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.