# Flavour Generation during Lactic Acid Fermentation of Brassica Vegetables--Literature Review
## 6 Changes in Flavour during the Fermentation of _Brassica_ Vegetables
### Amino Acid Metabolism  
During fermentation, extensive proteolysis occurs, which increases the pool of smaller peptides and free amino acids. Amino acids play a pivotal role in the formation of organoleptic features of fermented products. Amino acids themselves possess taste properties but are also substrates in the formation of aroma-active compounds. There are three main groups of amino acids that are distinguished by their taste: umami (aspartic and glutamic acid), sweet (alanine, serine, glycine, threonine, proline and asparagine) and bitter (valine, histidine, arginine, phenylalanine and leucine) [58]. In Brassica vegetables, the main amino acids are glutamine, asparagine, alanine and glycine in cauliflower [59], glutamic acid, aspartic acid, serine and alanine in broccoli [60], while in trochuda cabbage the main amino acids are arginine, followed by proline, threonine, glutamine, cysteine, and glutamic acid [61]. Glutamic acid, glycine and \(\gamma\)-aminobutyric acid were reported to be the main compounds responsible for the taste of paocai, the product obtained from LA fermentation of Chinese cabbage, cabbage, radish, mustard stems, long beans, peppers, daikon, carrots and ginger, in an electronic tongue study [62]. The authors underlined the role of LAB in the formation of flavour-active compounds.  
Degradation of amino acids during fermentation is done via Ehrlich and Strecker pathways as well as others. For instance, leucine, valine, isoleucine, methionine and tyrosine are converted to fused alcohols, which can be further oxidized to aldehydes, which can then be converted into the corresponding acid, which finally can be esterified [63]. The reaction starts with the formation of \(\alpha\)-ketoacid from the amino acid, which can then be converted to aldehydes, carboxylic acid and alcohols (Ehrlich pathway). Next, aldehydes are subjected to further conversions into alcohols by dehydrogenation or into carboxylic acids by hydrogenation, while carboxylic acids can be esterified by specific esterases, leading to the formation of (thio)esters (Figure 2) [64]. Some important products of LA fermentation of amino acids are aldehydes (2-methyl-propanal, 2-methyl-butanal, 3-methyl-butanal, etc.) and corresponding alcohols (2-methyl-1-propanol, 2-methyl-butanol, 3-methyl-1-butanol, etc.), which are responsible for the fruity and alcoholic flavour [65]. Notably, during fermentation, the activity of many enzymes derived from LAB and the l  
Figure 2: The metabolic pathways involved in the formation of aroma-active compounds during lactic acid fermentation.  
the ones belonging to transaminase and lyase pathways, is observed, which contribute to the formation of a wide range of aroma-active compounds [64].  
Among all of the amino acids, the metabolism of sulfur-containing amino acids such as methionine and cysteine is particularly important, as these are responsible for the formation of methanethiol, sulphides, thioesters and other sulfur-containing volatile compounds. Sulfur-containing aroma-active compounds are characterized by a very low odour threshold; thus, even their small concentration contributes to their cabbage-like, sulfury aroma [66]. It was repeatedly reported that these compounds are in the main responsible for the aroma of Brassica vegetables [45; 46; 67]. The formation of methanethiol from methionine can be achieved via a transaminase reaction with the formation of intermediate \(\alpha\)-keto methylthio-butyric acid, which can undergo decarboxylation into 3-methylthio-propanal, which are then converted to methanethiol by a chemical reaction [24]. Methanethiol can then be oxidized into dimethyl disulphide or dimethyl trisulfide or can be converted into thioesters via esterase (Figure 3).