An industry-focused study conducted by New Zealand researchers analysed the holding temperature of Sauvignon blanc machine-harvested grapes and the effect on the concentration of grape thiol precursors and that of free thiols in the resultant wines.1
Introduction
It is widely accepted that harvesting early in the morning (lower ambient temperatures) and cooling grapes before processing will better preserve the delicate flavours that characterise Sauvignon blanc wines. While there are several advantages to night-harvesting (such as better conditions for the harvesters, increasing efficiency and turnaround in the cellar and energy and cost savings), the exact effect of lower grape temperatures at the start of processing on the fruity volatile thiols have not been scientifically established.
Materials and methods
Machine-harvested Sauvignon blanc grapes were collected from a vineyard in the Marlborough region in New Zealand. The grapes were divided into three lots (4 L containers) and stored for two hours in three different temperature-controlled rooms operating at 6, 15 and 24°C, respectively. After one hour in the temperature-controlled room, the lots were mixed to ensure homogenous cooling/heating. The grapes were processed and the juice cold settled at 6°C for 16 hours. After racking, the clear juice was moved to a 15°C room to prepare for fermentation. Juices were inoculated with Lalvin yeast EC1118 and fermented to dryness (<3.8 g/L).The study was done in triplicate.
Results
The temperature of the grapes, before placing the grapes in the respective temperature-controlled rooms, was at an average of 18.9°C. After the two hours holding, the temperature of the grapes were 16.2°C (6°C room), 18.1°C (15°C room) and 21.3°C (24°C room).
The must composition of the samples taken from the grapes stored at varying temperatures showed only slight and mostly insignificant differences.
- The titratable acidity of the juice stored at 15°C was slightly lower compared to the other two treatments.
- The free and total sulphur dioxide of the must stored at the warmer room (24°C) was slightly lower compared to the musts stored at lower temperatures.
The composition of the resulting wines also did not differ significantly. The only routine parameter which showed statistically significant differences between the treatments was alcohol. Even though these differences were statistically significant, practically, the differences were still small with the highest concentration (13.75% v/v) reported for the wine made from must that was stored at 15°C as opposed to the lowest alcohol content (13.57% v/v) reported in the wine made from must stored at 6°C.
Volatile thiol precursors in the must
The concentration of two volatile thiol precursors, Cys-3MH and Glut-3MH, were measured before and after the holding period. The must stored at 15°C showed significant increases for both precursors and was the treatment delivering the highest precursor concentration. The musts stored at 6°C and 24°C only showed significant increases in Glut-3MH. The lowest precursor concentration was reported for the 6°C treatment.
It is important to note that previous studies have reported low conversion rates from these precursors to the respective aromatic volatile thiols.2,3 Other precursors and mechanisms can also contribute significantly to the volatile thiols in the resulting wines. It is thus advised to use the free thiol concentration as an indication of treatment effects as opposed to select precursors.
Volatile thiols in the wine
The average volatile thiol concentration of the three treatments ranged between 403 and 555 ng/L for 3-mercaptohexan-1-ol (3MH) and between 123 and 147 ng/L for 3-mercaptohexyl acetate (3MHA). The differences in concentration between the treatments were statistically insignificant and relatively small. Previous studies would suggest that it would not have a sensorial impact.4
Conclusion
The results from this study would suggest that the cooling of grape must before processing does not guarantee improved production and/or preservation of the volatile thiols in the resulting wines. The time, effort and costs associated with cooling technology might therefore not be worth the potential benefits associated with volatile thiols (which is proven in this study to be marginal if at all). Similarly, the producer can rest assured that mild temperatures during transportation will not alter the volatile thiol potential of the finished wines.
It is also evident that the amount of time needed to lower grape temperature before processing is underestimated. Two hours were not a sufficient amount of time for the grapes to reach the ambient temperature of the temperature-controlled rooms and consequently, the temperatures of the three treatments were more similar than what was originally planned. Considering that the study was conducted in small scale (4 L containers), it is likely that larger volumes would need even more time to reach the ambient temperatures. Placing the grapes or must in temperature-controlled rooms before processing would therefore not necessarily lower the temperature of the fruit, but it would certainly prevent further temperatures increases during hot days.
The benefits associated with lower temperatures should also be weighed against the impact of a longer skin contact time. The amount of time needed for the grapes to reach the holding temperature might negate the potential advantages of lower temperatures due to the extraction of unwanted compounds from the grape solids, especially with machine-harvested grapes.
Even though lowering the temperature of the must did not result in any significant benefits associated with volatile thiols, the impact of holding temperatures on other aroma compounds and the sensory perception of the resulting wine needs further investigation.
Abstract
A recent industry-focused study conducted by New Zealand researchers analysed the effect of varying holding temperatures on Sauvignon blanc must and wine composition. The study specifically focussed on volatile thiol precursors in the must and the free volatile thiols in the finished wines. Results would suggest that the time, effort and costs associated with cooling technology might not be worth the potential benefits associated with volatile thiols. Similarly, the producer can rest assured that mild temperatures during transportation will not alter the volatile thiol potential of the wine.
References
- Parish-Virtue, K., Herbst-Johnstone, M., Bouda, F., Deed, R.C., Grose, C., Martin, D. & Fedrizzi, B., 2021. Effect of holding temperature on the thiol potential of machine-harvested Sauvignon blanc grapes. Australian Journal of Grape and Wine Research. 12498. https://doi.org/10.1111/ajgw.12498.
- Subileau, M., Schneider, R., Salmon, J.M. & Degryse, E., 2008. New insights on 3-mercaptohexanol (3MH) biogenesis in Sauvignon blanc wines: Cys-3MH and (E)-hexen-2-al are not the major precursors. Journal of Agricultural and Food Chemistry 56(19): 9230 – 9235. https://doi.org/10.1021/jf801626f.
- Pinu, F.R., Jouanneau, S., Nicolau, L., Gardner, R.C. & Villas-Boas, S.G., 2012. Concentrations of the volatile thiol 3-mercaptohexanol in Sauvignon blanc wines: No correlation with juice precursors. American Journal of Enology and Viticulture 63(3): 407 – 412. https://doi.org/10.5344/ajev.2012.11126.
- Coetzee, C., 2021. Do smaller thiol increases have a significant sensorial effect? Winetech Technical, January.
– For more information, contact Carien Coetzee at carien@basicwine.co.za.
