3.6. Leaching Study
A leaching test was performed to assess the physical stability of the bigel formulations (Table 2). It was found that for all formulations, the amount of liquid lost remained below 10% with greater than 90% of leached liquid coming from the aqueous phase. In NMR T2-relaxation, the aqueous phase was found to have a more mobile population than the lipid phase. The observation that the aqueous phase accounts for most of the liquid leached from the system, is consistent with the fact that the aqueous phase is more mobile within the bigel and, therefore, water tends to percolate more than the oil from the bigel matrix. Data from the leaching study also found that formulations with the highest concentrations of MDG had a significantly lower amount of overall liquid leached from the matrix. Specifically, the amount of water that leached from the system reduced by 34% and 23% when comparing the highest and lowest MDG concentrations for OG:HG ratios 60:40 and 70:30, respectively. This may be the result of MDG interacting with the aqueous-lipid interface or the co-crystallization of MDG preventing the movement/leaching of water from the matrix. Additional studies are needed to confirm which mechanism is at work in the system. Although texture analysis showed a decrease in hardness, this does not necessarily correspond to a loss of physical stability.
4. Conclusion
The effect of mono-diglycerides (MDG) on a rice bran wax oleogel – gelatin hydrogel biphasic system was explored to improve its physical stability and tailor its mechanical and textural properties. MDG had the most significant impact on the 60:40 OG:HG bigel formulations, where CSLM images confirmed a phase inversion from oleogel-in-hydrogel to hydrogel-in-oleogel at 3% (w/w) MDG. These microstructural changes were reflected in FTIR to show that as the MDG concentration increased, the interactions within the oleogel phase increased and the interactions within the hydrogel decreased. The textural hardness of the bigel formulations also significantly decreased with the increase of MDG concentration while maintaining greater physical stability. Through the available evidence, it appears MDG serves as both a lipid modifier and emulsifier in this biphasic system and can be used to both tailor the mechanical properties and increase physical stability.