Microstructure and optical properties of sodium caseinate films containing oleic acid–beeswax mixtures

The effect of internal and surface structure of films obtained from sodium caseinate (NaCas), containing glycerol and lipid mixtures of oleic acid (OA) and beeswax (BW) (1:0.3:0.5 protein:glycerol:lipid ratio) on optical properties (gloss and transparency) was studied. Gloss was measured at different angles of the incident light (20, 60 and 85°) using a gloss meter, and transparency was evaluated through Kubelka–Munt coefficients obtained from the film reflection spectra (400–700 nm). Film structure was analyzed by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Control films (without lipid) exhibited the greatest transparency and gloss whereas lipid imparts opacity and loss of gloss to the film, especially as beeswax increased in the lipid mixture, due to the formation of greatest lipid aggregates in the internal and surface parts of the film. Good correlation was established among gloss, transparency and surface roughness parameters of the films.