3.4.1. Solid fat content (SFC)
SFC was measured to determine if MDG was contributing to the solid content of the bigel and played a role in promoting crystallization of the lipid phase. Fig. 4 shows normalized SFC for all bigel formulations. Normalization was achieved by measuring the SFC of a pure 7% (w/w) gelatin sample and subtracting its value from all bigel SFC measurements to ensure that the data only reflects the SFC of the sample. When comparing the expected theoretical SFC of the samples, accounting for the addition of MDG solids, with the experimental data obtained, it was found that the actual SFC values were lower than expected. For example, for the formulation 70:30(0) and 70:30(1), the expected SFC values were calculated to be 7.0% and 7.9%. The measured values for these samples were 6.0%±0.4% and 6.2%±0.2%, respectively. These results can be explained by the solubility of RBW in liquid oil, determined to be 93.05%, and the presence of resins which can affect the solid-fat content (Blake et al., 2014). The results of this study are in line with those obtained by Blake et al. (2014) for RBW.
In contrast, although the experimental SFC values were lower than theoretically expected, an additional increase in the experimental SFC values was observed with an increase of MDG concentration. For 60:40 OG:HG, there appears to be a break in the positive linear trend with an unexpected increase in SFC at 2% and 3% (w/w) MDG. There could be two plausible explanations for this. One explanation is that MDG at 2% (w/w) and higher concentrations is could be promoting crystallization of RBW. Emulsifiers can be used to modify crystallization properties of fats. They can act as heteronuclei and accelerate fat crystal growth through a catalytic action as impurities (Ribeiro et al., 2015). The SFC data provides evidence that this could be the case with RBW as well. Another explanation is that MDG could be co-crystallizing at these concentrations. Previous studies have shown that monoglycerides can co-crystallize with wax-based oleogel systems, producing highly birefringent structures (Rodriguez-Hernandez et al., 2021; Toro-Vazquez et al., 2013). In both cases, crystal growth of RBW and MDG can be hindered. If MDG is accelerating RBW crystal growth, it is well known that accelerated crystallization creates smaller crystals (Ribeiro et al., 2015). In the case of MDG co-crystallization, MDG crystal growth can be restricted once they reach the interface of the hydrogel phase or the RBW crystal network (Rodriguez-Hernandez et al., 2021). In particular, RBW requires long crystals to promote crystal-crystal interactions that are considered desirable for the formation of strong organogels (Blake et al., 2014; Dassanayake et al. 2012). Knowing that long crystals are desirable for stronger RBW organogel matrices, it can be hypothesized that the addition of MDG will reduce the strength of the overall bigel system which may impact its stability. This is discussed further with the results obtained from texture analysis and the leaching study.