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.