# Flavour Volatiles of Fermented Vegetable and Fruit Substrates: A Review
## 3 Results
### Acids  
Twenty-five studies out of the thirty-five reported that acids contributed to the flavour of fermented vegetables and fruits. An extensive range of short- to long-chain fatty acids, including acetic acid, propanoic acid (propionic acid), 2-methyl-propanoic acid (isobutyric acid), butanoic acid (butyric acid), 3-methyl-1-butanoic acid (isovaleric acid), 2-methyl-1-butanoic acid, caproic acid (hexanoic acid), enanthic acid (heptanoic acid), caprylic acid (octanoic acid), pelargonic acid (nonanoic acid), capric acid (decanoic acid), lauric acid (dodecanoic acid), palmitic acid (hexadecanoic acid), and oleic acid, were detected in the fermented fruits and vegetables, which were formed from sugars or amino acid catabolism [52]. Acetic acid, which is a key flavour compound of fermented juices, is produced mainly by heterofermentative LAB (_L. brevis_, _Limosiactobacillus fermentum_ (_L. fermentum_), _L. reuteri_, _L. plantarum_, _L. rhamnosus_, and _L. casei_), which first utilise sugars via the PK pathway and produce acetyl phosphate, which is subsequently converted into acetic acid by the acetokinase enzyme [13]. However, homofermentative LAB (_L. acidophilus_, _S. thermophilus_, and _L. heleticus_), with high glycolytic flux rates, ferment sugars into solely lactic acid, while under slow growth conditions and low glycolytic flux rates, the homofermentative LAB change to mixed acid fermentation (formic acid, lactic acid, ethanol, and acetic acid) [70]. Acetic acid can also be produced from citrate, which is found in fruit and vegetable juices [65]. The acetic acid concentration increased after fermentation of fruit and vegetable juices in 15 studies: (1) In Chinese wolfberry juice fermented by either _L. casei_, _L. paracasei_, _L. heleticus_, or _B. lactis_, the acetic acid concentration was 19,773.7, 16,093.9, 12,698.4, and 14,011.1 ug/mL, respectively, and it was not detected in the unfermented juice or juice fermented by _L. plantarum_ or _L. acidophilus_[50]; (2) In two varieties of jujube (Muzao and H retain) juices fermented by different LAB strains, the acetic acid concentration ranged from 161 to 234.3 ug/mL, and 211.2 to 278.1 ug/mL, respectively, compared to 90.8 ug/mL in Muzao and 104.2 ug/mL in Hetain unfermented varietal juices. Interestingly, _L. heleticus_increased the acetic acid concentration by 158.2% in Muzao varietal juice and _L. casei_ increased it by 166.9% in Hetian varietal juice [20]; (3) in elderberry juice fermented by either _L. plantarum_, _L. casei_, or _L. rhamn_ strains, the acetic acid concentration ranged from 205.9-1012.3, 62.3-122.1, and 47.2-132.1 ng/mL, respectively, compared to 0.3-12.2 ng/mL in the unfermented juice [22]; (4) in cherry juice fermented by various _L. plantarum_ strains, the acetic acid concentration ranged from 54.8 to 184.8 ng/mL, compared to 0.01 ng/mL in the unfermented juice [29]; (5) in non-pH-adjusted (2.7) sea buckthom juice, the acetic acid concentration was 1.0 ng/mL which increased after fermentation for 36 and 72 h by _L. plantarum_ to 2.1 and 3.5 ng/mL, respectively, where the pH of the juice was adjusted to pH 3.5, the initial acetic acid concentration of 0.8 ng/mL increased after fermentation with _L. plantarum_ for 36 and 72 h to 50.5 and 85.9 ng/mL, respectively [42], 6. In grape juice fermented by LAB, the acetic acid concentration was 25.5 ng/mL, and it was not detected in the unfermented juice [43]; (7) in jujube juice fermented by a mixture of _L. plantarum_, _L. rhamn_s, and _S. thermophilus_, the acetic acid concentration was 12.2 ug/g, compared to 2.8 ug/g in the unfermented juice [47]; (8) in horse gram sprouts fermented by _L. plantarum_ strains, the acetic acid concentration ranged from 4.8 to 5 ug/g, and it was not detected in the raw seed [37]; (9) in goji juice fermented by different combinations of bacterial strains (either _L. plantarum_, _L. rhamns_, _L. reuteri_, _B. velezensis_, or _B.