## 1 Introduction  
Soybean protein, including soybean protein concentrate and soybean protein isolate, is primarily derived from defatted soybean, offering an economical and efficient source of dietary protein [1]. Due to its balanced amino acid profile and significant nutritional value, soybean protein is widely regarded as an excellent substitute for animal proteins. Soybean protein are subjected to further processing (e.g., chemical/enzymatic hydrolysis, microbial fermentation, thermal treatments, or gastrointestinal digestion) to produce protein hydrolysates for various food applications [2]. Soy protein hydrolysates can exhibit beneficial physiological properties (e.g., solubility, emulsifying capacity, and digestibility), functional attributes (e.g., antihypertensive and antioxidative activities), and flavor characteristics (umami taste). However, some drawbacks of soybean protein hydrolysates, including the naturally occurring grassy or beamy flavor of soybean and potentially increased bitter taste, greatly restrict their further food applications considering consumer acceptance [3]. Accordingly, there has been an ongoing effort from researchers to develop novel processes or to optimize processing conditions for improving the flavor profiles of soybean protein hydrolysates. Some literature reported that an appropriate enzymatic hydrolysis could improve the taste of soybean hydrolysates (e.g., increase in umami taste and reduction in bitterness) for food seasoning [4]. A range of commercial enzymes has been demonstrated to effectively reduce the bitterness of soybean protein hydrolysates [5]. In terms of odor, the prevailing methods to mitigate undesirable aromas in soybean protein hydrolysates include physical treatments, chemical processes, activated carbon adsorption, and fermentation [6]. In our previous studies, fermentation was demonstrated to effectively elevate the contents of umami amino acids, sweet amino acids, and organic acids in soybean hydrolysates, thereby enhancing the taste profiles, including umami and sweetness [7]. Moreover, fermentation has been widely used to improve the aroma of foods, including grains and legumes, through the biochemical reactions, enzymatic actions, and metabolic processes of the microorganisms [8; 9]. Lactic acid bacteria (LAB) can break down protein, fat, and carbohydrates to produce flavor compounds, including aldehydes, alcohols, esters, and acids, as LAB are capable of producing enzymes (like protease, lipase, esterase, reductase, and peroxidase) during fermentation to hydrolyze proteins into peptides and free amino acids [10; 11]. Besides producing flavor precursors and offering probiotic properties, LAB can also convert the food components in raw materials to biologically active substances, such as bioactive peptides and phenolic metabolites [12]. Some halophilic aromatic yeasts (such as _Zygosaccharomyces rouxii_, _Candida versatilis_, _Candida etchellsi_, and _Pichia guillermondii_) are other types of microbial strains for the fermentation of soybean products and give soybean fermented products distinct flavors and a special texture [13]. During fermentation, yeasts can modify carbohydrates and convert them to various flavor-active metabolic by-products and also facilitate the biosynthesis of esters, alcohols, and other volatile compounds [14]. Nevertheless, relatively few studies have comprehensively analyzed how improvements in fermentation affect the aroma of soybean hydrolysates. Gas chromatography-mass spectrometry (GC-MS) is advantageous based on its wide detection range, high separation efficiency, and high capacity for identifying unknown compounds based on the National Institute of Standards and Technology (NIST) database [15]. However, a complex preconditioning procedure is required prior to GC-MS analysis, and it also has a relatively high cost due to the required vacuum and helium as the carrier gas.