## 4 Discussion
plantarum_ TUST-232 could reach 8.91 log CFU/mL at 14 h, related to the high contents of natural sugars and in the R-B mixed juice. After fermentation, the apparent decrease in sucrose content can be attributed to the fact that sucrose was consumed by sucrose phosphorylase and acid hydrolysis produced by _L. plantarum_ TUST-232, consistent with the observation of [PERSON], [PERSON], and [PERSON] [41] in the fermentation of durian with LAB strains. LAB strains vary in their ability to consume sucrose, leading to changes in the content of sucrose before and after fermentation, thus affecting the fermentation of the strain and the viable cell count in the beverage. Glucose and fructose can be utilized as carbon sources for the growth of LAB, especially glucose [42]. After fermentation, the contents of glucose and fructose had no significant changes. Combined with the significant decrease in sucrose content, this phenomenon may be due to the decomposition rate of sucrose being equivalent to the consumption rate of glucose and fructose during the fermentation [40]. In addition, the contents of glucose and fructose had no significant changes, which may be related to the lower pH value and the significant decrease in the malic acid content of the R-B mixed juice after fermentation. It has been reported that, under low-pH fermentation conditions, _L. plantarum_ would use a nonconventional carbon source such as malic acid and convert it into lactic acid, resulting in a significant decrease in sucrose content and no significant changes in glucose and fructose contents [43; 44]. The starter plays an important role in the flavor of a fermented beverage, and _L. plantarum_ TUST-232 was shown as a suitable starter for this fermented beverage, playing a key role in the metabolism of organic acids. Oxalic acid, citric acid, malic acid, succinic acid, lactic acid, and adipic acid are the primary organic acids in a beverage. During fermentation, _L. plantarum_ TUST-232 can reduce the acidity of the beverage by converting malic acid into lactic acid, and the final content of malic acid was reduced by 13.41%. [PERSON] et al. [45] used six different probiotics to ferment mango pulps, and LAB was shown to completely consume the malic acid in the mango pulps and convert it to lactic acid after fermentation. After fermentation by _L. plantarum_ TUST-232, malic acid was converted into lactic acid, leading to a reduction in the acidity of beverage. At the same time, the flavor of the fermented beverage from the R-B mixture was also improved, related to the conversion of malic acid to lactic acid, resulting in a decrease of 13.41% in the final malic acid content. [PERSON] et al. [46] studied a mulberry beverage coeultured with _Saccharomyces cerevisiae_ and _L. plantarum_. They found that _S. cerevisiae_ and _L. plantarum_ converted malic acid to lactic acid, which could not only reduce the acidity of beverage, but also improve the flavor through the generation of metabolites, thus enhancing the microbial stability and sensory quality of the final product. Meanwhile, some oxalic acid and adipic acid in the beverage could be metabolized by _L. plantarum_ as carbon and energy sources. During 48 h fermentation, the content of citric acid showed a continuous increase from 0-24 h, followed by a sharp drop. _L. plantarum_ TUST-232 can produce succinic acid instead of lactic acid, and the final content of succinic acid was increased by 2.08%. Flavor is one of the essential properties of a fermented beverage, and the peak area normalization method was used calculate the relative content of the volatile compounds. The pre-fermented beverage was relatively simple in flavor, mainly involving the smell of red jujube fruits and bamboo shoots. The fermentation of _L. plantarum_ TUST-232 released and produced some volatile compounds, such as acids, aldehydes, ketones, and other substances, thus improving the quality and flavor of the beverage. After fermentation by _L.