass jars and polymer plastics are also relatively stable and have also been used.
Why Not the Aluminum Lining?
Apart from oxygen permeability, the flexible packaging film materials must have other attributes such as heat and chemical résistance, strength, puncture resistance, and even printability in order to comply with various food legislations.
The aluminum lining was thus developed to meet with these requirements and serve as an effective oxygen barrier. However, its use requires a thickness of more than 25 micrometers to work effectively. A thickness of less than this increases susceptibility to cracks formation and pin holes leading to oxidation of products.
In addition, lining the packaging material with aluminum is expensive due to multiple lamination steps required. The waste product is also not recyclable and so this was not the ultimate solution.
Ethyl Vinyl Alcohol as an Oxygen Barrier Film
The invention of ethylene vinyl alcohol was a major breakthrough due to its ability to form a high oxygen barrier. Its permeability to oxygen is approximately 0.006cc.mil per 100 square inches per day at temperatures of 23 centigrade.
In addition it has very low moisture sensitivity as long as the atmosphere is kept dry. This has made EVOH to be the product of choice for various retort packaging requirements.
Manufacture of the Barrier film
This copolymer of ethylene and vinyl alcohol is made under the process of copolymerization. The process is widely adopted because it allows for modifications of the properties of the resulting product to suit the requirements of final package.
The production process involves polymerization of the free radicals followed by a process called saponification. It is important to note that the amounts of ethylene and vinyl alcohol used in manufacture determine the final properties including the mechanical strength and oxygen permeability.
Crystallization Process
This is the process of aggregation of polymer chains to form crystals which have a higher order than the amorphous phase. One of the reasons that for good barrier film properties is due to its high level of crystallinity which results to improved gas and moisture protection.
Scientific Research on Moisture Sensitivity
Despite the overwhelming benefits, this product displays some level of moisture sensitivity when exposed to high relative humidity. Research has shown however that moisture only affects the amorphous phase of the structure and the resulting oxygen permeability is low.
Conclusion
Heat processed foods suffer loss of quality due to packaging that allows oxygen permeability. The aluminum lining was one of the innovations developed to solve this problem but proved ineffective due to development of cracks and pinholes. This led to development of ethyl vinyl alcohol which is synthesized through a process of copolymerization. It has low oxygen permeability and thus more accepted as a barrier film in packaging.
