*6.2. Fat Replacer*

To mimic the properties of animal fat, soy-based emulsified systems have been researched as a promising alternative to replace animal fat [77,78]. Emulsion gel structures based on soy protein have the potentials to replace fat in sausages [79,80]. Pintado et al. [81] fabricated the soy protein stabilized emulsion gels that contained two different solid extracts of polyphenols obtained from grape seed and olive to be used as an animal fat replacer in the development of frankfurters.

The addition of SPI in ice creams may decrease the influence of heat on the recrystallization and melting rate of ice. Chen et al. [82] reported that larger-sized SPI nanoparticles with higher surface hydrophobicity and enhance the potential of packing at the oil–water interface are more suitable to form Pickering emulsions with enhanced freeze–thaw stability. The addition of soy protein in ice cream reduces the heat shock effect on the melting rate and recrystallization of ice. Functional optimization of the structure of soy protein is currently attracted grea<sup>t</sup> interest to achieve a high degree of fat globule partial coalescence in the preparation of ice cream. Soy protein under selective hydrolysis provides sufficient fat partial coalescence and good melt-down rates. Chen et al. [83] compared the effect of commercial soy protein isolate (CSPI), native soy protein isolate (NSPI), soy protein hydrolyzed by pepsin (SPHPe), and skim milk powder (SMP), and soy protein hydrolyzed by papain (SPHPa) on ice cream mix stability and melt-down properties. Among all these, the highest melt-down rates were observed in the ice cream made with SPHPe, which ranged from 1.23% min−<sup>1</sup> to 2.05% min−1. While the CSPI, SMP, and SPHPa based ice creams exhibited the lowest melt rate with no significant difference from each other. The highest fraction of protein at the fat globules during aging and freezing was SPHPa mix (22.6%) which was almost similar to SMP (21.8%).

Currently, the soybean has been considered as popular among all the plant-based sources used for yogur<sup>t</sup> production, because of its quantity, quality, and functional characteristics of the protein. Such plant-based systems upon acidification cause destabilization of soy proteins, which may result in the formation of a non-continuous, weak gel [84]. SPNPs stabilized emulsions have been studied as functional non-dairy yogurts. Sengupta et al. [85] found that the addition of SPNPs distinctly improved the quality of soy yogurts with enhanced radical scavenging activity and ferric reducing property. The SPNPs (72.42–586.72 nm) incorporated soy yogurts exhibited significantly enhanced oxidative stability against peroxidation of lipids, indicating the applicability in the development of functional yogurts.

Consumers are purchasing plant-based "milks" and "creams" frequently because of their environmental and perceived health benefits. Hence, the food companies have been developing a wide range of plant-based milk and creamer products including soy products. Chung et al. [86] applied soy lecithin (1–5%) to stabilize 10% O/W emulsions, which remained physically stable upon addition of an acidic hot solution of coffee and retard the phase separation or increment in particle size. The soy lecithin stabilized oil droplets exhibited the enhanced surface negativity, hence they exert the strong electrostatic repulsion among the droplet and retard their aggregation. Koo, Chung [63] found the ability of soy lecithin to replace the caseinate in coffee creamers. The model O/W emulsions stabilized with a mixture of emulsifiers based on sodium caseinate (0.5%) and soy lecithin (0.5%) showed physical stability over a pH range of 5.5 to 7.

#### **7. Challenges and Future Trends**

It should be noted that the commercial soy-based products are changing along with the development of pro-cessing technologies. The functional properties of soy-based emulsifiers are sensitive to the processing history, such as enzymatic, thermal, and jet cooking processes [87–89], which makes the soy ingredients used in literatures varied. Hence, understanding the physiochemical states of various soy-based emulsifiers is necessary for better application of that in food products. Lee, Ryu [88] compared the solubility of commercial soy protein from different manufacturers and they found the soy protein isolates

can be classified into three groups. One group had high solubility near the pl. Another group had low solubility near the pl, but high solubility at pH 11. The third group had low solubility even at pH 11. Recently, Zheng, Wang [89] selected 20 soy protein isolate samples from three manufacturers and they found that the best sample was Chiba tofu due to high hardness and springiness, and excellent quality. Hence, the researchers should identify their soy ingredients and select the most appropriate starting materials for their future academic or product development work.

Recently, various studies focused on the effect of novel technologies on the conformation, molecular interaction, and functional properties of soy proteins, soy polysaccharides, and soy lecithin, which provide fundamental understanding of the mechanism of their functional properties in various food processing conditions. However, there are still some challenges on the modification process on the soy-based materials. In future studies, the combination of the physical, chemical, enzyme, or biological methods to improve the emulsifying properties and other functional properties will be a trend for the development of novel food emulsifiers. However, the main challenge for the development of such novel emulsifiers is to select a stable fraction with high functionality under certain environmental conditions for the desirable formulated products. Therefore, more studies are needed to understand the relationship between the functional properties and molecular structure of these components before use in practice. The combination of soy based low molecular weight emulsifiers and high molecular emulsifiers (proteins and polysaccharides) will be a challenge in extending their applications to new fields.

**Funding:** This work was supported by the Natural Science Foundation of Hubei Province of China (Grant No. 2020CFB204), the Key Research and Development Program of Hubei Province of China (Grant No. 2020BAB078), and the Foundation of Xinjiang Uygur Autonomous Region of China (Grant No. 2020D01B08).

**Conflicts of Interest:** The author declares no conflict of interest.
