## 3 Results
### Stress Tolerance of _L. plantarum_
#### 3.2.4 Tolerance of \(L\). plantarum  
MLF is usually carried out in the complex environment of wine, which contains ethanol, organic acids, SO\({}_{2}\), amino acids, and others. These factors determine the growth and metabolism of LAB in wine. Therefore, investigating the stress tolerance of strains to the compound conditions of ethanol, pH, and SO\({}_{2}\) is essential for screening good MLB. With the increasing stress intensity of combination of factors, the growth of strains decreased continuously. Furthermore, the growth of A7, A50, B28, and B51 with good resistance to a single factor decreased significantly. The first group of orthogonal treatment had the lowest  
Figure 2: Growth of twenty \(L\). _plantarum_ and control strains in MRS-AJ, which contained 13, 15, and 17% (\(p\)/\(v\)) ethanol, respectively. Different small letters indicate significant differences according to the LSD Fisher test (\(p<0.05\)).  
stress, and the ninth group had the most potent stress. In the ninth group, the viable cell counts of A7, A50, B28, and B51 reached 1.0 \(\times\) 10\({}^{6}\) CFU/mL. They showed strong tolerance for the stress (Figure 5).  
The viable cell counts of nine groups were compared by analysis of variance. There was a significant difference in the growth of the isolated strains under combination of stresses (\(p\) < 0.05) (Figure 6). Among them, the viable cell count of B28 was the highest. The average value was 3.7 \(\times\) 10\({}^{8}\) CFU/mL, which was significantly better than the control strains (\(p\) < 0.05). A7, A18, A23, and A50 were relatively better. The growth of other strains was slightly weaker. The primary and secondary stresses affecting the growth of A7 were ethanol > pH > SO\({}_{2}\), and the primary and secondary stresses affecting the growth of A18, A23, A50, and B28 were ethanol > SO\({}_{2}\) > pH (Tables 3 and 4).  
Figure 4: Growth of twenty \(L\)_plantarum_ and control strains in MRS-AJ, which contained 20, 40, and 60 mg/L SO\({}_{2}\), respectively. Different small letters indicate significant differences according to the LSD Fisher test (\(p\) < 0.05).  
Figure 3: Growth of twenty \(L\)_plantarum_ and control strains in MRS-AJ, which contained pH3.4, 3.2, and 3.0, respectively. Different small letters indicate significant differences according to the LSD Fisher test (\(p\) < 0.05).  
Figure 5: Growth of twenty \(L\). _plantarum_ and control strains in MRS-AJ, which contained pH, ethanol, and SO\({}_{2}\).  
Figure 6: The average value of twenty \(L\). _plantarum_ and control strains under nine groups combination of stresses. Different small letters indicate significant differences according to the LSD Fisher test (\(p<0.05\)).  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Experimental Number**} & **Ethanol** & \multirow{2}{*}{**pH**} & \multirow{2}{*}{**SO\({}_{2}\)**} & \multicolumn{4}{c}{**The Viable Cell Counts (10\({}^{7}\) CFU/mL)**} \\ \cline{4-9}  & **(\% (\(\prime\)/\(\prime\)))** & & **(mg/L)** & **A7** & **A18** & **A23** & **A50** & **B28** \\ \hline
1 & 1 & 1 & 1 & 24.07 & 17.60 & 14.33 & 16.00 & 28.53 \\
2 & 1 & 2 & 2 & 13.33 & 22.53 & 24.07 & 19.13 & 24.00 \\
3 & 1 & 3 & 3 & 13.60 & 14.00 & 9.27 & 9.13 & 15.73 \\
4 & 2 & 2 & 1 & 0.51 & 0.82 & 0.99 & 0.68 & 0.86 \\
5 & 2 & 3 & 2 & 11.47 & 8.40 & 20.60 & 7.60 & 15.47 \\
6 & 2 & 1 & 3 & 0.23 & 0.08 & 0.09 & 0.40 & 0.17 \\
7 & 3 & 3 & 1 & 0.93 & 1.22 & 0.11 & 0.11 & 0.69 \\
8 & 3 & 1 & 2 & 0.49 & 0.14 & 1.05 & 1.18 & 0.61 \\
9 & 3 & 2 & 3 & 0.19 & 1.04 & 2.77 & 0.03 & 0.26 \\ \hline \hline \end{tabular}
\end{table}
Table 3: The viable cell counts of orthogonal experiment.