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.