# Quality Characteristics and Flavor Analysis of Five Mulberry Varieties
## 3 Results and Discussion
### Soluble Sugar and Organic Acid  
As an energy source, soluble sugar is a major fruit component and an intrinsic quality of fruits [32]. Glucose and fructose were the key sugars in the five mulberry varieties examined. The fructose content detected in all mulberries was greater than the glucose content, indicating that fructose contributes more to sweetness in these mulberries (Table 1). [PERSON] et al. found that the fructose content of three mature mulberry fruits accounted for the highest proportion. Our findings are consistent with theirs [33]. Although the fructose content of Zhongsang 5801 was not the highest, it was 38.89 \(\pm\) 6.55 g/kg (Table 1). The difference, however, was not significant compared with the variety with the highest fructose content (\(p\) > 0.05). The glucose content of Zhongsang 5801 was the highest (26.19 \(\pm\) 6.29 g/kg) (Table 1), and no significant difference was observed among the varieties with the second and third highest glucose contents (\(p\) > 0.05). The fructose content of five kinds of mulberries was measured to be in the range of 19.75-39.66 g/kg. There was a partial overlap with the 12.66-62.77 g/kg fructose content measured by [PERSON] et al. in five mulberry fruit varieties suitable for cultivation in tropical areas, which indicates the reliability of the results of soluble sugar in this experiment [33].  
Figure 2: Physical and chemical indices and nutritional indices of five mulberry varieties. (**A**) Titratable acidity (g/kg). (**B**) Soluble solid content (\({}^{\circ}\)Brix). (**C**) Total sugar content. (**D**) Total phenol content (g/kg). (**E**) Total flavonoid content (g/kg). Different lowercase letters indicate significant differences among varieties (\(p\) < 0.05).  
Organic acids play a pivotal role in forming fruit flavor [34]. Oxalic acid, tartaric acid, and malic acid were detected in all mulberry varieties, which indicated significant variations among the five mulberry varieties. Among these organic acids, the relative content of malic acid was the highest, and it served as the primary organic acid in mulberries. Malic acid is an indispensable intermediate of the tricarboxylic acid (TCA) cycle in plants and significantly influences the quality of fruit flavors [35]. The malic acid content was significantly higher in 2000-3 than in other mulberry varieties, measuring 1.85 \(\pm\) 0.05 g/kg (Table 1, \(p\) < 0.05), followed by White Shahtoot Mulberry, Yuesang 10, and Zhongsang 5801. By contrast, the lowest concentration was observed in Jialing 40 (1.26 \(\pm\) 0.07 g/kg) (Table 1). These findings suggest variations in the metabolic pathways of the TCA cycle among these five mulberry varieties, which leads to inconsistent accumulation of intermediates [36]. Citric acid, tartaric acid, succinic acid, lactic acid, fumaric acid, and acetic acid were detected in white mulberry, black mulberry, and red mulberry, among which the malic acid content was 13.23-44.67 g/kg [37]. The content of malic acid measured in this experiment was 1.26-1.85 g/kg, which is lower than the previously reported value. Furthermore, only malic acid, oxalic acid, tartaric acid, and acetic acid were detected. The organic acid content among species may vary because of genetic factors, cultivation practices, and environmental conditions such as temperature, light intensity, and humidity [38; 39].