## 4 Discussion  
Twenty strains of _L. plantarum_ isolated from petit verdot wines were analyzed by RAPD. It was found that the genomic DNA of the isolated strains was polymorphic. The _L. plantarum_ was divided into four clusters, and each category had a different number of branches. The diversity of _L. plantarum_ in the same producing area brings more possibilities for MLF. After combination of factors stress on twenty strains of _L. plantarum_, it was found that there were significant differences in stress tolerance among _L. plantarum_ isolated from the same producing area. Among them, A7, A18, A23, A50, and B28 grew better and could survive under low acid and high ethanol. Some studies have suggested that the growth limit of _L. plantarum_ is pH 3.5, and the ethanol is 8% (\(v/v\)) [30; 31]. However, some studies hold other views, saying that _L. plantarum_ cannot only survive in the 13% ethanol but also at pH between 3.2 and 3.5 [8; 11]. However, it was found that the growth of some _L. plantarum_ isolated from the Yinchuan region could be maintained at 10\({}^{6}\) CFU/mL under pH 3.4 and 15% ethanol, which was different from the above results. However, it can be proved that _L. plantarum_ has good ethanol tolerance. The strains showed strong tolerance to high ethanol and good tolerance to pH, SO\({}_{2}\), and combination of stresses. Volatile compounds perceptible to the human senses are not detectable in the glycosides that form in wine. Therefore, glycosidase has a major influence on the sensory characteristics of wine [32]. _L. plantarum_ is considered to be a new generation of new starter be  
Figure 9: Cluster diagram of volatile compounds for cabernet Sauvignon wines. cause of its richer enzymatic properties and is gradually used in MLF [33]. [PERSON] et al. [2] and [PERSON] et al. [19] used PCR to amplify genes to prove that \(\beta\)-glucosidase exists in most of _L. plantarum_. Although the existence of this gene does not guarantee the expression in the process of MLF, it can be used as a standard for screening strains and a basis for aroma changes. The environment in wine is complex, while ethanol, acid, and temperature will all affect the enzyme activity [13]. [PERSON] et al. [34] analyzed gene expression under several abiotic stresses, indicating that enzyme activity was significantly regulated by stress. At the same time, the enzyme activity of \(\beta\)-glucosidase is strain-dependent. Not all strains contain this enzyme, and there are differences in enzyme activity [35]. Therefore, screening strains with higher enzymatic activity for MLF has an important impact on aroma characteristics. All the strains could start MLF typically after being inoculated into the wine. Compared with the wine without MLF, it showed that the MLF of each strain played a role in reducing acid. According to the degradation of malic acid and the change of viable bacteria, it was found that the time of _O. oeni_ 31-DH and _L. plantarum_ BV-S2 were shorter, and the viable cell counts were higher during MLF. Although the fermentation rate of A7 is slow, it can be finished eventually. However, the MLF could not be finished for A50 and B28. [PERSON] et al. [36] inoculated _L. plantarum_ preadapted in MLO medium into the simulated wine containing 4.5 g/L malic acid. Malic acid consumption increased by 21%-43%. Malolactic enzyme (_mle_) is a key enzyme in MLF process. [PERSON] et al. [37] proposed that _mle_ expression was also affected by malic acid content, pH, and ethanol. Different strains show different enzyme profiles. In different environments, the expression levels of _mle_ for each strain are also different. Therefore, the rate of acid reduction is affected by many factors. Preconditioning culture and enzyme activity are the key factors affecting malic acid consumption. In part, the time required for MLF is determined by the recovery time of the viable cell counts. [PERSON] et al. [38] found that all strains showed a significant decrease in activity or death in the first few days after starting MLF.