## 3 Effect of Cryogenic Technologies on Physicochemical and Aroma Compounds of Grape Must and Wine
### Effect of Cryogenic Technologies on Varietal Thiols  
Varietal thiols are sulphur compounds found in grapes in a bound form that originate from fatty acids [6; 19]. These sulphur-containing compounds are originally associated with off-odours resulting from hydrogen sulphide (H\({}_{2}\)S), methylmercapan (methanethiol), ethanethiol, and methionol [58]. They are considered the main compounds involved in the aroma of wine and responsible for its archetypal flavour [30]. These volatile sulphur compounds are typically divided into two categories, i.e., highly volatile compounds, most of which are associated with aroma defects (carbon sulphide, ethanethiol, methanethiol, hydrogen sulphide), and low-volatile compounds, including the main desirable sulphur \begin{table}
\begin{tabular}{c c c} \hline \hline
**Aroma Development Stage** & **Compound** & **Origin** \\ \hline Varietal & Precursors (free or bound) & Grape berry (skin and pulp) \\ \hline Pre-fermentative & C\({}_{6}\) compounds & Enzymatic/catalytic reactions due to processing (crushing of berries) \\ \hline Fermentation & Ethyl esters, fusel alcohols, fatty acids, thiols & Microorganism metabolism (yeast and bacterial) \\ \hline Post-fermentation & Oxidation of volatile aroma compounds; increase in fatty acids, esters, aldehydes, ketones, and polyphenols & Wine aging (bottle, barrel, storage, aging on lees) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Aroma development stage, compounds, and their origin. compounds that contribute to the enhancement of the sensorial quality of wines. Thiols are \"bound\" with glutathione or cysteine and released by the yeast during the fermentation process via the carbon-sulphur lyase (C-S) enzyme. Therefore, the quantification of their natural precursors in the must is important and can help the wine producer determine the aromatic potential of the grapes. An accurate quantification of these natural and deuterated compounds, i.e., 4-sulfuryl-4-methylentan-2-one precursors (S-4-(4-methylpentan-2-one)-L-cysteine and S-4-(4-methylpentan-2-one)-glutathione), is achieved using SIDA (stable isotope dilution assay) that involves labelled analogues [6; 64]. Key thiols present in Sauvi-gnon blanc and responsible for its varietal aromas are 4-methyl-4-sulfurylentan-2-one (4-MSP), 3-sulfurylhexan-1-ol (3-SH), and 3-sulfurylhexyl acetate (3-SHA) with perception thresholds of 0.8 ng L\({}^{-1}\), 60 ng L\({}^{-1}\), and 4 ng L\({}^{-1}\), respectively (Table 3). They are predominantly responsible for the \"tropical\" (gooseberry, grapefruit, and passion fruit) characteristics associated with Sauvi-gnon blanc [56]. However, it is interesting to note that when present in excessive concentrations, they often impart less desirable strong, sweaty aromas resembling \"cat urine\" [54; 56]. Furthermore, research conducted on South African (SA) Chemin blanc revealed the presence of the varietal thiols 3-SH and 3-SHA in concentrations above their aroma thresholds, indicating that these two compounds also contribute significantly to the aroma of Chemin blanc wines [65; 66; 67]. Varietal thiols are present in grape juice in the form of aroma-inactive, non-volatile precursors and are released by yeast enzymes during the fermentation process [12; 38; 56; 68; 69; 70]. [PERSON] et al. [58] showed that the production of varietal thiols and other aroma compounds in Sauvi-gnon blanc wines is not necessarily only dependent on nitrogenous and sulphur compounds but is also influenced by other juice metabolites such as carboxylic and fatty acids. Their research demonstrated that concentrations of wine aroma compounds can be modified using pre-fermatative treatments to produce different wine styles from the same grape varietal based on the metabolic profile of the juice, thus altering the metabolite levels. In addition, juice modulation through new winemaking practices, i.e., metabolite supplementation or blending, could be seen as a useful tool to create new wine styles [58].