Ultrasonic Processing and Its Impact on the Rheology and Physical Stability of Flaxseed Fiber Dispersions

Featured Application

The knowledge gained from this investigation holds great potential for the development of new products containing flaxseed fiber through ultrasound processing.

Abstract

Ultrasonic homogenization is an emerging technique with significant potential to modify the structure and functionality of food ingredients. This study evaluated the effect of ultrasonic homogenization on the rheological behavior and physical stability of aqueous dispersions of flaxseed fiber. Flax mucilage, with health-promoting and techno-functional properties, is of growing interest in several industries. The samples were subjected to different ultrasonic treatments, varying in amplitude (from 40 to 100%) and duration (from 2 to 20 min), with and without preliminary rotor–stator homogenization. The rheological properties were analyzed using small-amplitude oscillatory shear (SAOS) tests and steady shear flow curves. Physical stability was assessed by multiple light scattering. The results revealed that short treatment under ultrasonic homogenization had minimal impact on the viscoelastic parameters and viscosity, regardless of the amplitude used. However, longer treatments significantly reduced both values by at least one order of magnitude or more, indicating the occurrence of microstructural degradation. The relevance of this research lies in its direct applicability to the development of functional foods, since it is concluded that control of the ultrasonic homogenization process conditions must be carefully selected to retain the desirable rheological properties and physical stability.

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) β-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 α-L-rhamnopyranosyl 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. The review works on these topics and the applications of fibers carried out by Spotti and Campanella (2017) [30] and, most recently, by Gan et al. (2021) are worth mentioning [31]. Ultrasonic homogenization is an innovative technique widely used in food, pharmaceutical, cosmetic, and biotechnology processes, especially on small- and medium-scale operations. The reason for this is due to high efficiency and safety operation [30]. An ultrasound operates by using high-frequency sound waves that generate pressure waves in a liquid, causing cavitation (the formation and implosion of microscopic bubbles) in the liquid medium. These cavitation phenomena create intense shear and turbulence forces, which cause larger particles or droplets to break up into smaller (even nanometer scale) and more uniform sizes. In dietary fibers, mechanical disruption may alter the surface’s hydrophilicity and weaken the fiber matrix, thereby enhancing the solubility of previously insoluble fibers. Additionally, this effect is also employed to increase fiber extraction yields or to improve the efficiency of subsequent biochemical treatments, such as enzymatic reactions. The extent to which fibers are modified depends on factors such as their molecular weight, the intensity of the applied ultrasound, or the duration of the treatment. In the literature, there are numerous studies using techniques based on high-pressure processes or even microfluidization. Some published works are about palm fruit bunch fibers [32], corn bran [33], wheat bran fibers [34,35,36], rye bran [37], arabinoxylan from wheat bran [38], soybean dietary fiber [39], papaya peel fiber [40], orange fiber [41], citrus fiber [42,43], or flaxseed fiber [44], as some examples. However, there are fewer studies on the impact of an ultrasound on the rheological properties of aqueous fiber dispersions (see, e.g., Li and Feke (2015) [45]) and, to our knowledge, none exist on the effect of ultrasonic homogenization processing parameters on the rheological properties and physical stability of aqueous dispersions of flaxseed fiber. For this reason, the aim of this study was to relate the amplitude and time of ultrasonic homogenization to macroscopic properties such as the viscoelasticity, viscosity, and physical stability of the aqueous dispersions of flaxseed fiber. The quality of the resulting samples is monitored by combining techniques such as rheology and multiple light scattering.

This study aims to highlight the importance of suitable processing for each fiber in order to guarantee their techno-functional properties and at the same time, to extend the knowledge of the use of fibers and ultrasonic homogenization.

2. Materials and Methods

2.1. Materials and Preparation of Samples

The aqueous dispersions of flaxseed fiber (HiFood, Parma, Italy) were prepared at a concentration of 2.6 wt%. Based on details shared by the manufacturer, this fiber was obtained through an extraction procedure developed using proprietary methods (HI-FOOD R&D Dept., Parma, Italy). It has a molecular weight of 1.47 kD and its chemical composition comprises mainly fiber (76 wt%) and a low proportion of carbohydrates (9 wt%) and proteins (4 wt%). In addition, it contains lipids (<2 wt%) and moisture (<7 wt%). Additionally, 0.1 wt% potassium sorbate (Sigma Aldrich, Madrid, Spain) was added as a preservative.

Batches of 200 g of the sample were obtained by weighing the appropriate amounts of water and potassium sorbate, to which the calculated quantity of fiber was added. The mixture was then subjected to different treatments: (a) treatment U: 2 min of ultrasonic homogenization using a VCX750W (Sonics, Newtown, CT, USA) device at an amplitude of 40, 60, 80, or 100%, with 5 s on and 5 s off (samples U-2min-40%, U-2min-60%, U-2min-80%, and U-2min-100%, respectively), and (b) treatment LU: 4000 rpm for 2 min using the Ultraturrax T50 (IKA, Staufen, Germany) rotor–stator device with a S50NG45F dispersion unit and subsequently, the resulting sample was submitted to ultrasonic homogenization at an amplitude of 100% and a time of 10 or 20 min (LU-10min-100% and LU-20min-100%, respectively), with 5 s on and 5 s off. A sample prepared with only the T50 device at 4000 rpm for 2 min (L-0min) was used as a control in the LU treatment samples (results published in [44]). After preparation, the samples were kept under storage at about 4 °C until characterization.

Table 1. Samples studied, processing variables, and energy per mass unit applied to the aqueous dispersions of flaxseed fiber.

Sample	Rotor–Stator Time (min)	Rotor–Stator Rate (rpm)	Ultrasound Amplitude (%)	Ultrasound Time (min)	Energy/Mass (J/g)
U-2min-40%	0	0	40	2	6.64
U-2min-60%	0	0	60	2	31.17
U-2min-80%	0	0	80	2	44.15
U-2min-100%	0	0	100	2	61.88
L-0min	2	4000	0	0	62.70
LU-10min-100%	2	4000	100	10	408.75
LU-20min-100%	2	4000	100	20	712.50

shows the treatment applied to the aqueous dispersions of flaxseed fiber and the energy per mass unit applied. The energy applied during the ultrasonic homogenization process was provided by the device. The energy applied during the rotor–stator homogenization process was calculated as indicated in a previous work [44].

2.2. Characterization of Samples

2.2.1. Rheological Characterization

The viscoelastic properties of the samples were determined by low-amplitude dynamic viscoelastic tests (SAOS) using an AR 2000-controlled stress rheometer (TA Instruments, New Castle, DE, USA) together with a 40 mm diameter aluminum plate–plate sensor (PP40). First, the extension of the linear viscoelastic zone (LVR) was determined by means of stress sweeps at a fixed frequency of 1Hz. Then, frequency sweep measurements were performed at a stress located within the LVR. The dependence of the storage modulus on frequency can be fitted to a potential law equation whose shape is as follows:

$${\mathrm{G}}^{\prime }=\mathrm{a}·{\mathsf{\omega }}^{\mathrm{b}}$$

(1)

where a is related to the value of the storage modulus and b to the degree dependence of G′ on frequency.

To determine flow behavior, steady state measurements were performed using a Haake Mars 40-controlled stress rheometer (Haake, Karlsruhe, Germany), with a geometry of coaxial cylinders with a sand-blasted surface (CC25 sensor, Ri = 1.25 cm and Re/Ri = 1.085). A multistep stress control protocol was applied with 120 s per point. The experimental data were fitted to the Carreau model:

$$\mathsf{\eta }={\displaystyle \frac{{\mathsf{\eta }}_0 }{{\left(1+{\left(\raisebox{1ex}{$\dot{\gamma }$}\!\left/ \!\raisebox{-1ex}{$\dot{{\gamma }_c}$}\right.\right)}^2\right)}^{\left(\frac{1-n}{2}\right)}}}$$

(2)

where η is the apparent viscosity, $\dot{\gamma }$ is the shear rate, and η₀, ${\dot{\gamma }}_c$ and n are the fitting parameters. η₀ is the so-called zero-shear viscosity, n is the flow index, and ${\dot{\gamma }}_c$ is the critical shear rate, related to the onset of shear thinning behavior. It should be noted that the Carreau model has been simplified by eliminating the apparent viscosity at a very high-shear rate (η_∞), since this Newtonian region has not been attained.

The intervals used in both sweeps, stress or frequency, and flow curves depended on the characteristics of the sample, encompassing values ranging from 0.01 to 100 Pa, from 10 to 0.01 Hz, and from 1 to 100 Pa, respectively.

All rheological tests were carried out 24 h after sample preparation, with a fresh sample, in duplicate, and at 20 °C ± 0.1 °C. After loading the sample into the sensor system and prior to rheological measurements, an equilibration time of 300 s was allowed.

2.2.2. Physical Stability

The physical stability of refrigerated aqueous dispersions of flaxseed fiber was determined by performing multiple light scattering measurements with Turbiscan Lab Expert equipment (Formulaction, Toulouse, France), at room temperature, as a function of the height of the sample and the aging time. This test allowed for the shelf life of these dispersions to be established. Two signal modalities, designated as transmission light (T) and backscattering light (BS), can be obtained. Typically, a transmission light value is employed for the analysis of clear samples, while a backscattering light value is utilized for non-clear or opaques samples. In this study, the latter parameter is presented because, although the samples are neither completely opaque nor completely translucent, it shows the highest intensity. Additionally, it is measured in reference mode (ΔBS%, subtracting the scan at time 0 from the remaining scans) because this approach better highlights the changes that occur.

The stability results were quantified by calculating the Turbiscan Stability Index parameter, TSI, according to the following equation:

$$TSI={\displaystyle \frac{1}{N_h}}{\sum }_{t_{min}}^{t_{max}}{\sum }_{z_{min}}^{z_{max}}\left|BS\left(t_i,z_i\right)-BS\left(t_{i-1},z_i\right)\right|$$

(3)

where t_max is the time t at which the TSI is calculated, z_min and z_max are the lower and upper selected height limits, respectively, and N_h the number of height positions in the selected zone of the scan. From this equation, it can be deduced that the fewer changes that occur in the sample and, consequently, the greater its physical stability, the lower the TSI value obtained [46].

2.3. Statistical Analysis

Statistical analysis was performed according to analysis of variance (ANOVA) and the least significant difference test (p < 0.05) using Origin 8.0. All data were expressed as mean ± standard deviation.

3. Results and Discussion

3.1. Rheological Properties of Aqueous Dispersions of Flaxseed Fiber Submitted to Ultrasonic Homogenization

A linear viscoelastic range (LVR) and the corresponding critical oscillatory stress amplitude values can be determined by means of stress sweeps at constant frequency. A linear viscoelastic range comprises the stress amplitude values for which the viscoelastic functions remain constant or do not vary significantly with increasing stress and strain values, with critical stress (τ_c) being the value of stress amplitude at which these functions cease to be constant. In this case, the storage modulus (G′), related to the elastic component, and the loss modulus (G″), associated with the viscous component, have been chosen as control viscoelastic functions. Figure 1 shows the variation of G′ and G″ with stress amplitude as a function of the ultrasonic treatment applied to the aqueous dispersions of fiber. In addition, the stress sweep for the sample corresponding to 0 min in an ultrasound (L-0 min) is also included. All samples, except LU-20min-100%, show a greater elastic than viscous character (G′ > G″). The aqueous flaxseed dispersions subjected to ultrasonic homogenization for 2 min and the increasing amplitude values exhibit similar viscoelastic moduli and critical stresses values (

Table 2. Critical stress to exit of the linear viscoelastic region and fitting parameters of the power law equation for the storage modulus dependence of frequency of the aqueous dispersions of flaxseed fiber under different treatments of ultrasounds. Temperature: 20 °C.

Sample	τc (Pa)	a ± SDa	b ± SDb	R2
U-2min-40%	12.6	24.51 ± 0.560	0.14 ± <0.01	0.999
U-2min-60%	12.6	22.01 ± 0.020	0.16 ± <0.01	0.999
U-2min-80%	12.6	21.59 ± 0.01	0.17 ± <0.01	0.999
U-2min-100%	12.6	21.21 ± 0.17	0.16 ± <0.01	0.998
L-0min	12.6	30.86 ± 0.05	0.13 ± <0.01	0.999
LU-10min-100%	3.9	8.95 ± 0.12	0.33 ± 0.01	0.995
LU-20 min-100%	0.4	0.20 ± 0.00	0.65 ± <0.01	0.998

). This result indicated that, under these conditions, the extension of the linear viscoelastic region is not affected. In this same fiber, the amplitude of the LVR is also not affected by the homogenization rate or the geometry employed during rotor–stator homogenization, as previously reported [44]. Under different treatments of homogenization (non-homogenized, slightly homogenized, or highly homogenized), other dispersions of fibers such as those from apple pulp, a carrot, or a potato also did not exhibit a significant effect of homogenization on the value of critical stress [47]. The application of pre-shear and a higher ultrasonic time, as can be observed in Figure 1, provoked a significant decrease in the values of G′ and G″, with a greater decrease occurring the longer the exposure time. At the same time, there is a clear and significant reduction in the extension of LVR, with the value of critical stress going from about 4 Pa for LU-10min-100% to 0.4 Pa for LU-20min-100% (Table 2). A smaller value of this parameter is associated with a lower structural strength and a lower degree of molecular association [48]. It is worth noting that the sample L-0 min exhibited a critical stress value similar to that of the samples submitted to different ultrasonic amplitudes, 12.6 Pa. Therefore, it is deduced that the most relevant effect on the extension of LVR is the time of exposure to ultrasonic homogenization.

Once the linear viscoelastic region has been determined, the influence of frequency is studied. It is worth noting the relevance of this test since the small amplitude oscillatory tests allow for understanding of the viscoelastic behavior of the samples without an alteration of the structure.

First, the influence of amplitude percentage (A%) on mechanical spectra was investigated (Figure 2). All the samples exhibited storage modulus (G′) values greater than their loss modulus (G″) values in the whole frequency range studied, which pointed out a higher solid character than liquid. The small difference in values between G′ and G″ and the slight dependence of both on frequency indicated that these samples showed a weak gel-like behavior in agreement with the results obtained in the literature with other dispersions of polysaccharides [43,49]. An increase in amplitude percentage (A%) from 40% to 60% caused a slight decreasing trend in both viscoelastic moduli, G′ and G″. A further rise of A% did not significantly influence their values.

Next, the effect of ultrasonic homogenization time at an amplitude of 100% on the dependence of G′ and G″ with frequency was studied. The investigated samples were submitted to a previous treatment with Ultraturrax T50. For the sake of comparison, the viscoelastic properties of a sample prepared only by rotor–stator homogenization were also determined as a control. The latter exhibited a mechanical spectrum typical of weak gels, like the fiber dispersions submitted to different ultrasonic amplitude percentages, but exhibited slightly higher values of G′ and G″. It is worth noting that this fact allows for a connection of the results obtained for samples U and LU. When the ultrasound time was 10 min, i.e., more energy was applied, a change in the shape of the mechanical spectrum occurred. G′ was still greater than G″, but both moduli increased their dependence on frequency and, in addition, the difference in values between them decreased. The change of the microstructure was more pronounced when the ultrasonic homogenization time was 20 min (L-20min) to such an extent that the sample exhibited predominantly liquid behavior, with the values of G″ exceed those of G′. Li et al. (2016) [50] studied the influence of ultrasound treatment on the molecular weight (M_w) and rheological properties of wheat bran arabinoxylans and found decreasing values of M_w and G′ with the extension of the ultrasound treatment time. According to these authors, this result was due to the intense cavitation and mechanical forces generated by the ultrasound treatment, which led to the disruption of the arabinoxylan molecular chains. From this analysis, it can be deduced that a similar phenomenon could have occurred in flaxseed fiber at high ultrasound exposure times.

The G′ curves have been fitted quite well (R² > 0.99) to the Equation (1), which was previously used by other authors [51,52]. The values obtained for parameters a and b are shown in Table 2.

Concerning slope “b”, the sample that was not subjected to ultrasonic homogenization; L-0min presented the lowest value, followed by U-2min-40%. Afterwards, U-2min-60%, U-2min-80%, and U-2min-100% showed similar slopes between them but slightly higher slopes than U-2min-40%. Next, L-10min-100% exhibited a higher value, and finally the sample L-20min-100% showed the highest slope. Regarding the variation of the parameter “a” with the treatment employed, a tendency to decrease with an increase in the time or amplitude of ultrasonic homogenization was observed.

A relation between the microstructure and the energy applied per mass unity during the preparation of the aqueous dispersion of flaxseed fiber (Table 1) can be established. The increase in amplitude percentage in ultrasonic homogenization was associated with a greater energy applied to the sample, which caused a small breakdown of the interactions between macromolecules, and therefore, a loss of part of the tridimensional network formed. As a consequence, a slight decrease in moduli was observed and a slight increase in dependence on the frequency occurred, according to a sample with shorter relaxation times. Nevertheless, at these energy levels, the ultrasound effects on fiber were reduced. The application of pre-shear with Ultraturrax T50 and a higher ultrasonic homogenization time (10 min) noticeably increased the energy applied which provoked not only a higher level of breakage, and consequently a higher loss of entanglements, but also probably a significant decrease of the molecular weight, hence the change in the shape of the mechanical spectrum. When the time was 20 min, the alteration of the microstructure was considerable to such an extent that the network was completely broken, and the sample behaved as a liquid. From these results, we can deduce that shear during the preparation of aqueous dispersions of flaxseed fiber can negatively influence their microstructure and therefore, potential applications. In addition, if L-0 min and U-2min-100% are compared, it is possible to conclude that the Ultraturrax T50 rotor–stator device was less effective than ultrasonic homogenization, since applying similar energy per mass unity results in a slightly stronger microstructure.

Regarding flow behavior (Figure 3), all the samples presented a shear thinning behavior, characterized by a decrease in viscosity as the shear rate increases and a tendency to reach a Newtonian viscosity at very low shear rates. The application of shear induces an alignment of the initially disordered polymer chains along the flow direction, thereby reducing entanglements and intermolecular interactions, which results in a lower viscosity of the solution. Other fibers also exhibit pseudoplastic behavior, such as aloe vera mucilage, which is considered a type of soluble fiber [53], microfluidized wheat bran suspensions [36], or citrus fiber [43]. The flow curves demonstrated negligible sensitivity to variations in ultrasound amplitude, but showed marked sensitivity to the duration of ultrasonic exposure. The application time of ultrasonic homogenization created a negative impact, causing a decrease in viscosity values and a change in the flow index toward a more Newtonian behavior.

The experimental results have been quite well adjusted to the Carreau model (Equation (2)) as can be deduced from the regression coefficient R² (>0.99). The fitting parameters of the experimental results of the model can be found in

Table 3. Fitting parameters of experimental data for the Carreau model. SD is the standard deviation.

Sample	η0 (Pa·s) ± SDη0	$\dot{{\mathit{\gamma }}_{\mathit{c}}}\left({\mathit{s}}^{-1}\right)\pm$$\mathbf{SD}\dot{{\mathit{\gamma }}_{\mathit{c}}}$	n ± SDn	R2
U-2min-40%	3200 ± <0.1	2.4 × 10−3 ± 77.4 × 10−4	0.17 ± <0.01	0.990
U-2min-60%	2500 ± <0.1	3.5 × 10−3 ± 1.2 × 10−4	0.15 ± <0.01	0.996
U-2min-80%	2450 ± <0.1	2.3 × 10−3 ± 5.6 × 10−4	0.18 ± 0.02	0.955
U-2min-100%	2700 ± <0.1	2.7 × 10−3 ± 4.9 × 10−4	0.16 ± 0.01	0.962
L-0min	3690 ± 69.4	3.5 × 10−3 ± 2.4 × 10−4	0.13 ± <0.01	0.990
LU-10min-100%	122 ± 2.8	9.2 × 10−3 ± 6.3 × 10−4	0.30 ± <0.01	0.990
LU-20min-100%	118 ± 1.3	3.5 × 10−7 ± <1 × 10−4	0.60 ± 0.01	0.990

.

As mentioned above, the amplitude variable did not influence the fitting parameters. Instead, a trend to decrease zero shear rate viscosity was observed with the homogenization time, this fact being significant after submitting the control sample to ultrasonic homogenization (compare L-0min and LU-10min-100%). In contrast, the flow index, which was less than one in all samples supporting the shear thinning behavior previously mentioned, increased. This fact indicated an increase in Newtonian character and therefore, a decrease in the structural complexity of the sample. These effects of ultrasounds were previously observed by Wang et al. (2022) [54]. They investigated the flow behavior of chia seed mucilage solutions extracted by a combined thermal-ultrasound process and they observed that an increase in ultrasound time provoked a decrease in viscosity and an increase in n. A similar conclusion to that obtained with the flow index can be drawn from the values of the critical shear rate. This parameter, which is inversely related to the terminal relaxation time, tended to increase significantly with the ultrasonic homogenization time, which means that the terminal relaxation time decreased, increasing the liquid character of the sample. The long ultrasound time provoked a drastic reduction in viscosity, probably related to a reduction in the molecular weight of the fiber polymer [54,55,56]. Notably, this result could be explained by the level of chain entanglement: a lower molecular weight leads to a lower density of entanglement, enabling chain mobility and decreasing the time needed to restore the entangled network after deformation [53]. The flow results are in agreement with those obtained in the SAOS tests. Different behaviors were found in fibers such as citrus or lemon peel fibers, both considered mainly insoluble fibers. In lemon peel fiber subjected to different homogenization pressures, Willensen et al. (2018) [57] observed that increases in pressure enhance particle hydration and facilitate a restructuring of the particle network, leading to an increase in G′, which reaches a plateau at the highest pressure tested. In the same way, in citrus peel fiber, Lupi et al. (2020) [43] also found a stronger three-dimensional network when higher levels of energy or power of mixing were used for the preparation of citrus fiber in aqueous suspension.

3.2. Physical Stability of Aqueous Dispersions of Flaxseed Fiber Submitted to Ultrasonic Homogenization

In Figure 4, the results of delta-backscattering percentage (%ΔBS) versus the height of the measurement cell containing the sample as a function of the aging time for the U-2min-40% and LU-20min-100% samples are shown as an example. In the mentioned figure, it is observed that the main changes occur during the first 24 h of aging, after which the scans undergo only minimal changes. This result suggests the need to wait at least one day to stabilize the sample’s structure. However, these initial changes seem to be more significant the less shear is applied. This result may primarily be due to the fact that the greater the applied energy, the greater the destruction of the microstructure, leading not only to lower viscosity values but also to a significant loss of elasticity. As a consequence, there is less structure to stabilize. It should be noted that there is a slight syneresis process, characterized by a decrease in ΔBS% in the upper zone of the turbiscan vial.

To quantify the physical stability of these aqueous flaxseed fiber dispersions, the global TSI value was calculated, considering the entire height of the sample and therefore, accounting for all destabilization processes occurring within it. Figure 5 shows these values as a function of aging time and the processing variables studied. An analysis of this figure reveals that the greater the energy applied to the sample, the higher its physical stability, as inferred from the lower TSI values. However, in general, it can be stated that all the samples studied are quite stable, since the ΔBS% values are below 5% [58,59].

4. Conclusions

The effect of the ultrasonic homogenization variables (amplitude and time) on the rheological properties and physical stability of the aqueous flaxseed fiber dispersions with and without preliminary rotor–stator homogenization was assessed. The most significant variable was the ultrasonic exposure time. Thus, short times at increasing amplitudes (2 min, 40–100%) do not significantly influence the mechanical spectra, but longer treatments (10 or 20 min at 100%) alter the fiber microstructure, reducing the values of G′ and G″ and the critical stress. This result pointed to microstructural degradation and a transition toward liquid-like behavior. All the samples exhibited shear-thinning behavior, typical of polysaccharide networks. Consistent with the results obtained by SAOS tests, amplitude was not a sensitive variable in a short period of time. Higher ultrasound exposure decreased zero shear viscosity and increased the Newtonian character (higher flow index) of the samples, indicating a loss of structural complexity. The hard conditions under ultrasound treatment probably reduced molecular weight as a consequence of chain scission. Most of the changes in backscattering occurred within the first 24 h, stabilizing thereafter. In general, all the samples were stable over the study period.

The results obtained reveal the importance of properly controlling ultrasonic processing variables (amplitude and duration) in order to maintain the desired rheological properties and, therefore, the techno-functional properties of flaxseed fiber, since high shear has a negative impact on microstructure. Nonetheless, future studies demonstrating the decrease of molecular weight after a long duration of ultrasonic treatment (by high-performance liquid chromatography, as example) and examining the effects of this emergent technique (ultrasonic homogenization) on the biological properties of this fiber would provide valuable insights of significant scientific interest.

These findings support the rational design of functional products using flaxseed fiber and expand the understanding of an ultrasound as a processing tool in fiber-based systems.