# Flavour Volatiles of Fermented Vegetable and Fruit Substrates: A Review
## 3 Results
### Esters
2-Phenylethyl acetate, which is formed by a reaction between 2-phenylethyl alcohol and acetyl CoA, was the third most commonly reported (3 studies) ester in juices after fermentation: (1) In horse gram sprouts fermented by _L. plantarum_, the 2-phenylethyl acetate concentration was 220 ng/g, where it was not detected in raw seeds [37]; (2) in grape juice fermented by a mixed culture of _L. plantarum_ and _L. brevis_, the 2-phenylethyl acetate concentration was 3.5 ng/mL, compared to its concentration in the unfermented juice (1.6 ng/mL) [43]; and (3) in durian pulp fermented by a combination of _L. casei_ and _W. saturnus_ yeast, 2-phenylethyl acetate was detected, whereas it was not detected during _L. casei_ only fermentation or in the unfermented pulp [30]. Propyl acetate, which is formed by a reaction between propanol and acetyl-CoA, was detected in cherry juice fermented by either _L. plantarum_, _L. rhamns_, or _L. paracasei_, where the propyl acetate concentration was 18.5-83.4, 1186.7, and 201.6 ng/mL, respectively, compared to about 0.01 ng/mL in the unfermented juice. In this study, the formation of propyl acetate during fermentation appeared to correlate with acetic acid production. As there was a low concentration of acetic acid after fermentation by _L. rhamns_ or _L. paracasi_, this was taken as evidence of the conversion of acetic acid to the corresponding ester. In contrast, in the same study, fermentation by _L. plantarum_ resulted in a high concentration of acetic acid and a lower concentration of propyl acetate [29]. Overall the total combined ester concentration increased in 6 studies after fermentation due to the availability of alcohol precursors [52]: (1) In Muzao jujube juice fermented by either _L. acidophilus_ or _L. plantarum_, the total combined ester concentration was 65.4 and 156.7 \(\mathrm{\SIUnitSymbolMicro g}\)/mL, respectively, compared to 4.7 \(\mathrm{\SIUnitSymbolMicro g}\)/mL in the unfermented juice [20]; (2) in mixed juices (apple juice, orange juice, carrot juice, and Chinese jujube juice) fermented by LAB mixed culture (_L. plantarum_, _Bifidobacterium breve_ (_B. breve_), and _S. thermophilus_), the total combined ester concentration was 415 ng/mL, compared to 239 ng/mL in the unfermented juice [35]; (3) in apple juice fermented by different LAB, the total combined ester concentration ranged from 81.8 to 92.9 ng/g, compared to 33.7 ng/g in the unfermented juice [21]; (4) in grape juice fermented by LAB, the total combined ester concentration increased by 83.76%, compared to its concentration in the unfermented juice [43]; (5) in mango slurry fermented by LAB and yeast, the total combined ester concentration increased,compared to the unfermented slurry, with the yeast _S. cerevisiae_ generating a significantly (\(p<0.05\)) higher number of esters present at a high concentration than LAB [41]; and (6) in pomergnate juice fermented by LAB, the total combined ester concentration increased, compared to the concentration in the unfermented juice [24]. On the other hand, 3 studies reported a reduction in the total combined ester concentration after fermentation, possibly due to hydrolysis into their corresponding acids and alcohols [47]: (1) In apple juice fermented by either _L. plantarum_, _L. helveticus_, _L. casei_, _L. paracasei_, _L. acidophilus_ or _B. lactis_, the total combined ester concentration was 1090, 1279.6, 787.5, 695.9, 702.3, and 643.1 ng/mL, respectively, compared to 1410.7 ng/mL in the unfermented juice [19]; (2) in jujube juice fermented by a mixture of _L. plantarum_, _L. rhamns_, and _S. thermophilus_, the total combined ester concentration was 1541 ng/g, compared to 5814 ng/g in the unfermented juice [47]; and (3) in tomato juice fermented by either _L. casei_ or _L. plantarum_, the total combined ester concentration was 1.6 times and 7 times lower, respectively, compared to the concentration in the unfermented juice [32].