Pectin/starch/glycerol films: Blends or composites?

The miscibility of pectin (P), starch (S), and glycerol (G) in films was investigated by dynamic mechanical analysis (DMA), tensile measurements, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), and film dissolution studies. DMA revealed that P/S/G films exhibit up to three thermodynamic transitions prior to film degradation when heated. Transitions appear to be induced or enhanced by the addition of glycerol. The lowest transition occurred below -50 degrees C, the second above room temperature and below 100 degrees C, and the third above 100 degrees C. DMA and SEM on P/S/G films containing starch solubilized for various lengths of time revealed that a tenfold difference in the particle size of starch produced no more than a 20% difference in the dynamic moduli. Tensile measurements revealed that changes in glycerol content or relative humidity induced greater changes in mechanical properties than time of starch solubilization (gelatinization). FT-IR spectra of P/S/G films revealed that the microstructure around ester and carboxylic acid side chains of pectin was changed significantly by the level of glycerol and relatively little by varying the starch level. Results from dissolution studies were consistent with the idea that P/S/G films are highly compatible composites of starch in pectin.