A significant amount of time can elapse between the production of food products and their sale. During this process, they spend most of their time packaged in the same container. It is therefore important that the product be able to withstand the environmental changes to which it is subjected, but it must also remain edible during this long period before reaching the consumer. Internal coatings are one of the primary ways to package food products and extend their shelf life. These are the various layers of alloy inside a metal package that create a barrier between the food and the metal.
The type of internal coating used is selected based on the region where it is produced, access to materials, and the metal to which it will be applied. These coatings vary widely around the world, but currently, epoxy-based coatings are the most common, accounting for up to 90% of the coatings used. They are widely used because they have been around since the 1950s and are very durable, flexible, and resilient. They contain numerous monomers that bond together to form a strong cross-linked structure. As a result, they are much more resistant to damage, precisely because so many bonds must be broken to cause damage.
Furthermore, these types of coatings are currently the least expensive to produce. They can be used for most foods [2]. The coatings in the remaining 10% are made from organosol, polyester, vinyl, and oleoresin alloys. These coatings are often made from organic plant molecules, which makes them more sustainable. On the other hand, they each have properties that make them less attractive than epoxy coatings.
Organosols are more expensive to produce; polyesters are unstable when they come into contact with acids; vinyl coatings do not adhere completely to metals and decompose at high temperatures; and oleoresins offer only moderate resistance to corrosion and take a long time to dry [1,2].
The drawback of epoxy-based coatings is that they are made from bisphenol A. Over the past 10 years, studies on bisphenol A have begun to identify its toxic effects on humans. The major problem that has been identified is its disruptive effect on the human endocrine system [1,4]. The endocrine system controls hormone production in the body, and when it is disrupted, it can impair bodily functions.
It has also been shown that certain older epoxy-based resins cause abnormally high levels of BPA to be absorbed by food. Newer epoxy resins have addressed this issue, and studies on them show that only 0.5% of the BPA found in human samples can be attributed to epoxy resins in food packaging. Research is currently focused on developing methods for producing alternative resins that are both less expensive and more environmentally friendly [2].
Although epoxy-based resins are the most versatile coatings in terms of functionality, they are not necessarily the best. Their potential impacts on human health are their main drawback. The European Commission and the FDA have recently tightened their regulations regarding epoxy resins, and certain low-molecular-weight resins—such as variants of Bisphenol A Digycidyl Ether (BADGE)—have even been declared illegal in some European regions [1]. When companies must choose which internal coating to use for a food product, it is advisable to review the product’s requirements to determine which coating is best suited for it.
References
[1] "Packaging Materials 7: Metal Packaging for Foodstuffs (2007)," by Peter K.T. Oldring and Ulrich Nehring.
[2] "Can Coatings (2016)," by Birgit Geueke.
[3] "Changes to Corrosion Protection Coatings in Food Packaging" (retrieved April 2019), by the American Coating Association (paint.org).
[4] "Food Contact Materials - Legislation" (retrieved April 2019) by the European Commission.
