Fresh soybean dregs can have a moisture content of between 80 and 85% (Chen et al., 2022), which affects the separation and purification of the dry matter in them directly. Based on the large amount of soybean dregs produced in practice, processors must dry the dregs as soon as possible to facilitate storage; otherwise, the high moisture content and unhygienic storage conditions would cause the dregs to quickly deteriorate. However, soybean dregs are different from ordinary food materials, their drying is extremely difficult, often accompanied by a large amount of energy consumption and long-time input (Lu et al., 2022). Even if the dregs are dried through higher processing costs, their rough palatability often makes them difficult for consumers to accept, which led to the frequent use of soybean dregs as animal feed (Jiang et al., 2019; Hu et al., 2023). To improve the processing performance of dreg as food, it is often modified using different methods, such as fermentation, chemical modification, etc., (Zhou et al., 2019; Wu et al., 2023), which undoubtedly diminishes the suitability of soybean dregs as a food ingredient for processing.

Soybean whey’s unsuitability for food processing can be shown in its inability to be utilized in food preparation. It is challenging to link this effluent to food processing because of its bitter flavor (Huang et al., 2022) and high concentrations of several constituents, such as trypsin inhibitors and lectins. The conventional way of processing soybean whey into food is to ferment it with probiotics to improve its potential functionality and processing characteristics (Tian et al., 2021; Yang et al., 2021), which gives some special potential functionality to the soybean whey but does not enhance its suitability for processing from a practical production point of view. Instead, it raises the cost of processing.

As shown in Table 1, soybean processing by-products do contain a large number of potential processing resources (Li et al., 2012; Chua and Liu, 2019). However, obtaining these processing resources comes at a significant expense due to processing.

The most worthwhile potential processing resources to be obtained from soybean dregs are soybean dietary fiber and functional peptides (Fang et al., 2021; Wang et al., 2021). Insoluble dietary fiber from soybean dregs, for example, has been shown to regulate the body’s glucose-lipid metabolism and intervene in the intestinal internal environment at high purity (Lyu et al., 2022b; Wang et al., 2023), which is considered a good resource for functional food. However, to obtain it, a combination of drying methods and the use of enzyme engineering techniques are required (Lyu et al., 2021), whose level of cost is unacceptable in industrial production.

Representative potential processing ingredients in soybean whey are oligosaccharides and isoflavones (Hu et al., 2024). As of today, the most efficient method recognized for separating these two components from soybean whey is membrane separation technology (Liu et al., 2015a,b). Complemented by larger yields than soybean dregs, it is more costly to achieve efficient separation of oligosaccharides and isoflavones from soybean whey in practical production. Again, no producers are doing this.

The process of extracting potential processing ingredients from soybean by-products is characterized by the fact that it is generally easy to succeed in the laboratory but difficult to scale up and apply in actual production. High processing expenses should be held primarily responsible for this.

Contamination and food safety risks of soybean by-products can be summarized in three categories: microorganisms, residual proteins, and metal ions.

Microorganism overload is a common problem in the treatment of soybean dregs and soybean whey. Among plant-based food wastes, microbiologically contaminated soybean dregs have the highest acidity (Wahab et al., 2022). Although fewer microbial species spoil soybean dregs in the natural environment, food microbial contamination is high in acidophilic microorganisms (Rawat, 2015), which are highly susceptible to risks such as Pseudomonas putida (Cunha et al., 2022). The microbial spoilage pattern of soybean whey is similar to that of soybean dregs, which will spoil within a day under conventional storage conditions (Chua and Liu, 2019). Conventional processing methods make it difficult to process soybean whey in 1 day. In addition, the odor of soybean whey is hard to receive after rotting and remains even after sterilization (Hao et al., 2023), which makes it no longer suitable for food processing.

Residual proteins in by-products are closely related to microbial contamination. Insoluble dietary fiber and soluble sugars provide a rich source of carbon for microbial reproduction, while, residual proteins provide an amount of nitrogen. Residual protein in soybean dregs comes from proteins that are not fully soluble in water, and conversely, the residual protein in the soybean whey stems from incomplete cross-linking or reactions during the production of soy protein products. More seriously, the removal of certain residual proteins is not achieved by changes in processing methods, such as certain acid-soluble proteins found in soybean whey (Liu et al., 2016). In short, residual protein increases the risk of microbial contamination of by-products.

Contamination by metal ions, on the other hand, occurs mainly in soybean whey, which mainly comes from excessive coagulants added during the processing of tofu. Common tofu coagulants include gypsum (main ingredient: CaSO₄), salt brine (main ingredient: MgCl₂, etc.), and glucono-δ-lactone (Li et al., 2022), the first two of which cause metal ion contamination. The root cause of metal ion residues is the excessive use of coagulants to ensure the yield of tofu, and the salt ions that are not bound to proteins in this process are present in the soybean whey. Again, this problem cannot be solved at this stage by adjusting the production process.

As mentioned above, the food safety risks of by-products include microorganisms, residual proteins, and metal ions, which can be avoided in the raw material processing environment provided by screw extrusion technology.

Any type of screw extruder can easily provide a processing environment above 120°C, especially during the production of STP using screw extrusion, where temperatures often exceed 200°C (Lyu et al., 2022a). This environment, coupled with the minutes-long processing time, is such that no microorganisms can survive, at least not at the moment the material leaves the extruder. The risk of microbial contamination is also non-existent.

Residual proteins are no longer considered by-products in the screw extrusion process but as raw materials. The central idea presented in this paper is to use by-products to replace some of the key raw materials in the STP production process, e.g., using soybean dregs to replace a portion of water and soybean meal, meanwhile using soybean whey to completely replace pure water. According to estimates, soybean dregs can replace at least 20% of the total amount of raw materials, and more for soybean whey. Therefore, in this substitution process, the residual protein is involved in the texturization process of the raw protein, which, together with the absence of microbiological risks, does not pose additional food safety risks.

As the by-products undergo the texturization process with the protein, the risk of metal ion residues is not present. Studies have shown that although some coagulants are toxic, they are rendered harmless when they bind or react with proteins (Ali et al., 2021). The violent reaction environment of the screw extrusion process causes the metal ions to fully cross-link with the proteins, and with a high content of proteins and a low content of metal ions, this reaction should be regarded as complete for the metal ions.

In addition to the feasibility of the basic principles, the most limiting factor for the high-value utilization of by-products is the high cost of treatment. As a technology that has long been used in food processing and soybean product processing (Shelar and Gaikwad, 2019), screw extruders are not high-end processing equipment and have very low acquisition and operating costs. Especially single-screw extruders, which are more suitable for processing rough materials, such as soybean dregs (Lyu et al., 2022a), whose running cost is lower than others. This means that manufacturers can upgrade their inherent production lines at a very low cost and realize the utilization of by-products without affecting the production of the original products.

As mentioned previously, screw extrusion technology is mainly used in soybean processing for the production of STP. Compared to SPI, STP is a high-value product in its own right. As long as the by-product can be successfully turned into one of the feedstocks for STP production, its economic value is already very high because there are no more by-product treatment costs. On this basis, the inclusion of by-products will increase the content of dietary fiber, oligosaccharides, and isoflavones in STP, which will result in a significant increase in the value of the end product. This will help to realize the upgrading of ordinary food ingredients into potentially functional foods and get more market attention. The hidden value is very significant.

From the perspective of sustainable food processing, the use of screw extrusion technology to treat soybean by-products has longer-term significance.

For the soybean industry, this will be an attempt to utilize soybeans for laddering and whole-seed processing. This will achieve high-value utilization of more than 20% of the dry matter in soybeans, an amount that cannot be ignored in countries and regions where there is a scarcity of high-quality plant-based food resources, which will also further reduce pressure on the planting side regarding increased soybean production.

As far as the environment is concerned, in addition to the basic advantages mentioned above, zero-emission processing technology represented by screw extrusion has long-term significance for soybean processing and even for the food processing industry. Due to emissions and pollution issues, in most countries and regions, food processing companies must be located in suburban areas and cannot produce in urban or densely populated areas, which undoubtedly increases the cost of food production, especially transportation and storage. With the promotion of zero-emission processing technology, the urbanization of food processing will be promoted, which will bring many advantages, such as reduction of production costs, alleviation of labor problems, application of new technology, improvement of consumer acceptance, and so on.

Primary soybean processing is a traditional and stereotypical industry, but screw extrusion technology is currently only used in soy protein producers. Getting traditional primary soy producers to recognize and embrace screw extrusion technology is a long process, especially since the industry is highly fragmented. At the same time, compared to other new processing technologies, screw extrusion technology has a smaller overall investment. However, it is not easy to get producers to voluntarily bear the added costs in the actual production process. In addition to economic costs, there is an increase in hidden inputs to producers, such as labor and policy inputs.

While the use of screw extrusion to process by-products has proven theoretically feasible (Wang et al., 2020, 2024), there is an imbalance in raw material and product throughput capacity for soybean processors. On the one hand, there is an imbalance between the amount of by-product output and the processing capacity of the screw extruder. The current capacity of large screw extruders is only a few tons per day (in STP or SPI), but conventional soybean producers produce much more than that as by-products. This means that producers need to introduce several screw extruders to solve the by-product handling problem, which undoubtedly raises production costs. On the other hand, the by-products are only co-materials of the STP production process. To realize the treatment of by-products, producers also need to purchase or produce SPI, gluten (wheat protein), soybean meal, pea protein, etc. as the main raw materials, which are exactly what the traditional soybean processing enterprises do not have. The above process again raises the cost of running it.

While the addition of soybean dregs and soybean whey to the STP production process certainly enhances the content of functional substances in the product, this process brings up many unresolved scientific questions, for example: reactions of proteins and functional substances in extrusion processes, the change of functional components in the extrusion process, conversion of dietary fiber from insoluble to soluble, conversion of isoflavones from aggregated to free state, evaluation of processing and flavor characteristics of end products, etc. Precise analysis of these issues is the key to scaling up the technology, which in turn provides a potential research direction for researchers.

The advantages, limitations, and further scopes of screw extrusion technology in soybean by-product processing are summarized in Table 2.