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.