Climate change is a worldwide issue which calls for effective and urgent actions from mankind. In this perspective, shifting towards a more plant-based diet can contribute to reduce global greenhouse gas emissions. However, the texture of plant-based foods is often associated with negative sensory experiences (e.g. grittiness and mouth dryness) representing a huge barrier for consumer acceptance for this product category. Texturizers of biopolymer nature like xanthan gum are often used to improve the oral textural perception of foods. Yet, the use of these molecules might cause the occurrence of undesirable textural sensations like sliminess which requires the investigation of alternative solutions to polymer-based texturization. Therefore, the objective of this PhD thesis is to characterize and design a particle-based texturizer i.e. fluid gel to impart smooth textures in flowable products and have potential to reduce negative in-mouth perceptions of fiber particles.

In addition to the review of the state of the art (Chapter 2), this thesis is based on four experimental chapters: rheological behavior and ageing of fluid gels (Chapter 3), impact of processing stirring speed on fluid gel properties (chapter 4), impact of calcium concentration and history of calcium addition on low acyl gellan gum fluid gels’ properties (Chapter 5), and design of gel coatings for oral soft perception of fiber particles (Chapter 6). The final conclusions of the PhD thesis (Chapter 7) and potential future perspectives (Chapter 8) are also discussed.

Fluid gels are challenging materials to characterize from a rheological point of view as they are subject to large fluctuations in their rheological properties as well as a time dependent rheology. The first study focused on the rheological behavior of fluid gels investigating the origin of the rheological fluctuations and time dependency. It was showed that fluid gels are multi-scale materials which behave as granular materials at the level of the particle core and as colloidal glasses at the level of the fluid gel particle arms. The granular matter nature of fluid gels is responsible for the significant fluctuations measured in the rheology of fluid gels. The time dependent rheology is instead due to the presence of physical ageing. It was found that physical ageing is not conditioned by the presence of calcium ions and it did not present gravitational contributions. Therefore, this suggests that the physical ageing observed is caused by structural rearrangements of the colloidal arms surrounding the fluid gel particle core.

Stirring is an essential step in the processing of fluid gels as it prevents the formation of a volumespanning gel network, creating discrete gel particles instead. However, knowledge regarding the effects of stirring speed on the properties of fluid gels manufactured with a rotor-stator mixer was still rather limited. In this perspective, the second study investigated the impact of processing stirring speed on the properties of low acyl gellan gum fluid gels manufactured using a rotor-stator mixer. It was found that fluid gels processed at a higher stirring speed exhibited an overall fluid gel particle elongation. The change in particle morphology might have affected the resulting rheological properties. Fluid gels processed at a higher stirring speed were found to have a lower G’ in the linear viscoelastic region as well as an increase in yield strain suggesting the formation of softer structures. In addition, fluid gels manufactured at a higher stirring speed exhibited faster ageing rates which might be caused by a larger surface area facilitating interparticle contact points. Finally, the elastic modulus of fluid gels manufactured at lower stirring speeds exhibited a decrease when exposed to an increasing number of shear cycles. Conversely, an increase in G’ with an increasing number of shear cycles was measured in fluid gels manufactured at a higher stirring speed. The opposite behavior might be explained by two counteractive phenomena namely: abrasion and intermittent compaction.

The rheological properties of fluid gels made of ionotropic hydrocolloids like low acyl gellan gum can be modulated by changing the ion type and ion concentration used in their formulation. The third study showed that the G’ of low acyl gellan gum displays an optimal calcium concentration before fluid gel formation. In addition, the calcium bound to the fluid gel particles was higher than the theoretical estimate suggesting that -COOCa+ complexes might be formed. In addition, fluid gels manufactured at a higher calcium concentration exhibited a lower ageing rate which might be caused by an increase in the height of the interparticle repulsive barrier with increasing calcium concentration present in the formulation before fluid gel formation. In addition, an increase in G’ was measured with increasing calcium concentration after fluid gel formation. It is hypothesized that the increase in G’ with increasing calcium concentration after fluid gel formation might be due to an increase in interparticle contact strength.

Dietary fibers notoriously negatively affect the palatability of liquid products causing grittiness, mouth dryness and particle detection. Typically, the solution adopted consists in increasing the viscosity of the continuous phase though changing the consumption experience. The fourth experimental chapter of the PhD thesis focused on investigating a novel approach based on creating a gel coating around the fiber particles to reduce negative textural attributes of fiber particles. The gel coating approach was able to significantly reduce the amount of perceived particles, grittiness, and mouth dryness. The gel coating was showed to be stress sensitive. However, more stress resistant coatings can be designed by increasing their gel strength.

Finally, the findings presented in this thesis contribute to significantly advance the knowledge on the understanding of fluid gels and how their properties can be modulated through processing and formulation. In addition, a novel and effective strategy to reduce negative textural perceptions of fibers was presented. The insights provided in this thesis can contribute to design the next generation of more sustainable, healthier, and more palatable foods.