## 1 Introduction  
Soy protein isolate (SPI) is an important food ingredient and is often used in human foods due to its nutritional value and technological properties, as well as its relatively low price [1]. The gel properties are important functional properties of SPI. SPI has the ability to form a gel when heated. Traditionally, SPI is added in the industrial production of emulsified muscle foods to improve product texture and yield [2,3]. However, this increases the cost because of the gel softening of natural soy protein isolate (N-SPI) with the extension of the storage period [1,4]. Furthermore, [PERSON] et al. (2015) reported that N-SPI had a negative effect on the gel properties of some meat proteins. The limitationson the functional properties of N-SPI affect its application in food processing. Therefore, research on how to improve gel strength and delay gel softening might be potentially beneficial.  
The literature identifies many factors that affect the gel properties of soy protein isolate, e.g., protein concentration, drying methods (spray drying, freeze drying, and vacuum drying), and environmental factors (pH, temperature, time, and salt ions) [5; 6; 7; 8]. Spray drying has been shown to be a simple, low-cost, reproducible process. It is widely used in the food industry [9]. [PERSON] et al. (2011) reported that spray-dried lentil protein isolates (LPI) powder had the highest solubility and better gelling properties when compared with that produced using freeze drying and vacuum drying [10]. [PERSON], [PERSON], and [PERSON] (2018) reported that the addition of Ca\({}^{2+}\) and Na\({}^{+}\) (0.005-0.01 M) increased gel strength, elasticity, viscosity, water holding capacity (WHC), and thermal stability [11]. [PERSON], [PERSON], and [PERSON] (2018) reported that when the Ca\({}^{2+}\) concentration during pre-aggregation was increased from 0 to 7.5 mM, the elastic modulus of the gels increased substantially. Compared with the normal \"one-step\" process, pre-aggregation promoted the formation of stronger gels with thicker strands and denser structure [12]. Moreover, [PERSON], [PERSON], and [PERSON] (2015) reported that stiffer gels of SPI were formed at lower salt concentrations (0.2 M NaCl) [13]. [PERSON], [PERSON], and [PERSON] (2018) reported that during the NaCl-induced gelation of aggregates, the activation energy was much smaller and the gels were more homogeneous than native soy proteins [14]. Thus, spray drying and the ionic strength have roles in the formation of gel products with an appropriate texture and WHC [15]. SPI gels can be classified into heat- and cold-induced gels depending on differences in gel formation conditions, and SPI heat-induced gels have been widely used in the food industry. The formation of cold-induced gels of SPI is relatively mild, which provides a way to develop new foods. The current research focuses on the formation of heat-induced gels in the presence of salt ions or gels induced with salt addition and their properties after spray drying. Little research has been done on the modifications of salt addition before spray drying on SPI and its heat- and cold-induced gel properties or on efforts to explain the mechanism of action of salt ions.  
Thus, the objectives of the presented research were to compare the heat- and cold-induced gel properties of SPI as affected by salt ions (Na\({}^{+}\), Mg\({}^{2+}\), Ca\({}^{2+}\)) at various concentrations (0.005, 0.01, 0.015 M) before spray drying. The properties measured included the texture, WHC, and microstructure of the SPI gels to hopefully improve SPI's gelation properties. The experiments were also designed so that we may begin to understand the mechanism(s) of any improvements.