*3.4. Glycation Modification*

Traditional chemical modifications of proteins, such as acylation, phosphorylation, and alkylation, are less researched in food applications due to safety and environmental concerns [30]. Protein non-enzyme glycation, commonly regarded as the initial stage of the Maillard reaction, has been widely researched in the food industry to modify the functional properties of protein due to the relatively mild and safe reaction conditions, and no extraneous chemicals were needed [31]. Thus, this makes glycation a promising method for protein modification in the food industry [32]. There have been emerging studies that focused on improving the stability of glycated soy protein hydrolysates stabilized emulsions. Recent studies that focused on the glycation of polysaccharides with soy proteins for improved emulsifying properties are listed in Table 1. Covalent attachment between proteins and polysaccharides may enhance the protein functionality to act as both emulsifier and stabilizer. Most of the investigations (Table 1) conducted on Maillard conjugates followed a similar trend that glycated conjugates increased the emulsification ability and emulsion stability. The main advantages of the soy protein-polysaccharide conjugates synthesized by the Maillard reaction include the increased functional characteristic and solubility over a wide range of environments, such as very low pH, and very high ionic strength [33]. In the case of larger molecular weight polysaccharides, the conjugate-stabilized emulsion may have a thicker stabilizing layer than the protein-stabilized ones (Figure 2). It has been reported that the well-prepared soy protein hydrolysate–polysaccharide conjugates substantially improve emulsifying and stabilizing properties as compared with soy protein hydrolysates and its native proteins [34]. Apart from polysaccharides, some low molecular weight carbohydrates such as glucose and maltose have also been reported to be conjugated with soy protein hydrolysates. Yang et al. [35] studied the effect of the chain length of the carbohydrate on the interfacial and structural characteristics of the conjugates of soy protein hydrolysates (Mw > 30 kDa) produced by the Maillard reaction. The results of the study revealed that increasing carbohydrate chain length increases the emulsion stability of the conjugates. Zhang et al. [36] reported that the soy protein hydrolysate–dextran conjugate-based emulsions exhibited better freeze–thaw stability compared with the SPIdextran conjugates, especially in the case of 3% DH. For the lowest creaming index and best freeze–thaw stability, the optimum wet Maillard reaction conditions included a soy protein hydrolysate/dextran ratio of 2:3 in which the dispersion of 40 g/L of soy protein hydrolysate was prepared in phosphate buffer (pH 8), which was then incubated for 1 h at 85 ◦C. The surface activity measurements indicated the closely packed soy peptide–dextran conjugates which formed a thick adsorbed layer at the oil–water interface. Even though the Maillard reaction has long been regarded as an effective method to improve the emulsifying properties of soy protein [31], there are still concerns about the conformational change, surface polarity, specific interactions among the components. Due to the complexity of the Maillard reaction, the productivity, stability, and repeatability of the conjugates are concerns for commercial use.


