Maximizing Wine Aroma

Many wine writers draw a distinction between “aroma” and “bouquet.” Typical is the discussion by Yair Margalit in his excellent book Concepts in Wine Chemistry. He says aroma “is the term for smell derived from the grape . . . Some varietal aromas are very powerful and recognized easily, and others are very weak and unspecific.” In contrast, he defines bouquet as “smell derived from the processing of the wine, which includes fermentation conditions, cellar operations and aging.” I have always found these distinctions awkward and not very helpful in wine appreciation. If a wine smells good, it smells good, and if it stinks, it stinks. Where that comes from doesn’t change my pleasure or disappointment. 

I tend to use “aroma” and “smell” interchangeably and rarely mention “bouquet.” In considering the word usage somewhat ambiguous, I note the writing of Jancis Robinson in her excellent The Oxford Companion to Wine. She says aroma is an “imprecise tasting term for a relatively simple smell such as that of a grape, fermenting must, or young wine.” After noting that wine professionals differ in distinguishing the point in a wine’s history where dominant aroma gives way to mature bouquet, she says the latter word is an “oft-ridiculed tasting term for the smell of a wine, particularly that of a mature or maturing wine.” While I don’t find the origin very useful in wine appreciation, it can be very important in winemaking, where we want to maximize desirable smells and avoid unpleasant ones.

Regardless of what we call them or where they come from, aromatics are central to the wine experience. While we have just the five basic tastes (salt, sour, bitter, sweet, and umami), experts estimate that humans can distinguish something like 10,000 aromas. Of those, about 1,000 have been isolated and identified in wine samples. In this article, I will address aromas in red and white wines, including some of the science behind where they come from and how to maximize wine aromatic quality. To do that, we will look at processes and conditions all the way from the vineyard to the wine glass.

Dissecting Wine Aromas

The hundreds of aromatic compounds in wine share some basic characteristics. First, they must be volatile. That is, only those molecules that evolve into the air at ambient temperature can be detected by a sense of smell. In the case of wine tasting, we can extend “ambient temperature” up to body temperature of 98.6 °F (37 °C). That is because while some of the aromatics are presented at the rim of the wine glass, others do not evolve in significant amounts until the wine warms up in the taster’s mouth. At that time, volatile compounds rise from the back of the palate and encounter the olfactory bulb in the nasal passage where odors are detected; these are referred to as retronasal aromas. While it can be interesting to note which come from the glass and which arise after sipping, the distinction does not really change the appreciation. Just like aroma vs. bouquet, it is all part of the overall impression of the wine.

Presence in significant amounts is the second key characteristic of wine aromatics. Modern analytical instruments can detect chemical compounds at very, very low levels. Unless the concentration of a compound in the wine is high enough to be detected by a human, it does not play a part in aroma perception (although sometimes closely related compounds can “add up” to a detectible aroma). While sensitivity varies a great deal from person-to-person and from time-to-time, the concept of odor threshold is useful in considering aromatics. This term refers to the lowest concentration of a compound in a particular medium (water, white wine, red wine, 10% ethanol in water as synthetic wine) that can be detected by a human subject. Since odor sensitivities are not predictable from chemical structure, they must be determined by repeated tests using humans. In a typical panel for odor threshold, the target compound is dissolved in the selected medium at levels that are expected to fall below and above the threshold. In an odor-free room, a test subject is presented with two or three samples, one of which contains the compound and the others do not. Asked, “which one is different?” successively higher concentrations are presented. The odor threshold is set at the level where 50% of test subjects provide correct answers above random predictions. Because results vary so widely, some aromatics need to be present in wine at rather high concentrations to have a detectible effect, where others can be quite low.

Among the many aromas of wine, four groups stand out as often providing pleasurable sensations. These are various terpenes, a range of esters, pyrazines in some varieties, and some sulfur compounds. Terpenes in grapes have floral aromas and are particularly prominent in Muscat wines and similar varieties like Riesling and Müller-Thurgau. The terpenes that are freely present in the fruit meet the classic definition of “aroma” as they are released into the juice and later manifest in the wine. Some fraction of the terpenes, however, is bound to sugar molecules in compounds called glycosides that make them non-volatile and odorless. Those terpene molecules must be released from the sugar molecules before or during fermentation if they are to contribute to wine aroma (more on that later). Very prominent among the terpenes are compounds named geraniol and linalool. Geraniol has a rose-like scent and is used commercially in a variety of fruit flavoring products. In addition to grapes, linalool is present in aromatic plants as diverse as cinnamon, lavender, and sweet basil. In wine, it is considered to impart general floral notes with some spiciness. While these two are prominent, they do not alone make up the aroma we think of as “Muscat;” there are several more terpenes that contribute to the overall aroma. Terpenes are not extremely volatile, with boiling points in the range of 400 °F (200 °C). Nonetheless, their high concentrations in grapes, from 100 to 1,000 µg/L, or parts per billion (ppb) combines with fairly low odor thresholds of 100 to 130 µg/L to make them prominent in the aroma of Muscat and other varieties. The individual terpenes are present in skins, pulp, and juice. It is because of the distribution of the compounds that manipulation of white grape must can significantly affect the floral aroma of the resulting wine, as discussed later.

If we assign terpenes the lead role in “floral” presentation, for “fruity” we look primarily to esters. Esters are chemical compounds formed from a carboxylic acid combined with an alcohol. In wine, ethanol is the most common alcohol but esters of isoamyl alcohol, isobutyl alcohol, and others occur. Prominent wine esters that contribute fruity aromas include isoamyl acetate that occurs in a range of 2 to 6 mg/L (or ppm) with an odor threshold around 0.2 mg/L. The aroma of isoamyl acetate is described as being similar to both banana and pear. Another is ethyl octanoate, occurring at 0.5 to 2 mg/L and detectable at around 0.5 mg/L. This ester is described as generally fruity and floral and is used commercially in synthetic flavoring products. Others occurring at or above threshold levels include ethyl hexanoate, described as having a pleasant pineapple smell, and ethyl decanoate, described as sweet, waxy, and apple-like. Other esters can contribute berry-like aromas to red wines. The vanilla and coconut aromas from barrels are esters, too, as are the hazelnut notes in sparkling wine aged on the yeast lees. It is worth noting that one ester we do not want in our wine is ethyl acetate. It is the first step in volatile acidity (VA) production that ultimately results in acetic acid and the wine becoming vinegar. A nail polish or fresh latex paint smell of ethyl acetate is usually the first indicator that VA is developing in a wine. Along with avoiding that, you will also want to avoid the next stage with overt development of vinegar aromas.

Wine pyrazines have structures that incorporate the hexagonal pyrazine molecular layout. This six-sided figure includes four carbon atoms, with two nitrogen atoms across the ring from each other. In grapes, the most prominent derivatives are methoxy pyrazines and all are described as “vegetal” in aroma. (This is illustrated easily when one notes the methoxy pyrazine abundance in asparagus, lettuce, and bell peppers.) That last vegetable reference, bell peppers, leads us to wine aroma. One of the methoxy pyrazines found in grapes has been reported with a threshold of only 2 ng/L (nanograms per liter or parts per trillion — ppt). That compound (3-isobutyl-2-methoxypyrazine or IBMP) has been reported in Cabernet Sauvignon wines at 4 to 55 ng/L and in Sauvignon Blanc wines at 0.5 to 40 ng/L. In Cabernet Sauvignon (and its family of grapes — Cabernet Franc, Merlot, and Carménère), the aroma usually comes across as bell pepper and many tasters don’t like it. At low enough levels, some wine tasters might find a faint pepperiness to be appealing, but for me it detracts from the fruitiness of the red wine. In Sauvignon Blanc, on the other hand, an experienced wine drinker anticipates some herbaceous character. If the pyrazines come across as grassy or asparagus-like, the effect may be considered pleasant and appropriate to the variety. I still prefer a fruitier profile, as described later in the section on vineyard advice.

In addition to the pyrazines, the herbaceous character of Sauvignon Blanc reflects the presence of a group of sulfur compounds. These compounds, called thiols, each contain a sulfur-hydrogen (S-H) group bound to a carbon in an organic molecule. The series found in Sauvignon Blanc wines impart aromas of just-cut wood, cat pee, passion fruit, and grapefruit. From those descriptions, it is easy to see that some presence may be desirable while others may constitute defects in the wine profile. The thiols are produced during fermentation by yeast from precursors in the Sauvignon Blanc juice. Sulfur compounds other than thiols may be produced during fermentation, also, usually yielding unpleasant aromas. If a fermentation is inhibited, the yeast may produce hydrogen sulfide, with the odor of rotten eggs. If not corrected, the hydrogen sulfide may go on to react and produce other foul-smelling reduced sulfur compounds know as volatile reduced sulfur (VRS). These include diethyl sulfide, described as rubbery; dimethyl sulfide, cooked cabbage; and methyl mercaptan, rotten cabbage. On the opposite end of the oxidation scale, sulfur dioxide (SO₂ or sulfite) is added by the winemaker to help prevent oxidation and spoilage. Used in excess, sulfite can impart a sharp, burning aroma like a just-struck match. Some sensitive individuals can smell sulfur dioxide as low as 50 ppm free SO₂ and it is best to go to bottle at or below that level (or wait a while before opening the wine if it was higher at bottling, since the level declines over time).

It Starts in the Vineyard

Now that we know the good, the bad, and the ugly — how does the home winemaker respond? If you grow your own grapes, it starts in the vineyard. I share the respect toward Dr. Richard Smart often mentioned by fellow WineMaker columnist Wes Hagen. Published in the early 1990’s, Dr. Smart’s book Sunlight Into Wine revolutionized canopy management around the world. If you grow grapes, you should have a copy. 

In terms of aromas, allowing sufficient sunlight to reach the grapes and keeping the crop load in balance with the vine are critical for reducing pyrazines and developing fruity aroma compounds. Fundamentally, achieving the best possible aroma in the vineyard comes down to getting the grapes fully mature at a sugar level that will produce enough alcohol to be a shelf-stable wine. Canopy management will include pulling leaves in the fruiting zone and research has shown that this can be done more aggressively in cooler climates. Research also shows that pyrazine levels are dramatically higher in Sauvignon Blanc and Cabernet Sauvignon grown in cool climates. Along with managing the canopy, be careful not to apply excessive fungicides, especially sulfur, too close to harvest. Yeast itself is a fungus and, as mentioned earlier, an inhibited fermentation can give rise to unpleasant VRS odors. 

Aroma Influence at the Crush

Once you harvest, the next influence is crushing. Since the skins of grapes contain many aromatic and aroma precursor compounds, breaking the skin can help with release. While some white wines are made without skin contact by whole-cluster pressing, aromatic varieties (and all reds) benefit from extraction of these compounds from the skins. 

One step to help with that extraction is cold soaking. Keep the crushed fruit below 55 °F (13 °C) as long as overnight for whites and possibly longer for reds (with appropriate sulfite protection). Pressing whites can also influence aroma, with press-fraction white juice showing significantly higher terpene (desirable) aroma compound concentrations than free-run juice. Do not hesitate to press those aromatic whites! In both whites and reds, extraction can be further enhanced with the use of enzymes. This is an opportunity to break the glycosides that are binding aromatic terpenes in skins, pulp, and juice to achieve their release. Most enzyme products are primarily pectinases. That is, they contain enzymes that break pectin to soften and burst grape cell structure. For aroma enhancement, look for products that contain significant amounts of glucosidases (they free terpenes bound to glucose molecules) and glycosidases (they free terpenes bound to other sugars).

Aroma Influence During Fermentation

At the beginning of fermentation, other products may be added to assist aroma development. Some yeast-derived nutrient products, like Stimula Sauvignon Blanc from Lallemand, have been developed specifically to help with yeast uptake of thiol precursors to help produce more of these desirable aromas. It is also important to add sufficient complex nitrogen yeast nutrient to avoid stressing the fermentation because insufficient nitrogen is one of the factors that contributes to yeast production of VRS. As mentioned earlier, oak esters may contribute favorable aromas. You can begin early with tannin additions to the fermentation. There are even some citrus-wood tannins that can be used in white fermentations to further build terpene-glycoside concentrations.

For the fermentation, choose a yeast strain that is reported to enhance varietal character and release aromas. Some of these strains have active glycosidase activity and can further release bound aromas in the must. Be sure to use an adequate pitch to avoid stressing your fermentation. Temperature control also influences aroma. We usually want to keep white fermentations cool to avoid volatilizing off the desirable floral and fruity aromas from the grapes. On the other hand, we want to ferment red wines warm to help the yeast develop the esters that are influential in red wine aromas.

Aroma Influence Post-Fermentation

After fermenting an aromatic wine, keep it fresh by avoiding oxygen exposure and managing your sulfite content during cellaring. Keep wines topped up and add sulfites regularly in accordance with the online sulfite calculator at Winemakermag.com/sulfitecalculator. Keep your temperature constant and cool to avoid losing volatile aromas. This period is another opportunity to add oak aromas. If a barrel is not part of your program, consider chips, sticks, beads, spirals, or other toasted oak alternative products. 

Your final stage of cellaring is in the bottle. Avoid excessive exposure to air on bottling day and make a final sulfite addition so you go to bottle protected against spoilage. Those mature “bouquet” smells often develop during bottle aging.

Finally, on the way to the wine glass, consider decanting. Wines with a low level of hydrogen sulfide odor may “blow off” that odor and present a fresher aroma when mixed with air in a decanter. Young wines may “open up” and seem more mature from the same process. For decanting older red wines, we will turn again to Margalit, who says of his own experience with aerating such wines, “The wine begins to be ‘opened’ on ‘breathing,’ which is followed by developing its bouquet to a maximum, and then the gradual decline of its quality to eventually become flat and tasteless.”

Now you can manage your wine aromatics from vineyard to glass!