## 3 Results and Discussion
### Effect of BZ25 Addition on the VOCs of Natto
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Number** & **Name** & **Aroma Description** & **RI** & **ID** & \multicolumn{3}{c}{**Threshold**} & \multicolumn{3}{c}{**OAV**} \\  & & & & & & **Value (ng/L)** & **SS** & **SFB** & **NMB** & **NMBE** \\ \hline F4 & 2-Methyl-butanol & Cocoa, almond & 916.3 & MS, RI & 1 \({}^{d}\) & — & — & 540 & — \\ F5 & Octan & Credo oil smell & 12846 & MS, RI & 0.91 & 316 & — & 120 & — \\ F6 & Phenylacetaldehyde & Fengnigat taste & 1647 & MS, RI & 4 \({}^{l}\) & 108 & 41 & 30 & — \\ F8 & Benzaldehyde & Cherry, nuts, bitter almond aroma & 1528.8 & MS, RI, Std & 85 \(\,\)8 & 80 & 25 & 26 & — \\ F9 & Hexanal & Grass, fat & 1082.9 & MS, RI & 230 \(\,\)s & 22 & \(<\)1 & \(<\)1 & — \\ \hline \hline \multicolumn{2}{c}{Acholobs} & & & & & & & & \\ G1 & 1-Octan-3-cl & Mashroom, green & 1447.3 & MS, RI & 10 \(\,\)s & 168 & 13 & 6 & 931 \\ G2 & Trans-gramal & Flowery, lemon aren & 1845.3 & MS, RI & 1 \({}^{f}\) & — & 208 & — & 189 \\ G3 & 3-Octan & Herbaceous, melon, citrus-like odor & 1389.8 & MS, RI & 18 \({}^{d}\) & — & 7 & — & 39 \\ G4 & 1-Hexanol & Green, fowery & 1390.3 & MS, RI & 34 \(\,\)s & — & 52 & — & 12 \\ G9 & 2-Methyl-3-benoand & — & 1370.8 & MS, RI & 46-41 \({}^{d}\) & — & — & 11-29 & — \\ G20 & Enculylyled & — & 1206.7 & MS, RI, Std & 4.6\({}^{d}\) & 73 & — & — & — \\ \hline \multicolumn{2}{c}{Aromatics} & & & & & & & & \\ H1 & 2-Methy-phenol & Spicy, medicine fragrance & 1869.1 & MS, RI & 1.5 \(\,\)s & 465 & 2200 & 569 & 495 \\ H2 & Biphenyl & — & 2005 & MS, RI & 0.5\({}^{d}\) & 9 & 11 & 5 & 65 \\ H3 & Phenol & Phenol & 2010.6 & MS, RI & 21 \(\,\)s & 25 & 19 & 30 & 37 \\ \hline \multicolumn{2}{c}{Furans} & & & & & & & \\ I2 & 2-Penyl-furan & Bean, green & 1230.1 & MS, RI & 5.8\({}^{l}\) & 502 & 126 & 35 & 78 \\ I2 & 2-3 Dhydro-benzofuran & — & 2399 & MS, RI & 48 \({}^{d}\) & 2 & 3 & 3 & 13 \\ I3 & 2-Ethyl-furan & Malt fragrance & 957.3 & MS, RI & 2.3\({}^{l}\) & 7694 & 2029 & 334 & — \\ \hline \multicolumn{2}{c}{Others} & & & & & & & & \\ J1 & Headacane & Alkane & 1405 & MS, RI, Std & 25\({}^{l}\) & — & — & 84 & 56 \\ J2 & Undacane & Alkane & 1090.5 & MS, RI, Std & 1578.8\({}^{d}\) & — & — & \(<\)1 & \(<\)1 \\ J3 & Styrene & Balasamic, gasoline & 1259.6 & MS, RI & 65 \({}^{d}\) & 2 & — & — & 16 \\ J6 & Naphthalene & — & 1757.7 & MS, RI & 6\({}^{d}\) & — & — & — & 43 \\ J7 & Anchelbe & Fenel, spicy, liorotles smell & 189 & MS, RI & 100 \({}^{f}\) & — & — & 4 & 20 \\ J13 & Dimethyl trisulfide & Sulfuric, strong onion odor & 1380.1 & MS, RI & 14 \(\,\)s & 19 & 19 & 2 & — \\ \hline \hline \end{tabular}  Note. \({}^{d}\) Other thresholds were determined in water. Other thresholds taken from Ref. [42]: \({}^{e}\) Older thresholds taken from Ref. [42]: \({}^{e}\) Older thresholds taken from Ref. was 4.25, while that of the NMB was 3.6. NMBB was presented with cocoa, sour, and roasted nutty fragrance. The NMB showed ammonia, fruity, and cocoa fragrances. During fermentation, the amounts of ammonia are released to give a negative influence on the flavor of natto [38]. The disappearance of amine volatiles and the decrease of bitter amino acids make NMBB more popular. The increase in the flavor score was attributed to the lactic acid produced by BZ25, which neutralized amines and gave natto a slightly acidic odor. BZ25 plays a major role in reducing the ammonia taste and bitter taste of NMBB. The sensory properties of fermented soybean products are affected by various microorganisms, and microbial enzymes also influence the characteristic taste and flavor of products [50]. Soy protein can be hydrolyzed by proteases to produce peptides and amino acids. The bitter taste of natto during chewing may be related to bitter peptides and amino acids such as leucine, histidine, and proline, especially peptides and tyrosines produced by casein hydrolysis [51]. From the data of FAAs (Table 2), it can be seen that the mixed fermentation of two bacteria can reduce the bitter taste of natto. The decrease in bitter amino acid content is also one of the reasons for the increase in the taste of NMBB.