Results and Discussions
Oleogelation using pulse protein stabilized foams in the
presence of CW or
MAG
Both FPC-XG and PPC-XG foams readily adsorbed the added canola oil about
30-times their weight with and without CW or MAG and produced a
self-standing solid-like structure after 24 h of storage in a
refrigerator. No oil came out from any oleogels while inverting the
tubes after storing the refrigerator, except the oleogels consists of
only less than 1% CW or less than 2% MAG as oleogelator. The critical
concentration (the concentration of oleogelator at which the oleogel
doesn’t flow while inverting the tube) of candelilla wax for gelation of
canola oil was previously reported as 1%
(Blake, Co and Marangoni, 2014), which
matched with the present study. However, no such report on the critical
concentration of a similar MAG in canola oil at the same condition was
found.
Microstructures of different oleogels (Fig. 1) demonstrated a crystal
network of CW and MAG in the foam-templated oleogels. Oleogels
stabilized only by CW exhibited network of finely dispersed small
grain‐like crystals (Fig. 1m), while MAG oleogel displayed a dense
network of fibril-like crystals (Fig. 1n). Similar microstructures of CW
in CO has been reported before (Blake, Co
and Marangoni, 2014). The microstructure of MAG oleogel was similar to
the oleogel prepared using safflower oil with 4% MAG (the same
ingredient used in the current study) as reported by
López-Martínez, Morales-Rueda,
Dibildox-Alvarado, Charó-Alonso, Marangoni and Toro-Vazquez (2014), but
a denser network and smaller crystals were found in the current study,
which might be due to a different cooling rate and the use of different
vegetable oil.
In the foam-templated oleogels with and without CW or MAG, dark rings of
protein foam boundaries were observed under non-polarized light (Fig.
1d-f, and Fig. 1j-l), and crystal network of CW or MAG, similar to that
of CW or MAG-only oleogels, were observed under polarized light (Fig.
1a-c, and Fig. 1g-i). This confirms that the formation of CW or MAG
crystal network formed within the foam-templated oleogels without
altering any protein foam network. Birefringent starch particles with
spherulitic microstructures were also observed in the oleogels
stabilized by pulse protein foams with and without CW or MAG under
polarized light. More starch granules can be seen in FPC oleogels (Fig.
1g-i) compared to PPC oleogels, as the former had more than double
starch content (See Table S1, Supporting Information for proximate
analysis of the pulse protein samples).
Oil loss from the
oleogels
Oil loss (OL) is an indicator of oil binding capacity (OBC) of oleogel,
higher the OL means lower the OBC. The OL of different oleogels is
displayed in Fig. 2a and 2b. As expected, the OL of both FPC-XG and
PPC-XG foam-templated oleogels significantly reduced with the addition
of both CW and MAG, indicating an increase in OBC of the foam-templated
oleogels with these additives. When there was no additive, the OL was
about 34.4 ± 5.2 % and 32.4 ± 7.1 % in the case of PPC-XG and FPC-XG
foam-templated oleogels, respectively. The OL became zero with more than
2% CW and remained at 4.5 ± 0.2 % and 3.3 ± 0.5 % with 3% MAG in
PPC-XG and FPC-XG foam-templated oleogels, respectively (Fig. 2a). It
should also be noted that CW or MAG alone cannot bind oil as good as in
combination with a protein stabilized foam. The oleogels made only with
1% or less than 1% CW or MAG displayed 100% OL (Fig. 2b), showing
complete disruption of crystal network made of CW or MAG under
centrifugation. No oil was released when 2 % or more CW was present in
CO; however, about 7.0 ± 0.6 % oil was released even when 3 % MAG was
present in the CO. Previously, a significantly higher oil loss was
reported in MAG-stabilized canola oil oleogels, where even with 8% MAG
about 50 % of the oil was lost from the oleogel during storage on a
filter paper for 15 days (Lopez-Martínez,
Charó-Alonso, Marangoni and Toro-Vazquez, 2015). This difference could
be attributed to the use of different methods and timescale to measure
the OL. Lower OL (higher OBC) of oleogels containing CW indicates that
the crystal network of CW plays a critical role in determining the OL.
Blake, Co and Marangoni (2014) showed that
lower OL could be obtained if the oleogelator can cover more surface
area in the oleogel. Although the dense crystalline network was found in
both CW and MAG systems (Fig. 1), higher OL in MAG oleogels compared to
CW oleogels indicates that needle-shaped MAG crystals (Fig. 1n) formed a
weaker network compared to granular and spherulitic CW crystals (Fig.
1m) in binding CO and preventing disruption during centrifugation.
Although most of the oil was released from the CW or MAG-only oleogels
during centrifugation (with 1% or below oleogelator), the presence of
protein foam alone significantly reduced OL (Fig. 2a), which was further
improved when the oleogels were stabilized by protein foam along with CW
or MAG.
Intermolecular interactions such as hydrogen bonding, hydrophobic
interactions and van der Waals interactions could be the factors that
helped both CW and MAG crystal network and the foam network to bind
canola oil. In a previous study, it was demonstrated that major
interaction that helped the protein foam network to bind canola oil was
hydrophobic and van der Waals interactions, evidenced by changes in
protein secondary structure and changes in corresponding interaction
bands in FT-IR spectra (Mohanan, 2020).
Weak intermolecular hydrogen bonding was detected in MAG-stabilized
oleogels (Lupi, Greco, Baldino, de Cindio,
Fischer and Gabriele, 2016, Ögütcü and
Yilmaz, 2014) as well as in natural wax stabilized oleogels (). The
strong crystalline network formed through intermolecular interactions
might have trapped the liquid oil presented in the protein foam
stabilized oleogels, thereby further reinforcing the overall oleogel
network and reducing OL.
Oleogel viscoelasticity
Rheological properties of the oleogels are important to understand their
gel strength and better define their application. Oscillatory
strain-dependent viscoelastic measurements of different oleogels were
performed at a constant frequency of 1 Hz to find the linear
viscoelastic region (Fig. S1, Supporting Information). At low strain %,
the storage moduli (G’) of all samples, except 0.5% CW, and 0.5% MAG,
were higher than that of loss moduli (G”), indicating an elastic
behaviour. An LVR was observed until about 0.1% strain. With an
increase in strain, G’ decreased, indicating yielding, followed by a
cross over between G’ and G”, representing a loss of gel structure.
Frequency-dependent viscoelastic measurements of all oleogels were
performed within the LVR and are shown in Fig. S2 (Supporting
Information). Storage moduli (G’) of all samples, except as that of
0.5% CW, and 0.5% MAG, were higher than loss moduli (G”), indicating
an elastic behaviour of the gels at the performed range. All oleogels
with CW (Fig. S2a, c, e) displayed similar changes in G’ and G” with
frequency, G’ increased linearly with frequency. In contrast, all
oleogels with MAG (Fig. S2b, d, f) displayed a slightly higher rate of
increase in G’ at a lower strain, which decreased, making G’ more
independent at a higher frequency. Overall, the slope of the G′ curve
was only slightly positive for both CW and MAG, indicating limited
rearrangement of the gel structure at a shorter time scale (or higher
frequency). For all oleogels, G” decreased first to a minimum and then
increased with frequency (Fig. S2a, c, e), and this behaviour was
evident in oleogels with more than 1% CW or MAG. The minima in G” as a
function of frequency indicates motion in the gel network from a more
rigid in-cage structure to a more fluid out-of-cage structure at a
shorter time scale (higher frequency) and weakening of the gel structure
(Zhou, Hollingsworth, Hong, Cheng and Han,
2014). Interestingly, with an increase in concentration, the minimum
values of G” shifted to a higher frequency in case of the MAG-based
oleogels (Fig. S2b, d, f), while it shifted to lower frequencies in case
of CW-based oleogels (Fig. S2a, c, e). Similar rheological behaviour was
observed for oleogels of vegetable oil stabilized by sorbitan and
glycerol monostearate and is common for network-based gelled dispersions
(Sánchez, Franco, Delgado, Valencia and
Gallegos, 2011).
To compare the gel strength of different oleogels, G’ and G” of
different oleogels at 10 rad/s were replotted in Fig. 3. Both G’ and G”
increased with an increase in CW or MAG content, except for FPC
foam-templated oleogels (Fig. 3b), where no change was observed at 2 and
3% CW or MAG concentrations. Although the viscoelastic moduli values
were much lower for the oleogels without foam, the rate of increase as a
function of CW and MAG concentration was much higher (Fig. 3c) compared
to the foam-templated oleogels. The increase in viscoelastic moduli
could be due to the increase in solid content and the strength of
crystal network with an increase in the concentration of CW or MAG. With
the presence of a protein foam network in the oleogels, the effect of CW
or MAG concentration on gel strength was less visible, especially at 2
and 3% concentrations. There were no significant differences between
the G’ and G” values when PPC-XG was replaced by FPC-XG
(p>0.05) in foam-templated oleogels at all CW or MAG
concentrations (Fig. 3a, b). In the absence of foam, the oleogels
containing CW displayed higher G’ and G” values than the oleogels made
of MAG at all concentrations (Fig. 3c), which indicates that the CW
crystal network was stronger than that of MAG and are less prone to
deformation. The order of magnitudes of G ’ and G” of 3 % MAG oleogel
was close to that of 4 % MAG oleogel created with safflower oil
(Lopez-Martínez, Charó-Alonso, Marangoni
and Toro-Vazquez, 2015). Surprisingly, the foam-templated oleogels
containing MAG displayed higher G’ and G” than the corresponding CW
containing oleogels at all concentrations (Fig. 3a, b). This might be
due to the higher oil loss of MAG containing oleogels (Fig. 2). In our
previous study, it was also demonstrated that when the oil content was
lower, the foam-templated oleogels displayed higher G’ and G” due to the
stronger effect of the protein network on oil gelation
(Mohanan, Tang, Nickerson and Ghosh,
2020).
Oleogel spreadability
The firmness and cohesiveness of all oleogels were measured using the
spreadability test and are shown in Fig. 4. Firmness and cohesiveness
indicate the easiness of spreading and handling of oleogels; the higher
the firmness, the harder it is to spread the oleogel (lower
spreadability), and higher the cohesiveness, the stickier the oleogel.
As shown in Fig. 4, both the firmness and cohesiveness of all oleogels
increased with an increase in CW and MAG concentrations as expected.
When there was no foam present (Fig. 4c and 4d), The firmness and
cohesiveness of the oleogels were much lower compared to the
foam-templated oleogels (Fig. 4a and 4b). Between these two oleogels
without foam, CW oleogels displayed higher firmness and cohesiveness
than MAG oleogels (Fig. 4c and 4d). This agrees with the rheology
results, where the G′ of CW oleogels was significantly higher than that
of MAG oleogels. Therefore, it is reasonable to assume that the firmness
of oleogels is associated with the strength of the crystal network and
its ability to retain oil. In both FPC-XG and PPC-XG foam-templated
oleogels, however, there was no significant difference in firmness or
cohesiveness when CW was replaced by MAG (p>0.05) at all
concentrations, except when the cohesiveness decreased at 3%
oleogelator concentration. Nonetheless, unlike what has been observed in
rheological properties, FPC-XG foam-templated oleogels displayed higher
firmness and cohesiveness than corresponding PPC-XG foam-templated
oleogels with and without CW or MAG.
Cake baking with oleogel
Conventionally, a highly saturated fat containing shortening is used in
cake baking to stabilize air bubbles in the batter and to provide the
right rheology of the batter to give a soft tendered final cake product
(Wilderjans, Luyts, Brijs and Delcour,
2013). In the present work, shortening was fully replaced with oleogels
to determine how much of these oleogels can mimic the functionality of
shortening. To do this, we used PPC-XG and FPC-XG foam-templated
oleogels with and without 3% CW and 3% MAG, and oleogels made of 3%
MAG and 3% CW alone. As a control, the cake was also baked using 100%
CO as a control. Note that the oleogels used in cake baking contained
more than 95% CO.
Properties of cake
batters
The specific gravities of the cake batters prepared using vegetable
shortening, CO and the various oleogels are shown in Table 1, and their
microstructures obtained using a light microscope are shown in Fig. 5.
The batter prepared using shortening displayed the least specific
gravity, indicating higher incorporation of air into the batter
(Wilderjans, Luyts, Brijs and Delcour,
2013). This is also evident in their microscopic image, where numerous
small air bubbles can be observed (Fig. 5). The conventional shortening
used in the research consists of solid fat crystals and mono and
diglyceride emulsifiers, which are capable of stabilizing air bubbles in
the batter (Wootton, Howard, Martin,
McOsker and Holme, 1967). Batter prepared using CO displayed the
highest specific gravity, which could be due to the inability of the
liquid oil to stabilize air bubbles in the batter. As illustrated in
Fig. 5, there were only a few air bubbles incorporated in CO batter.
Oleogels consist of only foams or CW or CW with foam displayed similar
but significantly higher specific gravity compared to shortening. The
microscopic images of these batters shown in Fig. 5 also confirmed the
incorporation of fewer air bubbles in the compared to shortening batter.
Among all the oleogels, those consist of MAG with or without foam
displayed lowest specific gravity; however, they were still higher than
that of shortening batter (Table 1). The oleogels consist of MAG and
protein foams displayed similar specific gravity as that of batters with
only MAG oleogel (p > 0.05), indicating the presence of the
protein foams did not help further air incorporation in the batter.
Microscopic images of batters containing MAG clearly showed the
incorporation of a greater number of air bubbles in the batter compared
to CO, only protein foams and CW batters (Fig. 5). It also shows that
the presence of solid fat crystals made of MAG was crucial for the
stabilization of air in the batter, which could be ascribed to the
surface activity of the MAG. Although CW was able to form crystals in
CO, the cake batters prepared using only CW displayed almost similar
specific gravity as that of CO batter (p > 0.05). The
addition of protein foams in the oleogels with CW decreased the specific
gravity, but it not significantly different than the batters prepared
using only protein foam-templated oleogels. These results show that not
only improvement on gel strength or OBC of oleogels can improve the
quality of batters, but the ability of the oleogelators to increase the
stabilization of air bubbles does matter.
It is known that the rheological properties of the batters determine the
functional properties of the final product
(Ronda, Oliete, Gómez, Caballero and
Pando, 2011). Therefore, both flow and viscoelastic behaviour of all
batters were determined (Fig. 6). All cake batters displayed
pseudoplastic behaviour, where viscosity decreased with an increase in
shear rate (Fig. 6a). The flow curves of all cake batters were fitted to
a power-law model (\(n=K{\dot{\gamma}}^{n-1}\)), where ɳ is the
viscosity, \(\dot{(\gamma)}\) is the shear rate, K is the consistency
coefficient, and n is the flow behaviour index. The fitting parameters
are shown in Table 2 (R2 values were >
0.83 for all). Flow behaviour indices of all batters were less than 1,
and shortening batter displayed the lowest flow behaviour index followed
by the oleogel batters consisting of both foam and MAG, indicating more
shear thinning and the formation of higher-order structure. Batters with
both foam and CW displayed the highest ‘n’ values (Table 2), indicating
less shear thinning behaviour. Moreover, the shortening batter displayed
the highest consistency index (K), indicating higher viscosity at 1
s-1 shear rate than that of all other batters (Fig.
6a). The higher low-shear viscosity of shortening batter might be
attributed to solid fat crystals and higher air incorporation. At higher
shear rates, the viscosities of all batters were close to each other,
where the fat crystal network present in the shortening could be
disrupted and the air bubbles collapsed, leading to a reduction in
viscosity. Due to the inclusion of a higher portion of liquid oil in the
oleogel and CO batters, they displayed lower viscosities than the
shortening batter. Reduction in viscosity while replacing shortening
with rapeseed oil was previously reported
(Hesso, Garnier, Loisel, Chevallier,
Bouchet and Le-Bail, 2015). The n and K values of the PPC and FPC
foam-templated oleogel cake batters were also comparable to that of
muffin batters prepared using HPMC foam-templated oleogel batters
(Lee, 2018), indicating similar
functionality of protein and HPMC foam in oleogels.
All batters displayed tan δ (ratio of G” to G’) values lower than 1
(Fig. 6b) and higher G’ than G” at the whole frequency range studied
(Fig. 6c and 6d), indicating elastic behaviour. Tan δ for shortening
batter decreased with an increase in frequency, indicating dominant
elastic behaviour. All batters displayed similar tan δ to the shortening
batter below 1 rad/s frequency, however, it raised beyond 1 rad/s and
exceeded the value of shortening batter, indicating replacement of
shortening with oleogels led to more contribution to the viscous nature
of the batter. Both G’ and G” also increased linearly with increase in
frequency, indicating structural changes with the lowering of the
timescale of the applied strain. The increase in G’ with frequency was
less in shortening batters compared to the others. When shortening was
replaced with oleogels or CO, both G’ and G” reduced significantly. The
batters prepared using wax-based oleogels (Lim et al., 2017, Kim et al.,
2017) and HPMC foam-templated oleogel
(Lee, 2018) also displayed similar
viscoelastic behaviour as that of the batters used in the current study.
Both G’ and G” increased with increase in frequency when shortening was
replaced with HPMC foam-templated oleogels
(Lee, 2018). Lower viscosity and
viscoelastic values of oleogel batters compared to shortening batter
could be related to partial replacement of solid fat by liquid oil and
lower air incorporation. An increase in specific gravity due to lower
air incorporation was led to a reduction in cake volume and an increase
in cake hardness (Sahin, 2008). The
ability of the batter to retain the air determines their rheological
properties and the quality of the final product.
Properties of cakes
The cross-sectional view of the final cake products and their specific
volumes (cm3/g) are provided in Fig. 7 and Table 1,
respectively. Shortening cakes displayed the uniform distribution of
open air cells and granular particles of flour solids, while cakes
prepared using CO and oleogel displayed the relatively non-uniform
distribution of large air bubbles dispersed in a dense network of cake
matrix. Oleogels prepared using both the foams with and without CW or
MAG displayed similar cakes appearance. However, contradictory to what
has been observed in the literature
(Sowmya, Jeyarani, Jyotsna and Indrani,
2009), there was no volume reduction observed when shortening was
replaced by liquid oil or oleogel, all cakes displayed similar specific
volume (p>0.05). These results confirm that the rheology or
specific gravity of the batters cannot be used to predict the final cake
volume. There might be interactions with other ingredients during
baking, which determined the volume of the final product.
Textural properties of cakes (hardness, springiness, cohesiveness, and
chewiness) are displayed in Fig. 8. Hardness is an indicator of staling
of baked products and can be determined using the force required to
compress the sample to a certain height. The shortening cakes displayed
the lowest hardness (p < 0.05), followed by the hardness of
cakes prepared by PPC and FPC foam-templated oleogels containing MAG,
which were lowest among all the oleogel and CO-added cakes (Fig. 8a).
Cakes prepared using CO and CW oleogel displayed similar, but highest
hardness among all the cakes where shortening was replaced with another
lipid. All other oleogel cakes (MAG, PPC-XG, FPC-XG, PPC-XG+CW, and
FPC-XG-CW) displayed similar hardness. The hardness of the cakes
correlated well with their batter specific gravity. Batters with lower
specific gravity displayed lower hardness, and these results are
supported by the literature (Sahin,
2008). Higher hardness also indicates higher protein-protein and
protein-gelatinized starch interaction
(Paraskevopoulou and Kiosseoglou, 1997).
The monoacylglycerols/diacylglycerols present in the shortening
interacted with starch and reduced its interaction with protein and
hence lowering of the staling process
(Cauvain, 1998,
Mattil, 1964), which led to the lowest
hardness of shortening cakes and lower hardness of MAG-based oleogel
cakes. In addition to this, shortening saturated fat crystals’ ability
to prevent the formation of gluten network might have also led to lower
hardness. Generally, springiness values (Fig. 8b) were found to be very
close to each other, although CO cake displayed slightly higher, and MAG
oleogel cake displayed slightly lower springiness than shortening cake
(p < 0.05). Springiness is an indication of the elasticity of
cake, and our results showed that all the cakes had almost similar
capacity to recover its deformation after exposed to any external force.
Cohesiveness is a direct function of work required to breakdown the
internal bonds between different ingredients in the cake matrix during
each chew or the internal resistance of food to traction, which was
determined from the ratio of the second to the first peak area of the
two-bite test. Shortening cakes displayed the highest cohesiveness
followed by all CO and all oleogels cakes, except the cakes made of
oleogels consist of CW, which displayed the lowest cohesiveness (Fig.
8c). This indicates that the replacement of shortening with liquid oil
and oleogels make it easy to disintegrate the cake matrix during
mastication. Both springiness and cohesiveness did not correlate well
with any batter characteristics studied, such as viscosity,
viscoelasticity or specific gravity. Therefore, springiness and
cohesiveness of cakes could be influenced by the changes in the
interaction between the ingredients and how they form the structure in
the cake matrix during baking. The chewiness of the cakes followed a
similar trend as that of hardness, except the CW oleogel cake, which
displayed lower chewiness than CO cakes (Fig. 8d). Chewiness indicates
the energy required for mastication, which was calculated from the
product of primary parameters hardness, cohesiveness and elasticity
(Friedman, Whitney and Szczesniak, 1963).
Chewiness values indicate that the CO cakes required the highest energy
to masticate, and the replacement of CO with all oleogels decreased the
chewiness to a level similar to shortening-based cake.