Quantitative Structural Analysis of Fat Crystal Networks by Means of Raman Confocal Imaging

Abstract

The techniques that are currently available to assess fat crystal networks are compromised with respect to invasive sample preparation and ability to quantify compositional and structural features. Raman confocal hyperspectral imaging coupled to analysis with multivariate curve resolution can address these bottlenecks, as it provides label‐free, noninvasive chemical information in three dimensions (3D). We demonstrate the ability to acquire compositional maps of dispersions of micronized fat crystals (MFC) in oil, which contain local concentrations of liquid oil and solid fat with submicron spatial resolution and with acquisition times in the order of 10 min. From the compositional maps, we can derive quantitative information on the size and porosity of fat crystal flocs, as well as the solid fat content of the embedding continuous phase. Furthermore, the fractal dimension of the fat crystal network could be determined from the compositional maps via the box‐counting method and via the porosities of the crystal flocs. This makes it feasible to assess the validity of the weak‐link network theory under industrial relevant conditions. The confocal imaging mode allows for straightforward acquisition of 3D compositional cubes by recording a stack of two‐dimensional (2D) images. The box‐counting fractal dimension analysis performed on 2D maps can be extended to 3D cubes, which allows for straightforward verification that MFC networks are self‐similar rather than self‐affine.

Oil Diffusion in Fat Crystal Matrices: Characterization by NMR Relaxometry and Diffusometry

“…This observation is in accordance with the weak‐link network of fractal fat crystal networks as shown by Martens et al. [ 32 ] and Nikolaeva et al. [ 32,33 ] Indeed, De Graef [ 34 ] observed a comparable morphological evolution during static crystallization and subsequent isothermal crystallization for palm oil.…”

“…Furthermore, Martens et al. [ 32 ] recently showed evidence of the presence of a crystalline phase in the oily inter‐spherulite space using Raman Confocal Imaging spectroscopy. Moreover, they showed that the spherical dense aggregates were in fact highly porous.…”

Networks of micronized fat crystals grown under static conditions

“…Full details of the experimental procedures can be found elsewhere. 33,34 In short, experiments were performed on a WITec confocal Raman (Alpha 300R+) microscope equipped with a Zeiss upright microscope with a 532 nm laser at 25.0 mW. The sample was placed on a 19°C water-cooled plate.…”

“…Fractal dimensions D were determined by the box-counting method, which assessed fractality with better precision compared to other approaches. 33 An image is overlaid with boxes of decreasing size, determining for each box size how many boxes contain ( part of ) a self-similar object. The relationship between the number of filled boxes (N b ) and the length of the boxes (l b ) provides the fractal dimension D:…”

“…We investigated the microstructure of MCF and MC dispersions by non-invasive confocal Raman imaging. Micron-scale compositional maps of solid fat 33,34 are shown in Fig. 2.…”