Rheo-NMR to investigate fat crystallization under shear

Abstract

It is well known that shear has an effect on fat crystallization. Whereas rheo-NMR has been used to study the impact of shear on the crystallization kinetics in the past, these methods mostly used a simple Teflon mixing shaft inside a sophisticated NMR instrument to apply shear to the sample. However, this method did not enable the determination of rheological parameters. In this work, a custom made low-field rheo-NMR device was evaluated, consisting of a commercial rheometer combined with a low-field permanent magnet to enable simultaneous rheological and NMR measurements. Two fats, i.e. partially hardened sunflower oil (PHSO) and soft palm mid fraction (sPMF), were submitted to several rheo-NMR experiments. The results of these experiments clearly indicated that these fats crystallized differently. First, PHSO crystallized faster than sPMF. Moreover, the latter seemed to crystallize in two steps. Initially a weak structure was formed when a low amount of solids was present, but this structure was replaced by a stronger network once more crystals were present. Both fats were studied under stagnant conditions, but also when submitted to low shear rates (1 s −1 and 5 s −1 ). It was shown that the amount of solids necessary to obtain a viscosity of 10 ​Pa ​s was higher when the shear rate was higher. The strength of the formed crystal network at a given percentage of solids was also weaker as the shear rate during crystallization increased. Whereas these experiments were done non-isothermally, it was shown that rheo-NMR can also perfectly be used for isothermal measurements.

Rheology of Gels and Yielding Liquids

“…Since gels are used in such a multiplicity of applications, it is crucial to correctly characterize the rheological behavior of these soft matter, bearing in mind that their properties are mainly determined by their microstructure. Various methods are used to study gel structures in order to explain their rheology, such as small-angle neutron scattering [ 31 ], rheo-NMR [ 32 ], fluorescence measurement [ 33 ], light-scattering [ 34 ], atomistic simulation [ 35 ], etc.…”

Interplay of Polymorphic Transition and Mixed Crystal Formation in Model Fat Systems

“…Breitschuh and Flöter have argued that multiple crystallization steps indicate the solid immiscibility of AMF triglyceride fractions . For both, PO and AMF, a two-step process was visible when shear was applied. − It is generally agreed that the applied shear facilitates the transformation from α to β′. − …”