# Flavour Volatiles of Fermented Vegetable and Fruit Substrates: A Review
## 3 Results
### Ketones
plantarum_ strains, the acetoin concentration ranged from 44 to 287.9 ng/mL, compared to 0.002 ng/mL in the unfermented juice [29]; (6) the acetoin concentration in a distillate prepared using vacuum distillation from orange pomace fermented by _L. rhamns_ was 450 ng/mL, compared to 110 ng/mL in the unfermented pomace distillate, where acetoin was not detected in distillates from either fermented or unfermented pomace using the simple distillation method [44]; (7) in mung beans fermented by two _L. plantarum_ strains, the acetoin concentration was 2.8 and 7.5 times higher, compared to the concentration in the unfermented mung beans [45]; and (8) in papaya juice fermented by either _L. plantarum_ or _L. acidophilus_, the acetoin concentration was 2.2 and 3.7 times higher, respectively, compared to the unfermented juice [23]. In 5 studies, acetoin was only detected after fermentation of juices: (1) In okara fermented by a LAB co-culture (_L. acidophilus_, _L. rhamns_, and _P. acidilactici_), the acetoin concentration was 166.3 ug/g, where it was not detected in LAB monocultures [46]; (2) in horse gram sprouts fermented by _L. plantarum_, the acetoin concentration was 440 ng/g [37]; (3) in goji juice fermented by a bacterial mixture of _L. rhamns_, _L. reuteri_, and _B. velezensis_, the acetoin concentration was 87.2 ng/g juice [25]; (4) in mango slurry fermented by _S. thermophilus_, acetoin was detected, where it was not detected after fermentation by yeast _S. cerevisiae_ or other LAB [41]; and (5) in durian pulp fermented by _L. casei_ in sequential co-culture with _W. saturus_ yeast, acetoin was detected, where it was not detected in _L. casei_ monoculture [30]. Note, that in apple juice fermented by either _L. acidophilus_, _L. helveticus_, or _L. paracasei_, the acetoin concentration was reported to have decreased to 0.8 ng/mL, 1.2 ng/mL, and 4.3 ng/mL, respectively, compared to 5.4 ng/mL in the unfermented juice, where it was not detected in _L. plantarum_, _L. casei_, or _B. lactis_ fermented apple juice [19]. The second most commonly reported ketone was 2,3-butanedione (diacetyl), which imparts creamy/buttery notes. It is produced by LAB from citrate present in juice (Figure 3). As discussed for the acetoin production pathway, \(\alpha\)-acetolate can be directly converted into diacetyl through nonenzymatic oxidative carboxylation in the presence of molecular oxygen [63; 65]. Diacetyl was reported to be increased by the presence of some LAB during fermentation of juices in 6 studies: (1) In Chinese wolfberry juice fermented by either _L. plantarum_, _L. casei_, _L. paracasei_, or _L. acidophilus_, the diacetyl concentration was 45.1, 51, 71.9, and 28.1 ug/mL, respectively, where it was not detected in the unfermented juice nor the juice fermented by _L. helveticus_ or _B. lactis_[50]; (2) in elderberry juice fermented by either _L. rhamns_, _L. plantarum_, or _L. casei_ strains, the diacetyl concentration ranged from 37-586.8, 16.2-400.7, and 221.9-276.6 ng/mL, respectively, compared to 3.3-12.2 ng/mL in the unfermented juice [22]; (3) in kiwifruit juice (Xuxiang, and Hongyang cultivars), the diacetyl concentration was 261.1 ng/mL in Xuxiang cultivar juice fermented by _L. helveticus_, where it was not detected in Xuxiang cultivar juice fermented by other LAB or in the unfermented juice. Interestingly, with the Hongyang cultivar juice, diacetyl was not detected in the unfermented juice or in any of the LAB fermented juices [49]; (4) in watermelon juice fermented by either _L. plantarum_, _L. brevis_, _L. casei_, or _L. rhamns_, the diacetyl concentration was 1.46, 1.47, 62.5, and 85.7 ng/mL, respectively, where it was not detected in the _P. pentosaceus_ fermented juice and the unfermented juice [48]; (5) in mango slurry fermented by _L. casei_, diacetyl was detected, whereas it was not detected in other LAB or yeast fermentations [41]; and (6) in pomergnate juice fermented by _L. plantarum_ strains, the diacetyl concentration increased, compared to the concentration in the unfermented juice [24].