# Flavour Volatiles of Fermented Vegetable and Fruit Substrates: A Review
## 3 Results
### Aldehydes  
Aldehydes were present at lower concentrations after the fermentation of fruit and vegetables in 26 out of 35 studies. During the fermentation process, aldehydes are generated via alcohol oxidation or acid decarboxylation. The main aldehyde compounds detected after fermentation included ethanal (acetaldehyde), phenyl methanal (benzaldehyde), 2-methyl butanal, 3-methyl butanal (isovaleraldehyde), pentanal (valeraldehyde), hexanal (caproaldehyde), (E)-2-hexenal, heptanal (enanthaldehyde), octanal (caprylaldehyde), (E)-2-ocenal, nonanal (pelargonaldehyde), (E)-2-nonenal, decanal (capraldehyde), dodecanal (lauraldehyde), 2,4-dimethyl-benzaldehyde, octadecanal (stearaldehyde), 2-undecenal, tridecanal, 3,5-dimethyl-benzaldehyde, and benzeneacetaldehyde/phenylacetaldehyde. Acetaldehyde provides fermented juices their distinct flavour, and it is produced by LAB from the amino acid threonine [68] or from sugars via the PK (phosphoketolase) pathway, and by yeast from sugars via the EMP (Embden-Meyerhof-Parnas) pathway [13]. At lower concentrations, acetaldehyde improves the flavour of fermented juice; however, at higher concentrations (200 \(\mathrm{\SIUnitSymbolMicro g}\)/g or 200 \(\mathrm{\SIUnitSymbolMicro g}\)/mL or above) [20; 21], it may negatively influence the flavour of fermented juices. Acetaldehyde was detected in 6 studies after LAB fermentation: (1) In Muzao jujube juice fermented by _L. acidophilus_, the acetaldehyde concentration was 19.9 \(\upmu\)g/mL, compared to 1.5 \(\upmu\)g/mL in the unfermented juice, with other LAB strains generating slightly higher or lower acetaldehyde concentrations compared to the unfermented juice [20]; (2) in kiwifruit juice (Xuxiang and Hongyang cultivars), Xuxiang cultivar juice fermented by either _L. plantarum_, _L. acidophilus_, or _L. heletic_s the acetaldehyde concentration was 1013.1, 136.1, and 124.3 ng/mL, respectively, compared to 109.5 ng/mL in the unfermented juice, whereas in the Hongyang cultivar juice fermented by either _L. plantarum_ or _L. acidophilus_, the acetaldehyde concentration was 1075.6 and 159.7 ng/mL, respectively, compared to 95.7 and 25.5 ng/mL in the unfermented juice and fermented juice by _L. heletic_s, respectively [49]; (3) in apple juice fermented by either _L. casei_, _L. rhamns_, _L. plantarum_, or _L. acidophilus_, the acetaldehyde concentration was 40.4, 15.0, 27.5, and 21.9 ng/g, respectively, whereas it was not detected in the unfermented juice [21]; (4) in apple juice fermented by either _L. plantarum_, _L. heletic_s, _L. casei_, _L. paracasei_, _L. acidophilus_, or _B. lactis_, the acetaldehyde concentration was 5.4, 2.1, 4.5, 2.4, 1.9, and 3.0 ng/mL, respectively, whereas it was not detected in the unfermented juice [19]; (5) in non-pH-adjusted (2.7) sea buckthorn juice, the acetaldehyde concentration was 10.6 ng/mL, which increased after fermentation for 36 and 72 h by _L. plantarum_ to 13.9 and 15.8 ng/mL, respectively. However, if the pH of the juice was adjusted to pH 3.5, the initial acetaldehyde concentration of 10.8 ng/mL decreased after _L. plantarum_ fermentation for 36 and 72 h to 1.1 and 1.2 ng/mL, respectively [42]; and (6) in watermelon juice fermented by either _L. plantarum_ or _P. pentosaceous_, the acetaldehyde concentration was 4.6 and 3.2 ng/mL, respectively, compared to 2.3, 2.0, 0.5, and 0.5 ng/mL in the unfermented juice, and _L. brevis_, _L. casei_, or _L. rhamns_ fermented juices, respectively [48]. The concentration of acetaldehyde in 6 studies reported here was still well below the concentration that has been reported to adversely affect flavour, indicating that acetaldehyde may have a positive impact on the overall flavour profile of fermented juices if it is above the minimum concentration required for perception. Note, in Chinese wolfberry juice fermented by either _L. plantarum_, _L. casei_, _L. paracasei_, _L. heletic_s, or _B.