Loading...
Cryogenic pre-treatment during winemaking practices: effect on white wine sensory and chemical profiles
Author(s)
Van Breda, Valmary Michelle
Date Issued
2026
Type
doctoral thesis
Publisher
Cape Peninsula University of Technology
Abstract
Sauvignon blanc (Vitis vinifera), one of the most extensively cultivated white wine grape varieties globally, is renowned for producing wines with characteristic “grassy” and “tropical” varietal aromas. These aromas result from aroma compounds, such as methoxypyrazines and varietal thiols, present in the grape skin and pulp, the latter being released by yeast during the alcoholic fermentation process. Similarly, Chenin blanc, another popular white wine cultivar that shares a genetic origin with Sauvignon blanc (both originating from Savagnin blanc or Traminer), was also shown to possess varietal thiols. Several anthropogenic factors have been investigated to increase varietal aroma compounds, specifically the varietal thiols; however, the reported results varied. Alternative technologies, such as low-temperatures and cryogenic pre-treatment, have been researched and shown promise. Additionally, the impact of harvesting technique (hand versus machine-harvested) has been researched, reporting that grape juice and wines from mechanically harvested grapes had higher levels of varietal thiol precursors and varietal thiols in the final wines. Therefore, this research aimed to investigate the effect of pre-fermentative cryogenic freezing (-20 ºC and -4 ºC) at four production stages (whole grapes [WG], macerated grapes [MG], turbid must [TM] and clear juice [CJ]), immediately (T0) and for a four-month (T4) storage period on the standard physicochemical properties, varietal aroma compounds (volatile thiols and methoxypyrazines) and the sensory profiles of two popular South African (SA) white wine varieties (Sauvignon blanc and Chenin blanc). Following complete defrosting, a standard white winemaking protocol was followed. The control wines were not subjected to any cryogenic treatments. Subsequently, all grape musts were analysed for physicochemical properties, and wines were analysed for physicochemical properties, varietal aroma compounds, and sensory properties. Physicochemical parameters in the grape must were generally unaffected by the cryogenic pre-treatments (total sugar, glucose/fructose ratio, and total soluble solids), except for the pH (higher in cryogenically pre-treated grape must) and total acidity (TA) (lower in cryogenically pre-treated grape must) when compared to the unfrozen control. Furthermore, the yeast assimilable nitrogen (YAN) was higher in the unfrozen control and cryogenically pre-treated grape must for all Producers in 2020 compared to 2021, suggesting that the differences were vintage-related. The physicochemical parameters of the final wines were generally within the legal limits for SA white wines, except for pH, which was slightly higher (> 3.4) in the wines made from the cryogenic pre-treatments. The concentrations of the varietal thiol, 3-sulfanylhexyl acetate (3-SHA), detected in the Sauvignon blanc wines exceeded the aroma perception threshold (4 ng L -1 ) and the reported range for SA Sauvignon blanc wines (23-151 ng L -1 ), whilst the concentrations of 3-sulfanylhexan-1-ol (3-SH) were generally within the aroma perception threshold (60 ng L-1 ) and reported range (178-904 ng L-1 ). Moreover, the concentrations of 4-methyl-4-sulfanylpentan-2-one (4-MSP) were found to be below the aroma perception threshold (0.8 ng L -1 ) and the typically reported range (0-21.9 ng L -1 ). For the Chenin blanc wines, the concentrations of 3-SHA were found to be higher than the typical reported range for SA Chenin blanc (5-253 ng L-1 ) and above the aroma perception threshold (4 ng L -1 ), whilst the concentration of 3-SH were generally below its reported range (99-1124 ng L -1 ) and aroma perception threshold (60 ng L -1 ). Furthermore, the concentrations of 4-MSP were detected above the reported range, not detected (n.d.), but below the aroma perception threshold (0.8 ng L -1 ). Methoxypyrazines were detected at concentrations above the reported aroma perception threshold (2-16 ng L-1 ) and range for 3-isobutyl-2-methoxypyrazine (ibMP) (2-30 ng L -1 ) and 2-methoxy-3-sec-butylpyrazine (sbMP) (< 10 g L -1 ) in Sauvignon blanc wines, for most cryogenic treatments compared to the control wines. Differences were also observed between producers from different regions and between vintages (2020 and 2021), with overall methoxypyrazine levels higher in 2021. Moreover, from a sensory perspective, the wines made from WG and MG subjected to cryogenic pre-treatment technologies yielded wines with higher tropical, thiol-type, pineapple and banana aromas as well as higher body, general quality and overall intensity when compared to most wines made from the control grapes as well as the remaining cryogenic treatments. This study highlighted that the production stage at which the cryogenic treatment was applied had the most prominent effect, whilst the effects of the cryogenic temperatures and storage times were negligible. Furthermore, vintage and regional differences also influenced the final wine sensory profiles. Differences in varietal thiol concentrations for Sauvignon blanc wines resulted mainly from wine region and vintage for the T4 wines made from cryogenically pre-treated WG and MG (-4 ºC). Although no definite trends were observed in terms of which cryogenic temperature, stage of production, or storage time yielded the most favourable levels in final wines, indications are that the region from which the grapes originated, the harvesting method, and the vintage contributed the most. Therefore, the industry recommendation would be to apply cryogenic pre-fermentative treatments to WG or MG at -4 ºC (energy efficient) for T0 (more economical), considering the region and vintage to achieve the desired outcome.
Additional information
Thesis (Doctor: Food Science & Technology)--Cape Peninsula University of Technology, 2026
File(s)![Thumbnail Image]()
Loading...
Name
Valmary_Van Breda_198100477.pdf
Size
6.45 MB
Format
Adobe PDF
Checksum
(MD5):3892c3711eda120167d58803b15bc966