# Dynamics of Microbial Community Structure and Metabolites during Mulberry Ripening
## 3 Results and Discussion
### Dynamic Changes in Amino Acids in WHDS during Ripening  
Amino acids are the basic units of proteins and the main flavour substances of many foods. There are nine essential amino acids, namely, Lys, Trp, Phe, Met, Thr, Val, Ile, Leu, and Tyr, and vegetables and fruits are the main sources of amino acid intake for the human body [42]. The 17 common amino acids are usually classified as umami amino acids (UAAs), sweet amino acids (SAAs), bitter amino acids (BAAs), and Others. Asp and Glu belong to UAAs; Gly, Trp, Ala, Pro, Ser, Met, and Thr belong to SAAs; Val, Leu, Phe, Lys, Ile, and Arg belong to BAAs; and the remaining Tyr and Cys belong to Others [43]. The composition of flavour-presenting amino acids directly affects the quality and taste of mulberry fruit, so changes in the flavour-presenting amino acid composition were examined during ripening [44]. As shown in Figure 3a, UAAs showed a slow decline from 26% to 23% as WHDS matured. The content of SAAs was not stable throughout the stage, but varied little from stage to stage, with a maximum of 36% at S2 and S4. There was no clear pattern in the variation in BAAs, with the peak of 38% occurring at S5. The contents of the other amino acids did not change throughout the ripening stage and were all 5%. Among the first three types of flavour-presenting amino acids, SAAs and UAAs are the most important components of mulberry flavour, and although the contents of the two flavour-presenting amino acids tended to decrease with the increase in ripening, the content of these two types of amino acids accounted for more than 56% of the total content at all stages and reached the maximum value of 61% in S2. The high proportion of SAAs and UAAs improves the flavour profile of WHDS as well as the nutritional value. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Phase** & **S1** & **S2** & **S3** & **S4** & **S5** \\ \hline Tartaric acid (mg/g) & 12.55 \(\pm\) 0.23b  & 16.45 \(\pm\) 0.24a  & 11.24 \(\pm\) 0.07c  & 9.64 \(\pm\) 0.08d  & 9.24 \(\pm\) 0.14e  \\ Lactic acid (mg/g) & 8.95 \(\pm\) 0.05b  & 10.34 \(\pm\) 0.13a  & 7.73 \(\pm\) 0.06c  & 6.53 \(\pm\) 0.18d  & 6.15 \(\pm\) 0.22e  \\ Acetic acid (mg/g) & 7.84 \(\pm\) 0.24b  & 9.94 \(\pm\) 0.16a  & 6.64 \(\pm\) 0.14c  & 5.83 \(\pm\) 0.19d  & 5.62 \(\pm\) 0.12d  \\ Malic acid (mg/g) & 2.14 \(\pm\) 0.17b  & 2.73 \(\pm\) 0.04a  & 1.96 \(\pm\) 0.21b  & 1.63 \(\pm\) 0.11c  & 1.72 \(\pm\) 0.10c  \\ Fumaric acid (mg/g) & 1.27 \(\pm\) 0.18a  & 1.34 \(\pm\) 0.12a  & 1.30 \(\pm\) 0.01a  & 0.89 \(\pm\) 0.03b  & 0.94 \(\pm\) 0.11b  \\ Oxalic acid (mg/g) & 0.74 \(\pm\) 0.22 ab  & 0.95 \(\pm\) 0.02a  & 0.60 \(\pm\) 0.07b  & 0.55 \(\pm\) 0.19b  & 0.50 \(\pm\) 0.10b  \\ \(\alpha\)-Ketoblutaric acids (mg/g) & 0.35 \(\pm\) 0.17a  & 0.47 \(\pm\) 0.01a  & 0.35 \(\pm\) 0.18a  & 0.23 \(\pm\) 0.11a  & 0.23 \(\pm\) 0.07a  \\ Citric acid (mg/g) & 0.47 \(\pm\) 0.01a  & 0.45 \(\pm\) 0.01a  & 0.34 \(\pm\) 0.11b  & 0.29 \(\pm\) 0.01b  & 0.33 \(\pm\) 0.04b  \\ Pyruvate (mg/g) & 0.33 \(\pm\) 0.05 ab  & 0.43 \(\pm\) 0.02a  & 0.35 \(\pm\) 0.11 ab  & 0.23 \(\pm\) 0.07b  & 0.23 \(\pm\) 0.05b  \\ Succinic acid (mg/g) & 0.11 \(\pm\) 0.01 & ND & ND & ND & ND \\ \hline \end{tabular} Note: ND indicates that the test result is below the device threshold, i.e. not detected, the same below Different lowercase letters represent significant differences in factors at \(p<0.05\).  
\end{table} 
Table 2: Dynamic changes in organic acid contents in WHDS during ripening. Figure 3: Alterations in the proportion of taste amino acids at different maturity stages of WHDS (**a**) and alterations in the amino acid composition (**b**). The composition of the three flavour-presenting amino acids is not sufficient to provide a full picture of the quality and taste of WHDS but needs to be evaluated in relation to the content of the individual free amino acids. The type and content of free amino acids in mulberries are affected by various factors, such as the variety, soil, and water [45; 46].