Shellac

Shellac is a natural resin secreted by the lac insect, Kerria lacca, and refined into flakes or into pharmaceutical glaze, a solution in ethanol. It is one of the oldest enteric materials in use: the free resin is insoluble in acid and dissolves in the alkaline conditions of the small intestine, so a shellac film will hold in the stomach and release beyond it. India is one of the principal producers of the raw resin. Shellac appears in the USP-NF as Shellac and as Pharmaceutical Glaze, and as E904 where it is used as a glazing agent.

Shellac forms a hard, glossy, acid-resistant film that is also a competent moisture barrier. Its weakness is well documented and it is a storage weakness rather than a coating one: the resin continues to polymerise and esterify after the film is laid down, so dissolution time lengthens as the coated tablet ages. A batch that met its gastro-resistance and release specification at manufacture can fail the same test months later without anything having been done to it.

Where a natural, non-synthetic material is a requirement of the brief rather than a preference: nutraceutical, ayurvedic and herbal products, and confectionery-style glazing, where shelf life is shorter and dissolution is not a release specification. For a pharmaceutical enteric coat held to a pharmacopoeial gastro-resistance and dissolution test across a two or three year shelf life, a methacrylic acid copolymer or a cellulosic enteric polymer is the sounder choice, because their release behaviour does not drift on storage in the same way.

Applied from an alcoholic solution rather than from water, so the run is a non-aqueous one with the solvent handling, recovery and flameproofing that implies. Bed temperatures are kept modest to avoid driving the film hard, and the coat is not over-dried, since heat accelerates the ageing the material is already prone to.