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.