Polymeric Coatings for Metal Food Packaging

Metal boxes for food preservation have a long history but, if in the past, they had deficiencies from the hygienic and toxicological point, especially in because of the welds that were made in Sn-Pb alloy, currently the quality of the manufactured products are much better.

Today the protection of food is mainly entrusted to the polymeric layer of inner coating , called coating , which stands between the metal wall and the contained food.

The primary function of this barrier is to protect food products from light, oxygen, enzymes, humidity, pollutants and microorganisms that would result in the modification of the structure of the food and its quality.

The aim is also to increase the useful life of the food or the drink which in normal, that is, not canned, it would deteriorate more quickly, as the biochemical and enzymatic reactions and the activity of microorganisms would normally run their course.

Therefore, to increase the life of the food, the metal packages are normally coated with synthetic resin film applied to the metal sheet again flat, film with a thickness of a few microns.

The choice of the type of resin depends on its mechanical, chemical or thermal characteristics based on the content they must host. Below we can list the main ones:

• Rosin consists mainly of abietic acid, which is normally added with ZnO to control the chemical reactions that are formed through the sulfur amino acids of proteins.

• Vinyl resins are from the family of thermoplastic resins, normally PVC, which have excellent resistance to acids, but have the defect to absorb food pigments.

• Phenolic resins are composed through the polymerization of formaldehyde and phenol which have excellent resistance to heat treatments, PH and to fats. Through the formaldehyde content we can identify two families of phenolic resins: Novolacche (thermoplastic) and Resoli (thermosetting).

• Epoxy Resins are thermosetting resins consisting of bisphenol A and epichlorohydrin which are the most common coating in canned foods, especially in fish-based foods in oil.

• Polyester resins are thermosetting resins obtained from different monomers such as phthalic anhydride, maleic anhydride or fumaric acid, integrated with vegetable oils and pigments. They have the characteristic of flexibility giving this characteristic to the metal wall layer.

• Epoxy-Phenolic Resins are the result of the polymerization of epoxy resins with phenolic ones through catalysts. They are used as a transparent coating for many metal cans that contain oil, vegetable or pet food preserves.

As regards the toxicological characteristics there are specific legal regulations that place limits on the possible migration of packaging substances into food, in which both specific migration and global migration are considered.

However, the scientific community has given new impetus to studies and research on the toxicological aspects relating to plastics used in the food industry, with particular attention no longer to the single element that constitutes the packaging, but takes into consideration the cocktail effect that is given by all the elements that come into contact with food translated over time and with different thermal characteristics.

Undoubtedly the food or drink contained in the packaging at the time of packaging has certain characteristics, but over time and in different climatic conditions , the quality of the food that arrives on the table could be different.

Therefore it would be advisable to verify it through a chemical analysis, on a sample, with an instrument composed of a gas chromatograph and a spectrometer ion mobility that, in a simple and rapid way, will give the photograph, analytical, of the quality of food or drinks.

Automatic translation. We apologize for any inaccuracies. Original article in Italian.