Controlling lipid crystallization across multiple length scales by directed shear flow.

The crystallization behavior of lipids is relevant in many fields such as adipose tissue formation and regeneration, forensic investigations and food production. Using a lipid model system composed of triacylglycerols, we study the formation of crystalline structures under laminar shear flows across various length scales by polarized light-, scanning electron-, and atomic force microscopy, as well as laser diffraction spectroscopy. The shear rate during crystallization γ̇cryst influences the acyl-chain length structure and promotes domain growth into the flow direction thereby transforming the crystallites from oblate into prolate particles. Concentration dependent aggregation of crystallites into clusters is the rate limiting step for floc and floc network formation. At high γ̇cryst, fast crystallite cluster formation at smaller equilibrium diameters is promoted. The high crystallite cluster concentration induces their aggregation into flocs which form weak networks. At low γ̇cryst, floc generation is limited by the low amount of crystallite clusters leading to slow growth of larger flocs and forming of strong networks. The findings in this work have potential implications ranging from the design of injectable soft tissue fillers for adipose tissue regeneration, to the crystalline network formation in microorganism derived lipids, up to a more energy-efficient production of chocolate confectionery.

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 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.