It often takes food products a long time to go from being produced to sold. During this process they spend a majority of their time packaged in the same container. It is important that the product not only withstands the changing environment it is moved through, but also that it remains edible over the long time period before it reaches consumers. One of the main ways that products are packaged to increase their shelf life is through the use of internal coatings. These are layers of different compounds on the interior of metal packaging that creates a barrier between the food product and the metal.
The type of internal coating used is based upon the region it is being produced, access to materials, and metal it is being used on. These vary largely around the world, but currently epoxy-based coatings are the most common, making up 90% of all coatings used. Epoxy-based coatings are common because they have been around since the 1950’s, are very strong, flexible, and resistant. They contain many crossed-linked monomers that join together to form a strong lattice of bonds. This makes them resistant to damage because they many bonds that need to be broken.
Additionally, they are currently the cheapest form of coating to make. They can be used for most types of food products [2]. The remaining 10% of coatings are based on organosol, polyester, vinyl, and oleoresin compounds. These coatings are often made with organic molecules from plants, which makes them more sustainable. The downside is they each have aspects that individually makes them less desirable than epoxy coatings.
Organosols are more expensive to produce, polyesters are unstable when they come into contact with acids, vinyl coatings do not adhere well to metals and break down under high temperatures, and oleoresins have limited corrosion resistance and take a long time to dry [1,2].
The downside to epoxy-based resins is that they are made with bisphenol-A. In the past 10 years studies on bisphenol-A have started to find links between them and toxic effects for humans. The main issue that has been discovered is that they possibly disrupt the human endocrine system [1,4]. The endocrine system controls hormone production in the body and if it is altered it cause harm to normal bodily function.
Some older epoxy-based resins have been found to cause abnormally high levels of leaching of BPA into food products. More recent epoxy resins have corrected this issue and studies on new coatings show that only around 0.5% of BPA found in human samples can be directly traced back to epoxy resins used in food containers. Research is currently focused on developing ways to produce alternative resins in cost and environmentally sustainable ways [2].
Even though epoxy-based resins are the most well rounded internal coatings in terms of functionality they are not necessarily the best. The possible human health issues they cause is a serious negative factor to consider in their use. The European Commission and FDA have recently stepped up their regulation of epoxy resins and some low molecular weight resins, like variations Bisphenol A Diglycidyl Ether (BADGE), have been outlawed in some areas of Europe [1]. When companies are deciding on which internal coating to use for a food product it is best practice to assess the requirements of the product and determine which coating best meets its needs.
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 American Coating Association (paint.org).
[4] "Food Contact Materials - Legislation" (retrieved April 2019) by European Commission.
