# Flavour Volatiles of Fermented Vegetable and Fruit Substrates: A Review
## 3 Results
### Aldehydes
lactis_, the acetaldehyde concentration was reduced to 52.8, 124.2, 123.3, 23.4, and 13.3 \(\upmu\)g/mL, respectively, compared to its concentration in the unfermented juice (155.9 \(\upmu\)g/mL), where in the juice fermented by _L. acidophilus_, the acetaldehyde concentration was 188.2 \(\upmu\)g/mL [50]. Further, the initial acetaldehyde concentration in a distillate prepared using simple distillation from unfermented melon by-product was 1320 ng/mL, which was reduced to 470 ng/mL in the _L. rhamns_ fermented melon by-product distillate. Moreover, when using the vacuum distillation method, in the unfermented melon by-product distillate, the acetaldehyde concentration was 160 ng/mL, where it was only 20 ng/mL in the distillate from _L. rhamns_ fermented melon by-product. Some LAB can convert acetaldehyde to ethanol and acetic acid, which could explain the decrease in acetaldehyde concentration in some fermentations [44]. Another important aldehyde from a flavour perspective as it imparts a pleasant aroma to fermented juices is benzaldehyde, which is generated by LAB from the amino acid phenylalanine. The conversion of phenylalanine to benzaldehyde by LAB is initiated by the aminotransferase enzyme. The resulting phenyl pyruvate acid is chemically converted to benzaldehyde in the presence of oxygen and manganese [58; 69]. The benzaldehyde concentration increased after fermentation of vegetable and fruit juices in 7 studies: (1) In Chinese wolfberry juice fermented by either _L. plantarum_, _L. paracasei_, or _L. acidophilus_, the benzaldehyde concentration was 117.2, 68.1, and 40.7 \(\upmu\)g/mL, respectively, where it was not detected in the unfermented juice nor in juice fermented by other LAB strains [50]; (2) in kiwifruit juice (Xuxiang and Hongyang cultivars), Xuxiang cultivar juice fermented by _L. acidophilus_, the benzaldehyde concentration was 490.3 ng/mL, compared to 369.7 ng/mL in the unfermented juice. In contrast, it was not detected in the unfermented Hongyang cultivar juice nor in the Xuxiang cultivar juice fermented by _L. heletic_s or _L. plantarum_ and all LAB fermented Hongyang cultivar juices [49]; (3) in bog bilberry juice fermented by two _L. plantarum_ strains, the benzaldehyde concentration was 55.5 and 62.3 ng/mL, compared to 41.8 ng/mL in the unfermented juice [34]; (4) in non-pH-adjusted (2.7) sea buckthorn juice,the benzaldehyde concentration was 2.7 ng/mL, which increased after fermentation for 36 and 72 h by _L. plantarum_ to 5.4 and 7.9 ng/mL, respectively. However, if the pH of the juice was adjusted to pH 3.5, the initial benzaldehyde concentration of 2.3 ng/mL decreased after _L. plantarum_ fermentation for 36 and 72 h to 1.1 and 1.7 ng/mL, respectively [42]; (5) in goji juice fermented by different combinations of bacterial strains (either _L. plantarum_, _L. rhamn_, _L. reuteri_, _B. velezensis_, or _B. licheniformis_), the benzaldehyde concentration ranged from 55.5 to 101.4 ng/g, compared to 46.3 ng/g in the unfermented juice [25]; (6) in durian pulp fermented by _L. casei_ monoculture, the benzaldehyde concentration was 2.9 times higher, compared to its concentration in the sequential co-culture with yeast _W. saturus_, and it was not detected in the unfermented pulp. This difference is because LAB can convert phenylalanine amino acid to benzaldehyde; however, yeast preferentially convert phenylalanine amino acid to phenylethyl alcohol via the Ehrlich pathway, resulting in a higher quantity of benzaldehyde in LAB fermentations [30]; and (7) in papaya juice fermented by _L. plantarum_, the benzaldehyde concentration was 2 times higher, compared to the concentration after the _L. acidophilus_ fermentation or in the unfermented juice [23]. Though the benzaldehyde concentration increased after LAB fermentation, it also reduced in 4 studies: (1) in Hetain jujube juice fermented by LAB, the benzaldehyde concentration ranged from 22.1 to 29.7 ug/mL, compared to 33 ug/mL in the unfermented juice [20]; (2) in jujube pulp fermented by a mixture of _L. plantarum_, _L. rhamn_, and _S.