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 [italic small gamma, Greek, dot above](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 τ(0) is observed and the viscosity η scales linearly with Φ(SFC). Above Φ(c), a non-linear dependency of both τ(0) and η as function of Φ(SFC) is apparent. For AMF and CB, the increase in [italic small gamma, Greek, dot above](cryst) leads to a decrease in η and τ(0) 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 Φ(eff)(SFC) 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.