Coupling of dielectric and mechanical relaxations with glass transition and stickiness of milk solids

Dielectric and mechanical α-relaxations of milk solids with varying milk protein contents were determined by dielectric (DEA) and dynamic-mechanical (DMA) analysis, respectively. The frequency dependence of α-relaxations occurring around and above the glass transition was modeled by the Vogel–Fulcher–Tamman (VTF) relationship. The α-relaxations were governed by the amorphous lactose and shifted to higher temperatures when protein contents of milk solids were increased at all water activities, aw. Increasing protein contents lowered the overall molecular motions of milk solid components which were shown by small changes in dielectric and mechanical properties above the glass transition. The α-relaxations were strongly frequency-dependent. At temperatures around and above the glass transition, the primary α processes were recorded and the corresponding relaxation times were determined. The temperature dependence of the relaxation times was affected by glass transition, solid composition and water. The relaxation times decreased with increasing temperature and water content as a result of thermal and water plasticization. Higher protein contents increased the relaxation times measured for milk solids. This was associated with small changes in α-relaxation characteristics and reduced powder stickiness. Data on the compositional effects on the relaxation times of milk solids were related to flow characteristics and powder stickiness around and above the glass transition.