# Flavour Volatiles of Fermented Vegetable and Fruit Substrates: A Review
## 3 Results
### Aldehydes
thermophilus_, the benzaldehyde concentration was 3516 ng/g, compared to 4672 ng/g in the unfermented pulp [47]; (3) in cherry juice fermented by various _L. plantarum_ strains, the benzaldehyde concentration ranged from 15.3 to 33.4 ng/mL, compared to 100.5 ng/mL in the unfermented juice, and in cherry juice fermented by _L. rhamn_, the benzaldehyde concentration was 76 ng/mL, compared to 90.5 ng/mL in the unfermented juice [29]; and (4) in okara fermented by LAB monocultures of _L. rhamn_ or _P. acidilactici_ and a co-culture (_L. acidophilus_, _L. rhamn_, and _P. acidilactici_), the benzaldehyde concentration was 22.4, 45, and 46.2 ug/g, respectively, compared to 114.6 ug/g in the unfermented okara, and in okara fermented by _L. acidophilus_, the benzaldehyde concentration was only slightly reduced to 109.9 ug/g [46]. Hexanal and nonanal are two other important aldehydes that may influence the flavour profile of fermented juices. Hexanal, which is produced from linoleic fatty acid, imparts a fresh, green, grassy, waxy, fatty, and unpleasant aroma to fermented fruit and vegetable juices [54]. After fermentation, the hexanal concentration was reduced in goji juice [25], mump bean [45], watermelon juice [48], sea buckthorn juice [42], kiwifruit juice [49], and okara pulp [38; 46]. In addition, it was not detected after the fermentation of Chinese wolfberry juice [50]. [PERSON] et al. [41] reported that in mango slurry fermented by LAB, the nonanal was detected, where it was not detected after yeast fermentation. Further, the nonanal concentration was reduced in fermented yarn juice [33], okara pulp [46], mung bean [45], watermelon juice [48], kiwifruit juice [49], and sea buckthorn juice [42]. It was also not detected during the fermentation of goji juice and durian pulp [25; 30]. Overall, the total combined aldehyde concentration was reduced after the fermentation of vegetable and fruit juices in 8 studies. This was probably due to the conversion of aldehydes either by reduction to alcohols or oxidation to acids [23]: (1) In Chinese wolfberry juice fermented by either _L. plantarum_, _L. casei_, _L. paracasei_, _L. acidophilus_, _L. helveticus_, or _B. lactis_, the total combined aldehyde concentration was 332.8, 680.1, 500.5, 983.3, 422.1, and 492.6 ug/mL, respectively, compared to 1841.2 ug/mL in the unfermented juice [50]; (2) in unfermented jujube (Muzao and Hetain varieties) juice, the total combined aldehyde concentration was 102.8 and 121.3 ug/mL, respectively, compared to 65.4-94.7 and 61.9-83.9 ug/mL in the fermented Muzao, and Hetain varietal juices by different LAB strains, respectively. However, the total combined aldehyde concentration increased by 38.3% in Hetain varietal juice fermented by _L. plantarum_ and 158% in Muzao varietal juice fermented by _L. acidophilus_[20]; (3) in kiwifruit juice (Xuxiang and Hongyang cultivars), Xuxiang cultivar juice fermented by either _L. acidophilus_, _L. heleticus_, or _L. plantarum_, the total combined aldehyde concentration was 2411.9, 4248.7, and 4915.8 ng/mL, respectively, compared to 15,316.3 ng/mL in the unfermented juice, where in the Hongyangcultivar juice fermented by either _L. acidophilus_, _L. heleticus_, or _L. plantarum_, the total combined aldehyde concentration was 2466.1, 2528.9, and 3866.4 ng/mL, respectively, compared to 23,477.9 ng/mL in the unfermented juice [49]; (4) in cherry juice fermented by various _L. plantarum_ strains, the total combined aldehyde concentrations ranged from 43.5 to 78.2 ng/mL, compared to 208.5 ng/mL in the unfermented juice [29]; (5) in okara fermented by _L. acidophilus_, _P. acidilactici_, or _L. rhamnosus_, and a co-culture (_L. acidophilus_, _P. acidilactici_, and _L. rhamnosus_), the total combined aldehyde concentration was 866.1, 282.9, 25.3, and 68.3 ug/g, respectively, compared to 1450.6 ug/g in the unfermented okara [46]; (6) in watermelon juice fermented by either _L. plantarum_, _L. rhamnosus_, _L. casei_, _L. brevis_, or _P.