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.