Characterisation of zein-oleic acid films and applications in fruit coating

Pure zein films are known to be very hydrophobic, but are inappropriate for edible coating applications because of their brittle nature. In an attempt to improve the flexibility of these coatings, the influence of low concentrations of oleic acid (OA) as a plasticiser on mechanical, topographical and wetting properties of zein thin films was evaluated. Films were first obtained by casting from aqueous ethanol solutions with 4.0% in mass of zein and additions of 0%; 0.25%; 0.50%; and 1.0% (w/w) of OA. The results indicate an improvement in mechanical properties with increasing plasticiser leading to a reduction in the elastic modulus. An increase in the elongation at break has been observed, but with minor influence on tensile strength. All plasticised zein films have similar initial contact angle (approximately 69 degrees) with a time-dependent receding drop behaviour. An increase in plasticiser concentration increases film's affinity towards water. As measured by atomic force microscopy, a consistent linear relation (R-2 = 0.991) was estimated between film composition and surface adhesion and consequently on the hydrophilicity. Surface topography also varied with plasticiser addition, becoming smoother as the OA concentration increases. When tested as an edible coating on pears (Pyrus communis L.), a formulation with 0.25% wt of plasticiser achieved the best results in preserving the pear mass as measured during the evaluated storage time (12 days) at room temperature. A 0.5% concentration of plasticiser had no influence and higher amounts resulted in a reduction in fruit protection.