Gloss and transparency of hydroxypropyl methylcellulose films containing surfactants as affected by their microstructure

Optical (gloss and transparency) and microstructural properties of nine edible films prepared with hydroxypropyl methylcellulose (HPMC) and surfactant mixtures (Span 60 and Sucrose ester P-1570) with different hydrophilic-lipophilic balance (HLB) were evaluated. Gloss was measured at 20, 60 and 85° using a gloss meter and transparency was evaluated through Kubelka-Munk coefficients obtained from film reflection spectra (400-700 nm). Film microstructure was analysed using light microscopy (LM), atomic force microscopy (AFM) and scanning electron microscopy. Fractal analysis was applied to the microscopy images to quantify the film microstructure complexity. Film transparency and gloss increased as the hydrocolloid-surfactant ratio (H/S) increased. The greatest HLB values gave rise to the lowest film transparency, but increased gloss. A great impact of the film microstructure (internal and surface arrangement of the different phases) on film optical properties was observed. The complexity of the developed structures showed a self-similarity in the analysed scale. Apparent fractal dimensions from LM and AFM images were well correlated with transparency and gloss parameters, respectively.