**4. Soy Polysaccharides**

Soybean soluble polysaccharide (SSPS), a by-product of isolating soybean proteins, has been reported to be used as an emulsifier for the emulsification of beverages owing to the acidic nature of polysaccharide. Rhamnogalacturonan backbone is present in the SSPS structure, which is branched by β-1,4-galactan and homogalacturonan, α-1,3-, or α-1,5-arabinan chains [47] (Figure 3). It has been reported as a source of dietary fibers in fortified foods as well as a functional ingredient for food and pharmaceutical applications. The conformation of SSPS is not easily affected by pH and ionic strength which results in environmental stability of the SSPS stabilized emulsions. The structure of glycoprotein present in SSPS is almost similar to that of the Wattle Blossom Model suggested for gum arabic. The attachment of carbohydrate functionality of the polysaccharide on the oil–water interface is mainly due to the protein fraction of SSPS. Therefore, the hydrophilic portion of SSPS forms a 30 nm thick hydrated layer that retard the chance of coalescence and stabilizes the oil droplets by steric repulsion [48] (Figure 3).

**Figure 3.** The chemical structure of soybean soluble polysaccharide (SPSS) (**left**) and a schematic diagram of SSPS-stabilized emulsion droplets (**right**).

Generally, the emulsifying ability of SSPS is affected by the protein fraction, molecular weight, and extraction conditions. Nakamura, Takahashi [47] studied the emulsifying potential of three different types of SSPS and observed that all the soy polysaccharide stabilized emulsions showed stability against creaming for 30 days at pH 3.5–5.0 when the polysaccharide concentration was above 4%. The soy polysaccharide that was extracted at pH 3.0 and 120 ◦C for 2 h showed the best emulsifying ability, in which the protein fraction had an inevitable effect. Nakamura et al. [49] separated the SSPS into two fractions, i.e., high molecular and low molecular, and found that the 2.2% protein facilitated the emulsifying properties of the high molecular fraction but did not exhibit a similar effect on

the low molecular fraction of SSPS. The emulsions stabilized by the high molecular fraction showed no change when heated at 90 ◦C and pH 3.0–7.0 or in the presence of <10 mM CaCl2, while the low molecular fraction stabilized emulsions undergo aggregation when heated at pH 7.0 [50]. The SPSS was further enzyme-digested by pectinases (polygalacturonase (PGase), hemicellulases (galactosidase (GPase), and rhamnogalacturonase (RGase)), and arabinosidase (Afase). The Rgase digested SSPS showed improved emulsifying properties while the others compromised the emulsifying potential [51]. The additions of SSPS to the protein-stabilized emulsions have also been reported to progress the stability against thermal treatment, low pH, and under simulated gastric conditions. Yin et al. [52] fabricated the stable nano-sized emulsions with the help of soy protein and SPSS complexes, which formed the interfacial films under the influence of the temperature by the process of electrostatic complexation of the denatured protein and soy polysaccharide. The interfacial fixed polysaccharide chains are also able to stabilize the oil droplets in an aqueous medium even in the unfavorable condition of soy protein in which they undergo aggregation.

Recently SPSS has shown potential applications in food emulsion products, such as beverages and mayonnaise. Nakamura et al. [53] used SSPS as a stabilizer in the dispersions of acid milk and suggested the comparable stabilizing potential of SSPS with pectin. It was also found that SSPS did not show interaction with casein at pH > 4.6, but exhibited better stabilizing ability at pH < 4.2 than high methoxyl pectin [54]. Chivero et al. [55] examined the ability of SSPS to produce O/W mayonnaise-like emulsions and observed that SSPS could stabilize emulsions with a maximum oil content of about 60 wt%, and the emulsions remained stable after 30 days. The improved stability was observed when SSPS was combined with a thickening agen<sup>t</sup> (xanthan gum) to induce a stronger network. Xu et al. [56] fabricated casein and SSPS compact complex aggregates of 133 nm (Figure 4a), leading to the stabilized emulsions having the stability of more than 500 days with a curcumin loading efficiency of 99.9% and droplet diameter of about 324 nm (Figure 4b). It was also found that the absorption of curcumin was more effective compare with the absorption of the curcumin/Tween 20 suspension group, resulting in 11-fold higher oral bioavailability of curcumin in the emulsion group (Figure 4c). Zhan et al. [57] studied the SSPS effect on the functional characteristics of pea protein isolate (PPI), and found that SSPS adhered to PPI by means of hydrophobic interaction and hydrogen bonding which resulted in decreased hydrophobicity of the surface of reconstituted PPI particles and enhanced the stability of emulsions. It was also suggested that the incorporation of SSPS rearranged and interconnected the modified particles, resulting in the improved interfacial and rheological properties of the emulsions.

**Figure 4.** Diagrammatic representation of formation of SSPS-casein complex (**a**), the stabilized emulsion with a long-term stability (**b**), and the significantly improved loading and bioavailability of SSPS-casein stabilized curcumin emulsion compared to that stabilized by Tween 20 (**c**) [56].

Apart from SSPS, the insoluble soy polysaccharide (ISP) containing cellulose, hemicellulose, lignin, and a protein fraction, has attracted attention due to its potential as a Pickering emulsion stabilizer. Porfiri et al. [58] performed the acidic extraction at pH 3.5, 120 ◦C and extracted insoluble soybean polysaccharide (ISPS) from insoluble okara. The pretreatments (high-pressure homogenization or sonication) are assumed to expose the internal site of the structure of protein and polysaccharide to enhance the superficial hydrophobicity. This in turn facilitates the formation of the outer layer and/or absorption of the macromolecules at the oil–water interface, hence providing increased rigidity of the interfacial film. Particularly, the molecules that resulted from high-pressure homogenization exhibited promising emulsifying potential and showed stability against the pH variation of the emulsions. Mwangi et al. [59] found that ISP dispersions under a high power ultrasonication treatment result in the breakdown of polysaccharide fibers and allow the preparation of the nanoparticles with a size range of 127–221 nm. The fabricated nanoparticles exhibited remarkable potential towards emulsification and allow the formation of Pickering emulsions. It was further reported Yang et al. [60] also fabricated ultrasound-induced insoluble soy polysaccharide nanoparticles with a size of about 160 nm, which can be served as an outstanding Pickering stabilizer for the emulsions having a high internal phase due to the formed gel network (Figure 5). The high internal phase Pickering emulsions showed high stability against environmental stress. The gel structure can be maintained over the pH range 2.0–12.0 and ionic strength range 0–500 mM. All the high internal phase Pickering emulsion gels exhibited excellent stability against prolonged storage and heating, as well as unique reversibility of freeze–thawing-destabilization/re-emulsification.

**Figure 5.** Synthesis of ultrasound-induced insoluble soy polysaccharide (ISP) based nanoparticles for developing and edible O/W high internal phase Pickering emulsion gels [60].
