# Flavour Volatiles of Fermented Vegetable and Fruit Substrates: A Review
## 3 Results
### Alcohols
Benzyl alcohol was detected after bacterial fermentation in 7 studies [19; 25; 29; 34; 35; 47; 45] with concentrations ranging from 2.3 ng/mL to 270 ng/mL. 3-Methyl-1-butanol and 2-phenylethyl alcohol were the most common amino acid-derived alcohols detected after fermentation. 3-Methyl-1-butanol and 2-phenylethyl alcohol are synthesised from the catabolism of the amino acids leucine [57] and phenylalanine [58], respectively. 3-Methyl-1-butanol has a malt/alcoholic/whiskey odour that is considered unpleasant when present in concentrations greater than 400 \(\mu\)g/mL [59]. In fermented fruit and vegetable juices, 3-methyl-1-butanol was detected in 10 studies [21; 22; 23; 24; 29; 31; 38; 44; 45]. However, in fermented fruit and vegetable juices, the 3-methyl-1-butanol oncentrations was reduced in 5 studies: (1) In Chinese wolfberry juice fermented by either _L. casei_, _L. paracasei_, _L. acidophilus_, _L. heleticus_, or _B. lactis_, the 3-methyl-1-butanol concentration was 825.9, 674.0, 833.6, 799.6, and 820.6 \(\mu\)g/mL, respectively, compared to 2065.3 \(\mu\)g/mL in the unfermented juice, and it was not detected in the juice fermented by _L. plantarum_[50]; (2) in non-pH-adjusted (2.7) sea buckthorn juice, the 3-methyl-1-butanol concentration was 122.9 ng/mL, which was reduced after fermentation for 36 and 72 h by _L. plantarum_ to 98.9 and 91.4 ng/mL, respectively. However, if the pH of the juice was adjusted to pH 3.5, the initial 3-methyl-1-butanol concentration of 121.4 ng/mL increased after _L. plantarum_ fermentation for 36 and 72 h to 217.9 and 233 ng/mL, respectively [42]; (3) in apple juice fermented by different LAB strains, the 3-methyl-1-butanol concentration ranged from 4.5-16.9 ng/mL, compared to 73.2 ng/mL in the unfermented juice, among the strains studied, _L. acidophilus_ fermented juice had a 93% reduction in 3-methyl-1-butanol concentration [19]; (4) in apple juice fermented by the yeast _S. cerevisiae_, the 3-methyl-1-butanol concentration was 644 ng/mL; however, the concentration of 3-methyl-1-butanol reduced to 42.1 ng/mL after _L. plantarum_ sequential fermentation [51]; and (5) in cashew apple juice fermented by _L. acidophilus_, the 3-methyl-1-butanol concentration reduced by 2 times, compared to other LAB strains (either _L. plantarum_ or _L. casei_) studied [26]. In 10 studies, the concentration of 2-phenylethyl alcohol, which has a flowery smell, was increased or it was detected after fermentation: (1) In Chinese wolfberry juice fermented by _L. plantarum_, the 2-phenylethyl alcohol concentration was 246.4 ug/mL, where it was not detected in the unfermented juice or the fermented juice by other LAB strains [50]; (2) in non-pH-adjusted (2.65) bog bilberry juice fermented by two strains of _L. plantarum_, the 2-phenylethyl alcohol concentration was 1731.2 and 1775.8 ng/mL, compared to 663.5 ng/mL in the unfermented juice. However, if the pH of the juice was adjusted to pH 3.5, the initial 2-phenylethyl alcohol concentration of 617.5 ng/mL was decreased after fermentation by two strains of _L. plantarum_ to 459.7 and 463.4 ng/mL [34]; (3) in grape juice fermented by LAB, the 2-phenylethyl alcohol concentration was 40.6 ng/mL, where it was not detected in the unfermented juice [43]; (4) in horse gram sprouts fermented by two _L. plantarum_ strains, the 2-phenylethyl alcohol concentration was 1290 and 780 ng/g, compared to 40 ng/g in raw seed [37]; (5) in goji juice fermented by different combinations of bacterial strains (either _L. plantarum_, _L. rhamns_, _L. reuteri_, _B. velezensis_, or _B. licheniformis_), the 2-phenylethyl alcohol concentration ranged from 362.6 to 494.0 ng/g, compared to 103.1 ng/g in the unfermented juice [25]; (6) in juibube pulp fermented by a mixture of _L. plantarum_, _L. rhamns_, and _S. thermophilus_, the 2-phenylethyl alcohol coh concentration was 283 ng/g, where it was not detected in the unfermented juice [47]; (7) in okara fermented with a monoculture of _Rhizopus oligosporus_ (_R.