# Flavour Volatiles of Fermented Vegetable and Fruit Substrates: A Review
## 3 Results
### Acids
licheniformis_), the acetic acid concentration ranged from 25.4 to 88.9 ng/g, where it was not detected in the unfermented juice [25]; (10) in mung bean fermented by _L. plantarum_ strains, the acetic acid concentration ranged from 0.17 to 0.29 ng/g, where it was not detected in the unfermented mung bean [45]; (11) in papaya juice fermented by either _L. acidophilus_ or _L. plantarum_, the acetic acid concentration was 5.7 and 2.4 times higher, respectively, compared to the concentration in the unfermented juice [23]; (12) in mango slurry fermented by either _L. plantarum_ or _S. thermophilus_, the acetic acid concentration was 2.4 and 2.7 times higher, respectively, compared to the concentration in the unfermented mango slurry, and in addition, it was not detected in _L. casei_ or yeast fermentations [41]; (13) in tomato juice fermented by _L. plantarum_, the acetic acid concentration was 8.6 times higher compared to the concentration in the unfermented juice, where it was not detected in _L. casei_ fermented juice [32]; (14) in yam juice fermented by LAB, the acetic acid concentration was 1.5 times higher compared to the concentration in the unfermented yam juice [33], and 15. in durian pulp fermented by _L. casei_, the acetic acid concentration was 1.4 times higher compared to the concentration in a co-culture of _L. casei_ and yeast _W. saturus_, where it was not detected in the unfermented pulp [30]. 3-Methyl-1-butanoic acid is an important acid flavour compound in dairy foods, which is produced from the amino acid leucine by an aminotransferase enzyme [57]. However, when present at high concentrations, it may negatively impact on the flavour of fermented juices [26]. The 3-methyl-1-butanoic acid concentration increased in 3 studies after LAB fermentation: (1) In non-pH-adjusted (2.7) sea buckthorn juice, the 3-methyl-1-butanoic acid concentration was 19.2 ng/mL, which increased after fermentation for 36 and 72 h by _L. plantarum_ to 25 and 39 ng/mL, respectively, where the pH of the juice was adjusted to pH 3.5, the initial 3-methyl-1-butanoic acid concentration of 13.1 ng/mL increased after _L. plantarum_ fermentation for 36 and 72 h to 129.3 and 185 ng/mL, respectively [42]; (2) in tomato juice fermented by _L. plantarum_, the 3-methyl-1-butanoic acid concentration was 3 times higher compared to the concentration in the unfermented juice, where, it was not detected in _L. casei_ fermented juice [32]; and (3) in papaya juice fermented by _L. plantarum_, the 3-methyl-1-butanoic acid concentration was 4.1 times higher compared to the concentration in _L. acidophilus_ fermented juice, where it was not detected in the unfermented juice [23]. Butanoic acid, which is produced from the fermentation of sugars through the fatty acid biosynthesis pathway [71] and can confer a dairy/cheesy aroma to fermented juices, was detected in 3 studies after fermentation: (1) In jujube juice fermented by a mixture of _L. plantarum_, _L. rhamns_, and _S. thermophilus_, the butanoic acid concentration was 1487 ng/g, compared to 345 ng/g in the unfermented juice [47]; (2) in apple juice fermented by different LAB, the butanoic acid concentration ranged from 2.3 to 4.7 ng/g, where it was not detected in the unfermented juice [21]; and (3) in okara fermented by a combination of _R. oligosporus_ and _Y. lipolytica_, the butanoic acid concentration was 34 times higher compared to the concentration in the juice fermented by _R. oligosporus_ monoculture [38]. However, in mango slurry fermented by the yeast _S. cerevisiae_, the butanoic acid concentration was 4.4-5.6 times lower compared to the concentration in the unfermented mango slurry, where the butanoic acid concentration was slightly higher after LAB fermentation compared to the unfermented mango slurry [41]. Hexanoic acid, which can be produced from the fermentation of sugars through the fatty acid biosynthesis pathway or from the cleavage of linoleic acid via hexanal, may impart a fatty/cheesy/sour flavour to fermented juices [22; 54; 72; 73].