Dynamic mechanical analysis of food powders: Direct determination of the mechanical properties and glass transitions in native faba bean meal at different hydration states

The liberation of protein bodies and starch granules as the compartments of legume cotyledon cells during milling is a crucial prerequisite for the dry fractionation process by air classification or triboelectric separation. Besides the intensity and type of the acting forces, the mechanical properties of the legumes’ compartments determine their fracture behavior. Selective comminution aims to control the crack propagation in a system by exploiting differences in its components’ mechanical properties. Since protein bodies and starch granules are known to show the behavior of amorphous polymer networks, their physical state influences their mechanical properties. Thus, a method based on dynamic mechanical analysis (DMA) for the simultaneous determination of the thermo-mechanical behavior of faba bean meal and physical transitions of the meal’s protein and starch fractions is developed. The DMA method is validated by comparisons with DSC measurements for lactose as a model system, and the influence of the applied strain amplitude and frequency on the measured moduli is evaluated. DMA measurements with amorphous and partly-crystalline lactose and faba bean meal revealed DMA’s high sensitivity for changes in the mechanical behavior of amorphous samples during their glass transition. Therefore, the detection of glass transitions in complex and multi-component systems is possible by the evaluation of the samples’ mechanical characteristics. Furthermore, the glass transition temperatures (T g ) of faba beans’ protein and starch fractions are determined simultaneously in their native environment. For both fractions, a decrease in T g is shown with increasing moisture content, whereat the plasticizing effect of water is stronger for the faba bean’s protein fraction. The established state diagram enables the link between the beans’ physical state and their mechanical properties. It is shown that certain temperature-moisture combinations lead to different physical states of the faba bean’s protein and starch fraction, assuming a differing fracture behavior during milling and possibly enhancing selective comminution and liberation of the fractions.