1. Introduction
Soybean (
Glycine max) remains one of the most valuable agricultural commodities in the world [
1]. Although soybeans can be processed into a wide variety of foods such as tofu, soy milk, soy flour, and soy nut butter, more than 80% of soybeans produced in the United States are crushed into edible oil [
2]. Soybean oil, primarily harnessed to produce renewable diesel, constitutes approximately 20% of the weight of a bushel of soybeans (or ~12 lbs./1.55 gal); the remaining is soybean meal (SM), a by-product of soybean processing [
3]. With a growing number of soybean processing facilities across the U.S., the supply of SM is on the rise, making it an accessible resource for a diverse range of applications, including animal feed, feedstock for organic fertilizer, nutritional supplements, and bio-based products. Moreover, the protein content in SM is regarded as high-quality protein due to its abundance of essential amino acids such as lysine, methionine, and tryptophan [
4]. Effective management and valuation of SM can significantly impact protein availability [
5]. Similarly, efficient valorization of SM could improve resource efficiency, reduce adverse environmental effects, and offer an avenue to extract economic value from waste streams [
6]. Due to its versatility and high protein content, SM plays a crucial role in food processing [
7] and is a valued ingredient in a wide range of food products, including bakery goods and meat substitutes [
8]. SM is equally a notable alternative to animal proteins in plant-based diets, making it a necessary part of diets for vegetarians and vegans [
9].
Protein is the most vital component of SM, making up about 47% to 49% of its total composition. This highlights the significance of SM as a bioresource in a wide variety of applications that require high quantities of protein, particularly animal feed formulations. Other resourceful components in SM include essential amino acids, dietary fiber, sugars, and starch [
10]. These components vary in concentration among samples due to factors such as geographic location of soybean production, processing methods of soybean, soybean variety processed amongst others [
11]. For instance, U.S. SM is known to have a more consistent and higher digestibility, better protein quality (based off essential amino acid levels), and lower fiber than SM from Argentina, India, and Brazil, whereas the oil content of SM from South American countries was higher than in U.S. SM [
12]. Several studies have investigated the nutritional profile of SM, revealing that it contains high levels of lysine and limited levels of methionine and cystine [
13,
14,
15].
Efforts to enhance the nutritional value of SM to improve the digestibility of its amino acids by reducing anti-nutritional factors (ANFs) include enzyme addition and fermentation [
16,
17,
18]. Moreover, fermentation has shown promise in increasing SM’s concentration of easily digestible proteins and essential amino acids [
19]. The research iteration to fully harness SM has mostly focused on reducing ANFs, characterizing and enhancing its nutritional profile at the detriment of other untapped fundamental properties and applicability of SM. While its use in livestock nutrition is undisputed, SM’s broad applicability suffers from insufficient exploration of its functional and structural properties. This gap restricts the utilization of SM within the conventional bound of animal feed, neglecting prospective value as human dietary supplements, organic fertilizers, and environment-friendly bio-based products. To utilize SM protein (SMP) for biochemical production, [
20] used a reusable solvent to extract SMP. Progressively, SM-extracted protein could be fermented to produce sizeable quantities of biological ammonia [
21]. Herein, we evaluated SMP’s functional and structural properties, focusing on its digestibility, proximate composition, functional group, and biomolecular structures. Specifically, this study delved into the extraction of SMP, hydrolysis of SMP using various proteases, activities of SMP antinutritional factors such as trypsin inhibitor, proximate analysis, and FTIR characterization. Collectively, these properties can provide useful insights into ways to valorize SMP for economic value creation, environmental sustainability, and improvement of global food systems.
4. Conclusion
Soybeans and its by-product soybean meal are rich sources of protein for animal feed. They can also be utilized to supplement human protein requirement and as feedstock for biochemicals production. The heat treatment involved in soybean processing during oil extraction is hypothesized to impact the protein composition of the resulting soybean meal. Through multiple analytical parameters, including proximate composition, digestibility, FTIR spectroscopy, and SDS-PAGE banding patterns, this study is a comparative analysis of the protein profile of soybean meal protein isolates (SMPI) and soybean protein isolate (SPI). Low concentrations of ammonium hydroxide (NH4OH) ranging from 0.25–1% was shown to extract about 25% protein from soybean meal (SM). Contrary to previous opinions that heat treatment would increase the protein digestibility of soybean meal protein, this study established that SPI had higher digestibility than its meal. Relative to unprocessed soybean protein, proximate analyses showed that soybean meal is more protein dense. However, the observation that both isolates of soybean and soybean meal proteins had similar molecular structure and gel banding pattern revealed that heat treatment of soybeans during processing has limited impact on the protein quality of soybean meal. These findings provide insights into the utilization of soybean meal protein towards improving global food systems and environmental sustainability. Future studies should explore the optimization of extraction methods and the digestibility of soybean meal protein to broaden its application scope as a resource in the food and various industrial sectors.