Thermal and tensile properties of corn semolina-protein blends in the glassy state

In the present work, the effect of adding whey protein concentrate (WPC) (3%, 6%, and 9%) on the tensile properties of corn semolina (CS) is studied. Differential Scanning Calorimetry measurements (DSC) were performed to determine glass transition temperatures (Tg) of specimens in a moisture content range of 2-25%. All materials are glassy at room temperature according to their Tg values regardless of their moisture content. Most specimens exhibit fragile behavior according to tensile tests, except for neat CS and CS-WPC-3%, with 25% moisture content, which display a fragile-ductile transition in the glassy state. Only small differences are observed among materials with different WPC contents, in terms of glass transition temperatures and mechanical properties. Nevertheless, our results indicate that WPC acts as a plasticizer for neat CS in terms of both, glass transition temperatures and mechanical behavior. Such plasticization effect is probably caused by Maillard reactions. Water, on the other hand, causes an anti-plasticization effect at low moisture contents, as revealed by increases in stress at break up to a maximum value. At higher moisture contents, water induces a plasticization effect and the stress at break decreases. Since no significant changes in strain at break are observed with moisture content, toughness values behave in a similar way to the stress at break exhibiting a maximum value at intermediate moisture contents.