3.2. Physical characteristics of oleogels and emulgels
Thermograms for oleogels and emulgels are shown in Fig. 2. The final
melting completion temperatures for algal oil and emulsion were roughly
-30.8 °C and 11.1 °C, respectively. These values are much lower than the
gelators included in this experiment, with monolaurin and MAG/DAG both
possessing a melting completion point at roughly 67.0 °C for gelators
alone. The thermograms in Fig. 2 show the improvement made in melting
points for each treatment compared to the non-gelated controls. All gels
exhibited a much higher melting completion temperature when compared to
bulk algal oil or emulsion alone. Additionally, increasing gelator
content seemed to have an effect, although not significant, on the
melting completion and crystallization completion temperatures as well.
Increasing monolaurin content in gels exhibited higher melting
completion temperature, 12% monolaurin (w/w) had the highest melting
completion at roughly 59.38 °C. The same trend was seen with MAG/DAG
oleogels, except that MAG/DAG oleogel exhibited an even higher melting
completion point of roughly 62.19 °C. Emulgels exhibited a similar
pattern, although monolaurin and MAG/DAG were reversed, with the 12%
monolaurin and MAG/DAG emulgels exhibiting the highest melting
completion points, at 65.62 and 60.18 °C, respectively. This pattern
would suggest that increasing gelator content helps to develop more
stable gels, and that MAG/DAG gelator is better at developing a
physically stable oleogel while monolaurin gelator produces a more
physically stable emulgel.
The SFC data shown in Fig. 3 exhibits a similar effect to the thermogram
results in Fig. 2. SFC was highest at 0 °C and as temperature increases
the SFC drops until the fat content in samples has melted completely.
When temperature was set at 0, 30, or 60 °C, oleogels made with 12%
MAG/DAG showed the highest SFC value (12.50, 10.27, and 0.46%,
respectively). As the control group, algal oil exhibited SFC of 2.03%
at 10 °C which decreased sharply to 0.67% at 15 °C, and was completely
melted by 40 °C. This melting pattern exhibited with 12% MAG/DAG
oleogel also matches the data from the thermograms shown in Fig. 1 as
well.
For emulgels, the highest SFC exhibited was from 12% monolaurin with an
average SFC at 0 and 30 °C of 14.93 and 10.40%, respectively. As the
control group, emulsion exhibited an average SFC of 0.7% at 30 °C and
was completely melted by 40 °C. Again, these results are in line with
the thermograms of melting and crystallization shown in Fig. 2. The
MAG/DAG oleogels seemed to be more thermally stable, while monolaurin
emulgels were more stable than MAG/DAG emulgels. Results of SFC data
indicate that 12% and 10% monolaurin oleogels could be used as
potential butterfat analogs, as the SFC for these samples were similar
to what others have found in butterfat at certain temperatures (Zhang,
Willett, Hyatt, Martini, & Akoh, 2021).
Fig. 4 shows the XRD data of oleogels and emulgels as well as bulk algal
oil and emulsion alone. Algal oil sample exhibited only β′ short spacing
peaks at 4.26, 3.99, 3.74, and 3.52 Å. Emulsion sample exhibited two β′
peaks at 3.75 and 3.72 Å. Monolaurin oleogels exhibited β′ short spacing
peaks at approximately 4.5, 4.12, 3.86, 3.63, and 3.42 Å. MAG/DAG
oleogel exhibited both strong β and β′ peaks at 4.68 and 4.64 Å (β), as
well as 4.28, 4.26, 4.16, and 3.85 Å (β′). This result is in line with
thermographs and SFC data as MAG/DAG oleogels were the most stable, and
the β crystalline form is regarded as having the highest stability
(Ribeiro et al. 2015).
Emulgels for both monolaurin and MAG/DAG exhibited strong β′ peaks as
seen in XRD data shown with graphs in Fig. 4d and 4e. However,
monolaurin emulgels exhibited more short spacing peaks at 4.43, 4.16,
4.18, 3.85, 3.71, and 3.67 Å, compared to MAG/DAG emulgels only
exhibiting peaks at 3.95, 3.92, 3.68, and 3.71 Å. The higher number of
short spacing peaks may correlate to a higher stability in product due
to a more needle like morphology (Sato & Ueno, 2005). This is supported
by the morphology, which was examined with polarized light microscopy
and is shown in Fig. 5 with micrographs.
The micrographs shown in Fig. 5 are all at the 400x magnification level.
Additionally, the micrographs have been converted to 8-bit images using
ImageJ software for better visualization of crystal structures. The
algal oil and emulsion alone exhibited little to no crystalline
structure, while the monolaurin and MAG/DAG oleogels and emulgels
exhibited a strong needle like morphology with a trend that crystal
clusters became denser as the gelator content increased. For oleogels,
the 12% monolaurin and MAG/DAG samples exhibited the highest density of
crystals within a given micrograph, however, it seemed that all of the
MAG/DAG oleogels possessed a more tightly packed needle like morphology
than the monolaurin oleogel counterparts. This would also agree with
previous data on physical structure as it suggests the more densely
packed needle like morphology is why MAG/DAG oleogels exhibited a higher
physical stability.
The smaller more needle like structure of MAG/DAG oleogels may lend the
gel to be more stable as they may possess a stronger oil binding ability
(Sato & Ueno, 2005). Micrographs of emulgels were not as consistent as
those for oleogels. Upon visual inspection it appears that the density
of crystals increases as gelator content increased for both monolaurin
and MAG/DAG emulgels. However, there doesn’t appear to be a noticeable
difference in the density of crystal morphology between the two types of
emulgels. The thermographs, SFC, and XRD data suggests that monolaurin
emulgels are more physically stable.
Images of both oleogel and emulgel samples can be found in supplementary
material Fig. S1. The rheological properties for both oleogel and
emulgel samples can be seen in supplementary material Fig. S2. Fig. S2
depicts heating sweeps where rheology was measured under a heating
program of 0 – 60 °C. The two lines displayed are the storage (G′)
modulus and the loss (G′′) modulus. The cross point between the two
lines is called the cross-over modulus and is indicative of a phase
change state with rubbery or pseudo-elastic properties . Typically, at
this point the microstructure of a compound is beginning to “flow” and
possibly breakdown as well (Gonzalez-gutierrez & Scanlon, 2018). This
also correlates to previous data on physical characteristics for
oleogels, as the monolaurin oleogels undergo a cross-over point much
earlier than the MAG/DAG oleogels. However, once again, the emulgels did
not correlate as well since both monolaurin and MAG/DAG emulgels have
earlier cross-over points at similar temperatures.
Overall, MAG/DAG oleogels exhibited a stronger physical stability over
monolaurin oleogels. This trend was reversed with emulgels, which may be
explained by the HLB values of the given gelators and the polar paradox
theory, which may allow for more complete hydrogen bonding in different
sample matrices (Marangoni & Garti, 2018). For oleogels, the
thermograms, SFC, XRD, micrographs, and rheological data all correspond
to MAG/DAG producing a more physically stable gel than the monolaurin
counterpart. For emulgels, the thermograms, SFC, and XRD all correlate
to monolaurin producing more stable gels, but the micrographs and
rheological data does not correlate as strongly as the oleogel data.