Professor Chen Fusheng of Henan University of Technology, et al.: Rheological properties and microstructure of peanut protein-pectin composite emulsion gels

3.
Effect of oil phase and its addition amount on the rheological characteristics of PPI-HMP composite emulsion gel

Apparent viscosity

As shown in Figure 3, when the amount of oil phase addition increased from 40% to 70% by volume, the initial viscosity of the system gradually increased, and the samples with peanut oil, olive oil and 24 degrees palm oil all exhibited pseudoplastic fluid behavior
.

As can be seen from Table 2, with the increase of oil phase addition, the viscosity coefficient of the compound system gradually increases
.
The emulsion gel samples prepared at the highest addition amount of each oil phase correspond to the maximum viscosity coefficient K (representing the apparent viscosity of the system) and the smallest fluid index n
, respectively.
This shows that in the emulsion gels prepared in high oil phase, the oil droplets not only participate in the formation of the gel network as "active fillers" but also as structural units, so that the mechanical properties of the gel network are enhanced and the apparent viscosity is increased
.

Storage modulus and loss modulus

As shown in Figure 4, during the 45 °C holding process after TG enzyme was added for 1 h, the G' values of all samples slowly increased with time, and the G' values of all samples (except PPI-HMP hydrogels) were higher than the G" values
.
The hydrogel G' was first lower than G", lengthened over time and eventually higher than G", and there was a gel transition point
at 76 minutes.
PPI is slowly crosslinked under the action of TG enzyme to form a network structure, and after adding the oil phase, the emulsion becomes more viscous, less fluid, and has a higher G' value, thus showing a certain rigidity
at the beginning of the test.
During the temperature increase from 45 °C to 85 °C, both the G' and G" of the hydrogel and the emulsion gel decreased
over time.
It may be that both hydrogels and emulsion gels have higher viscosity, while high-viscosity fluids decrease in viscosity with increasing temperature, which in turn causes G' and G" to decrease
.
When the temperature was raised to 85 °C and kept warm for 30 minutes, the G' and G" of both hydrogels and emulsion gels increased, especially hydrogels
.
This is due to the high temperature denaturation of PPI and the further formation of the gel structure
under the influence of hydrogen bonding and hydrophobic interactions.
When the temperature drops from 85 °C to 25 °C, both G' and G" have an upward trend and remain at high values
.
This is mainly due to the entropy effect and the enhancement
of PPI hydrogen bonding and intermolecular forces at lower temperatures.
It can be seen from Figure 4B~D that the G' value of PPI-HMP composite emulsion gel increases
with the increase of oil phase addition.
This indicates that the increased amount of oil phase addition can make the structure of the emulsion gel stronger
.
When the amount of oil phase was 60%, the G' values of the emulsion gel under different oil phases were: palm oil> olive oil > peanut oil at different oil
phases.
Under the same conditions, compared with peanut oil and olive oil, the emulsion gel prepared from 24 degrees palm oil has the strongest rigidity and the greatest
gel strength.
This is consistent with
Section 2.
2 texture test results.

Viscoelastic properties

As shown in Figure 5, the G' of each emulsion gel sample is greater than G' and increases
with frequency.
PPI-HMP hydrogels begin to be greater than G' after a frequency of 0.
40 Hz, and since in strong gels, the modulus is frequency-independent and does not intersect, and in weak gels, a frequency-dependent occurs
.
The G' and G" of emulsion gels containing different oil phases were larger than those of hydrogels, and emulsion gels with olive oil as oil were larger than emulsion gels
with peanut oil as oil.
The results show that the stability of the gel network structure of the emulsion gel is greater than that of the hydrogel, and that of the emulsion gel network structure with olive oil as the oil phase is greater than that of the emulsion gel
with peanut oil as the oil phase.
This may be related
to the chemical affinity between the adsorbent layer on the droplet surface and the gel matrix and the mechanical properties of the packing particles.
With the increase of the oil phase addition, the G' and G" of each emulsion gel increased, which indicates that the mechanical properties of the emulsion gel can be enhanced
by the increase of the oil phase addition.

4.
Effect of oil phase and its addition amount on the microstructure of PPI-HMP composite emulsion gel

As shown in Figure 6, the emulsion gel in this study is formed
by droplets wrapped in a gel matrix.
Figure 6A shows that a similar gel network has
emerged in the PPI-HMP suspension.
TG enzyme addition catalyzes the acyl transfer reaction between (or within) proteins, resulting in covalent crosslinking
of proteins.
The formation of the emulsion gel network structure under low oil phase is mainly formed
by cross-linking between proteins that are not adsorbed to the surface of the oil droplet.
With the increase of oil phase addition, the droplets gradually change from dense spherical-like droplets to irregular structures
.
This indicates that as the amount of oil phase addition increases, most of the droplets begin to accumulate with each other, and the size and distribution become uneven
.
This is because with the increase of the ratio of the oil phase, the hydrophobic interaction between the oil droplets leads to the merger of the oil droplets, which gradually reduces the protein adsorption capacity, and the viscosity of the high oil phase emulsion system is too high, which is not conducive to the interaction
of proteins and enzymes.
Therefore, the formation of the emulsion gel network structure under high oil phase is the result
of the covalent crosslinking of enzymes and the interaction of high viscosity between the PPI-HMP composite interface film wrapped on the surface of the oil droplets.
For active-filled droplets, the size and surface properties of the droplets determine the deformability of the droplets, which determines the effect of
the droplets on the gel matrix.
When the oil phase was added at 60%, the network structure of the emulsion gel prepared from 24 degrees palm oil was denser, and the degree of oil droplet aggregation was relatively weak
.
It is shown that the network structure of the emulsion gel with palm oil at 24 degrees Celsius as the oil phase is more rigid
at this time.
Different oil phase compositions can affect the size and distribution of oil droplets, so that the G' and hardness of the emulsion gel can be changed, and ultimately the structure of
the emulsion gel.
This is consistent with
the results of texture tests and rheological tests.

Conclusion

The effects of
different oil phases and their addition amounts on the apparent properties, texture, rheological properties and microstructure of PPI-HMP composite emulsion gels were studied.
The results showed that the emulsion gel formed at the protein-pectin ratio of 5:2 and pH 6.
0 had uniform color and hard texture.
It cannot be made into a stable emulsion gel
under neutral and alkaline conditions.
With the increase of the amount of oil phase addition, the surface of the emulsion gel became more delicate and smooth, and its hardness, adhesion, adhesion, recovery and chewiness increased.
At 60% (V/V) at 24°C palm oil, the emulsion gel has the highest hardness, adhesion, recovery and chewiness values
.
The rheological results showed that the G' and G" of different oleophase emulsion gels were significantly higher than those of hydrogels, and the G' value and apparent viscosity value of the emulsion gel increased with the increase of the oil phase addition, and the apparent viscosity and G' values of the emulsion gel were 24 degrees palm oil> olive oil > peanut oil
at the same oil phase addition amount, respectively.
The microstructure results of PPI-HMP composite emulsion gel showed that with the increase of oil phase addition, the droplets began to aggregate and were unevenly distributed.
When the oil phase is added at 60% (V/V), the 24-degree palm oil-filled emulsion gel has the best ability to stabilize oil droplets and the densest network structure
.
The results show that the oil droplets interact with the gel matrix composed of peanut protein-pectin as active fillers, and the increase of the amount of oil phase addition contributes to the formation of the gel network, which enhances the mechanical properties of the gel network, increases the apparent viscosity, and makes the gel structure more rigid
.
The results of this study provide ideas
for the development and utilization of PPI-HMP composite emulsion gels in dairy products and baked goods.

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