Novel designs for the inclusion of hydrogels in monoacylglycerol-based oleogels
Creators: name: Habibi, Asieh
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 300602
subject: Food chemistry and food sensory science
classificationCode: 300607
subject: Food technology
classificationCode: 300699
subject: Food sciences not elsewhere classified

description: High consumption of solid fat has been documented to impact human health adversely. Oleogelation is considered a feasible strategy to substitute solid fat by gelling edible oils with a better nutritional profile and high unsaturated fatty acid concentration. Incorporating water into oleogel is a promising approach to reduce calorie intake further. Thus, it is vital to develop novel gel designs containing water components that can mimic the physical properties and functionality of solid fat. This thesis aims to examine the inclusion of varied hydrogel particle sizes into oleogels via various bigel designs and assess their influences on the microstructure and physico-functional properties of the resultant bigels.

The first study fabricated hydrogel-in-oleogel by mixing 20% (w/w) hydrogel phase in an oleogel phase (80% w/w). The former contained -carrageenan (0.4% w/w) or whey protein concentrate (WPC; 15% w/w) as gelling agents, whereas the latter was structured by monoacylglycerol (MAG, 10%) to be mixed with canola oil (90% w/w). Both -carrageenan + MAG and WPC+MAG bigels formed the self-sustained gel network. Using a rotor-stator mixer and high-intensity ultrasound (HIU), different hydrogel particle sizes at a constant hydrogel phase in bigel were obtained. The microstructure and rheological behaviour of the resultant gel were affected by the differed hydrogel particle sizes. The reduction of ĸ-carrageenan hydrogel particle size as a polysaccharide to form hydrogel behaved differently in the bigel network compared to the WPC hydrogel. The smaller hydrogel particle size (36-64 µm) with a more spherical shape was observed in ĸ-carrageenan based bigel, while WPC based bigel had a wide range of particles in the range of 64-150 µm with irregular shapes. Also, the formation of WPC hydrogel during the bigel setting reduced the size of MAG crystal clusters in the WPC-based bigel network. The reduction of hydrogel particle size into medium and small particle sizes, regardless of the type of hydrogel agent used, improved the gel strength, texture, and oil binding capacity of the formed bigels. Also, the reduced hydrogel particle size in both bigel systems prevented the instant disruption of the gel network at high temperatures by increasing the Tgel-sol transition temperature.

The second study employed HIU treatment to fabricate new double emulgel (DEG), oleogel-in-hydrogel-in-oleogel, which comprised three phases, all in gel state upon setting. This design was studied with various water proportions from 6.8% to 27.2% (w/w). The HIU treatment effectively stabilized the primary emulgel (PEG) (hydrogel-in-oleogel) by reducing the particle size of the inner oil phase from 56.1 µm to 6.5 µm. It also caused the formation of a weak WPC gel structure as a continuous aqueous phase in the PEG. The increase in water fraction in the final DEG altered the PEG particle size, rising from 48 µm to 87 µm in DEG containing 6.8% (w/w) (DEG10) and 27.2% (w/w) (DEG40). Increasing water content by including 20.4% (w/w) of stable PEG-UT-HIU in DEG30 reinforced the hardness value (0.48 N) and gel strength by illustrating ~ 70% critical strain in the amplitude sweep test.

The third study further examined the DEG investigated in the second study with alteration of PEG particle sizes. The DEG30, which showed the reinforcement of hardness and gel strength, was treated with various levels of HIU. The particle sizes of DEG-I, DEG-II, DEG-III, and DEG-IV samples were 104 µm, 67 µm, 88 µm, and 49 µm, respectively. The smallest PEG particle size (49 µm) extended the LVR region in the amplitude sweep test to 73 %, indicating the greater mechanical strength in the small deformation test. The Tgel-sol transition temperature of DEG samples in the temperature ramp test fluctuated from 54.7-56.2 °C. There was no significant change in hardness in the differed particle size DEG samples. However, the smallest PEG particle size caused a slight reduction in cohesiveness value.
 
The combined results of the second and third studies in the frequency sweep test showed that the water content incorporated in DEG had a more significant effect than particle size on the resistance of DEG to low frequency in the frequency sweep test. DEG10 (48 µm) with a lower water content of 6.8% (w/w) showed greater resistance to the applied low frequency at 0.01 Hz compared to DEG-IV (49 µm) with 20.4% (w/w).

This thesis suggests that the inclusion of 20-20.4 % water as a hydrogel in the oleogel phase improves the viscoelastic properties of oleogel in both (hydrogel-in-oleogel) bigel and DEG designs. The physical and functional properties of the resultant gels are affected by the reduction of hydrogel particle size. Applying different water incorporation methods in oleogel, such as bigel and DEG, accompanied by the decrease in particle size of the aqueous phase, can be considered an approach to fine-tune the desired physico-functional features of the resultant gel network for the development of low-fat products purposes.