In two research publications appearing in February and March 2016, Rafael Paseiro-Cerrato and colleagues from the Center for Food Safety and Applied Nutrition of the U.S. Food and Drug Administration (FDA) report on the identification of oligomers migrating from polyester-based metal can coating, and characterize the long-term migration of monomers and oligomers from this type of coating.
Several alternative coatings for metal cans have been introduced on the market, aiming to replace those based on epoxy resins containing bisphenol A (BPA, CAS 80-05-7). Along with polyvynilchloride (PVC)-based coating (FPF reported), polyester coatings are frequently used. They are manufactured by reacting carboxylic acids and alcohols, using substances approved for use in food packaging. However, the resulting material may also contain non-intentionally added substances (NIAS), such as oligomers. Their identity, as well as their potential migration into food, remain poorly characterized.
In an article published on March 16, 2016 in the peer-reviewed Journal of Chromatography A, the substances potentially migrating from polyester can coating were analyzed with a variety of mass spectrometry (MS)-based techniques. The cans for analysis were obtained directly from an unspecified manufacturer. Cans were extracted for 24 hours with acetonitrile at 40°C or with ethyl acetate at 20°C; the former condition was found to be superior in terms of the number of compounds and intensity of responses.
Twenty nine non-volatile oligomers, including 22 linear and 7 cyclic ones, could be identified tentatively. Of these compounds, all but one expectedly corresponded to the reaction of carboxylic acids and diols. In addition, several unknown oligomers with high MS signal were observed. The proposed identity of the oligomers suggested that the analyzed material was a polyester coating based on the monomers isophthalic acid (CAS 121-91-5), terephthalic acid (CAS 100-21-0), and nadic acid (CAS 3853-88-1). In the U.S., these compounds are approved as components of polyester can coating. The EU does not have a specific list for substances allowed for use in coatings intended for food contact. In the Union list for plastics, both isophthalic and terephthalic acids are listed as monomers, and salts of nadic acid are allowed as plastics additive.
The second article, published on February 25, 2016 in the peer-reviewed Journal of Agricultural and Food Chemistry, describes the long-term migration of the three monomers and thirteen oligomers identified in the study described above. To simulate high temperature processing and long-term storage of products such as metal cans, FDA guidance recommends a migration test with a retort step for food sterilization at 121°C for 2 hours followed by 10 days at 40°C. However, it is not known whether this short-term testing accurately simulates migration over longer time periods, which are typical for many canned foods.
To investigate this, FDA scientists tested migration from retorted (i.e. heat-sterilized) and non-retorted polyester-coated cans into several food simulants, water, 3% acetic acid, 10% ethanol, 50% ethanol, and isooctane. The cans were stored at 40°C for a period from 1 day to 515 days. After long storage times in alcoholic food simulants, an increase in several migrants, including monomers and low molecular oligomers, was observed. As this was accompanied by a decrease in some high molecular weight oligomers, the authors hypothesized that these substances could be subject to slow hydrolysis, yielding compounds with lower molecular weight. These observations indicate that a revision of FDA migration protocols may be needed in order to enable capturing the changes in migrants occurring during long-term storage. The authors indicate that, before such a revision may be initiated, similar analyses of long-term migration should be carried out in real foods.
Paseiro-Cerrato, R. et al. (2016). “Identification of unknown compounds from polyester can coatings that may potentially migrate into food or food simulants.” Journal of Chromatography A (published online March 16, 2016).
Paseiro-Cerrato, R. et al. (2016). “Evaluation of long-term migration testing from can coatings into food simulants: Polyester coatings.” Journal of Agricultural and Food Chemistry 64:2377-2385.