# Flavour Generation during Lactic Acid Fermentation of Brassica Vegetables--Literature Review
## 5 Flavour of Brassica Vegetables  
Brassica vegetables are an important group of products and are commonly consumed in all parts of the world. Brassica vegetables are part of the genus _Brassiaceae_ and include, among others, broccoli (_Brassica oleracea_ var. _italica_), Brussels sprouts (_Brassica oleracea_ L. var. _gemmifera_ (DC.) [PERSON]), cabbage (_Brassica oleracea_ L. var. _capitata_ L.), cauliflower (_Brassica oleracea_ var. _botrytis_ L.), horseradish (_Armoracia rusciana_), rapeseed (_Brassica napus_ subsp. _nupus_) and many others. Another name used for Brassica plants is cruciferous, a name that is related to the shape of their flowers, the four petals of which resemble a cross. The flavour of Brassica vegetables is unique and is not always tolerated by a significant part of society, which is caused mainly by genetic predisposition [14]. The compounds responsible for their characteristic bitter taste, as well as their sulfurous aroma, are mainly sulfur-containing substances, such as glucosionolates (GLS), their degradation products isothi cyanates, as well as other sulfur-containing volatile organic compounds (sulfides, thiols, etc.) [31]. Some Brassica vegetables, such as horseradish, are used mainly to flavour dishes due to the high content of allyl isothiamate [32]. Despite the multiple literature sources that describe GLS as bitter and isothi cyanates as pungent, it has been proven so far that this is not always true, since not all GLS and isothi cyanates impart the negative sensory traits [31; 33]. As has been noticed in recent studies, some of the GLS were correlated with pleasant taste attributes, such as 4-hydroxyglucobrassicin in rocket salad [34]. On the other hand, in Brussels sprouts, the high GLS concentration caused consumer rejection. A major GLS in Brussels sprouts is sinigrin, which has a taste threshold of 106 mg/L [35], and in different cultivars of Brussels sprouts, the content of this GLS was higher [33]. To date, there has been no published record focused on the taste thresholds of other GLS than progoitrin and sinigrin [31; 36], which is probably caused by the extremely high prices of their standards, which are dictated by their problematic extraction and purification. Therefore, the majority of research has rather focused on the correlation studies integrating data from sensory studies with the results of quantification of GLS and other phytochemicals, such as sugars, phenolic compounds, etc. [33; 34]. Even though knowledge of the role of GLS in flavour creation is still under investigation and more experiments are necessary to fully explore this subject, their fate in the processing of the Brassica plant cannot be overlooked. GLS are not contributors to flavour by themselves, but they are also important flavour precursors. Products of GLS degradation are the main odorants in rapeseed oil, accounting for 80% of volatiles [10]. The enzymatic pathway leading to GLS hydrolysis involves an enzyme called myrosinase (EC 3.2.1.147). The enzyme usually associated with Brassica plants might also occur in certain insects, bacteria, or fungi [37; 38]. Myrosinase and GLS are stored in different compartments in the plant cell [39] and the reactions between them occurs when the cell structure is broken. Possible products of GLS degradation are isothi cyanates, thiocyanates, nitriles, or epithionitriles [14] as presented in Figure 1. The biological role of the GLS-myrosinase system is to defend plants against being eaten by insects or herbivores, since products of GLS hydrolysis are toxic to predators [40]. In general, the direction of the GLS hydrolysis and the formed products depend on the structure of native GLS, the conditions of the reaction, as well as the species of the plant. All these possible products of GLS hydrolysis differ significantly in terms of flavour. Usually, isothi cyanates are the main products that influence the final aroma, due to relatively low odour thresholds, compared with nitriles or thiocyanates [41].