## 1 Introduction  
Natto is a traditional soybean product fermented by _Bacillus subtilis_ that contains various functional components, such as nattokinase (NK), vitamin K, isoflav, superoxide dismutase, phospholipids, saponins, and linoleic acid [1, 2]. Natto is a low-cost but high-nutrition product made using a simple procedure. Presoaked soybeans are cooked until tender, drained, cooled to 40 \({}^{\circ}\)C, inoculated with _Bacillus_ natto, and incubated at 30-40 \({}^{\circ}\)C for 12-36 h. The production process of fermented soybeans mainly includes selecting soybeans, soaking, sterilization, steaming, inoculation fermentation, and after-ripening [3, 4]. The active substances in natto exert preventive effects on thrombosis, high blood cholesterol, and cancer [5, 6]. The health function of natto has always been the interest of consumers. Past research focused on extracting and increasing the functional components of natto, and there were few reports on improving the flavor and taste of natto. The flavor of natto is related to volatile compounds, and amines are the main cause of the unpleasant smell of natto [7]. Given that _Bifidobacterium animalis_ BZ25 may produce lactic acid and acetic acid that mask amines [8, 9], adding _B. animals_ may improve the VOCs of natto. The advantage of two-strain mixed fermentation could be attributed to the synergistic effect between different strains that improved the flavor of natto [10]. [PERSON] et al. [11] found mixed _H. uvarum_/_S. cerevisiae_ fermentation leads to the enhancement of wine's fruity aroma. [PERSON] et al. [12] found that the co-fermentation of _Saccharomyces Boulardi_ and _Lactiplantiobacillus plantarum_ improved and modulated the flavor of green tea. In general, mixed fermentation/multi-strainstarters are considered to have advantages over a single strain contributing to complexity, taste, and flavor [13]. Therefore, adjusting the formula of the start recipe of natto can enhance the sensory characteristics and general acceptability of the product and improve the content and activity of nattokinase. Currently, how _Bacillus subtilis_ single bacteria and multiple strains affect the characteristics of natto is unclear [14]. Determining such an effect helps understand the role of strains in soybean fermentation for the preparation of high-quality natto. Free amino acids (FAAs) are organic compounds that comprise basic amino and acidic carboxyl groups and exist only in a free state in organisms. Protein hydrolase degrades soybean protein to produce FAs during soybean fermentation. FAs can be divided into four categories by their sensory properties: the monosodium glutamate-like FAAs (Asp ad Glu), the sweet FAAs (Met, Ala, Gly, Ser, Pro, and Thr), the bitter FAAs (Arg, His, Ile, Leu, Phe, Trp, Lys, and Val), Astringent FAAs (Tyr) and the tasteless (Cys) [15]. The bitter amino acids in natto greatly affect the taste of natto, giving it a light, bitter taste. Excessively high contents of bitter amino acids worsen taste [16]. The composition of FAAs can be modified by using different food-processing technologies [17]. FAAs are one of the key quality indicators of natto taste, and FAA content affects the quality of final food products [18]. Therefore, this study investigated whether changing free amino acid content could improve natto taste through two-bacterium co-fermentation. Biogenic amines (BAs) are a general term for a low-molecular-weight organic compound with biological activities and amino groups. They are important organic bases and participate in the metabolism of microorganisms. They are formed by the decarboxylation of FAAs, the amination of aldehydes and ketones, or the conversion of ammonia [19]. Histamine and tyramine are considered toxic. High histamine concentrations in plasma may lead to food poisoning that is characterized by diarrhea, dyspnea, or hypotension [20]. Spermine is involved in regulating cell growth and proliferation, as well as DNA transcription, RNA translation, protein biosynthesis, and immune response [21; 22].