Avrami Parameters
Table 4 shows the parameters obtained with the Avrami model for the
different oleogels (R2>0.9). It was found
that all the systems present a linear growth (1-2) according to the
values of n , with a one-dimensional growth geometry (Toro-Vázquez
et al., 2000). A non-integer value indicates the formation of
irregularly shaped crystals. This aforementioned growth is
characteristic of fibrillar-type structures found in monoglyceride
systems. Fractional values have already been explained in other
investigations where they are attributed to the formation of structures
from different types of nuclei (heterogeneous nucleation)
(García-Andrade et al., 2020). It was found that values of ngreater than 1 correspond to systems with lower MY concentration (Table
4). This can be understood by taking into consideration what was
observed in Table 3, where oleogels with lower MY content showed longer
nucleation times and consequently slower growth or branching times (z).
A slower growth rate allows an almost two-dimensional growth due to a
greater distance between the already formed nuclei and the freedom they
present in the transfer of energy and interaction to form such
structures. The concentration of structuring agents is a determining
factor for the growth rate in oleogel systems.
Figure 5 shows the time evolution of the crystalline mass (F × 100), for
different MY concentrations in CA and CN oleogels. The curves followed
the same general sigmoidal trend of behavior, with a time difference of
≈4min, the crystallization rate increased beyond the TCSvalue until reaching zero crystallization rate at F × 100 = 100.
The times where the increase from the baseline is presented corresponded
to the TCS values shown in Table 2. The times obtained
by DSC (tSC~TSC)
(CAMGC=14.52min, CASAT=10.31min,
CNMGC=14.24min and CNSAT=10.07min) were
similar to those obtained by spectrophotometry (tn) and
also to that reported in rheological analyses of the cooling process
(Palla et al., 2019). However in the spectrophotometry graph (Fig. 4) it
can be observed that there is an increase in absorbance during the
60min, while in by DSC (Fig. 5) a maximum of crystallization is reached
in less time. This is due to two situations, the first is that in the
spectrophotometry kinetics when reaching the ambient temperature
(≈20°C), the programming was set so that the temperature remained
constant due to the limitations of the equipment. This execution,
together with the temperature differential generated between the
spectrophotometer chamber and the recirculating water bath, resulted in
a slower (almost isothermal) crystallization in the final stage of the
experiment. In the DSC evaluation, it was possible to reach 0°C by
maintaining a constant cooling rate and a fixed TSTOP for each exotherm.
It should also be noted that very low regression coefficients
(R2˂0.70) were obtained in obtaining the values of z
and n by DSC. These R2 values are low compared to
those obtained by Toro-Vázquez et al. (2000),. However, it should be
noted that the analysis developed by them was under isothermal
conditions (Toro-Vázquez et al., 2000), while in this work,
non-isothermal conditions were used, which results in obtaining less
symmetrical exotherms and an irregular distribution of the crystalline
fraction. However, the values of the initial temperature versus
nucleation temperature allow us to detect a range of temperatures where
there is an initial crystallization process (nucleation) that could be
omitted by conventional DSC. Therefore, nucleation kinetics from
spectrophotometry is a good alternative to study the initial stage of
microstructuring in OGs, which by DSC is not persived.