# Flavour Volatiles of Fermented Vegetable and Fruit Substrates: A Review
## 3 Results
### Alcohols
oligosporus_) fungi, the 2-phenylethyl alcohol concentration increased by 20 times, compared to its concentration in the unfermented okara, whereas with a mixed culture of _R. oligosporus_ fungi and _Yarrowia lipolytica_ (_Y. lipolytica_) yeast, the concentration increased by 8.5 times [38]; (8) in mango slurry fermented by yeast _S. cerevisiae_, the 2-phenylethyl alcohol concentration was 4 to 23 times higher, compared the concentration after LAB fermentation, where it was not detected in the unfermented mango slurry [41];(9) in papaya juice fermented by _L. plantarum_, the 2-phenylethyll alcohol concentration was doubled, compared to the concentration in _L. acidophilus_ fermented juice, where it was not detected in the unfermented juice [23]; and (10) in durian pulp fermented by _L. casei_ mixed with the yeast _W. saturus_, 2-phenylethyll alcohol was detected, where it was not detected in the unfermented pulp, or in the pulp fermented by a _L. casei_ monoculture [30]. Furthermore, compared to unfermented juice, one study reported that almost half of the alcohols detected decreased in _L. casei_ fermented tomato juice, likewise in _L. plantarum_ fermented tomato juice, most of the alcohols detected decreased. However, due to the generation of new alcohols, the relative peak area (RPA) for total alcohols increased to 59.9% and 49.7% in juice fermented by either _L. casei_ or _L. plantarum_, respectively, compared to a 49.3% RPA in the unfermented juice [32]. On the other hand, LAB fermentation increased the overall combined alcohol concentration of fruit and vegetable juices in 4 studies: (1) LAB fermentation of apple juice increased the overall combined alcohol concentration by 10 times compared to its concentration in the unfermented juice, demonstrating that most of the alcohols were produced during fermentation [21]; (2) LAB fermentation of grape juice increased the total combined alcohol concentration by 102.4% [43]; (3) LAB fermentation of kiwifruit juice (Xuxiang and Hongyang cultivars) increased the total combined alcohol concentration by 39, 107, and 56% in Xuxiang cultivar juice fermented by either _L. acidophilus_, _L. helveticus_, or _L. plantarum_, respectively, and by 25, 30, and 26% in Hongyang cultivar juice fermented by either _L. acidophilus_, _L. helveticus_ or _L. plantarum_, respectively [49]; and (4) LAB fermentation increased the total combined alcohol concentration of jujube juice (Varieties of Muzao and Heftian) by 66.5% in _L. acidophilus_ fermented Muzao juice and 33.7% in _L. casei_ fermented Hetian juice [20]. In another study, the total combined alcohol concentration of mango slurry fermented with yeast strains was nearly 10 times higher compared to mango slurry fermented with LAB strains [41]. Overall, alcohols such as 1-hexanol (11 papers), 3-methyl-1-butanol (10 papers), 2-phenylethyll alcohol (10 papers), benzyl alcohol (7 papers), ethanol (7 papers), and 1-octanol (5 papers) have been reported to have increased or were only detected after the fermentation of fruit and vegetable juices, mainly by LAB. However, it is important to note that for some substrates, the concentration of ethanol (6 papers), 3-methyl-1-butanol (5 papers), 1-hexanol (3 papers), and 1-octanol (1 paper) has been reported to have decreased after fermentation.