# Flavour Generation during Lactic Acid Fermentation of Brassica Vegetables--Literature Review
## 4 Lactic Acid Fermentation--State-of-Art  
Among the types of fermentation, the application of lactic acid bacteria (LAB) is the most commonly used methods for preserving dairy and vegetables, due to its simplicity, low cost and sustainability to maintain [21]. In addition to extending shelf life, lactic acid (LA) fermentation also modifies the sensory properties of products and increases their health-promoting effects [22]. Preservation by LA fermentation occurs mainly due to the synthesis of a wide range of metabolites including organic acids, fatty acids, bacteriocins, carbon dioxide, diacetyl and others [23]. Notably, a low pH and a relatively high concentration of LA provide the conditions that help inhibit the growth of competing, pathogenic and spoilage microbes. Therefore, LA fermentation not only extends the shelf-life but also secures the microbial safety of fermented foodstuffs. Moreover, in addition to providing safety and extending the shelf-life, LAB generate various secondary metabolites which affect the texture and nutritional value [24] and also possess a range of flavour activities [25].  
LAB ferment the products alone or in combination with other groups of microbes, including other bacteria, yeast or fungi. LAB constitute a group of Gram-positive, non-sporulating bacteria belonging to the _Firmicutes_ phylum. Compared to other bacteria, LAB are characterized by rapid acidification due to the fast consumption of carbohydrates. LAB can be further divided into homo- and heterofermattative, depending on the main products of their fermentation. Homofermattative bacteria convert glucose into LA with around an 80% theoretical efficiency rate [26] and include genera such as _Lactococcus_, _Pedicoccus_, _Streptococcus_ and _Enterococcus_. In turn, heterofermattative LAB, which include genera _Leuconstoc_, _Oenococcus_ and some _Lactobacillus_, except for LA also produce other acids, ethanol and carbon dioxide [27]. Additionally, some bacteria can be classified as facultatively hetero-erformative depending on the environmental conditions [28]. It is worth underlining that a new taxonomic classification for the genus _Lactobacillus_ was introduced in 2020 [29]. Based on the genome sequencing, the authors proposed the union of _Lactobacillus_ and _Leuconostocaceae_ and 23 new genera of _Lactobacillus_. However, in this study, the original nomenclature from cited articles was applied.  
LA fermentation of vegetables can be carried out by means of two main approaches. The first is the spontaneous fermentation done by the autochthonous bacteria present in the raw material. In this case, fermentation takes place under favourable conditions of temperature, salt concentration and oxygen availability. This type of fermentation is conducted in most of the fermented food consumed in developing countries. The second type is controlled fermentation, with the use of the starter cultures containing selected strains of bacteria, which secure the consistency, reliability and reproducibility of the final product [23]. Importantly, the area of the application of starter cultures in LA fermentation of vegetables is less developed compared to dairy and meat production, but some attempts have been conducted [30].