Physicochemical and antioxidant properties of fermented tomatoes
Changes in physicochemical and antioxidant properties of tomatoes fermented with six-different LAB strains are presented in Fig. 1. After 3 days of fermentation, soluble solid contents decreased to 1.42–1.82°Brix. The pH value and TA in tomatoes before inoculation were 4.26 and 0.7%, respectively. Differences in soluble solid contents, pH, and TA were dependent on the fermentation strains used, indicating that the quality of fermented tomatoes varied depending on the LAB strain used.
Total phenol and flavonoid contents were increased in tomatoes fermented by LM and LP, respectively. These fermented tomatoes also showed higher DPPH radical scavenging activities compared to other samples, suggesting that some new antioxidant components might have been produced during the fermentation of tomatoes by LM and LP. Differences in phenolic compounds after fermentation might be associated with changes in the sensory characteristics of foods such as color and flavor as well as differences in antioxidant effects .
Metabolites changes of fermented tomatoes
Tomato metabolites were analyzed using GC–MS to determine metabolic changes in the fermented tomatoes and the effect of different LAB strains. After data processing using XCMS, a total number of 2554 signal features were obtained. PCA score plot derived from the GC–MS data of the tomatoes is shown in Fig. 2a. There was a clear separation by PC1 between tomato samples before and after fermentation. Cumulative R2X and Q2 values were 0.663 and 0.635, respectively. These results indicated dramatic metabolic changes after 3 days of fermentation by LAB.
Among the 2554 features detected by GC–MS, a total of 18 metabolites were identified. Table 1 summarizes the metabolites identified in this study and their changes after fermentation. Levels of lactic acid, succinic acid, alanine, methionine, aspartic acid, glutamic acid, and erythritol were increased, while levels of malic acid, citric acid, serine, threonine, and fructose were decreased after tomato fermentation. However, changes in some amino acids, such as valine, leucine, and phenylalanine, showed different patterns with different LAB strains.
Metabolite profiling of fermented tomatoes by different LABs
To determine metabolic differences of tomatoes fermented by the different LAB strains used, PCA modeling was performed on samples after fermentation without samples before fermentation (Fig. 2b). Interestingly, PCA score plot showed a clear separation into three groups. The first group included tomatoes fermented with PP, LF, and BL. These samples were located close to each other in the PCA score plot, indicating that the metabolic profiles of these samples were more similar than those of other samples. The second group included tomatoes fermented with LP and LM, which were not fully separated in score plot. The last group was tomato samples fermented with LB, implying that metabolites of tomato samples fermented with LB were very different from the other samples. Interestingly, different or similar fermentation behaviors were observed for each LAB strain, even if the same tomato material was used.
Metabolite differences of fermented tomatoes by different LABs
To identify the effects of different LAB strains on metabolites identified in fermented tomatoes, PCA biplot was generated (Fig. 3). Similar to the results shown in Fig. 2b, PCA biplot showed clear separation into three groups. Two groups (LF, BL, and PP vs. LP and LM) were clearly separated by PC1. Citric acid and malic acid were clustered in LF, BL, and PP. In addition, lactic acid, succinic acid, and fructose were related to LP and LM groups. Meanwhile, LB was associated with erythritol on PC2.
Figure 4 shows the relative differences of the metabolites identified in tomatoes fermented by different LAB strains. Tomatoes fermented by LF, BL, and PP were found to have the highest levels of glyceric acid, malic acid, citric acid, alanine, and serine (p < 0.05), whereas levels of succinic acid and glutamic acid were significantly higher in tomatoes fermented by LP and LM (p < 0.05). The highest values of valine, leucine, threonine, methionine, phenylalanine, and erythritol were found in tomatoes fermented with LB (p < 0.05).
Metabolic pathway analysis of LAB strains
To determine the most relevant metabolic pathways affected by different LAB strains, metabolic pathway analyses were performed (Fig. 5). Metabolites identified in fermented tomato samples from the two most distinct groups (LF, BL, and PP vs. LB) were used for metabolic pathway analyses. Comparing LF, BL, and PP versus LB groups, impact values of aminoacyl-tRNA biosynthesis, cysteine and methionine metabolism, glycine, serine and threonine metabolism, citric acid metabolism, glutamine and glutamate metabolism, and glutathione metabolism were 0.18, 0.12, 0.25, 0.12, 0.17, and 0.11, respectively. On the basis of p and impact values, aminoacyl-tRNA biosynthesis and metabolism of cysteine and methionine were identified as pathways affected by different LAB groups (LF, BL, and PP vs. LB groups).