## 1 Introduction  
The concept of industrial symbiosis, which arises from industrial waste, is within the circular economy area in which industrial waste from one process is used as raw material for another. This strategy is relevant against the excessive generation of waste [1]. In 2019, FAO estimated that 14% of the post-harvest food is lost before reaching the consumer, and in 2015, the contribution of large food groups to food waste and loss was analyzed, where the legume and cereal industry accounted for approximately 50% [2]. Soybean (_Glycine max_) is widely grown worldwide and is one of the most valued seed crops from which edible oil is obtained [3]. During oil extraction, the seeds provide 20% of oil, whereas 80% of soybean cake (meal) is generated as a by-product that is then heat-treated and milled to produce the co-product, soybean flour [4, 5], which has been used as a protein source since it contains around 50% protein [6]. According to North Carolina Soybean Producers Association (2021), 97% of the soybean meal is marketed for livestock feeds, and just 3% is processed for human consumption [5]. The application of soybean flour in the food industry may be important since this co-product is used to fortify different foods like soup, cookies, snacks, bread, powdered drink, and hamburger patties, among others [7; 8; 9; 10; 11; 12]. On the other hand, soy protein isolate (SPI) is the soy extract with the highest protein content; it is prepared by removing most of the non-protein components and must contain at least 90% protein content, according to the American Association of Official Food controls [13]. Therefore, soybean protein isolate (SPI) may be considered a valorized product because of its applications in drug delivery systems, packaging food, antimicrobial food coatings, tissue engineering, food formulation and fortification (the most used), and other innovative applications [14; 15; 16; 17; 18; 19]. The functional properties of a protein provide information about its behavior in food systems, and they can be classified into three groups according to the type of interactions: protein-water (solubility, water absorption, viscosity, and others); protein-protein (coagulation, precipitation, and others), and protein-interface (emulsifying, foaming capacity and others) interactions. The applications of SPIs in the food industry depend on these properties [20; 21]. The most common protein extraction methods are alkaline, acidic, enzyme-aided, and NaCl-aided methods, and recently, basic or acidic electro activation (NaCl electrolysis) [22; 23; 24; 25; 26]. Nevertheless, the traditional and most efficient extraction process is alkaline because it gives a protein yield of ~70% when virgin raw material is used [24]. The extraction of soy protein from soy flour through alkaline-mediated treatment is one of the simplest, fastest, cheapest, and easily adaptable methods to obtain SPI. In this process, the pH of the solvent (generally water) is adjusted to 8.0-12.0, and alkaline solution liberates and solubilizes proteins by breaking down the plant matrix, in which protein exists. First, the insoluble plant residues are separated by centrifugation, and then solubilized proteins are precipitated by adjusting the pH to 4-5, followed by a second centrifugation step [27]. Alkaline pH induces changes in the secondary, tertiary, and quaternary structure of the proteins and exposes their sulfhydryl groups, making them more soluble. At this condition, protein polymerization may appear due to an increase in the reactivity of some amino acid residues. Moreover, due to strong alkaline pH, Maillard reactions and protein hydrolysis may occur. All these reactions change the functional properties of the proteins, which are more susceptible when combining alkaline with other physical and chemical treatments [28]. Other alkaline sources have gained a scope over the last years, but none have used by-products of another industry, such as the chemical industry.