Influence of glycerol on the molecular mobility, oxygen permeability and microstructure of amorphous zein films

The effects of glycerol on molecular mobility, oxygen diffusion and microstructure in amorphous zein matrix were studied using phosphorescence and atomic force microscopy (AFM). Films containing various amounts of glycerol (0, 5, 10, 20 and 30 wt%) were cast from solutions of 0.5% (w/v) zein in 70% ethanol/water (v/v). Erythrosin B (Ery B) phosphorescence was used to monitor the molecular mobility of these matrices over the temperature range from 0 to 100 °C. Analysis of Ery B emission peak frequency and bandwidth and intensity decay provided information about thermally-activated modes of molecular mobility in the matrices. Dipolar relaxation around the triplet state of Ery B was weakened and the extent of relaxation was decreased at low concentration glycerol (≤10%), indicating a role as antiplasticizer. The rate of non-radiative decay from the Ery B triplet state indicated that glycerol only performed as a plasticizer and increased the local mobility of the zein matrix at and above ∼20 wt %, while in films with glycerol at ≤20 wt % the local mobility remained nearly constant or only slightly increased compared with pure zein. Though transitioning from antiplasticizer to plasticizer at higher content, glycerol dramatically suppressed the oxygen permeability of the film in the whole concentration range tested. AFM images indicated that glycerol induced aggregation of zein complexes. These results indicate how the addition of glycerol to zein films could affect the physical properties, structure and thus functional properties in ways that influence their eventual use.