*3.1. Physicochemical Characterization of Samples*

*Psyllium* gel prepared with autohydrolysis (AH) liquor (PsyL) was visually more fluid than the *Psyllium* gel prepared with water (control). This is confirmed by texture and rheology measurements, so this issue will be discussed in Sections 3.3 and 3.4.

As seen in Figure 1, the *Laminaria-Psyllium* 75.25 gel samples presented syneresis (water in the filter paper, not measured) more evident in the sample not submitted to ultrasonic treatment. According to the previous study in which the purée-like mixtures were optimized [17], the authors did not find syneresis in the systems. This could be due to the alga high sodium content that bonds to *Psyllium*, and at the 25% ratio was not able to retain all the water in the system, causing the *Laminaria-Psyllium* 75.25 gels to contract and release part of the water previously enclosed. This phenomenon did not occur in samples with higher *Psyllium* gel ratios. A recent study by Figueroa and co-workers [21] reported the positive influence of *Psyllium* on the absence of syneresis of fruit jellies enriched with dietary fiber.

In Table 1, a chemical characterization of *Psyllium* husk and the developed gels is presented.


**Table 1.** Moisture, ash, and sulfate content of *Laminaria ochroleuca*, *Psyllium* husk, and the developed gels.

Psy (control), *Psyllium* gels prepared in water; PsyL, *Psyllium* gels prepared in autohydrolysis liquor; *Laminaria-Psyllium* gels: Lo.Psy\_25.75, Lo.Psy\_50.50, and LoPsy\_75.25; *Laminaria* with ultrasonic treatment-*Psyllium* gels: LoUS.Psy\_25.75, LoUS.Psy\_50.50, and LoUSPsy\_75.25. Data are presented as the mean ± sd. Different letters in a column show significantly different data values at the *p* < 0.05 level.

The moisture content of the *Laminaria-Psyllium* gels depends greatly on the proportion of both components, and decreased with the increase in *Laminaria* content. Ash content revealed the reverse trend, with the *Laminaria* proportion being crucial for the final gel mineral content. This is due to the high ash content (35%, d.b.) of this alga, as we previously determined [17]. It is also noteworthy that the ultrasonic treatment applied to the purée-like mixtures had a significant (*p* < 0.05) negative influence on the total ash content of the gel, especially in the 25.75 and 50.50 samples.

Sulfate presence in the *Laminaria-Psyllium* gels is a clear indication of the presence of fucoidan, a sulfated polysaccharide with reported activity against stomach-gastric adenocarcinoma cells and lung carcinoma cells [22].

The most important chemical feature of *Psyllium* husk is its high soluble fiber content (80%, d.b., as previously determined by the authors in the same sample [10]. However, aside from its fiber content, *Psyllium* husk has a mineral composition of 10.3 g K/kg, 0.88 g Na/kg, 1.36 g Ca/kg, and 94.7 mg Fe/kg, (present study), the last two minerals having a higher content than most cooked pulses [23].

Another important feature is *Psyllium*'s ability to retain sodium at physiological important conditions (pH 1.2—stomach; pH 6.8—intestine), being potentially active in reducing the bioavailable fraction of ingested sodium in the body [24].

Considering both the *L. ochroleuca* [17] and *Psyllium* husk's mineral content, one can conclude that the partnership between this alga and *Psyllium* husk could be advantageous for the development of new food products.

Water activity was very high and ranged between 0.999–1.000 for all samples (data not shown) and these are typical values for gels, although some reduction was expected in gels with a high proportion of *Psyllium*.

#### *3.2. Color Evaluation of Samples*

From the color evaluation results presented in Table 2, the *Psyllium* gels (control and PsyL) only differed in terms of their chromatic parameters, which could be of importance depending on the desired application.


**Table 2.** Color parameters (L\*, a\*, b\*) and ΔE\* of *Psyllium* husk, *Laminaria ochroleuca* and its AH liquor, and the gels developed.

Psy (control), *Psyllium* gels prepared in water; PsyL, *Psyllium* gels prepared in autohydrolysis liquor; *Laminaria-Psyllium* gels: Lo.Psy\_25.75, Lo.Psy\_50.50, and LoPsy\_75.25; *Laminaria* with ultrasonic treatment-*Psyllium* gels: LoUS.Psy\_25.75, LoUS.Psy\_50.50, and LoUSPsy\_75.25. Data are presented as the mean ± sd. Different letters in the same column correspond to significant differences (*p* < 0.05).

As expected, the lightness (L\*) of the gel samples decreased with the incorporation of *Laminaria*, although significant differences were only registered between the *Laminaria-Psyllium* 25.75 and *Laminaria-Psyllium* 75.25 samples. It is noteworthy that the color parameters of the gel samples with the highest *Laminaria* content were like those obtained in the purée-like mixtures [14]. Moreover, the color parameters L\* (Lo.Psy: r <sup>=</sup> <sup>−</sup>0.928, *<sup>p</sup>* <sup>=</sup> 1.3 <sup>×</sup> <sup>10</sup><sup>−</sup>9; LoUS.Psy: r <sup>=</sup> <sup>−</sup>0.920, *<sup>p</sup>* <sup>=</sup> 9.3 <sup>×</sup> <sup>10</sup><sup>−</sup>9) and a\* (Lo.Psy: r = <sup>−</sup>0.849; *p* = 1.2 <sup>×</sup> 10−6; LoUS.Psy: r = <sup>−</sup>0.913, *p* = 2.0 <sup>×</sup> 10−8) were strongly negatively correlated with *Laminaria* content in the gel formulations.

The ΔE\* between the *Laminaria-Psyllium* gels and the control ranged from 4.5 to 8.2, increasing with the increase in the *Laminaria* content in the system. These values indicate that the consumer would distinguish between the two samples compared [25].

It should be pointed out that the gels proposed here are not finished products. In this sense, the color of the final food gel product could be optimized by considering these findings and according to the desired final color.

## *3.3. E*ff*ect of Laminaria-Psyllium Ratio on the Dynamic Viscoelasticity*

Figure 2 shows the elastic behavior of *Laminaria-Psyllium* gel systems. As the alga fraction increased, a structuring effect was observed in the gels. This behavior was markedly noticed in the highest alga concentration (Lo.Psy\_75.25 and LoUS.Psy\_75.25). This interaction is probably due to physical entanglements between the polymers present in both the alga and *Psyllium*, causing the reinforcement of the gel. However, in the absence of the *Psyllium* gel, the alga purées showed a huge decrease in G to values similar to the control (Psy), but more frequency dependent (b = 0.209) [17].

**Figure 2.** Elastic modulus at 0.1 Hz, 1 Hz, and 10 Hz of *Laminaria-Psyllium* gels with (LoUS.Psy) and without ultrasonic treatment (Lo.Psy), control, and *Laminaria* purées (Alga Purée; Alga Purée.US) [17].

This synergistic interaction was also found in other polymeric systems, namely between the locust bean gum (LBG) and xanthan gum, where the latter did not form gel, rather a shear-thinning solution, but combined with LBG to form a strong gel structure [26].

The mechanical spectra of the developed gels are depicted in Figure 3.

**Figure 3.** Mechanical spectra of the *Psyllium* gels prepared in water (control) and autohydrolysis liquor (PsyL).(**a**) *Laminaria-Psyllium* gels (Lo.Psy\_25.75, Lo.Psy\_50.50, LoPsy\_75.25); (**b**) *Laminaria* with ultrasonic treatment-*Psyllium* gels (LoUS.Psy\_25.75, LoUS.Psy\_50.50, LoUSPsy\_75.25; (**c**) G , closed symbol; G", open symbol.

To quantify the impact of the different combinations of *Laminaria* and *Psyllium* on the viscoelastic moduli, the variation of G and G" with gel composition was obtained from the respective mechanical spectra (Table 3).


**Table 3.** Power law parameters (α , α", b , and b") of the gel samples with *Laminaria ochroleuca* and *Psyllium* husk, control, and PsyL.

The goodness of fitting (R2) ranged from 0.995–0.997 for G and from 0.936–0.997 for G".

The mechanical spectra Psy (control) and PsyL exhibited similar viscoelastic performance typical of well-structured weak gels, with slight frequency dependence (Figure 3a). This result is consistent with the rheological study by Haque et al. [27], in which it is also reported that *Psyllium* husk forms gel even at low temperature, this being an important feature to consider in food design. Moreover, the developed gels were more stable at higher frequencies than the ones produced with 10–15% chia flour at 90 ◦C (Ramos et al., 2016), reinforcing *Psyllium*'s potential as a valuable and sustainable biopolymer.

As mentioned earlier, PsyL appeared to be more fluid than the control (Psy), which was confirmed by the mechanical spectra, with both viscoelastic moduli of the control being higher than those of PsyL, and by the decrease of α (Table 3). Autohydrolysis promotes the solubilization of minerals [17] and the depolymerization of polysaccharides, namely alginate, fucoidan, and laminarin present in brown algae [28], rendering a liquid extract with an acidic pH (≈5, [19]). Since *Laminaria* liquid extract is a multicomponent matrix, its effect on the *Psyllium* gel properties are more complex, depending not only on the solution pH, but also on the type of ions in the solution, and even on the presence of peptides and other molecules resulting from the depolymerization of the polysaccharides. These polymer fractions can interfere with the gel matrix and exert an antagonism, leading to the reduction of gel links and de-structuring the material.

The ultrasonic pre-treatment applied to *L. ochroleuca* did not affect the rheological behavior of the *Laminaria-Psyllium* gels. The weak-gel like behavior was maintained in all *Laminaria-Psyllium* gel samples, and was more noticeable as the alga fraction increased, as can be observed by the increase in the b parameter of the resulting power law (Table 3). As the incorporation of the alga solid fraction increased, the dependence of the material on frequency increased; however the value of the viscoelastic moduli increased probably due to the reduction of the number of links, but also stronger ones, which may be due to interactions between the alginate from the alga and fiber from the *Psyllium* husk reinforced by calcium ions [29].
