PHOTO: Hendrik Holler (Wosa Library).

Techniques can be used to increase thiols or ensure preservation.

 

Harvesting methods

New Zealand winegrowers use mechanical harvesting as a norm. This is probably out of necessity, however, as luck would have it, it is by far the harvesting method delivering the highest concentration of thiols. This is contradicting to what has been believed for years to be best practice when producing top quality wines. Research studies comparing harvesting methods, such as handpicked with whole bunch pressing, handpicked with crushing and destemming and machine harvesting showed an increase in thiol concentration as the “roughness” of the method increased.10 This observation also falls in line with the scientific explanation involving oxygen, enzymes and C6 compounds together with a sulphur donor. The actual increase of some of the C6 compounds when comparing hand harvested grapes to machine harvested grapes has also been observed. With greater maceration (skin contact) of the fruit and greater enzyme activity, the levels increase. The use of maceration enzyme additions and sufficient time given for the reaction to take place can also increase the thiols formed during the process. Wine producers can mimic the effect that mechanical harvesting has on the grape berries. Some of these tools are listed below, but the fundamental aspect would be to cause some berry damage with sufficient protection to avoid too much oxidation.

 

Presence of antioxidants

When it comes to the volatile thiols, the importance of the presence of antioxidants cannot be overstated. The -SH group of the volatile thiols makes these compounds extremely susceptible to oxidation. It is therefore important to protect the wine from oxygen exposure and ensure sufficient presence of antioxidants, such as sulphur dioxide, which will help preserve the aromatic compounds.11 The earlier the addition of the antioxidants after harvest, the better. Studies have shown that increasing the amount of antioxidants also increased the amount of volatile thiols formed and maximum thiol potential seemed to be reached so long as a moderate level of free SO2 was maintained prior to fermentation.2 This does have some limits due to fermentation difficulties in the presence of too much sulphur dioxide, as well as the inhibition of the enzyme responsible for increasing the C6 compounds (lipoxygenase) during harvesting. However, this effect has not been conclusively demonstrated. Excessive SO2 concentrations (300 mg/ℓ) also led to lower 3MHA concentration probably due to the acetylation pathway converting 3MH to 3MHA being interrupted. The timing of SO2 additions also had an influence on 3MH and 3MHA production. Delaying the SO2 additions with two hours resulted in wines containing thiol concentrations of about half compared to wines where SO2 was added as early as 10 – 15 minutes after mechanical harvesting.2 Finding practical methods for early SO2 additions can be challenging. New Zealand wine companies have used methods, such as drip-feed of a concentrated SO2 solution, however, care needs to be taken over corrosion issues with equipment. In another New Zealand study, the addition of increasing amounts of SO2 also led to increases in volatile thiol production obtaining levels of up to 12 000 ng/ℓ which would be considered to be a high thiol wine.2 Interestingly, this wine was not Sauvignon blanc, but rather Pinot gris. These results show the necessity to investigate the contribution of the volatile thiols to other varieties and again also highlights the importance of the use of antioxidants. Should the winemaker want to minimise the concentration of volatile thiols in any wine, the limited use of SO2 could be employed.

Other antioxidants that have been under investigation are ascorbic acid and glutathione. Ascorbic acid can be added to juice and wine as a supplement to SO2 in order to increase antioxidant capacity. Glutathione, on the other hand, is not (yet) registered as a permitted additive, however, glutathione levels can possibly be increased by the addition of some commercially available inactive dry yeast products. In a study where 30 mg/ℓ SO2 was added to grape must together with either 100 mg/ℓ ascorbic acid or glutathione, the levels of all three important volatile thiols increased with 4MMP increasing significantly with the addition of glutathione.12 The addition of 30 mg/ℓ SO2 together with 100 mg/ℓ glutathione showed 3MH and 3MHA concentrations of at least double the value compared to treatments where glutathione was added alone. It would thus seem that the combined protective effect could be more effective than the individual antioxidants.

After fermentation the presence of sulphur dioxide is of utmost importance as the instability of the volatile thiols leads to drastic decreases if not protected.13 The thiol most susceptible to degradation and oxidation is 3MHA. During oxidation, very reactive quinones are formed which will readily bind to wine constituents including the volatile thiols rendering them odourless. Sulphur dioxide binds these quinones rapidly and thus prevents further oxidation reactions from taking place.

The methods used for the addition of antioxidants also need to be controlled for efficiency. Concentrated SO2 is expected to mix quickly with the must in the bins as grapes are transferred into trucks/winery hopper. The efficiency of powder should still be investigated and it is important to check the uniformity and distribution of the antioxidant by taking samples from the juice arriving at the winery. Pockets of juice with low free SO2 are likely to have lower antioxidant potential.

 

Cold soaking/freezing of grapes

In a study where Sauvignon blanc grapes were frozen (to -20°C) using dry ice and then thawed over 24 hours, the results showed an increase in 3MH and 3MHA content.14 Wines made from hand harvested grapes that underwent this cryogenic maceration contained about three times more 3MH and 3MHA compared to wines that did not undergo cryogenic maceration. The levels of thiols obtained were comparable with the same grapes that were harvested using the mechanical harvester. The explanation for this could be the increased leaching of precursors and enzymes into the grape must due to berry damage from ice crystal formation. The ice formation thus not only increases contact between reactants, but it also concentrates the reactants in the available liquid thereby facilitating the reaction. The expense involved in using this freeze/thaw cycle may not be economical when considering larger volumes of must, however, this technique could be applied in smaller batches to obtain larger diversity in wine styles.

 

Pressing and oxidation of juice

Studies have shown that wines made from juice obtained from the press (1 bar) contained either the same or a lesser amount of thiols when compared to wines made from the free run juice.15,16 This could be due to the higher potential for oxidation due to larger phenolic extraction as the pressing pressure increases. It is thus advised to ensure that sufficient SO2 is present prior to pressing and to obtain a reductive atmosphere (using carbon dioxide or nitrogen gas) during the pressing process. Higher pressing fractions might contain higher concentrations of thiol precursors (including C6 compounds), however, you run the risk of increasing the potential for oxidation (due to higher phenolic content). This could be risky seeing that an increase in the conjugated precursors would not necessarily lead to an increase in free thiols in the corresponding wine.

Juice oxidation (measured by absorbance at 420 nm) can also influence the volatile thiol concentration of the corresponding wines. Higher concentrations of 3MH were obtained from juices with lower 420 nm measurements,17 however, a low 420 nm measurement did not guarantee high thiol concentrations as other important factors could have a greater effect on the formation of precursors and volatile thiols. The oxidation of juice is an important factor to consider, however, the addition of sufficient SO2 at this stage has shown to minimise negative effects occurring due to oxygen addition delivering volatile thiol levels equivalent of juices that were not exposed to oxygen.18

It is advisable to keep juice fractions that might be in advanced stages of oxidation, separate due to lower potential for volatile thiol formation. The wine can then be bottled or blended if proven to have sufficient volatile thiols present after fermentation. Another option would be to eliminate phenolic compounds through the use of a specialised fining agent on the juice. This prevents formation of quinones at a later stage and preserves thiol containing compounds.19

 

Fermentation conditions

The yeast strain is extremely important as that would determine the amount of precursors converted during fermentation. However, the yeast strain will only have a limited effect. The juice composition needs to be of a certain standard and composition for the yeast to be able to convert and form the volatile thiols. Thus juice composition is still the main determinant when it comes to volatile thiol production in wine,20 with yeast strain selection having a secondary effect.

The strains can, however, have an important effect and multiple fold increases have been seen when comparing yeast strain volatile thiol production.

3MHA is not formed directly from a precursor, but rather due to an esterification reaction that takes place during the fermentation process. Not all strains of S. cerevisiae have the same capacity to express these compounds. Some strains are good thiol producers in that it releases 3MH and 4MMP from corresponding precursors or it can create the volatile thiols from other compounds. Other yeast strains can also be good converters in that it can efficiently convert 3MH to 3MHA. A mixture of these yeasts can be inoculated to maximise both production and conversion.

Of course S. cerevisiae is not the only strain capable of releasing/producing the volatile thiols from the precursors and other yeast strains, such as Pichia kluyveri, have also been proven to be effective in increasing volatile thiol production.21

Higher fermentation temperatures (irrespective of yeast strain used) also resulted in increased volatile thiol concentration when compared to lower temperatures, however, in some cases the extended higher temperature during fermentation led to a decrease towards the end.22 It is therefore advised to commence fermentation at 17 or 18°C for about 30 g of sugar fermented and then depending on the yeast used, whether it is cold tolerant or not, gradually lower the fermentation temperature to about 15 to 16°C in order to preserve the released volatiles.

 

Storage temperature

It is absolutely vital to keep the wine at a low temperature.23 Higher temperatures will not only accelerate the oxidation reaction, but it will also encourage hydrolyses of 3MHA to form 3MH (in some cases even leading to an increase of 3MH concentration). This way you will lose some of the aroma potency due to the higher perception threshold reported for 3MH, as well as a change in aroma quality. 3MHA is by far the volatile thiol most affected during storage of wine, while much smaller losses were seen for 3MH. Some studies have shown the effect of temperature to be even more important than oxygen exposure during storage of a wine. It would thus be advised to keep wine at a temperature as low as possible, not only until bottling, but also up to the point of consumption. Differences in storage temperature of only 3°C (15°C vs 18°C) could already have a massive impact on thiol preservation. Of course the cost of refrigeration should also be considered and a workable compromise of 10 – 12°C has been identified.2 Temperature logging during exports should also be considered to ensure preservation of all aroma compounds and especially the thiols. It might be worth the cost and effort to ensure the transport temperature and condition is of certain standard to guarantee better preservation. Unfortunately, these parameters are more often than not controlled by the shipping company.

 

References

2. Kilmartin, P., 2016. Thiols found in Sauvignon blanc and their significance. In: New Zealand Society for Viticulture and Oenology Sauvignon blanc Workshop, pp. 24 – 80.

10. Jouanneau, S., 2011. Survey of aroma compounds in Marlborough Sauvignon blanc wines. Regionality and small scale winemaking, University of Auckland.

11. Coetzee, C. & Du Toit, W.J., 2015. Sauvignon blanc wine: Contribution of aging and oxygen on aromatic and non-aromatic compounds and sensory composition – a review. South African Journal of Enology and Viticulture.

12. Rosales del Prado, D., 2015. Effect of glutathione and inactive yeast additions to Sauvignon blanc at harvest on wine aroma, University of Auckland.

13. Herbst-Johnstone, M., Nicolau, L. & Kilmartin, P.A., 2011. Stability of varietal thiols in commercial Sauvignon blanc wines. American Journal of Enology and Viticulture 62(4), 495 – 502.

14. Olejar, K.J., Fedrizzi, B. & Kilmartin, P.A., 2015. Influence of harvesting technique and maceration process on aroma and phenolic attributes of Sauvignon blanc wine. Food Chemistry 183, 181 – 189.

15. Maggu, M., Winz, R., Kilmartin, P.A., Trought, M.C.T. & Nicolau, L., 2007. Effect of skin contact and pressure on the composition of Sauvignon blanc must. Journal of Agricultural Food Chemistry 55(25), 10281 – 10288.

16. Parish, K.J., Herbst-Johnstone, M., Bouda, F., Klaere, S. & Fedrizzi, B., 2016. Pre-fermentation fining effects on the aroma chemistry of Marlborough Sauvignon blanc press fractions. Food Chemistry 208, 326 – 335.

17. Allen, T., Herbst-Johnstone, M., Girault, M., Butler, P., Logan, G., Jouanneau, S., Nicolau, L. & Kilmartin, P.A., 2011. Influence of grape-harvesting steps on varietal thiol aromas in Sauvignon blanc wines. Journal of Agricultural Food Chemistry 59(19), 10641 – 10650.

18. Coetzee, C., Lisjak, K., Nicolau, L., Kilmartin, P. & Du Toit, W.J., 2013. Oxygen and sulfur dioxide additions to Sauvignon blanc must: Effect on must and wine composition. Flavour Fragrance Journal 28(3), 155 – 167.

19. Moine, V., Murat, M.-L., Arfeuillère, C. & Thibon, C., 2011. Collage des jus de presse blanc. Influence sur leurs teneurs en composes phénoliques, en glutathion et précurseur d’arômes. Revue des œnologues 138, 45 – 47.

20. Lee, S.A., Rick, F.E., Dobson, J., Reeves, M., Clark, H., Thomson, M. & Gardner, R.C., 2008. Grape juice is the major influence on volatile thiol aromas in Sauvignon blanc. Australian and New Zealand Grapegrower and Winemaker 533(6), 78 – 86.

21. Anfang, N., Brajkovich, M. & Goddard, M.R., 2009. Co-fermentation with Pichia kluyveri increases varietal thiol concentrations in Sauvignon blanc. Australian Journal of Grape Wine Research 15(1), 1 – 8.

22. Masneuf-Pomarède, I., Mansour, C., Murat, M., Tominaga, T. & Dubourdieu, D., 2006. Influence of fermentation temperature on volatile thiols concentrations in Sauvignon blanc wines. International Journal of Food Microbiology 108(3), 385 – 390.

23. 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 Wine Research 18(1), 91 – 99.

 

– Originally written for the IGWS. For more information, contact Carien Coetzee at carien@vinlab.com.

 

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