Sweet wines, or what many in America refer to as “dessert wines” even though they pair well with more than just dessert, are made in many different styles and in many corners of the world. From Canadian ice wine to French Sauternes, these wines are created by employing more complex methods prior to and sometimes during fermentation that make them stand out and differ from your typical red or white table wine. Of course, there are some less-than-quality ways of making sweet wine, which simply involve adding sugar back to the wine after

fermentation is complete, however, for the purpose of today’s post, we are only talking about high quality sweet wines, the sweetness by which is a function of the natural sugar in the grapes/wine rather than added after the fact.

For nearly all sweet wines, sugar levels inside the grape are much higher than typical red or white grapes destined for table wines, which occurs by several different mechanisms. For Sauterne wines from France and Tokay wines from Hungary, the grapes are left to be infected by the fungus Botrytis cinerea, which while in these sweet dessert wines is preferred, it is actively combated against for table wines. Other sweet wines are created by allowing grapes to remain on the vine for a lengthened period of time or by dehydrating them in the open air. Finally, ice wines (eiswein) are created by freezing the grapes, which effectively freeze the water inside the grape but not the sugar, so that the only thing that gets pressed out of the grape is juice that is highly concentrated in sugar.

When grapes undergo the dehydration process, they are often laid out onto mesh mats (or something similar) and allowed to dry for 7 to 10 days, or however long it takes to reach around or higher than 45oBrix (450g/L sugar).

This increased level of sugar in the grapes can be problematic during the fermentation process, as many yeasts are not equip to function in high concentrations of sugar, resulting in incomplete or stuck fermentations or other fermentation problems. To combat this problem, some regions (particularly the Montilla-Moriles region of Spain) practice a fortification method, by adding 8% v/v ethanol to the must in order to achieve the appropriate levels of alcohol in the wine without having to undergo fermentation.

In these situations where ethanol is added to kill the yeasts and avoid fermentation all together, the wine takes on the characteristics of the grapes themselves. However, with fermented wines, the wine takes on the characteristics not only of the grapes, but also of the interaction of the grapes and the yeasts, adding more complexity to the wine than if it remained unfermented and fortified with ethanol. Wines that are made from dried grapes that don’t undergo fermentation also sometimes have problems with appropriate acid levels in order to achieve a satisfactory acid-sugar balance in the finished wine.

In order to solve the fermentation problems that are created when using dried grapes with high levels of sugar, the study presented today aimed to examine the fermentation of the musts made from these grapes using yeasts that are capable of withstanding the high sugar levels and to determine the contribution of these yeasts to the volatile and aromatic composition of the finished wines. Today’s study also aimed to determine if using these “specialized yeasts” create higher quality wines than wines created from the traditional fortification methods.

Methods

The grapes used in this experiment were Tempranillo from the Montilla-Moriles region of Spain. Grapes were harvested around 25oBrix (250g/L) and sun dried until the grapes reached about 45oBrix (450g/L). After pressing, must was split into 8 different batches of 1.5L each for treatment.

Treatments:

The first two batches were treated with Saccharomyces cerevisiae yeast of the strain CECT 13014.

yeast of the strain CECT 13014. The second two batches were treated with S. cerevisiae yeast of the strain CECT 13015. Both of these S. cerevisiae yeast strains are known to be tolerant of high sugar levels during fermentation.

yeast of the strain CECT 13015. The next two batches were treated with native yeasts that are known to undergo spontaneous fermentations.

The last two batches were treated using the traditional approach of fortification with 12% v/v ethanol.

Partial fermentation took place in 2L flasks at 22oC until alcohol levels in each treatment was 8 % v/v. Once this alcohol level was achieved, all fermentations were stopped by adding 12% v/v of ethanol.

Volatile compounds were measured in all treatments using GC-MS. Odor activity values (OAVs) of these compounds were also calculated (a value over 1 indicates it contributes to the aroma of the wine)

Sensory analysis of each wine was performed by a panel of 15 “expert tasters”. There were no other details given in the paper in regards to how the analysis was performed, and what qualified the panelists “expert tasters”.

Results

The most influential volatile compounds in the traditional wine (i.e. wine fortified with ethanol and no fermentation) were: Acetoin, 2,3-butanedione, pantolactone, guaiacol, acetaldehyde, ethyl acetate, and 4-valerolactone.

The most influential volatile compounds in wines treated with natural yeasts were: Guaiacol, butyrolactone, and pantolactone.

The most influential volatile compounds in wines treated with S. cerevisiae strain CECT 13014 were: Acetoin, 2-phenylethanol, propanoic acid, ethyl hexanoate, isoamyl acetate, acetaldehyde, ethyl acetate, and 4-valerolactone.

strain CECT 13014 were: The most influential volatile compounds in wines treated with S. cerevisiae strain CECT 13015 were: Propanoic acid, 2-phenylethanol, ethyl hexanoate, butryolactone, and 4-valerolactone.

strain CECT 13015 were: The aromas of chemical, ripe fruit, milky, and toast were present in all wines. Those wines partially fermented with S. cerevisiae strains were higher in these aromas, in addition to green fruit aromas, than the other wines. Milky aromas were lower in traditionally made wines.

Wines made from the two strains of S. cerevisiae were very similar to one another. Wines made from native yeasts were similar to wines made from S. cerevisiae treated wines. Traditionally made wines (i.e. fortified with ethanol and no fermentation) were very different from the wines treated with yeasts and that underwent partial fermentation.

were very similar to one another.

Sensory Analysis

Sensory analysis indicated that the traditionally made wines scored worse than all other wines. The order of acceptance of the four wines was the following: Traditional Wines < Wines treated with native yeasts < Wines treated with S. cerevisiae strain CECT 13015 < Wines treated with S. cerevisiae strain CECT 13014.

The highest scoring wine in the sensory analysis was the wine treated with S. cerevisiae strain CECT 13014. This was the only wine deemed “desirable”, while all other wines were deemed “acceptable”.

CECT 13014.

Conclusions

The results of this study showed that by partially fermenting sun-dried Tempranillo grapes with S. cerevisiae strains that have the ability to function under high sugar levels during fermentation, finished wines were more complex and more desirable in terms of sensory characteristics than wines made in the traditional fashion. As a result of these findings, the researchers claimed that partially fermenting sun-dried Tempranillo grapes with the appropriate strain of yeast is an effective alternative to the traditional approach.

Overall, this was a straightforward study that provided interesting and potentially useful and applicable results for the wine industry. I would have

liked to have seen the experiment carried out in larger vessels, particularly vessels that are similar to those tanks and barrels used it the wine industry, instead of using small 2 liter flasks. It’s possible that increasing the volume could change the outcome, though I wouldn’t be surprised if we saw the same results in the larger vessels as well did with the small.

I would love to hear what you all think about this study. Please feel free to leave you comments!

Source: López de Lerma, N., García Martínez, T., Moreno, J., Mauricio, J., and Peinado, R. 2012. Sweet wines with great aromatic complexity obtained by partial fermentation of must from Tempranillo dried grapes. European Food Research and Technology 234: 695-701.