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).