White wine aromas
FIRST PUBLISHED BY THE INSTITUTE OF OENOLOGY, BORDEAUX (DENIS DUBOURDIEU). THE COPYRIGHT LIES WITH THEM.
We have come to a greater understanding of the nature of varietal aroma, how to preserve it, and how to reveal it during fermentation and maturation. The typical varietal aromas of Sauvignon Blanc are box tree, grapefruit, asparagus, and peach. Since the 1980s, we have had good Sauvignon Blanc aromas in Bordeaux and elsewhere in France, but there was little knowledge of the chemical compounds which created these aromas. We began our research on this topic toward the end of the 1980s. Our first important realization was that the actual aroma of fresh Sauvignon Blanc grapes does not explain aroma of Sauvignon Blanc wine. We also noticed that adding small quantities of copper into Sauvignon Blanc wine very quickly decreases the characteristic Sauvignon Blanc varietal aroma. This happens because the copper readily reacts with a family of complex sulfur-containing compounds, called thiols. Thiols are largely responsible for the box tree, grapefruit, asparagus, and peach aromas of Sauvignon Blanc wine. In the last ten years, we have identified almost ten thiol compounds in Sauvignon Blanc wine. Each thiol compound has a different odor, which participates in the final total aroma.
This was not an easy work because, though thiols give off much odor, they are present in the wine at very low concentrations. This study has shed new light on the study of varietal aromas of all grape varieties. It has been discovered that thiols can participate in the aromas of Gewurztraminer, Riesling, Cabernet Sauvignon and other varietal wines. Because the thiol family is so large, we are still working on this subject. But, with respect to winemaking elaboration, what is very interesting is that those odors are never present in the grape juice.
If thiols are not present in Sauvignon Blanc grapes, how can they be present in the final wine?
The juice contains chemical precursors, which are transformed into thiols during the alcoholic fermentation. Within two or three days of the inception of fermentation, cellars can develop a strong smell of grapefruit. This aroma then decreases, because the fermenting juice becomes more and more reduced from this point on. Yet the grapefruit smell is still present, and you can find it in freshly bottled or young white wine. This concept of precursors was very new to enology. We knew that the yeast creates aromas, but not that it could reveal potential varietal aromas. At the same time as we were making this discovery, we demonstrated that the use of different yeast strains could have different effects on the creation of thiols, and hence significantly alter varietal wine aroma profile.
This work had important practical implications. One yeast strain can endow wine with typical Sauvignon Blanc aroma, another cannot. We believe that the precursors find their way inside the yeast cells. Inside the yeast cells the important chemical reactions take place. We also found that yeast can modify some grape compounds in more complicated ways. For example, the aroma of grapefruit can be transformed into that of box tree by an esterification mechanism catalyzed by yeast.
We also discovered that Saccharomyces cerevisiae, the yeast that predominates in most winemaking environments, reveals only 30 % of the aromatic potential of Sauvignon Blanc during alcoholic fermentation. These discoveries sent us back into our labs to find yeasts with different potentials for revealing the aromatic potential of the grape. We discarded yeasts which cover varietal character by leaving a strong fermentation character, such as banana. On the other hand, we identified strains that are very effective in transforming thiol compounds from precursor compounds. Our research has been very useful for laboratories which breed selected yeast strains, and to the winemakers who use these strains. Because chemical transformations during wine maturation in the cellar and aging in bottle can never increase varietal aromatic potential, the revelation step for this variety is the alcoholic fermentation period and never after.
Once Sauvignon Blanc's varietal aromas are revealed they are vulnerable to degradation by chemical reaction. Thiols can react very easily with copper. Copper sulfate spraying has been the traditional spray against mildew. We therefore need to stop spraying at some time between the setting of the fruit and the harvest. Another solution is to selectively spray only the leaves. This is acceptable because the leaves are mildew's primary target. If the residual concentration of copper in the juice is greater than the concentration of thiols, the thiol aroma will never be revealed. In addition, we must protect the juice from oxygen from the period of the crushing of the grapes to the initiation of the alcoholic fermentation.
If the crushed grapes and must are exposed to oxygen, oxidasic attack results in oxidative phenolic compounds, which easily react with the thiol compounds. Essentially, these compounds are traps for the thiols and stifle development of Sauvignon Blanc varietal character. This research showed us why oxidative settling and other forms of hyperoxidation initiated before fermentation are disastrous for Sauvignon Blanc elaboration.
At the time of the introduction of hyperoxidative methods in the early 1990s, we couldn't understand why the use of these techniques resulted in a negative outcome for Sauvignon Blanc wine, while they worked well in the production of other varietal wines. It was only later, due to our research on thiols, that could we understand the problem.
We now know that we must use sulfur dioxide before the fermentation phase of Sauvignon Blanc. The correct technique is to use only as much as is needed. Sulfur dioxide not only protects the juice from oxidation but also destroys oxidases on the grapes. We must use SO2 for the white wine, particularly for Sauvignon Blanc but also for Semillon elaboration, starting with the crushing of the grapes. In France we are not allowed to use certain other chemical antioxidants, such as ascorbic acid, which can be used in the United States. Use of ascorbic acid, however, without concomitant additions of other anti-oxidants, can lead to problems. Under certain circumstances, ascorbic acid can be transformed into an aggressive oxidizer. Ascorbic acid should therefore be used in conjunction with sulfur dioxide. Sulfur dioxide is an essential compound for the evolution of the white wine technology, but we need to use it at the lowest concentrations which prevent oxidation. Recent studies show that it is not a dangerous product in the concentrations used in enology.
I have so far discussed prefermentation and fermentation issues relating to expressing and preserving compounds responsible for varietal character. At the end of the alcoholic fermentation of Sauvignon Blanc, you must use sulfur dioxide to halt the malolactic fermentation because malolactic fermentation mitigates varietal expression. You must also use sulfur dioxide to prevent the oxidation of the thiols. Sulfur dioxide alone, however, can only protect the aroma for several months. To preserve the Sauvignon varietal character for a longer period, it is essential to mature the wine on its total lees. The lees have antioxidant properties. The method of maintaining substantial sulfur dioxide levels while aging on the lees is a new approach. In Burgundy, they have been maturing wines on the lees without substantial sulfur dioxide levels for many years. They have no need to stop the malolactic fermentation, because malolactic fermentation gives positive flavors to Chardonnay wine, and Chardonnay aromatic compounds are not as easily degraded by oxidation as Sauvignon Blanc ones.
There are, however, always problems when you develop new methods. When sulfur dioxide comes in contact with the lees just after the alcoholic fermentation, unpleasant hydrogen sulfide aromas usually result. The anti-oxidative enzymatic activity of the yeast reduces the sulfur dioxide, and the result is hydrogen sulfide and its pungent, stinky smell. This enzymatic activity, however, decreases very quickly within the first few days after the completion of alcoholic fermentation. In addition, the hydraulic pressure that is on the lees is at the origin of the liberation of much of the hydrogen sulfide and other sulfur compounds. Tall tanks exacerbate this problem. We recommend two procedures. We advise winemakers to add sulfur dioxide to the whites from one to two weeks after the end of fermentation, at the period when the enzymatic activity of the yeasts is much decreased.
We also recommend that they frequently conduct batonage, particularly if they use metal tanks. Batonage lessens the hydraulic pressure on the yeasts by distributing the yeast cells throughout the tank. Batonage also homogenates the reductive potential of the yeast within each barrel or tank, because reduction is concentrated around concentrations of yeast cells. It is not easy to conduct batonage in large tanks - small tanks give better results. Batonage protects Sauvignon Blanc varietal aroma for up to three years. It also adds quality-enhancing toasty smells, and thickens the body of wine. The flavor and texture enhancement is due to monoproteins, which are liberated from the disintegrating walls of decaying yeast cells. These monoproteins have the additional benefits of stabilizing the wine proteins, hence reducing bentonite additions by 50% to 80%. Bentonite fining can strip wines of color and compounds which give positive aroma and texture to wine. Batonage also stabilizes tartaric acid salts in solution, thus eliminating the need for cold stabilization. These are benefits, since bentonite additions and cold stabilization are not positive procedures with respect to increasing wine quality.
Maturation on the lees for wine kept in oak barrel also limits the impact of oak extracts on the wine. Hence, the container has less impact on wine color, aroma, and taste. We are now sure that for the elaboration of quality dry white wines in Bordeaux, less new oak should be in contact with the wine. The percentage of new oak barrels has recently dropped at many estates.
Although I could show you examples of Bordeaux white wines that have kept their typicity after 8 years of aging, dry Bordeaux white wines are best drunk young. Dry white wines, which have been fermented and aged in stainless steel, should be drunk within two years after the bottling. White wines fermented and matured in oak may keep their varietal character for up to four years after bottling.
Semillon has better development during long aging. Its varietal character has a toasty character similar to that given by oak barrels, and hence Semillon wine marries better with barrique than does Sauvignon Blanc. Semillon, however, rarely dominates white wine blends in Bordeaux.
This discussion eventually leads to how to maximize the original aromatic potential of the grape. Now that we have discovered how to reveal and preserve varietal character, we must find ways to increase the varietal potential of the grape. This is not so easy. The first thing that we can say is that, for producing quality varietal wine, yields have to be low. The legal yield for producing quality Sauvignon Blanc in Bordeaux is 60 hectoliters per hectare or less. The quality of these yields, though, also varies with the quality of the canopy management. We have much to learn about the effect of the canopy on grape quality. If, for example, you have a limited canopy, 60 hectoliters per hectare is too great a yield for our quality standards. If you have a big canopy with a lot of leaves, 60 hectoliters per hectare is fine. We have to understand the evolution of the precursors of aromas during the maturation of the grape. We know that the aromatic potential evolves in a different way than the evolution of the sugar and the acidity. We know also that the aromatic potential of Sauvignon Blanc is not very stable during the grape's maturation. We have a window of not more than 15 days to 3 weeks during the grape maturation period to make a harvest which maximizes the potential of the vine. Sometimes the optimum harvest window only exists for one week. If the harvest is done outside this window, some varietal character is sacrificed. We also give advice where varieties should be planted. Clay soils seem to be best for revealing Sauvignon Blanc varietal character. These soils are particularly beneficial in hot, dry growing seasons because they keep the vines cool and moist. On the other hand, in wet, cool vintages, gravelly soils are superior. The gravel retains radiant and thermal heat and radiates that heat back to the vine. The gravel also assists rapid drainage.
We have conducted much research on the viticulture, fermentation, and maturation of the dry white wines of Bordeaux. We must continue to find ways to better reveal and protect the maximum varietal expression of our regional grapes, and to stabilize their character and quality once the wine is bottled."
NOTE Those wishing to know in detail the science supporting the preceding discussion should consult the following recently published book. "Handbook of Enology" by P. Ribereau-Gayon, Y. Glories, A. Maujean, and D. Dubourdieu., 2 volume set. Copyright 2000. ISBN: 0-471-49865-3.