# Flavour Generation during Lactic Acid Fermentation of Brassica Vegetables--Literature Review
## 6 Changes in Flavour during the Fermentation of _Brassica_ Vegetables
### Carbohydrate Metabolism  
Sugars are primary substrates for LAB in fermented Brassica vegetables. Depending on the LAB species present in the raw material, sugars undergo homo- or heterorefmentation with the formation of various products. Homogenization, as mentioned before, results in the formation of mainly LA (Figure 2). Pyruvate is often an intermediate compound in many metabolic pathways that occur during fermentation, which can then be converted into many aroma-active compounds [52]. When carbohydrates are available in the fermenting matrix, the conversion of pyruvate into LA is the main metabolic pathway. Thus, the LA is quantitatively the most dominant acid in the fermented product. LA is, however, odourless and does not directly contribute to the flavour of fermented products. Heterofermentative LAB, except for LA, also produce acetic acid and other compounds, which can have aroma-active properties or can be precursors of aroma compounds. Ethanol formed by heterofermentative bacteria possesses a rather neutral aroma; however, its presence can affect the retention of other flavour compounds. An important product of LAB is acetic acid, which is responsible for its cider-vinegar flavour and can be formed by the oxidation of pyruvate. The ratio between LA and acetic acid decreases if there is an increase in the oxygen input during fermentation. The pool of acetic acid is usually built in the first 48 h of fermentation of cabbage [53; 54], increasing the sourness of the sauerkraut. Both LA and acetic acid can be precursors for ester formation, ethyl lactate and ethyl acetate, respectively, which are responsible for the fruity aroma. The formation of these esters is highly dependent on the LAB strains. According to [PERSON] et al. [54], the selection of specific LAB strains can result in a higher production of ethyl acetate, which can be useful in the formation of starter cultures for obtaining the products of certain sensory properties.  
Another carbohydrate substrate is citrate, which can be co-metabolized with sugars by citrate-utilizing LAB, such as _Lactococcus lactis_ and _Leuconostoc mesenteroides_. Fermentation of citrate leads to the formation of 2,3-butanedione, 2-hydroxy-3-butanone, acetaldehyde and 2,3-butanediol, which significantly contribute to the aroma of fermented vegetables [55]. Acetaldehyde can be directly formed by pyruvate decarboxylase or pyruvate oxidase and can be produced indirectly by the intermediate product acetyl-CoA with pyruvate dehydrogenase [56]. 2,3-Butanedione is produced by the metabolic intermediate \(\alpha\)-acetolactate through oxidative decarboxylation and at the same time, \(\alpha\)-acetolactate can be converted to 2-hydroxy-3-butanone by \(\alpha\)-acetolactate decarboxylase or through non-oxidative decarboxylation [55]. Apart from their aroma activity, these compounds are important aroma precursors for a variety of odour compounds, which can be formed from Maillard and Strecker reactions with amino acids, similar to those observed in wine [57].