## 3 Results and Discussion
### Effect of BZ25 Addition on the VOCs of Natto  
Given that strain types and production processes can affect fermentation and then change the composition of aroma components. SS, SFB, NMB, and NMBB were identified by HS-SPME-GC-MS. As shown in Table S1, 133 kinds of VOCs were identified. Pyrazines, ketones, and acids are abundant in NMB. The main VOCs in NMBB were pyrazines, ketones, acids, and alcohol. The difference between the main flavor substance in the NMB and the NMBB was alcohol, which was caused by the addition of BZ25. Compared with similar reports, it is found that the main VOCs of natto are the same but also different, reflecting both commonness and individuality. For example, [PERSON] et al. [35] found the volatile profile of the natto was dominated by alkylpyrazines and ketones. [PERSON] et al. [7] found that the main VOCs in natto were esters, ketones, pyrazines, and phenols. The different strains and fermentation conditions lead to the difference in natto volatiles components. To further examine the contribution of these 133 volatile compounds to the overall aroma profile of natto, their OAVs were calculated according to their concentrations and odor thresholds (Table S2). As shown in Table 1, VOCs with OAV > 10 in Table S2 are listed separately to more clearly find the substances that contribute significantly to the overall flavor of the sample. Trimethyl-pyrazine, 2,3-butanedione, decanal, nonanal, 1-octen-3-ol, acetic acid, and 2-methoxy-phenol were the main VOCs in NMBB. Pyrazines were an important type of VOCs in natto. Pyrazines are formed by _Bacillus subtilis_ or _natto_ bacteria [36]. [PERSON] et al. [1] found that the key VOCs in natto were 2,3-butanedione, 5-methyl-2-hexanone, furaldehyde, acetic acid, and 2,5-dimethylpyrazine, etc. Ketones can be generated by amino acids or by the Maillard reaction [1]. [PERSON] and [PERSON] [37] found that the key VOCs in natto were 2,3-butanediol, geranylacetone, isobutyl acetate, isovarelic acid, 2,5-dimethylpyrazine, and trimethylpyrazine, etc. The key VOCs in the NMB were ben-zeneacetic acid methyl ester, 2-ethyl-3,5-dimethyl-pyrazine, 2-heptanone, acetic acid, 2-ethyl-butanoic acid, 2-methyl-butanal, 2-ethyl-furan, and 2-methoxy-phenol. According to the VOCs of NMB, NMBB, and the natto reported in these articles, it was found that  
Figure 1: **(A)** Changes in the total viable count of SFB, NMB, and NMBB samples from 0 to 30 h. NMB: viable count of GUTU09 in NMB; SFB: viable count of BZ25 in SFB; NMBB: GUTU09: viable count of GUTU09 in NMBB; NMBB: BZ25: viable count of BZ25 in NMBB. Means with different characters (ai-hi, i = 1, 2, 3 or 4) on the same curve are significantly different (\(p\) < 0.05), the same below. **(B)** Changes in tittatable acid of SFB, NMB, and NMBB samples from 0 to 30 h. **(C)** Changes in the pH of SFB, NMB, and NMBB samples from 0 to 30 h.  
pyrazines were the main VOCs of natto, which indicated that _Bacillus subtilis_ or _natto_ bacteria could synthesize pyrazines during the fermentation of natto. The differences in VOCs also reflect personality differences. Most amines have a stimulating taste, which was the main reason for the ammonia flavor of natto. \(N\),_N_-dimethyl-methylamine, formamide, 3-methyl-butanamide, and ethosuximide were detected in NMB but not in NMBB. Natto fermented by a single bacterium produces an ammonia smell, which is consistent with the report from [PERSON] et al. [38]. The disappearance of volatile amine indicated that the ammonia taste of natto was effectively reduced. After the addition of BZ25, volatile amine disappeared, volatile aldehydes and ketones species decreased, and volatile alcohol increased. Alcohols may be produced from the metabolism of unsaturated fatty acids of microorganisms [39]. The VOCs of natto were usually related to microbial metabolism [40]. Compared with the sensory evaluation results, it is the disappearance of some irritating volatile substances that makes the NMBB sample more popular. [PERSON] et al.