The aim of this study was to evaluate the stability of commercial white wines aroma using both analytical chemistry and sensory evaluation approaches, with a focus on the stability of thiols. The results of the study will be presented in a two-part article, each covering one aspect of the work. Part 1 is focused on the evolution of some relevant classes of aroma compounds, namely thiols and major volatiles (esters, organic acids and higher alcohols) and of antioxidant-related parameters (glutathione, phenolics and colour).
Wine aroma stability
The availability of information on the stability of wine aroma and aroma compounds is dependent on the cultivar and compounds of interest. Work done on New Zealand Sauvignon blanc can offer a basis for what to expect during wine storage in different conditions. For Chenin blanc, some hypotheses can be proposed based on the research done on Sauvignon blanc. While there is other work published internationally that is of general relevance, the research usually deals with the overall aroma stability, especially as affected by oxygen exposure during storage. There is still a need to elucidate the fate of thiols in Chenin blanc and the effect storage conditions have on the wines both from a chemical and sensory point of view.
There is always an interest in the elucidation of various aspects linked to wine stability using both sensory and chemical approaches. The results of studies focused on this type of evaluation can help the industry by looking at the positive and negative aspects resulting from storage, thus indicating what to be avoided and what to be optimised. On the academic side, efforts are aimed at better analysis methods, more efficient and robust sensory techniques, and generally more knowledge related to wine and the transformations it goes through as time passes.
One of the aspects relevant to wine aroma is its stability during storage. Even though some extrapolations can possibly be made from other white cultivars, there is no data at the moment that elucidates the fate of various compounds of interest present in Chenin blanc wines for example. Thiols in particular are sensitive to oxidation and it can be hypothesised that the matrix effects will play a role in the degradation (or stability) of these compounds. The phenolic composition of a wine can play a role in the protection against oxidation, to give only one example.
The evaluation was done in parallel for Chenin blanc and Sauvignon blanc, since there is some chemical data available for the stability of thiols in Sauvignon blanc (from New Zealand), but the findings were not correlated with sensory assessment.
We have selected Chenin blanc and Sauvignon blanc wines from six wineries. The wines were stored for three and nine months in various conditions – 15°C and 25°C (temperature controlled environment), and room conditions (temperature not controlled). A control set was also included (time 0/T0 and no storage). Once the wines reached the designated storage time, they were moved to -4°C until further evaluation. The chemical analyses were focused on thiols (3MH, 3MHA and 4MMP),1 major volatiles (esters, higher alcohols and acids), and general antioxidants (glutathione and phenolic compounds). UV-Vis was used for the evaluation of phenolics (total phenolics at 280 nm, phenolic acids at 320 nm and browning at 420 nm). Additionally, the colour of the wines and the changes in colour were evaluated using CIE Lab, a system that decomposes the UV-Vis measurements into colour-related characteristics similar to the perception of the human eye. CIE Lab Colour Space system expresses colour as three numerical values, L* for the lightness, 0 = black, 100 = diffuse white, a* and b* for the green-red and blue-yellow colour components.
Thiols and major volatiles
Even though generally 3MH and 3MHA levels were comparable between cultivars, Sauvignon blanc (SB) wines had more 4MMP than Chenin blanc (CB), a result we have noticed before (Figure 2 – 4).2 SB samples stored at higher temperature for longer had more 3MH, whilst those at lower temperature for shorter had more 3MHA. This corresponds to the findings from New Zealand and to the hypothesis that 3MHA hydrolyses and produces 3MH. This did not, however, correspond to an increase in the 3MH-associated attributes in the sensory results, most probably because of other oxidation reactions that masked the increase in 3MH.
T0 samples were consistently different from the other treatments and a gradual trend was observed from T0 to T3 and T9 samples. Among the volatile compounds measured, the varietal thiols concentrations were associated with storage time/temperature combination for both SB and CB. The controls had higher 3MHA concentrations compared to the extreme treatment (T9/25). Wines stored at high temperatures for longer (T9/25 as the extreme) had higher 3MH and 4MMP, which in previous studies on New Zealand Sauvignon blanc was found not to have changed throughout storage.3
Major volatile composition was different between wineries and cultivars, in both levels and profile. The changes in the major volatile composition were not as marked as for thiols. This type of results is similar to findings on Spanish white wines,4 where the researchers found differences only after more than one year of storage; this may explain the results found in the current study, since the storage was only over nine months.
Taking into account all the aroma compounds analysed, the pattern of evolution was different between the wines tested. This can be due simply to the limited number of compounds measured (34 aroma compounds) or to the differences in the T0 wine profiles. In other words, the storage conditions (time and temperature) did not affect the volatile composition of all the wines similarly, possibly due to the different initial compositions.
FIGURE 1. Concentrations of 3MH (ng/L) for SB (orange) and CB (green) for one of the wineries participating in the study. Storage time is colour-coded on the outline (green – control, blue – T3 and red – T9).
FIGURE 2. Concentrations of 3MHA (ng/L) for SB (orange) and CB (green) for one of the wineries participating in the study. Storage time is colour-coded on the outline (green – control, blue – T3 and red – T9).
FIGURE 3. Concentrations of 4MMP (ng/L) for SB (orange) and CB (green) for one of the wineries participating in the study. Storage time is colour-coded on the outline (green – control, blue – T3 and red – T9).
Antioxidants and related measurements
The wines were bottled between a month and a few days before the start of the experiment. Therefore, at the time of initial storage, the wines had very different levels of dissolved O2, ranging from 0.04 to 1.23 mg/L.
As expected, glutathione levels decreased with time and temperature. After nine months, the wines stored at 15°C retained the highest level of GSH (Figure 4 and 5).
FIGURE 4. CB GSH concentration (mg/L) during storage for one of the wineries participating in the study. Samples are colour-coded according to storage time.
FIGURE 5. SB GSH concentration (mg/L) during storage for one of the wineries participating in the study. Samples are colour-coded according to storage time.
There was an increase in absorbance at 420 nm with temperature and time, indicating browning of the wines with storage temperature and duration. Also considering the transformation of the UV-Vis measurements into CIE Lab parameters, wines stored at higher temperature for longer corresponded to higher b* (increase in yellow colour), while wines stored at lower temperature and with shorter storage time corresponded to higher a* and L* (higher green and clarity). In most cases, the differences in colour components given by the CIELab parameters were below perception threshold by the human eye. In other words, the changes were too subtle to be perceived using sensory evaluation of the colour. There were some exceptions, all of them for SB wines, which saw a substantial increase in the green colour (a*) with temperature vs time.
Take home message
For the first time, a parallel study was done for Sauvignon blanc and Chenin blanc, with a focus on whether these two cultivars go through similar changes during storage. The changes reported in this study were over a nine month period of storage in conditions that varied from controlled to normal room temperature fluctuations that can be expected when a wine is on a shelf or at home. The results of the chemical evaluation of aroma and antioxidant-related parameters showed that the duration of storage has a great influence on a wine’s profile. The storage at higher temperatures creates less change in the wine when stored for shorter periods. The evolution of the chemical composition of the wines was clear for most of the parameters considered (thiols and antioxidants), with time being the most relevant factor. Taking into account that six different Sauvignon blanc and Chenin blanc wines were used for the testing, we demonstrated that the evolution of the wines during storage was generally not dependent on the characteristics of the initial wine. In other words, regardless of the composition of the wine at time 0, the chemical changes were similar for most wines.
Part 2 of the article will focus on the aroma changes that occurred during the testing period.
Wine composition is susceptible to change due to oxidation that may occur during storage and transportation, especially at high temperatures. In a recent study at the Department of Viticulture and Oenology of Stellenbosch University, we investigated changes in South African Sauvignon blanc and Chenin blanc wine sensory profile, volatile composition and antioxidant-related parameters, resulting from storage of unwooded wines under different temperatures (room temperature, 15°C and 25°C) and durations (0, 3 and 9 months). Statistical data analysis was used to assess the correlations in the sensory and chemical evolution of the wines relative to the control (time 0).
- Mafata, M., Stander, M., Thomachot, B. & Buica, A., 2018. Measuring thiols in single cultivar South African red wines using 4,4-dithiodipyridine (DTDP) derivatization and ultraperformance convergence chromatography-tandem mass spectrometry. Foods 7(9): 138. DOI: 10.3390/foods7090138.
- Coetzee, C., Schulze, A., Mokwena, L., Du Toit, W.J. & Buica, A., 2018. Investigation of thiol levels in young commercial South African Sauvignon blanc and Chenin blanc wines using propiolate derivatization and GC-MS/MS. South African Journal of Enology and Viticulture 39(2): 180 – 184.
- Makhotkina, O., Pineau, B. & Kilmartin, P.A., 2012. Effect of storage temperature on the chemical composition and sensory profile of Sauvignon blanc wines. Australian Journal of Grape and Wine Research 18(1): 91 – 99. DOI: 10.1111/j.1755-0238.2011.00175.x.
- Pérez-Coello, M.S., González-Viñas, M.A., Garcia-Romero, E., Diaz-Maroto, M.C. & Cabezudo, M.D., 2003. Influence of storage temperature on the volatile compounds of young white wines. Food Control 14(5): 301 – 306. DOI: 10.1016/S0956-7135(02)00094-4.
– For more information, contact Astrid Buica at email@example.com.