## 4 Conclusions  
We developed a fermented dairy product enriched with bioactive compounds by a stepwise co-fermentation using _B. subtilis_ HA and _L. plantarum_ EJ2014. MSG was added  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Free Amino Acid Content (\%)**} & \multicolumn{4}{c}{**Permentation Time (Days)**} \\ \cline{2-5}  & \multicolumn{2}{c}{_B. Subtilis_ HA} & \multicolumn{2}{c}{_L. Plantarum_ EJ2014} \\ \cline{2-5}  & **0** & **1** & **0** & **5** \\ \hline Glutamic acid & 2.62 \(\pm\) 0.15 b  & 1.88 \(\pm\) 0.04 c  & 3.05 \(\pm\) 0.10 a  & 0.01 \(\pm\) 0.03 d  \\ GABA & 0 \(\pm\) 0.00 b  & 0 \(\pm\) 0.00 b  & 0 \(\pm\) 0.00 b  & 2.61 \(\pm\) 0.04 a  \\ \hline \hline \end{tabular}
\end{table}
Table 1: UPLC/MS analysis of glutamic acid and GABA in co-fermented milk.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Retention Time (min)**} & \multirow{2}{*}{**Volatile Compounds**} & \multicolumn{3}{c}{**Peak Ratio (\%)**} \\ \cline{3-5}  & & **Before** & **Fermentation** & **After** & **Formation** & **Odor** \\ \hline
30.976 & Glycolic acid & 25.70 & 2.55 & N/A \\
19.723 & Cyclopentasiloxane & 23.46 & 2.90 & N/A \\
12.773 & 1,3,5-cyclohexutriene & 9.33 & - & N/A \\
64.965 & Hexanoic acid & - & 24.49 & Sweaty, cheesy, fatty \\
43.467 & 2,3-Butanediol & - & 20.53 & Fruit, onion \\
27.113 & Acetoin & - & 14.21 & Butter, cream \\ \hline \hline \end{tabular}
\end{table}
Table 2: Comparison of the volatile compounds before and after co-fermentation. Odor descriptors were sourced from The Good Scents Company [31] and Flavornet [32].  
as a precursor to producing a higher amount of both \(\gamma\)-PGA and GABA. After the first alkaline fermentation, milk supplemented with 3% MSG and 2.5% glucose contained \(\gamma\)-PGA and was highly viscous due to the incomplete bioconversion of MSG. During the co-fermentation with _L. plantarum_ EJ2014, MSG was completely bioconverted, resulting in higher production of GABA (up to 2.61%). The first alkaline fermentation of the milk provided protease activity, generating peptides with low molecular weight through the digestion of milk proteins. The co-fermented milk was effectively fortified with bioactive compounds such as \(\gamma\)-PGA, GABA, and peptides, as well as probiotics. In addition, the co-fermented milk exhibited a unique aroma similar to that of aged cheese. This co-fermented milk could be used as an ingredient in producing novel healthy products. In conclusion, the serial co-fermentation of milk in the presence of MSG could be a novel strategy to develop multi-functional ingredients and enhance flavor.  
Conceptualization, S.-P.L.; methodology, J.L. and S.-P.L.; investigation, W.-K.Y.; data curation, W.-K.Y.; writing--original draft preparation, W.-K.Y. and C.V.G.; writing--review and editing, C.V.G. and S.-P.L.; visualization, W.-K.Y.; supervision, J.L. and S.-P.L.; project administration, S.-P.L.; funding acquisition, S.-P.L. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Not applicable.  
**Conflicts of Interest:** The authors declare no conflict of interest.