3.3. Stability of the W/O emulsions developed by the CW
The stability of the W/O emulsions was evaluated through freeze-thaw
cycles between 25°C and -70°C using a DSC. This methodology is
particularly useful when considering that many types of edible W/O
emulsions, such as whipped toppings and table spreads, are frozen to
improve long-term storage and then thaw for further processing or
consumption. Considering the concepts described by Clausse et al. 2005
and Ghosh and Rousseau (2009), after freeze-thaw cycles stable emulsions
ought to crystallize developing just one exotherm. For simplicity
purposes we discuss just the behavior of the cooling thermograms
obtained from the second freeze-thaw cycle (see section 2.5. Emulsion
stability through differential scanning calorimetry) of the W/O
emulsions after 20 days of storage at 25°C. Within this context, Figure
4 shows the cooling thermograms for the W/O emulsions developed at the
different water to oleogel ratios used. The thermograms show the
corresponding CW concentration in the emulsion. For comparative purposes
Fig. 4 includes the cooling thermograms of the water and of the
vegetable oil, obtained under the same time-temperature conditions as
for the W/O emulsions. The corresponding exotherms had peak
crystallization temperatures of -19.6°C (± 0.3°C) and -45.1°C (± 1.2°C)
for the water and the vegetable oil, respectively (Fig. 4E). These
thermograms were used as references to establish the tentative position
of exotherms associated with the water or the oil released (i.e.,
“free”) from the microstructure of unstable W/O emulsions because of
the freeze-thaw cycles. Within this context, the results shown in Fig. 4
indicated that the W/O emulsions having CW concentrations between 1.5%
and 3% at water to oleogel ratios of 40:60 and 50:50, were the only
ones that showed just one well-defined crystallization exotherm. This
crystallization exotherm had, in all cases, a peak temperature at ≈
-40°C (Fig. 4). The rest of the emulsions (i.e., 0.75% CW emulsions at
all water to oleogel ratios, and the 1.5%, 2.25% and 3% CW emulsions
at the 60:40 water to oleogel ratio) also showed the major exotherm with
peak crystallization temperature ≈ -40°C (Fig. 4). However, independent
of the %CW, the emulsions developed with the 60:40 water to oleogel
ratio also showed the presence of a large shoulder at temperatures above
the major exotherm (indicated with a black arrow in the Fig. 4). In some
emulsions, i.e., the emulsions with 0.75% CW at all water to oleogel
ratios, we also observed a small shoulder at temperatures below the
major exotherm (indicated with a doted arrow in Fig. 4A). Considering
the crystallization behavior of the water and the vegetable oil (Fig.
4E) and the concepts discussed for the characterization of W/O emulsions
by DSC (Clausse et al., 2005; Ghosh and Rousseau, 2009), we associated
the shoulder observed at a temperature above the major exotherm with
“free” water, while the shoulder observed below the major exotherm
with “free” oil. These “free” water and oil, released from the
emulsion microstructure during the freeze-thaw cycles, were now
dispersed throughout the still stable water droplets of the emulsion.
From here and considering the results discussed for the PLM photographs
(Figs. 1 and 1SM) and for WDD97.5% (Fig. 2), we
concluded that the W/O emulsions formulated with water to oleogel ratios
of 40:60 and 50:50 and with CW concentrations between 1.5% and 3%,
were the most stables even after two freeze-thaw cycles applied to the
emulsions after storage for 20 days at 25°C.