Abstract
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 ΦSFC is investigated by in-line ultrasound velocity profiling – pressure difference (UVP-PD) rheometry. Measurements up to ΦSFC = 8.8% are presented. Below the percolation threshold Φc, no yield stress τ0 is observed and the viscosity η scales linearly with ΦSFC. Above Φc, a non-linear dependency of both τ0 and η as function of ΦSFC is apparent. For AMF and CB, the increase in cryst leads to a decrease in η and τ0 as function of ΦSFC, 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 aeff 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.
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