Formulation of zein coatings for apples (Malus domestica Borkh)

High gloss coatings are used to improve apple fruit (Malus domestica, Borkh) appearance and sales. The industry standard has been shellac-based formulations, which have problems with whitening, low gas permeability, and association with non-food uses. Zein, a natural corn protein, was used to formulate alternative, shiny coatings by dissolving zein in aqueous alcohol with propylene glycol (PG). Gloss levels on ‘Gala’ apple surfaces varied due to zein and PG content in coating formulations from that of controls to levels observed for shellac-coated fruit. At least 4% (by weight) PG was necessary for adequate gloss. However, increasing levels of both compounds resulted in increased gloss. Whitening, which occurred on the coated fruit surface upon wetting, was reduced by decreasing zein content to less than 11%. Permeability to CO2, O2, and water vapor was strongly dependent on the zein content in the coating. Internal CO2 and O2 in zein-coated ‘Gala’ fruit ranged 4–11 and 19–6 kPa, respectively, by increasing zein content in the coatings. An optimum formulation with 10% zein and 10% PG was developed, applied to ‘Gala’ apple, and was found to maintain overall fruit quality comparable to a commercial shellac coating.