# Flavour Volatiles of Fermented Vegetable and Fruit Substrates: A Review
## 3 Results
### Terpenes and Norisoprenoids  
Terpenes, which are comprised of isoprene (C5) units, impart floral, rose, and fruity flavours. In 13 studies reported here, terpen were classified as being terpen or norisoprenoids; interestingly, in 14 studies, some terpene compounds (\(\beta\)-linalool, \(\alpha\)-terpineol, \(\beta\)-citronellol, geraniol, \(\beta\)-damascenone, D-limonene, and trans-\(\beta\)-ionone) were classified as either alcohol/hydrocarbon/ketone, or as alkenes in 2 studies. Terpenes are classified based on the number of isoprene units they contain: hemiterpenoids (C5), monoterepenoids (C10), sesquiterpenoids (C15), triterpenoids (C20), esterterpenoids (C25), tr triterpenoids (C30), tetraterpenoids (C40) (\(\beta\)-carotene), and polytrepenoids (C \(>\) 40) [12; 75]. \(\beta\)-carotene can be further oxidized into \(\beta\)-ionone, \(\beta\)-damascenone, and \(\beta\)-ionol. Bacteria biosynthesize terpenids via the 2-C-methyl-D-erythitol-4-phosphate (MEP) pathway [76], while yeast produce them via the mevalonate pathway [75]. (\(\beta\))-Myrcene, D-limonene (1-Methyl-4- (prop-1-en-2-yl) cyclohex-1-ene), ocinene, (\(\beta\))-linalool (3,7-Dimethyl-1,6-octadien-3-ol), camphene, p-cymene, \(\alpha\)-terpinolene, (\(\alpha\))-terpinene, 1,8-cineo, (\(\pi\))-terpinene, (\(\alpha\))-terpineol (2- (4-Methylcyclohex-3-en-1-yl)propan-2-ol), CitronellyI formate, (\(\beta\))-damascenone ((E)-1-(2,6,6- Trimethyl-1-cyclohex-1,3-dienyl) but-2-en-1-one), (B)-citronellol (3,7-Dimethyloct-6-en-1-ol), geraiol ((2E)-3,7-Dimethylocta-2,6-dien-1-ol), trans-(\(\beta\))-ionone (4-(2,6,6- Trimethylcyclohex-1-en-1-yl) but-3-en-2-one), m-cymene, phenol, (\(\alpha\))-ionene, D-germacrene, valencene, cedrol, phytol, myrtenol, eugenol (2-Methoxy-4-(prop-2-en-1-yl) phenol), and \(\beta\)- phellandrene were the major terpene compounds detected in the reviewed papers. Linalool, an important flavour terpene, was detected after fermentation of juices in 7 studies: (1) The linalool concentration of a distillate prepared using simple distillation from unfermented orange pomace was 1.6 \(\mu\)g/mL and after fermentation by \(L\). _mramosus_ of the orange pomace, the concentration in the resulting distillate was 2.3 \(\mu\)g/mL. Similarly, in distillates prepared using vacuum distillation, the linalool concentration in the unfermented orange pomace distillate was lower than that in the distillate from \(L\). _mramosus_ fermented orange pomace, at 0.1 \(\mu\)g/mL and 1.1 \(\mu\)g/mL, respectively. In the same study, in the unfermented melon by-product distillate prepared using simple distillation, the linalool concentration was 0.5 \(\mu\)g/mL and in the \(L\). _mramosus_ fermented melon by-product distillate, the concentration was 4.4 \(\mu\)g/mL. However, linalool was not detected in the distillates of fermented and unfermented melon by-products prepared using vacuum distillation [44]; (2) in elderberry juice fermented by either \(L\). _plantarum_ or \(L\). _mramosus_, the linalool concentration was 328.5 and 339.7 ng/mL, respectively, compared to 79.3 and 189.3 ng/mL in the unfermented juices, respectively [22]; (3) in cherry juice fermented by either \(L\). _mramosus_, \(L\). _paracasei_, or \(L\). _plantarum_, the linalool concentration was 39.6, 21.5, and 19.6-29 ng/mL, respectively, compared to 13.7-15.7 ng/mL in the unfermented juice [29]; (4) in goji juice fermented by a bacterial mixture (\(L\). _plantarum_, \(L\). _mramosus_, \(B\). _velezensis_, and \(B\). _licheniformis_), the linalool concentration was 22.3 ng/g, where it was not detected in the unfermented juice [25]; (5) in apple juice fermented by different LAB, the linalool concentration ranged from 3.0 to 4.5 ng/g, compared to 2.0 ng/g in the unfermented juice [21]; (6) in yam juice fermented by \(L\). _plantarum_, the linalool concentration was 21 times higher compared to the concentration in the juice fermented by a combination of \(L\). _plantarum_ and \(S\).