*3.1. Particle Size and Microstructure*

In Table 1, the particle size of the mayonnaise samples, expressed as both D4,3 and D3,2, which are related to the size of the bulk of the droplets constituting the mayonnaise and the size of small droplets [22], respectively, are reported together with the specific surface area (SSA).

**Table 1.** Particle size parameters of mayonnaise samples after preparation, as affected by olive leaf phenolic extract (OLE) and alginate/pectin (Alg/Pec) + OLE enrichment.


Values are means ± SD. Different superscript letters in the same column indicate significant differences (*p* < 0.05).

The results showed a reduction of the volume mean diameter (D4,3) and the surface mean diameter (D3,2) values for both of the enriched samples (Mayo + Alg/Pec and Mayo + OLE), with respect to the control, and a corresponding increase of the specific surface area with values of 4272, 3663, and 3270 for Mayo + Alg/Pec, Mayo + OLE, and the control, respectively. In general, the particle size distribution curves of the samples showed a polymodal distribution with two main populations and a shift towards a lower size in both of the enriched systems, which was more evident in the Mayo + Alg/Pec sample (Figure 1).

**Figure 1.** Particle size distribution curves of the differently enriched mayonnaise samples after preparation.

The decrease of the particle size in both of the enriched systems can be associated with the presence of olive leaf amphiphilic phenolic compounds, which have been proven to lower the oil/water interfacial tension and exert emulsifying properties, especially when tested in acidic conditions (acetate buffer, pH 4.5) [7]. In the case of Mayo + Alg/Pec, the presence of microparticles could have further improved the formation and stabilization of the emulsified structure, likely due to a Pickering stabilizing mechanism. Indeed, particles are able to stabilize oil/water interfaces, making them mechanically stronger, and are able to provide a sufficient steric repulsion force to inhibit droplet coalescence during emulsification [23]. The positive effect of OLE on the dispersion degree of the oil droplets is in accordance with other authors who prepared mayonnaise samples enriched with increasing amounts of olive phenolic extract [24]. In particular, the mayonnaise prepared with sunflower oil (SO) with an amount of oil (≈ 80%) comparable to the systems under investigation in this study, showed quite similar values of D4,3 with respect to the control samples. However, when the enrichment was carried out at a similar concentration (200 ppm) to the one used in this work, a less broad particle distribution with a lower degree of polydispersity was obtained. Moreover, in the same study, olive oil mayonnaise-like emulsions formulated with different naturally phenolic-rich extra virgin olive oils displayed similar results when a low phenolic olive oil (270 mg/kg) was used as the source of phenolic compounds. Therefore, such results strengthen the argument for the use, for the formulation of high oil content emulsions, of a phenolic-rich extract recovered by olive by-products, with respect to a more expensive ingredient, such as extra virgin olive oil.

Optical microphotographs were captured to characterize the microstructural features of the samples (Figure 2a–c). In general, a well-dispersed oil phase could be observed in all of the systems, in agreement with the particle size results. While the presence of larger droplets was evident in samples a and b, defining a higher degree of polydispersity, a more finely dispersed structure and a closely packed distribution of oil droplets were visible in the mayonnaise enriched with OLE-loaded alginate/pectin microparticles.

**Figure 2.** Optical micrograph images of mayonnaise samples just after preparation. Control (**a**), Mayo + OLE (**b**), and Mayo + Alg/Pec (**c**).

#### *3.2. Color Properties*

Color has a main impact on consumers' choice, as it is one of the most important sensory features that affect the willingness to purchase or taste a food. In general, the typical pale yellow color of mayonnaise originates from the egg yolk and oil and may be further influenced by the addition of mustard, additives, or some other spices with coloring effects (i.e., Annatto E160 and Turmeric). The enrichment of mayonnaise with unconventional ingredients, which are different with respect to those used in a standard recipe, could lead to physical and chemical changes that may affect, in turn, the color of the final products. In Figure 3, the chromatic indices of lightness and hue angle (L\* and *h*◦, respectively) of mayonnaise samples, prepared with the addition of free OLE or OLE encapsulated in alginate/pectin microparticles, are reported, along with the control sample.

**Figure 3.** Color properties in terms of the lightness (L\*) and hue angle (*h*◦) of the mayonnaise samples under investigation. Different letters for parameters are significantly different by Tukey's HSD test (*p* < 0.05).

The lightness value (L\*), which is the colorimetric parameter referring to the ability of the system to reflect and scatter the light, was, in general, similar to data reported in the literature for full-fat mayonnaise [25–27]. Significant differences (*p* < 0.05) between the L\* value of the control and Mayo + Alg/Pec sample with respect to that of the Mayo + OLE sample were observed and this result may reflect the different dispersion degrees of the systems. In particular, the Mayo + OLE product displayed a significantly lower lightness due to the color of the added OLE extract, which, in the case of Mayo + Alg/Pec samples, was mitigated by the encapsulation.

On the contrary, the results of the hue angle were close to 90◦, confirming the yellow color, without significant differences (*p* > 0.05) among the samples. Overall, these results highlight that the addition of the olive leaf extract, either as a pure or encapsulated form, did not alter the visual appearance of the emulsion.

#### *3.3. E*ff*ect of OLE Enrichment on Flow Behavior and Lubricant Properties*

The viscosity properties of concentrated emulsions, such as mayonnaise, are strictly related to the close packing of the dispersed oil droplets as they interact with one another in the matrix: The closer the droplets, the higher the viscosity, as a consequence of the higher droplet–droplet interaction [24].

The flow curves of mayonnaise samples are reported in Figure 4. In general, all of the mayonnaise samples exhibited a non-Newtonian shear-thinning behavior and a yield point related to the initial resistance of the systems to flow. Control and Mayo + OLE samples are both characterized by a similar trend, with a tendency to break-down at a higher shear rate (300 s<sup>−</sup>1). At a similar shear rate, OLE-enriched mayonnaise displayed lower shear stress values with respect to the control, while the Alg/Pec mayonnaise samples showed higher yield stress and shear stress values. This latter behavior may be related to several factors, including the higher packing degree of the lower size oil droplets (Figure 2c) and the thickening effect of beads swelling due to the presence of alginate and pectin in the aqueous continuous phase. Recent studies observed an increase in viscosity for mayonnaise enriched with carbohydrate-based capsules due to the thickening effect of the polymers (i.e., dextran, pullulan, glucose syrup, and zein) used as shell material upon their disintegration [28,29]. Moreover, it must be pointed out that the pH values of the samples were 3.87 ± 0.06, 3.92 ± 0.01, and 3.89 ± 0.01 for the control, Mayo + OLE, and Mayo + Alg/Pec, respectively, which are values close to the average isoelectric point of egg yolk proteins, when the protein charge is minimized and both the viscoelasticity and stability of mayonnaise are generally enhanced [30].

**Figure 4.** Flow curves of the mayonnaise after preparation.

Mayonnaise flow curves were fitted with the Herschel–Bulkley model, which is commonly used to describe the flow properties of concentrated emulsified systems such as mayonnaise [31]; the flow parameters yield stress τ<sup>0</sup> (Pa), consistency index K (Pa sn), and flow index *n* of the samples after preparation are listed in Table 2.

**Table 2.** Herschel–Bulkley viscosity parameters of the mayonnaise samples.


Values are the median ± SD. The flow parameters are the yield stress τ<sup>0</sup> (Pa), consistency index K (Pa sn), and flow index *n*. Different superscript letters in the same column indicate significant differences (*p* < 0.05).

When compared to the control, both of the enriched samples were statistically different (*p* < 0.05) in terms of K, τ0, and *n*, with Mayo + Alg/Pec showing the highest τ<sup>0</sup> and *n* values. As reported in the literature, yield stress values are an index of the strength of the attractive forces between the oil droplets [24,32] and this is in agreement with the results related to the Mayo + Alg/Pec sample, which exhibited the finest oil dispersion (Table 1).

Moreover, all of the mayonnaises presented a flow index (*n*)—the parameter that describes a pseudoplastic shear thinning behavior—lower than 1. It is interesting to note that the flow index tended to increase from the control to the enriched samples, with Mayo + Alg/Pec mayonnaise showing the highest value, corresponding to a lower pseudoplastic behavior. A shear-thinning profile in an oil-in-water emulsion is an indication that the system flows more readily as the dispersed particles align with the direction of flow; the enrichment with alginate/pectin OLE-loaded beads, which undergo swelling upon hydration, caused an increase of solids in the dispersing phase that likely hindered the alignment of the oil droplets.

It is known that flow behavior can affect the mouthfeel of food emulsions, while other sensory attributes, such as creaminess, can be correlated with the lubrication properties of the emulsified structure. Overall, the perception of a food emulsion in the mouth is initially dominated by the bulk viscosity at the front of the oral cavity, where the gap between the tongue and palate is rather large (>10 μm) [33]; upon chewing, the product mixed with the saliva is squeezed between moving surfaces, such as the tongue and palate (or food substrate and palate), and the oral perception depends on its thin film rheological behavior. To better analyse the rheological properties of the mayonnaise samples in the form of thin films, tribological investigations were carried out. The lubrication properties of the

control and differently enriched mayonnaise samples were represented in the form of a Stribeck curve (Figure 5), where the friction coefficient (−) was plotted against the sliding velocity (mm s−1) in the regions from 1 mm s−1, generally associated with mouth-like conditions during food consumption, up to 200 mm s<sup>−</sup>1, which is the maximum speed that the human tongue can move [34].

**Figure 5.** Stribeck curves of mayonnaise samples.

For all samples, it is possible to observe the occurrence of a typical Stribeck curve where the different regimes can be distinguished: In particular, the boundary regime, at low sliding speeds, with surface–surface interactions dominating the friction response with high friction factors; the mixed regime, at mid-speeds, when the sample starts to separate from the contact surfaces, decreasing the friction; and, finally, the hydrodynamic regime, where the surfaces are completely separated by the fluid with an increase of the friction values. For all of the mayonnaise samples, a similar behavior against the entertainment speed was noticed, with some differences in the friction factor values with the following ranking: Mayo + Alg/Pec < Mayo + OLE < control, and a clear discrimination among samples. With the exception of the highest sliding speeds, where all of the samples present similar trends, the mayonnaise enriched with Alg/Pec microparticles showed the lowest friction values, probably associated with a higher content of solids in the formulation and the finest oil droplet dispersion, with a corresponding better lubricant behavior. The highest lubricant properties of Mayo + Alg/Pec could also be attributed to the swelling of the hydrogel particles, which made them more deformable, allowing a decrease in the friction [35]. Another interesting result is related to the fact that Mayo + Alg/Pec sample exhibited a shift of the transition from a mixed to hydrodynamic regime toward a higher speed with respect to control and Mayo + OLE samples.

Despite the general effect of the addition of OLE, in both the free form and encapsulated in Alg/Pec beads, on the lubricant properties, which decreased the friction factors obtained by tribological measurements, additional investigations are needed to describe the behavior of such systems during oral processing, where other parameters, including the presence of saliva and mouth conditions (e.g., temperature), may play a key role.

#### *3.4. E*ff*ect of OLE Enrichment on the Sensory Profile*

Mayonnaises enriched with pure OLE or OLE encapsulated in alginate/pectin microparticles were assessed by a group of 15 panelists using a quantitative descriptive analysis. Before each session, the assessors were trained on each attribute, the scale used, and how to score the samples. The average scores of each selected attribute are reported in the spider plot shown in Figure 6. As expected, polyphenols addition, either in a pure or encapsulated form, significantly affected the bitterness perception with respect to the control sample, while no marked differences were reported for the

two enriched mayonnaise samples, which were both perceived with a medium bitterness intensity (score ≈ 5.8) not appreciated by the assessors. The enrichment of mayonnaise did not influence the color, astringency, consistency, creaminess, and graininess parameters, which could be related to the presence of OLE or microcapsule powder. The saltiness, sourness, and spreadability were partially influenced by the addition of alginate/pectin microcapsules. It can be hypothesized that the saltiness increase may have been caused by the presence of calcium alginate of the microgels structure; moreover, in the literature, it is reported that bitter compounds such as caffein can enhance the effect of salt [36]. However, to the best of the authors' knowledge, to date, no literature data has been reported on the interaction between the saltiness and bitterness of olive leaf phenolic compounds. The enriched samples scored low in spreadability with respect to the control, indicating some resistance to spread, which is a result that is in accordance with the higher values of yield stress observed in the flow curves. Eventually, the sensory evaluation highlighted that both of the enriched mayonnaise samples, but in particular the system fortified with OLE-loaded microparticles, displayed the lowest overall acceptability from the panel. The results reflect the low ability of alginate/pectin microparticles to mask the bitterness of an OLE extract and present new challenges for further investigations, especially in terms of alternative encapsulation methods, in order to overcome such issues.

**Figure 6.** Spider plot of specific sensory attributes of the mayonnaise formulations. Lowercase letters for each attribute indicate significant differences (*p* < 0.05).
