## 3 Effect of Cryogenic Technologies on Physicochemical and Aroma Compounds of Grape Must and Wine
### Effect of Cryogenic Technologies on Grape Aroma Compounds  
Aroma compounds or odourants can be classified according to the different stages of wine grape and/or juice processing that they originated from (Table 2), namely varietal (cultivar), pre-fermentative (processing), fermentation (yeast and bacteria during alcoholic and malo-lactic fermentation), and post-fermentation (aging and maturation in wood, wine bottle storage, and preservation) aromas [6; 7; 57]. These compounds are often present in the grape as odourless or non-volatile precursors and are released during winemaking, specifically during the alcoholic fermentation process [57; 58].  
The three major classes of odourants that contribute significantly to varietal characteristics in wines are monoterpenes, methoxypyrazines, and volatile thiols [28; 59]. Volatile thiol compounds, also considered impact odourants, can either have a positive (tropical-, passionfruit-, guava-like) or negative influence (rotten egg, cooked vegetables, onion, cabbage) depending on their concentration in wines [60]. Other compounds also present in wine and found to play a significant role in its aroma are esters, fatty acids, higher alcohols, and aldehydes [28; 57; 61].  
Over the past three decades, there has been considerable interest and research into the volatile thiol aroma compounds and their precursors [57; 60; 62]. Grape aroma compounds are predominantly located in the grape skin and require an extraction process to be released. The extraction of a compound is dependent on the nature of the compound, the concentration in the berry, the location within the berry, and the method used during processing [63; 64; 65]. Winemakers usually achieve this by using a maceration step whereby the compounds are transferred from the solid components to the juice. Pre-fermentative cold maceration is another technique that has gained popularity during the white wine production process and was shown to enhance the varietal character of the wines produced [29; 31; 34].  
Research conducted by [PERSON] et al. [36] showed significant differences in phenolic acid (gallic, cafaric, coutaric, caffeic, and syringic acid) and polyphenol concentrations between wines produced from frozen and fresh grapes. [PERSON] et al. [25], [PERSON] et al. [51], and [PERSON] et al. [31] had similar observations regarding polyphenol concentrations, which confirms the earlier findings. [PERSON] et al. [31] found significant differences in acetaldehyde concentrations between their wines, which supported earlier research. [PERSON] and [PERSON] [63] investigated the impact of frozen storage on 22 free volatile compounds of two table grape varieties, i.e., Thompson Seedless and Flame Seedless. The free volatile compound profile of the frozen grapes and juice was significantly different from the fresh juice [63]. It was further shown that the different freezing treatments affected the volatile profiles of juice differently and that the grape variety played a role. When analysing the volatile profiles of grape juice, fresh juice is preferable to frozen juice [63]. However, should long-term storage be required, storage under liquid N\({}_{2}\) or at a temperature of \(-80\)\({}^{\circ}\)C is advised to reduce biochemical reactions from altering the free volatile compounds [63]. [PERSON] et al. [29] and [PERSON] et al. [34] found that extractable anthocyanins, total phenolics, and intergenic alcohols were among the compounds most affected by the freezing process. [PERSON] et al. [29] further found that the differences observed in the concentration of aroma compounds in their research between vintages could be more related to climatic and geographical factors.