# Flavour Generation during Lactic Acid Fermentation of Brassica Vegetables--Literature Review
## 5 Flavour of Brassica Vegetables
New topics such as thermal degradation of GLS in the model system are under investigation. [PERSON] and [PERSON] [42] performed an experiment aimed at observing the odorants resulting from the thermal treatment of sinigrin itself without any potential reaction partners which might be present in the food matrix. GC-O was used to determine the aroma-active substances occurring after thermal treatment. A total of 29 aroma active spots were detected after using GC-O. The examples of odours detected on GC-O were: garlic-like (allyl methyl sulfide), onion-like (thiophene), sweaty-like (2/3-methylbutanoic acid), coffee-like (2-furan-methanenthiol), caramel-like (4-hydroxy-2,5-dimethylfuran-3(2\(H\))-one, butter-like (2,3-butanedione), popcorn-like (2-acetyl-1-pyrroline), etc. As seen, many pleasant aroma impressions were perceived, which suggests that thermal, non-enzymatic degradation of sinigrin might lead to the occurrence of different class components other than isothiyanates, nitriles, or sulfides. Similarly, [PERSON] et al. [43] investigated the thermal degradation behaviour and volatile products of progoitrin. The authors observed 33 different compounds originating from the thermal degradation of progoitrin. Among others, there were, for example, 2,3-butanedione, dimethyl disulfide and 1,5-hexanediol, and the majority of identified components were aroma active. Opposite to GLS, isothiyanates are volatile substances, and therefore their role in flavour creation was mainly associated with the aroma. However, it was proven that due to the presence of thiocyanate moiety, this group has a predisposition to affect taste receptors as well [44]. So far, the majority of research has considered the impact of isothiyanates on the aroma rather than on the taste of vegetables. [PERSON] and [PERSON] [41] determined odour thresholds, as well as odour quality descriptors, for 19 isothiyanates that are frequently found in Brassica plants. They found that the odour thresholds of investigated isothiyanates are within the range of 0.005-0.2 mg/L. The main descriptors for standard substances were: garlic, sulfur, and cabbage. Not many papers involving a sensoric approach in the analysis of key odorants in Brassica plants were published to date. [PERSON] et al. [45] analyzed key odorants in raw and cooked kohlrabi. The authors identified 55 odour-active compounds in raw and cooked kohlrabi by using GC-O. Twenty eight of these odorants were characterized with high FD values > 64; therefore, these compounds were quantified and their OAVs were determined. Based on the obtained results, it was concluded that the backbone of kohlrabi aroma constitutes: dimethyl trisulfide, 2-isopropyl-3-methoxypyrazine, hexanoic acid; 1-isothiocyanato-3-(methylsulfonyl) propane; and 1-isothiocyanato-4-(methylsulfonyl)butane. In another study, which aimed at the detection of odorants in raw and cooked broccoli, 33 aroma-active compounds were detected. The majority of those were sulfur components, and the highest FD (1024) values were related to methanethiol and 1-penthanethiol,  
Figure 1: Degradation of glucosinolates in plant cells. followed by methyl trisulfide, dimethyl tetrasulfide, 2-methyl methanethiosulphonate, 4-methylpentyl isothiamate and hexyl isothiamate (all with an FD of 256) [46]. Although GLS and their degradation products are important for the flavour of Brassica vegetables, other groups of non-volatile compounds are also necessary to consider, such as sugars, phenolic compounds, amino acids, or fatty acids. These substances were recognized widely as contributors to the flavour of many food products, as they both contain the taste and are also the precursors of multiple odorants. Sugars, which are components of the majority of food products, have a dual role in flavour creation. On the one hand, they introduce the sweet taste to the product, while at the same time they might be a masking agent for bitter molecules.