# Flavour Volatiles of Fermented Vegetable and Fruit Substrates: A Review
## 3 Results
### Ketones  
A number of ketones were identified in 26 studies investigating the fermentation of vegetable and fruit juices. 3-Hydroxy-2-butanone (acetoin), 2,3-butanedione (diacetyl), 2-propanone (acetone), 1-hydroxy-2-propanone (hydroxy acetone), 2-butanone (methyl ethyl ketone), 2-pentanone, 2-hydroxy-3-pentanone, 3-methyl-4-methylene-2-hexanone, 2-heptanone, 4-heptanone, 4-methyl-2-heptanone, 6-methyl-5-hepten-2-one (sulcotone), 2-octanone, 2-nonanone, 2-dodecanone, 2-undecanone, 2-tridecanone, 2-tetradecanone, (E)-6,10-dimethylundeca-5,9-dien-2-one (geranyl acetone), 4-cyclopentene-1,3-dione, and 1-phenylethanone (acetophenone) were identified in the reviewed papers. 3-Hydroxy-2-butanone (acetoin), which imparts a creamy/buttery note, was the most frequently detected ketone produced during the fermentation of vegetables and fruits. Citrate in vegetable and fruit juices can be directly converted to acetonitrile (Figure 3) by some LAB strains exhibiting citrate permease and citrate lyase activities. Citrate can be converted by LAB to pyruvate via oxalocate, then to acetaldehyde-thiamine pyrophosphate (TPP) through a decarboxylation process, and finally to acetaldehyde-TPP through an enzymatic reaction involving \(\alpha\)-acetolactone synthase, resulting in the synthesis of \(\alpha\)-acetolate. \(\alpha\)-Acetolate synthase has a low affinity for pyruvate; therefore, an excess of pyruvate is required to favour this reaction. In the presence of citrate and sugars, homofermentative LAB will convert pyruvate directly to \(\alpha\)-acetolate when less NADH is generated than pyruvate. Heterofermative LAB will, however, accumulate pyruvate at low pH when citrate is the sole carbon source. Further, due to the instability of \(\alpha\)-acetolate, it is readily decarboxylated enzymatically or chemically to yield acetoin. Acetoin can also be synthesised from diacetyl via the enzyme diacetyl acetoin reductase [63; 64; 65; 66]. In addition, when the pH of the medium is between 5 and 8, _Lactococcus lactis_ can also produce acetoin from the catabolism of aspartic amino acid [67] (Figure 4). The acetoin concentration increased after LAB fermentation of fruit and vegetable juices in 8 studies: (1) In Chinese wolfberry juice fermented by either _L. plantarum_, _L. paracasei_, _L. helveticus_, or _B. lactis_, the acetoin concentration was 346.3, 267.4, 528.1, and 422.1 ug/mL, respectively, compared to 29.1 ug/mL in the unfermented juice, where it was not detected in the juice fermented by _L. casei_ or _L. acidophilus_[50]; (2) in Muzao jujube juice fermented either _L. plantarum_ or _L. helveticus_, the acetoin concentration was 29.9 and 30.8 ug/mL, respectively, compared to 17.5 ug/mL in the unfermented juice. However, the acetoin concentration was reduced to 10.5 ug/mL in _L. acidophilus_ fermented juice and acetoin was not detected in _L. casei_ fermented juice [20]; (3) in kiwifruit juice (Xuxiang, and Hongyang cultivars), the acetoin concentration was 2621.6 and 1348.9 ng/mL in the Xuxiang cultivar juice fermented by either _L. helveticus_ or _L. plantarum_, respectively, where it was not detected in the _L. acidophilus_ fermented juice or in the unfermented juice. The acetoin concentration was 8431.7 and 4390.6 ng/mL in Hongyang cultivar juice fermented by either _L. helveticus_ or _L. plantarum_, respectively, where it was not detected in the _L. acidophilus_ fermented juice or in the unfermented juice [49]; (4) in elderberry juice fermented by either _L. plantarum_, _L. casei_, or _L. rhamns_, the acetoin concentration was 83.1-496.7, 90.7-314.5, and 41.4-456.2 ng/mL, respectively, compared to 1.4-22.1 ng/mL in the unfermented juice [22]; (5) in cherry juice fermented by either _L. rhamns_ or _L. paracasei_, the acetoin concentration was 260.7 and 5.9 ng/mL, respectively, compared to 0.001 ng/mL in the unfermented juice, and in cherry juice fermented by different _L.