Effect of two different lipid sources on glass transition temperatures and tensile properties of corn semolina

Abstract

The effect of adding corn oil (CO) and corn germ (CG) on the thermal and mechanical properties of corn semolina (CS) was investigated. The lipids concentration employed were 2% and 3% of CO or CG. The thermal transitions were studied in a moisture content range of 3–27% (d.b.), by means of dynamic mechanical thermal analysis (DMTA) and differential scanning calorimetry (DSC) techniques. Wide angle X-ray scattering (WAXS) experiments showed that type-V amylose–lipid complexes were formed with the native lipids within the corn semolina during thermo-moulding and that re-crystallization to a type-A structure, favored by the added lipids, occurred upon storage in high relative humidity (RH) environments. Two thermal events were observed during the first DSC scan and tanδ DMTA curves. The first one appearing at around 55–60°C was attributed to a possible phase separation or ageing phenomenon taking place during storage of the samples, while the second was ascribed to the glass transition temperature of CS. It was found that addition of both, CO and CG, reduced the moisture content required for these transitions to appear. The expected decrease in the glass transition temperatures with moisture content was well described by the Gordon–Taylor equation. The addition of CO and CG also causes plasticization of CS. Hence, corn germ meal can be used as a lipid source that is more convenient to handle than liquid fats. However, the CS Tg values were independent of the lipids content. Tensile properties of CS and CS-lipid systems were found to be influenced by the moisture content, the physical state of the samples, and their composition.