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What's Under the Coat of Section 175.300?

The U.S. Food and Drug Administration's food additive regulations for indirect food additives are, in general, difficult to understand and daunting to interpret. Among the more complicated of the food additive regulations is the regulation that addresses "Resinous and polymeric coatings," found at 21 C.F.R. §175.300. However, an examination of the history of the enactment and an understanding of its context may provide substantial insight into FDA's regulation of coatings.

FDA's food additive regulations generally provide generic clearances for well-defined chemical substances, which are identified in the regulations by chemical name. In the case of coatings, however, the cleared substance typically is extremely complex, often cross-linked, and of no clearly defined chemical composition. Thus, under Section 175.300, FDA has resorted to listing the precursor materials as a means of defining the cleared product. It is the coating itself, however, not the method of manufacture, that is the subject of regulation.

Regulation Starts with Industry Wish-List

Interestingly, many of the reactants that are listed under Section 175.300 are not listed as a result of the typical food additive petition process. Shortly after the Food Additives Amendment of 1958 was added to the Federal Food, Drug, and Cosmetic Act (the Act), FDA surveyed industry to compile a list of substances that were being used to manufacture can coatings.

FDA was comfortable seeking industry input because the agency generally believed that these coatings were applied as a very thin layer over a metal substrate, such that there was only a small reservoir, if any, of the substance available to migrate to food. Additionally, at the time, can coatings were invariably baked on the metal substrate to form an enamel from which essentially nothing was expected to migrate to food.

These "unwritten" assumptions provided FDA with the necessary measures of safety it needed to allow industry to submit a list of substances that it desired to use in coatings. Furthermore, at the time the can coatings regulation was promulgated, FDA imposed a 50 parts per million (ppm) limit on the overall migration of non-volatiles from the coating to food (i.e., the "global" migration limit that is still in place today).

In response to FDA's survey, members of the Can Manufacturers Institute provided the agency with an omnibus petition containing a list of common materials used in can coatings. Rather than subject these substances to the usual substance-by-substance food additive petition review process, industry's "wish list" was simply reviewed by FDA's toxicologists, who deleted from the list any substance that posed a potential safety concern.

The revised list was then reviewed and agreed to by industry, and subsequently published as a regulation. Section 121.2514 (now designated as Section 175.300) became known and commonly referred to as the "can coatings" regulation, since the original drafters of the regulation were all members of the Can Manufacturers Institute. This may be a bit of a misnomer, since Section 175.300 does, in fact, permit coatings to be used on "any suitable substrate," provided the application of interest is limited to repeated-use. Single service applications, however, are restricted under Section 175.300 to metal substrates.

After the industry "wish list" was published, new substances were added to the coatings regulation pursuant to the required food additive petition process. Notably, as of January 2000, the food additive petition process was replaced with FDA's Food Contact Notification (FCN) program. Thus, currently, the appropriate regulatory vehicle for clearing coatings and other food-contact substances is via the filing of an FCN.

Other Considerations

The coatings regulation also anticipates the reaction of substances listed in the regulation pursuant to a permitted curing process for the finished coating. Specifically, Section 175.300(a)(1)(3) permits the use of coatings characterized by one or more of the following:

  1. cured by oxidation;
  2. cured by polymerization, condensation, and/or cross-linking without oxidation; and
  3. prepared from prepolymerized substances.

Accordingly, reaction products of substances listed in the coatings regulation are covered by the regulation for use in the coatings, provided the reaction products can properly be attributed to one of the permitted curing processes for the finished coating. This being the case, the use of pre-reacted components in the manufacture of coatings that would otherwise be produced during the manufacture of the finished coating also is permitted since this regulation is generic in nature (i.e., it does not require specific manufacturing procedures).

Section 175.300 also permits the use in coatings of substances that are generally recognized as safe (GRAS), prior sanctioned, or the subject of another food additive regulation, provided that the substance, when used in coating applications, complies with the specifications and limitations listed in the cross-referenced regulation.

For example, an antioxidant or stabilizer explicitly cleared under Section 178.2010 may be used as an antioxidant in coating applications under Section 175.300 because Section 178.2010 clears, broadly, antioxidants and/or stabilizers in food-contact applications. On the other hand, a defoamer listed under Section 176.200 may not be cross-referenced for use as a defoamer in can coating applications because the clearance for substances listed under Section 176.200 is limited to their use in the preparation and application of coatings on paper and paperboard. If, however, the application of interest is for repeated use, and the substrate to which the coating will be applied is made of paper or paperboard, then a substance listed under Section 176.200 may be cross-referenced to Section 175.300 (as discussed above, Section 175.300 also clears coatings applied to "any suitable substrate" in repeated-use applications). Of course, in that case, any other limitations stated under Section 176.200 would also have to be met.

Even if a substance may be said to be cleared under Section 175.300, the material, like all substances used in food-contact applications, must be of a purity suitable for the intended use. This requirement flows from Section 402 of the Act, which states that a food is adulterated if it contains an added substance that may render the food injurious to health or otherwise causes the food to be unfit for consumption.

Section 174.5 ("General provisions applicable to indirect food additives") of the food additive regulations implements this as a good manufacturing practice (GMP) requirement when it states that "[a]ny substance used as a component of articles that contact food shall be of a purity suitable for its intended use." The regulation restates a well-established FDA requirement that any material that is not of suitable purity is unacceptable for use in contact with food, even if the material otherwise has a suitable regulatory status.

Thus, an explicit listing under Section 175.300, the existence of a prior sanction, or determination that a compound is GRAS, does not satisfy all of the requirements under the Act. To fully comply with the Act, the substance must be of suitable purity for its intended use.

In addition to reactants, solvents frequently are used in the manufacture of coatings. Solvents are particularly common because they allow a coating to be applied as an emulsion. FDA, however, typically does not regulate solvents used in polymeric formulations, mainly because such substances, as well as other production aids, due to their generally volatile nature, are not expected to be present in the finished product at levels that are "significant" from a public health and safety standpoint.

As discussed above, however, it is necessary to establish that use of a solvent (or any other food-contact material or component) complies with FDA's GMP requirements in that the finished product is suitably pure for its intended use. In the case of solvents in coating applications, it is important to determine the suitable purity of the finished coating with which the solvents will be used. Thus, it is necessary to assess the potential exposure to the solvent from its intended use, and to ensure that the potential dietary exposure, based on publicly available information on the solvent's toxicity, is low enough to be considered safe.

In most cases, the solvent will volatilize away during the curing process and, thus, not be present in the finished coating. In some cases, though, there is the potential for low residual levels of the solvent to be present; therefore, it is important to ensure these levels are safe. One way to determine whether the solvent will volatilize away is to compare the solvent's boiling point to the coating's curing temperature. Oftentimes, the curing temperature will exceed the solvent's boiling point and, thus, the solvent will either be fully removed or present at such low levels that it would not present a safety concern. The safety of these levels, however, must be evaluated on a case-by-case basis.

Finally, it is important to remember that the finished coating must meet the end tests set forth under Sections 175.300(c)-(e). This includes, as discussed above, the 0.5 mg/in2 (or 50 ppm) of food-contact surface area overall migration limit of total non-volatiles from the coating in its finished form to food. While companies are free to create their own tests, the testing must yield results equivalent to those listed in the regulations.