Quantification of crystallinity and rheological properties of coconut fat via a unique and novel in-situ combination of rheology and 1H NMR

Abstract

Crystallization of fat has received much interest in research over the last decades due to the effect of microstructure on physical, specifically mechanical properties [1]–[4]. Manufacturing of the fat-based product consists of a controlled interference to preserve, alter or destroy a microstructure during processing [3]. During processing, various parameters change at the same time e.g. cooling rate and applied shear and elongation deformation. These have a large impact on the fat crystallization behavior such as polymorphic transitions, oil migration, fat bloom development, aggregation of crystals, differing crystal size, and solid fat content [5], [6]. Correlating the influences of processing parameters on fat crystallization is advantageous for scientists to understand and predict the principles for the fat and food industry, to control final product characteristics: the stability of the processed food, and thus, safe, diversified, convenient, and good quality fat products are produced [5]. Advanced characterization techniques provide a major contribution to the engineering sciences in fat research, with an investigation in micron length scales, such as those found in self-assembly colloidal structures, crystal networks, crystals of many types, and interfaces. These micron-sized (1 -10 µm) identities together with the solid fat content (SFC) are critically involved in the final mechanical properties, rheological and processing behavior, textural and sensory properties of food [3]. These properties can be altered by varying shear flow and/or temperature profiles during crystallization to attain customer needs. Commonly used characterization techniques to investigate fat crystal structure formation and their fractal properties are microscopy techniques e.g. polarized light microscope, environmental scanning electron microscopy (ESEM), and other physical methods e.g., X-ray diffraction (XRD), differential scanning calorimetry (DSC), nuclear magnetic resonance (NMR), light diffraction, and rheology.[3], [5] All of the mentioned methods provide quantitative analysis. Nevertheless, a direct correlation of the investigated properties for different methods is not possible due to the nonidentical calibration (shear rate, cooling rates, etc.). Therefore, multidisciplinary approaches have been used for better understanding the impact of processing conditions on the fat microstructure. Mazzanti et al. [7], [8] used a Couette shear cell with the combination of XRD to study shear-induced crystallization of cocoa butter, milk fat, stripped milk fat, and palm oil. They observed accelerated crystallization and oriented crystals along the shear direction by applied shear. For their later study [9] they used Rheo-NMR, comprising a 10 mm NMR tube with a Teflon shaft, creating a mini Couette cell, to investigate the SFC and its impact on crystallized fat properties. They used the free induction decay (FID) signal to determine the SFC of the blends of canola stearin (CS) in canola oil (CO) under different shear rates (58 -460 1/s), however, did not see a significant difference between sheared and nonsheared samples. In previous studies, shear was applied within the NMR spectrometer, while rheological parameters were not determined. The studies in the past twenty years conducted by Rheo-NMR setups rely dominantly on the NMR part to determine SFC and simplified rheology part with the application of steady shear flow via Couettegeometries for studying limited to low viscosity complex fluids [10, 11] without quantification of the steady-state viscosity or the complex viscosity under oscillatory deformation. Therefore, a reliable, robust, fast method to determine the SFC correlation with rheological quantities under applied large deformation would be desirable. Only very recently the newly developed low-field RheoNMR setup simultaneously measures the full rheological shear characterization allowing for the application of large amplitude oscillatory deformation “LAOS” and the related data analysis via I3/1 [12], FT Rheology [13], [14]) etc. via a high end rheometer and the quantitative molecular mobility by 1H NMR [15]–[18] and T2 relaxation time determination. Thus, a precise investigation of sample crystallization under identical and identified conditions has become possible with this unique Rheo-NMR setup. Additionally, this technique provides a correlation between the time evolution of the sample’s macroscopic material properties and microscopic molecular dynamics under applied nonlinear deformation. To demonstrate the capabilities of this new low-field RheoNMR technique, coconut fat was chosen as a model substance for a fat isothermal crystallization kinetic study. Although coconut fat is extensively used in several applications from food to industrial purposes, the kinetic study of coconut oil crystallization has rarely been published. The easy accessibility of coconut fat also makes it favorable for this study.