## 1 Introduction  
Following decades of extensive research, dietary fiber is now recognized as an essential component of a balanced and health-promoting diet [1, 2, 3, 4]. The definition of dietary fiber has undergone significant evolution, with multiple conceptual frameworks proposed to account for its various physicochemical properties and physiological functions [5, 6, 7]. According to the Association of Manufacturers and Marketers of Food Additives and Supplements (AFCA) or the CODEX Alimentarius Commission, fibers are technically defined as substances found in foods of plant origin that the small intestine cannot digest or absorb, so they reach the large intestine intact [8, 9]. Dietary fibers include the following: (a) polysaccharides other than starch (for example, cellulose, hemicellulose, gums, mucilages, and pectins), (b) oligosaccharides (for example, inulin), (c) lignin, (d) associated plant substances (waxes and suberin), and (e) the term fiber also includes a type of starch known as resistant starch. There are different criteria for classifying dietary fibers. One way is based on its solubility in water, resulting in two primary types: soluble and insoluble fibers. Most plants contain both fibers, but in different amounts. However, they have their own benefits. Soluble fibers dissolve in water to form viscous gels. In contrast, insoluble fibers do not dissolve in water and mainly promote intestinal transit [7]. Mucilages are a type of fiber extracted from plant seeds or tender stems, such as flax seed. Flax mucilage contains, among others, carbohydrates, proteins, and ash in varying amounts depending on the variety of raw material and the extraction method [10]. The main component of flax mucilage consists of a neutral polysaccharide and an acidic one. Depending on whether the flaxseed fiber contains a more neutral or more acidic polymer, it can be classified as more neutral or more acidic. The neutral polymer is composed of an arabinoxylan with a main chain of (1/4) \(\beta\)-D-xylan, to which arabinose and galactose side chains are attached in positions 2 and/or 3. The acid polymer has a main chain of residues of \(\alpha\)-L-rhammopyranosyl acid (1/2) and D-galactopyranosyluronic acid (1/4) attached, with fucose and galactose side chains [11; 12; 13; 14]. Additionally, flax mucilage can be classified as soluble mainly. The use of fibers and mucilages in the food sector is not new, since they have interesting techno-functional properties. Therefore, they have traditionally been used as stabilizers, thickeners, and gelling agents or to reduce the fat content of foods [15; 16; 17; 18; 19; 20]. However, in recent years, they have attracted substantial interest by considering them as functional ingredients due to their potential to prevent or manage certain diseases. In the specific case of flax mucilage, scientific evidence has demonstrated its beneficial effects on health, notably through reductions in cholesterol and triglyceride [21; 22; 23] and blood glucose levels [24; 25], as well as cardiovascular protection [26; 27; 28; 29]. It has been shown that the physical treatment, and also chemical treatment, of dietary fibers can cause structural modifications that impact their techno-functional properties--either enhancing or diminishing them. Knowing the impact of these treatments on the rheological properties and physical stability of more simple systems, such as aqueous fiber dispersions, before their incorporation into commercial products, is of great importance. The rheological properties along with studies of physical stability allow for the obtainment of information about their flow properties, microstructure, and potential stabilizing, thickening, or gelling characteristics. All these properties depend on the particle size, water/oil retention, molecular weight, and solubility of the fiber. During the processing of products, different physical treatments can be used, like extrusion, ultrasounds, microfluidization, or high shear homogenization.