The rheology and foamability of crystal-melt suspensions composed of triacylglycerols

The rheology of triacylglycerol (TAG) crystal-melt suspensions (CMSs) consisting of anhydrous milk fat (AMF), cocoa butter (CB), and palm kernel oil (PKO) as function of crystallization shear rate _cryst and crystal volume fraction Φ_SFC is investigated by in-line ultrasound velocity profiling - pressure difference (UVP-PD) rheometry. Measurements up to Φ_SFC = 8.8% are presented. Below the percolation threshold Φ_c, no yield stress τ₀ is observed and the viscosity η scales linearly with Φ_SFC. Above Φ_c, a non-linear dependency of both τ₀ and η as function of Φ_SFC is apparent. For AMF and CB, the increase in _cryst leads to a decrease in η and τ₀ as function of Φ_SFC, whereas for PKO based CMSs the opposite is the case. Scanning electron microscopy (SEM) and polarized light microscopy (PLM) relate these rheological findings to the microstructure of the investigated CMSs by taking the effective aspect ratio a_eff and the concept of the effective crystal volume fraction ΦeffSFC into account. Foam formation by dynamically enhanced membrane foaming (DEMF) is performed directly after crystallization and reveals that depending on the CMS rheology and crystallite-, crystallite cluster- and crystal floc microstructure, a wide range of gas volume fractions between 0.05-0.6 are achievable.