3.1. Oxidative stability of oleogels and emulgels
TOTOX values were calculated for oleogel and emulgel samples by combining PV and p AV and reported because they better represent the overall oxidation status of oleogel and emulgel samples. These TOTOX values are shown in Fig. 1. Additionally, changes in ω-3 FA composition for oleogel and emulgel samples are displayed in Table 1. The ω-3 FA composition focused mainly on EPA and DHA contents and starting levels of EPA and DHA were in line with reported values for theSchizochytrium spp. algal oil from the company’s certificate of analysis.
The OIT values determined with DSC are found in Table 2 and correlate with the results on TOTOX values and the change in ω-3 FA composition measured with GC-FID for both oleogel and emulgel samples. OIT measured with DSC has been used to evaluate antioxidant efficiency in samples as demonstrated in our previous studies and has been correlated with the oxidative stability of oil (Hyatt, Zhang, & Akoh, 2021; Zhang, Willett, Hyatt, Martini, & Akoh, 2021). For our purposes, algal oil is compared against oleogel samples while emulsion is compared against emulgel samples.
Algal oil without gelator exhibited an OIT value of 21.43 ± 0.25 min while the highest OIT value exhibited was from the 12% (w/w) monolaurin oleogel (27.03 ± 0.47 min). Table 2 shows a significant difference with Tukey’s HSD test between every oleogel and algal oil by itself, suggesting that gelation helped improve the oxidative stability of algal oil present. This pattern is repeated within comparison for emulgels. Emulsion without gelator exhibited an OIT value of 20.30 ± 0.25 min while the highest OIT value was again exhibited by 12% (w/w) monolaurin emulgel (25.01 ± 0.23 min). The trend shows that increasing gelator content may improve oxidative stability as it increases OIT, and monolaurin exhibited higher OIT values than the MAG/DAG counterpart for both oleogel and emulgel samples.
These trends suggest that gelation may have a positive effect on the oxidative stability of oil, and that monolaurin as a gelator could protect against oxidation more effectively than the MAG/DAG mixture. The trends observed with OIT were also seen when examining the TOTOX results as well as the change in ω-3 FA content in Figure 1 and Table 1, respectively. The highest TOTOX value was from algal oil and emulsion without gelator for both the oleogel and emulgel comparisons, respectively. The TOTOX value of algal oil after 14 days was 251.32 ± 1.25 while the TOTOX value for emulsion was 297.08 ± 2.69. In both sample types the lowest TOTOX value after 14 days was the 12% (w/w) monolaurin sample gel, with a value of 202.07 ± 2.33 for oleogel and 251.53 ± 1.79 for emulgel. The highest average decrease in TOTOX value for algal oil was approximately 19.59% while the average decrease in TOTOX value of emulsion was approximately 15.33%.
Additionally, the change in ω-3 FA content seen in Table 1 agrees with the pattern discussed above. In order to better compare between gels that have less oil due to the increased gelator content, all values were presented in concentration (mg/g) and normalized percentage. For oleogel samples the average decrease in EPA and DHA content was approximately 25.11% and 30.25%, respectively. The only significant difference in EPA protection among oleogels was seen with the 12% (w/w) monolaurin sample with an average decrease of 7.15% EPA content. While 10% (w/w) monolaurin was the second lowest, it was not statistically different than algal oil alone with an average decrease of 19.15% EPA content. The EPA content was not significantly protected by the MAG/DAG mixture in oleogel.
The lowest decrease for DHA content in oleogels was again the 12% (w/w) monolaurin sample with an average decrease of 9.82%. Multiple treatments exhibited significant differences in the change of DHA content, with the next best sample being the 12% (w/w) MAG/DAG oleogel followed closely behind by the 10% (w/w) monolaurin sample. Overall, the OIT, TOTOX, and change in FA composition agree on a trend of protective effect for both oleogels and emulgels, where 12% M > 12% MD = 10% M > 10% MD = 8% M > 8% MD > algal oil, (where M and MD stand for monolaurin and MAG/DAG, respectively).