Effects of cooling temperature profiles on the monoglycerides oleogel properties: A rheo-microscopy study

The oleogelation process has become in a great interest area for the food sector. The aim of this study was to understand the effect of cooling temperature profiles (CTP) applied during oleogelation on microstructure and some macroscopic properties of monoglycerides (MG) oleogels. To this purpose, oleogels from MG and high oleic sunflower oil were produced using programed CTP corresponding to the actual temperature evolution of the samples when they are left at rest to progress in a specific ambient temperature (AT). In order to evaluate the crystal formation during the gelation process, a torsional rheometer equipped with a rheo-microscope (RM) module was used. This allowed us to carry out simultaneously rheological measurements and record images of the gels during their formation process. Overall, microstructural characteristics were determined: fractions of crystalline material and oil, crystal length and shape, the Avrami index, and the fractal dimension. Although crystal formation took place during a similar range of temperatures (~55-46 °C), significant morphological differences in the distribution and size of crystal and aggregates were observed depending on the applied CTP, and the area occupied by the crystals and oil phase did not depend on CTP used. RM images were useful to follow the kinetics of crystallization as well as to identify a more restricted time domain in the rheological behavior allowing to find more accurate Avrami index values. Furthermore, the analysis of RM images turned out to be an efficient approach to obtain accurate measurements of the fractal dimension. High fractal dimension values were associated with gels exhibiting high number of homogeneous small crystals. Oleogels composed by this network generated a material with high capacity to retain oil. A weak-link regime approach applied to the dynamic systems was appropriate to describe the relationship between the elastic modulus and the crystal formation during the oleogels structuration. In conclusion, these findings may serve to the food industry to achieve a better understanding of the oleogelation process that allows it to control the quality of obtained oleogels, which could be utilized to replace and/or reduce the trans and saturated fats in food formulations.