# Flavour Volatiles of Fermented Vegetable and Fruit Substrates: A Review
## 3 Results
### Ketones
However, in tomato and pepper pomace fermented by either _Trichoderma atroviride_ (_T. atroviride_) or _Aspergillus sojae_ (_A. sojae_), the diacetyl concentration was reduced compared to the concentration present in the unfermented pomace [40]. Figure 4: Catabolism of aspartic amino acid by _Lactococcus lactis_. ODC: oxaloacetate decarboxylase, LDH: lactate dehydrogenase, PDH: pyruvate dehydrogenase, PDC: pyruvate decarboxylase, TPP: thiamine pyrophosphate, ALS: \(\alpha\)-acetolactate synthase, ALDC: \(\alpha\)-acetolactate decarboxylase, DAR: diacetyl acetoin reductase, and BDH: 2,3-butanediol dehydrogenase [67]. Figure 3: Citrate metabolic pathway by LAB. CL: citrate lyase, ODC: oxaloacetate decarboxylase, LDH: lactate dehydrogenase, PDC: pyruvate decarboxylase, TPP: thiamine pyrophosphate, ALS: \(\alpha\)-acetolactate synthase, ALDC: \(\alpha\)-acetolactate decarboxylase, DAR: diacetyl acetoin reductase, and BDH: 2,3-butanediol dehydrogenase [63; 64; 65; 66]. Overall, after LAB fermentation, the total combined ketone concentration increased in 5 studies: (1) In okara fermented by LAB co-culture with _L. acidophilus_, _L. rhamnus_, and _P. acidilactici_, the total combined ketone concentration was 2355.6 \(\mathrm{\SIUnitSymbolMicro g}\)/g, compared to 116.1 \(\mathrm{\SIUnitSymbolMicro g}\)/g in the unfermented okara; however, in okara fermented by monocultures of either _L. acidophilus_, _L. rhamnus_, or _P. acidilactici_, the total combined ketone concentration was 98.8, 64.3, and 57.8 \(\mathrm{\SIUnitSymbolMicro g}\)/g, respectively. During a monoculture fermentation, unstable aldehydes and ketones may be reduced to primary and secondary alcohols, whereas in a co-culture fermentation, synergic interactions between strains may instead result in the production of higher levels of ketones, which could be linked to the oxidation of alcohols [46]; (2) in cherry juice fermented by either _L. rhamnus_ or _L. paracasei_, the total combined ketone concentration was 285.1 and 11.3 \(\mathrm{\SIUnitSymbolMicro g}\)/mL, respectively, compared to 7.2 \(\mathrm{\SIUnitSymbolMicro g}\)/mL in the unfermented juice. In the cherry juice fermented by different _L. plantarum_ strains, the total combined ketone concentration ranged from 48.3 to 292.4 \(\mathrm{\SIUnitSymbolMicro g}\)/mL, compared to 6.9 \(\mathrm{\SIUnitSymbolMicro g}\)/mL in the unfermented juice [29]; (3) in apple juice fermented by either _L. plantarum_, _L. heleticus_, _L. casei_, _L. paracasei_, _L. acidophilus_, or _B. lactis_, the total combined ketone concentration was 17.0, 27.6, 29.6, 56.6, 22.5, and 26.4 \(\mathrm{\SIUnitSymbolMicro g}\)/mL, respectively, compared to 16.6 \(\mathrm{\SIUnitSymbolMicro g}\)/mL in the unfermented juice [19]; (4) in another study, using apple juice fermented by different LAB strains, the total combined ketone concentration ranged from 10.1 to 11.7 \(\mathrm{\SIUnitSymbolMicro g}\)/g, compared to 2.6 \(\mathrm{\SIUnitSymbolMicro g}\)/g in the unfermented juice [21]; and (5) LAB fermentation of kiwifruit juice (Xuxiang, and Hongyang cultivars) increased the total combined ketone concentration by 2.6, 5.2, and 2.6 times in Xuxiang cultivar juice fermented by either _L. acidophilus_, _L. helveticus_, or _L. plantarum_, respectively, and by 6.3, 75, and 37 times in Hongyang cultivar juice fermented by either _L. acidophilus_, _L. helveticus_, or _L. plantarum_, respectively [49]. Moreover, in mango slurry fermented by LAB, the total combined ketone concentration was 1.2-1.8 times higher compared to the concentration in the unfermented juice, whereas in mango slurry fermented by the yeast _S. cerevisiae_, the total combined ketone concentration was 1.4-2.7 times lower compared to the concentration in the unfermented juice [41]. Overall, ketones, which are key contributors to dairy notes, such as acetoin (13 papers) and diacetyl (6 papers) have been reported to increase or were only detected after the fermentation of fruit and vegetable juices.