2.2. Selection of gelators and emulsifier
Monolaurin and MAG/DAG mixture were selected as gelators through initial screening tests against other plausible gelators. Typically, a combination of gelators with different HLB values is ideal due to the improved physical properties seen in developed gels (Co & Marangoni, 2012). However, combining different gelators such as sucrose stearates, DAG and TAG combinations, and select phytosterols all exhibited a lowered ability to adequately form gels with algal oil and oil-in-water (O/W) emulsions. Other gelators including monolaurin and the MAG/DAG mixture were tested at a higher concentration than those used for the purpose of this study, at 15% w/w. This cursory test acted as a screening to find the best gelator for algal oil and O/W emulsion. Monolaurin and the MAG/DAG mixture not only developed a stable gel at 15%, w/w, concentration but also formed promising oleogels and emulgels at decreased concentrations as well.
The selection of emulsifier and emulsifier concentration for O/W emulsions followed parameters set forth elsewhere (Hyatt, Zhang, & Akoh, 2021). First, the interfacial and surface tension of four selected emulsifiers at three concentrations, 0.1, 0.5, and 1.0% were determined via a CSC-DuNouy Interfacial Tensiometer Model 70545 (CSC Scientific Company, Fairfax, VA) following parameters discussed in a previous study (Akoh, 1992). Interfacial and surface tensions were measured in order to determine potential stability of different emulsifiers at varying concentrations for developed emulsions (Bourrel, Graciaa, Schechter, & Wade, 1979). Additionally, particle size analysis was performed on sample emulsions following parameters mentioned elsewhere (Kowalska & Żbikowska, 2016). Particle size analyses were carried out using a Malvern Panalytical Mastersizer S (Malvern Panalytical Ltd, Malvern, UK) with a small volume sample dispersion unit. Data analysis was completed using the Mastersizer software (Malvern Panalytical Ltd, Malvern, UK).
Overall, the best performing emulsifier and concentration was S1170 at a concentration of 0.5% (w/w/w) with 5% algal oil and 94.5% deionized water. Emulsions developed using S1170 at 0.5% concentration had the smallest initial particle size with an average of 0.481 µm, and the lowest interfacial and surface tensions with averages of 7.9 and 46.6 dynes/cm, respectively. These values were significantly different from the results of other combinations of emulsifier and concentration.