1. Introduction
Bigels are biphasic gels systems in which both phases are structured
semi-solids. Bigels contain both an aqueous hydrogel phase and a lipid
organogel phase. Interest is growing in these biphasic gel systems
because of their ability to address the limitations and issues
associated with hydrogels, organogels and emulgels (Shakeel et al.,
2018). Some of the advantages attributed to bigels include their ability
to encapsulate both lipophilic and hydrophilic bioactive compounds
(Behera et al., 2015); the capability to change both the aqueous and
lipid phases to improve gel properties; and their enhanced physical
stability due to a structured internal phase (Sahoo et al., 2015).
Currently, bigels are mostly being considered for controlled delivery in
pharmaceutical application, nonetheless, there is effort in
investigating edible bigel formulations for food application.
Specifically, there have been bigel formulations developed for
encapsulation of probiotics to be incorporated into dairy products
(Bollom et al., 2021; Zhuang et al., 2021). Other unique formulations
have been developed to encapsulate sensitive bioactive reagents for
application in functional food products (Zhu et al., 2021).
Even though bigels can be prepared without the use of surfactants or
emulsifiers due to the semisolid nature of both phases, they may be
susceptible to destabilization at higher temperatures. Destabilization
at higher temperatures can occur when the sol-gel temperature is
reached. At this temperature, both phases gain mobility and, similar to
traditional emulsions, the mixture is thermodynamically unfavorable
(Singh et al., 2014). In this case, there is most likely phase
separation. To further enhance the stability of the biphasic gel
systems, emulsifiers can be considered. The choice of emulsifier is
important to promote successful stability of an emulsion and multi-phase
systems with incompatible phases. There is a wide variety of emulsifiers
available and their nature cannot only determine the stability of the
system, but also impact emulsion formation and functional properties.
In the food industry, mono-diglycerides (MDG) and their derivatives are
some of the most important food emulsifiers as they account for the
majority (~70%) of emulsifier production. They are
applied in foods such as bread, cake, spreads, ice cream and more. They
exhibit more lipophilic behavior with an HLB value between 3-6 and are
therefore more effective in stabilizing water-in-oil (W/O) emulsions
(Norn et al., 2015). As MDG exhibit more lipophilic behavior, they are
known to interact with the lipid phase. In addition, solid emulsifiers
can affect nucleation and/or growth of fat crystals, accelerating or
retarding these processes (Ribeiro et al., 2015). Monoglycerides, in
particular, have been extensively studied as organogelators, as they are
known to self-assemble into an inverse lamellar phase with close-packing
when heated above its melting temperature and cooled again in oil. From
these studies, it has been shown that the crystal morphology and role of
monoglycerides in a system is dependent on the concentration, the type
of oil it is added to and the presence of other components such as
co-emulsifiers and/or co-gelators (Valoppi et al., 2017; Wang et al.,
2022; Rodriguez-Hernandez et al., 2021).
Although there has been much work done on the effect of monoglycerides
in organogel/oleogel and emulsion-gel systems, there are few
publications that have focused on their effect on bigel systems. The
addition of MDG to a bigel may assist in extending physical stability,
as well as providing another way to tailor and control the mechanical
properties of the system. The goal of this study was to explore how MDG,
a common food emulsifier, alters the interfacial interactions between
the aqueous and lipid components of an edible rice bran wax (RBW) –
gelatin bigel system. More specifically, in this study, it will be
determined how different concentrations of MDG affect the RBW – gelatin
bigel’s microstructure, physical stability and mechanical properties at
various oleogel-to-hydrogel (OG:HG) ratios.