INTRODUCTION
Oleogels have proved to be an important alternative for the replacement of hydrogenated fat in food matrices in recent years, as well as vehicles with good stability in the release of nutraceuticals (Puşcaş et al., 2020). The positive impact on improving the nutritional and health profile as well as some other applications have already been addressed and discussed extensively (Alvarez et al., 2021). Oleogels can be defined as ”pseudoplastic” and thermoreversible semi-solid systems composed of a liquid phase (organic solvent) that is immobilized by a three-dimensional network formed from the self-assembly of gelling molecules (Zeng et al., 2021). The three-dimensional network in physical oleogels is formed by supramolecular self-assembly that is governed by non-covalent interactions such as hydrogen bonds, π-π stacking, Van der Waals interactions, and electrostatic interactions. Obtaining oleogels with low molecular weight molecules, such as monoglycerides, and governed by the aforementioned interactions, are known as physical gels. The obtaining of oleogels starts by solubilizing the gelling agent in a heated organic solvent, so that once the maximum solubility of gelling agent-solvent is exceeded, the system is cooled to a temperature below the solubility limit (Krafft temperature). This starts a reorganization of the gelling molecules leading to a nucleation process (Li & Liu, 2010). Once nucleation occurs in the oleogels, a process of crystallization and growth of structures by the gelling agent begins. This process resembles the crystallization of fat (Bayés-García et al., 2015). The crystalline structures that develop from the newly formed nuclei arise from a series of dimensionality patterns and crystal growth geometry. Transition temperatures, gelation and final properties of oleogels depend not only on the nature of the organic solvent and gelling agent, but also on the cooling rate and storage conditions,i.e ., they depend on different thermal and kinetic parameters (Ashkar et al., 2019). Nucleation and dimensionality parameters of crystal growth during oleogelation are important factors in determining the microstructure and crystallinity properties in oleogels. The initial crystallization (microstructuring) process of oleogels can be studied from a fully melted system by minitoring their thermal and kinetic properties during crystallization, as well as the resulting microstructure. This crystallization process is currently evaluated at the macroscopic level by thermal techniques such as differential scanning calorimetry (DSC), which allows to obtain clear crystallization profiles of oleogels (Puşcaş et al., 2021). Microstructure on the other hand can be evaluated by almost any microscopy equipment that allows the identification of crystalline structures. However, the use of this highly efficient and most common equipment in laboratories studying oleogel systems focuses mainly on the crystallization process and the final crystal structure. This leaves the initial stage of the microstructuring process ”nucleation” unable addressed during the characterization of new oleogel systems. However, spectrophotometric techniques give the possibility to obtain an absorbance response proportional to the solute concentration and solid formation.
The Avrami equation is used to model crystallization kinetics under isothermal conditions, however, it can be accommodated within non-isothermal evaluations (Rogers & Marangoni, 2009). Experimental reality limits the rate at which a system reaches a set crystallization temperature, and at the industrial level in the development of edible fat products, crystallization takes place under non-isothermal conditions. These conditions imply having considerations on the cooling rate that have already been well addressed by Marangoni et al. (2017),. The Lambert-Beer law states that the absorbance is directly related to the intrinsic properties of the analyte, to its concentration and to the path length of the radiation beam as it passes through the sample. Toro-Vázquez et al. (2000), indicate that the birefringence of the oleogel crystals can influence the results obtained by spectrophotometry in comparison with those obtained by DSC. This is because heterogeneous, sporadic nucleation and secondary crystallization are more as time progresses, so the amount of crystals is not constant with time. Therefore, the starting point of ”microstructuring” could be studied by non-isothermal nucleation kinetics by spectrophotometry, which is outside the limits of DSC. This would allow a broader perspective of the oleogelation process. Therefore, the objective of the present work was to perform a comparison in the study of the initial microstructuring process of oleogels, using non-isothermal nucleation kinetics by spectrophotometry versus a DSC analysis. Identifying the scopes and limitations of the different techniques.