3.4.2. Spin-spin Relaxation (T2)
T2 relaxation spectra were obtained to determine the effect of MDG on molecular mobility in the bigel matrices. T2 relaxation times reflect the mobility of proton populations within the matrix, where a shorter T2indicates a less mobile population and a longer T2indicates a more mobile population (Luyts et al., 2013). In 60:40 OG:HG bigels, regardless of the MDG proportion added, three populations with varying degrees of mobility were observed (Fig. 5). To confirm the identity of these populations, spectra of pure 10% (w/w) oleogel and 7% (w/w) gelatin sample were obtained (results not shown). The least mobile proton population with a T2,1 of ~44 ms is associated with the matrices of both the aqueous and lipid phases. These protons are most likely more tightly bound and strongly associated with the components of the oleogel and hydrogel phases. The second, more mobile population with a T2,2 of ~180 ms is likely associated with the lipid phase. This may involve the oleogel and the MDG molecules. The third proton population, the most mobile, has a T2,3 of ~525-630 ms that can be attributed to protons of the aqueous phase, or hydrogel. This population shows to be affected the greatest by the addition of MDG. When comparing the most mobile proton population, T2,3, 60:40(0.5) does not show much difference compared to the control, 60:40(0); however there is a noticeable difference at higher MDG concentrations. The samples 60:40(1) and 60:40(2) continually decreased in overall molecular mobility which is depicted by the low values of T2,3. This decrease in mobility is most likely related to the increased SFC (Fig. 4) in the lipid phase. If the MDG are increasing the solids content and promoting crystallization of RBW, then the solids may be restricting the movement of the aqueous phase. Breaking from the trend of decreasing mobility in T2,3, 60:40(3) is less mobile than 60:40(0) but is more mobile than 60:40(1) and 60:40(2). The 60:40(3) sample may have this increase in mobility because of its change in microstructure. The 60:40(3) sample was found to have a different microstructure than the other 60:40 OG:HG formulations (Fig. 2) where it was a hydrogel-in-oleogel type of bigel rather than an oleogel-in-hydrogel type. It was also found by FTIR (Fig. 3) that 60:40(3) has a decrease in interactions within the hydrogel phase. These decreased interactions within the hydrogel phase and discontinuation of the hydrogel matrix, may lead the aqueous phase in 60:40(3) to have more mobility.
On the other hand, 70:30(0) was found to have four populations. The first three populations are hypothesized to have the same identities as in the 60:40 OG:HG formulations. The T2 values for the first three peaks in all the 70:30 OG:HG bigels are ~45 ms, ~150 ms and ~500 ms. It should be noted that the T2,3 peaks in the 70:30 OG:HG formulations were less mobile than those in 60:40 OG:HG formulations. Similarly to what was observed fro the 60:40 OG:HG formulations, the addition of MDG influenced the T2,3 values although at much lesser extent. This may be due to a dilution effect or it may have a connection to the microstructure where 70:30 OG:HG formulations consistently exhibited a bi-continuous matrix, whereas there were greater microstructural changes in 60:40 OG:HG formulations (Fig. 2). The higher SFC content in the 70:30 OG:HG ratio compared to 60:40 OG:HG may also be contributing to restricted movement of the aqueous phase in the third population. The fourth population in 70:30(0) with a T2,4 of ~1320 ms is most likely attributed to the aqueous phase because the pure gelatin sample was found to have a T2 as high as ~850 ms while the pure oleogel sample did not surpass a T2 of ~230 ms. The T2,4 value is greater than that of what was found in pure gelatin and the reason for its appearance is still unclear. Further investigation is needed to understand how this fourth peak is related to the microstructure of this formulation. Overall, MDG seem to have the most impact on the mobility of molecules associated with the aqueous phase.