# Flavour Volatiles of Fermented Vegetable and Fruit Substrates: A Review
## 4 Summary  
Overall, a large array of VOCs was detected after the fermentation of fruit and vegetable juices. The major VOCs identified were ethanol, octanol, hexanol, benzyl alcohol, 3-methyl-1-butanol, and 2-phenylethyll alcohol (alcohols); ethyl acetate, 2-phenylethyll acetate, and ethyl butanoate (esters); acetoin, and diacetyl (ketones); acetaldehyde, and benzaldehyde (aldehydes); acetic acid, 3-methyl-1-butanoic acid, hexanoic acid, butanoic acid (acids); linalool, D-limonene, \(\beta\)-damascenone, \(\alpha\)-terpineol, eugenol, and geraniol (terpenes and norisoprenoids).  
The use of different fruit and vegetable substrates, micro-organisms, and fermentation conditions are all likely to have had an impact on the production of fermentation VOCs. With the exception of a few studies that used bacteria other than LAB, fungi, or yeast either as a monoculture or in combination with LAB, most studies used LAB either as a monoculture or in mixed cultures. The LAB most frequently used for producing desirable VOCs were _L. plantarum_, _L. casei_, _L. acidophilus_, _L. rhamnus_, and _L. helveticus_. In studies that used two cultivars of a fruit, there were notable differences in the resulting fermented VOCs present and their concentrations [20; 49]. Additionally, there were variations in the types and concentrations of VOCs between the pH-adjusted and non-adjusted juices of sea buckthorn [42] and bog bilberry [34]. Moreover, there were observable differences in the detected VOCs from distillates (simple or vacuum distillation) of LAB-fermented orange pomace and melon by-product, assessed by SPME [44]. The results demonstrate how VOCs detected after being produced during fermentation are greatly influenced by the substrate (species and cultivar) being fermented, the LAB strain being used, and the fermentation conditions.  
Overall, in LAB-fermented fruit and vegetable juices, the concentrations of the main dairy flavour VOCs, namely acetoin and diacetyl, ranged between 0.04 and 528.1 ug/mL, and 0.01 and 71.9 ug/mL, respectively. It was apparent that LAB fermentation can yield high concentrations of acetoin and diacetyl from plant-based substrates. However, a wide variety of VOCs, including desirable and undesirable compounds, were detected in all of the studies reviewed. Once desirable dairy flavour components have been produced, extracting and purifying them from the other components present will be the next challenge. Current research is focusing on metabolic engineering techniques that involve overexpressing rate-limiting enzymes that produce desirable VOCs or inactivating the enzymes that produce undesirable VOCs in order to improve or create new metabolic pathways in micro-organisms. For instance, pyruvate is a crucial intermediate in the synthesis of the dairy flavours acetoin and diacetyl. Pyruvate in excess can be converted to \(\alpha\)-acetolactate by modifying the metabolic flux of pyruvate. If acetoin production is of particular interest, \(\alpha\)-acetolactate decarboxylase can be designed to be overexpressed, whereas when diacetyl production is of interest, NADH-oxidase can be overexpressed and \(\alpha\)-acetolactate decarboxylase expression can be inactivated [66] (see acetoin/diacetyl production pathway in the ketones section/Figure 3). However, due to the complexity of plant matrices, the action of different metabolic processes, and the factors influencing the fermentation, such as temperature, pH, and aeration, desirable VOCs might be metabolised, or their presence masked by undesirable VOCs. To meet these challenges, research on the metabolic pathway analysis of various micro-organism(s) on complex matrices of plants is required.