# Flavour Volatiles of Fermented Vegetable and Fruit Substrates: A Review
## 3 Results
### Esters  
Esters, which have sweet and fruity notes, are formed when carboxylic acids linked with coenzyme-A (CoA) are esterified with alcohols [60]. Volatile esters were found in fermented fruit and vegetable juices in 27 of the 35 papers reviewed in this report. The sensory detection threshold for esters is lower than that of the corresponding alcohol or acid [61]. The majority of esters reported were either ethyl esters or acetate esters. Ethanol and acyl-CoA derivatives of fatty acids combine to form ethyl esters. Acetyl-CoA and alcohols, such as ethanol or higher alcohols, produced from amino acid metabolism, combine to form acetate esters [62]. The key ester compounds reported in the reviewed studies were the acetate esters: ethyl acetate (ethyl ethanoate); 3-methylbutyl acetate (isoamyl acetate/isopentyl acetate); 2-phenylethyll acetate and hexyl acetate, and ethyl esters: ethyl butanoate (ethyl butyrate); ethyl-3-methyl butanoate (ethyl isovalerate/ethyl isopentanoate); ethyl hexanoate (ethyl caproate); ethyl octanoate (ethyl caprylate); ethyl dodecanoate (ethyl laurate); ethyl propanoate (ethyl propionate); ethyl-2-methyl-butanoate; ethyl hexadecanoate (ethyl palmitate) and others: hexyl formate; methyl 3-methylbutanoate (methyl isovalerate/methyl isopentanoate); 3-methylbutyl 3-methylbutanoate (isoamyl isovalerate/isopentyl isopentanoate) and methyl 2-hydroxybenzoate (methyl salicylic). Eleven of the papers stated that ethyl acetate, which is formed by alcohol acetyltransferases from the reaction between acetyl Co-A and ethanol [20], was primarily responsible for the fruity flavour of fermented fruits and vegetable juices. A variety of LAB strains [19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 33; 44; 45; 50] were used in all studies except two; one used LAB combined with a yeast [30], and the other used fungi combined with a yeast [38]. In these studies, it was reported that for Chinese wolfberry juice fermented by either _L. plantarum_, _L. paracasi_, _L. acidophilus_, or _L. helicticus_, the ethyl acetate concentration was 6931, 4827.1, 4925.4, and 7323.3 \(\mathrm{\SIUnitSymbolMicro g}\)/mL, respectively, compared to 774.5 \(\mathrm{\SIUnitSymbolMicro g}\)/mL in the unfermented juice, where it was not detected in the juice fermented by _L. casei_ or _B. lactis_[50]. In Muzao jujube juice fermented by either _L. plantarum_ or _L. acidophilus_, the ethyl acetate concentration was 111.7 and 64.2 \(\mathrm{\SIUnitSymbolMicro g}\)/mL, respectively, where it was not detected in the unfermented juice or juice fermented by other LAB [20]. Further, in durian pulp fermented by _L. casei_ combined with a yeast _W. saturnus_, ethyl acetate was detected, whereas it was not detected in sole _L. casei_ fermentation or unfermented pulp [30]. However, [PERSON] et al. [32] found that in tomato juice, prior to fermentation, ethyl acetate was detected, where it was not detected after fermentation by LAB, and in mango slurry fermented by either yeast or LAB, the ethyl acetate concentration was reduced by 1.2-1.8 times, compared to its concentration in the unfermented slurry [41]. In 3 experiments, ethyl butanoate, which is formed by a reaction between ethanol and butyryl-CoA during LAB fermentation [21], was the next most common ester compound: (1) The ethyl butanoate concentration in a distillate prepared using simple distillation of _L. rhamns_ fermented melon by-product was 8760 ng/mL, compared to 700 ng/mL in the unfermented melon by-product distillate, and the ethyl butanoate concentrations of both the fermented and unfermented by-product distillates produced using vacuum distillation were at least 10 times lower than in the simple distillation distillates [44]; (2) in apple juice fermented by different LAB, the ethyl butanoate concentration ranged from 16.3 to 23.1 ng/g, compared to 2.1 ng/g in the unfermented juice [21]; and (3) in papaya juice fermented by LAB, ethyl butanoate was detected, where it was not detected in the unfermented juice [23].