Beyond cause and effect, impact compounds are a smaller subset of aromas that a student absolutely must learn and even own, so to speak. These compounds are derived from grape chemistry, vineyard environment, and winemaking techniques. Needless to say, the compounds are key markers for identifying classic grapes and wines. I can’t emphasize enough how important they are to the learning curve of becoming a professional taster, much less a necessity for exams. No surprise that I strongly recommend any student work on their memories for all the compounds listed below far more often than actually tasting. Otherwise, here is a list of most of the important compounds.
Disclaimer: I am NOT a chemist. In fact, I’ve never even impersonated a chemist much less taken chemistry in high school or college. I am a product/victim of a ‘60’s Catholic school education. By the time I arrived at a public high school, my math and science skills were beyond woefully inadequate. But I could diagram a sentence with the best of them. In all seriousness, the intent of mentioning chemical compounds here is all about sharpening the Occam’s Razor of deductive tasting.
Pyrazines: a chemical compound that smells and tastes like green bell pepper, asparagus, and cut grass. Recognizing pyrazines is important for identifying Cabernet family grapes including Cabernet Sauvignon, Cabernet Franc, Sauvignon Blanc, and other Bordeaux varieties.
Terpenes: intensely aromatic compounds with pronounced floral and sweet citrus notes. Common terpenic compounds include:
- Rose oxide: found in roses and rose oil; responsible for Gewürztraminer’s rose and litchi notes
- Linalool: highly floral and ginger-spice notes in Muscat grapes
- Geraniol: the scent of geraniums and lemon
- Nerol: the scent of roses and orange
- Alpha-terpeniol: grapey, pine-like aromas found in Torrontés
Grapes/wines displaying terpenic qualities include the following:
- Medium terpenes: Albariño (with lees contact and phenolic bitterness) and Riesling (with TDN and possible botrytis)
- Medium terpenes and elevated alcohol: Viognier (with possible diacetyl and oak) and Torrontés
- High terpenes: Gewürztraminer (lower acidity and elevated alcohol) and Muscat (higher acidity and less alcohol)
Rotundone: a chemical compound called sequiterpene that accumulates in grape skins, increasing in concentration between véraison and harvest. Rotundone accounts for the peppery aroma found in certain white and red grapes.
White grapes with rotundone: Grüner Veltliner from Austria
Red grapes with rotundone: Syrah/Shiraz
- Old World: wines from Northern Rhône Valley appellations of Côte-Rotie, St.-Joseph, Crozes-Hermitage, Hermitage, and Cornas
- New World: Syrah/Shiraz wines from Australia, California, Washington, South Africa, and others
TDN: the source of the petrol or kerosene character most commonly associated with Riesling. Technically, TDN is a chemical compound called 1,1,6-trimethyl-1,2-dihydionaphthalene. It was first isolated by German scientists in 2006. However, a study by the Australian Wine Research Institute found that high concentrations of TDN could also be the result of fruit grown in a region with a shorter, warmer, ripening cycle. It’s also thought that the combination of water stress, soil nitrogen deficiency, specific yeast strains, and clonal selection all affect the development of TDN.
Thiols: are sulfur-containing compounds that are present as odorless precursors in ripe grapes. During fermentation, yeasts can cause thiols to become volatile. These compounds are known as mercaptans and smell and taste like rubber, garlic, or onions. Thiols can be incredibly pungent but can also provide certain varietal aromas such as grapefruit, blackcurrant, passion fruit, and lemongrass.
Botrytis: the botrytis mold is responsible for many of the world’s great non-fortified dessert wines including Sauternes, Hungarian Tokaji, and Trockenbeerenauslese from Germany and Austria. Botrytis influence in wine smells and tastes like honey/honeysuckle, ripe/over-ripe stone fruits, marmalade, toffee, ginger, and button mushroom. It’s important to note that botrytis character can also often be found in non-dessert dry table wines including Alsace Pinot Gris and Riesling, Loire Valley Chenin Blanc (Vouvray), and German Grosses Gewächs Riesling.
Phenolic bitterness: phenolic bitterness in certain white wines is derived from the must being in contact with grape skins before, during, or after fermentation. Grapes/wines displaying phenolic bitterness include the following:
- Albariño and Grüner Veltliner: moderate alcohol with phenolic bitterness
- Alto Adige Pinot Grigio: medium aromatic intensity with phenolic bitterness
- Alsace Pinot Gris: phenolic bitterness on the finish along with riper fruit, elevated alcohol, possible botrytis, and residual sugar
- Viognier and Gewürztraminer: highly aromatic grapes/wines with elevated alcohol and pronounced phenolic bitterness
Diacetyl: a by-product of malo-lactic fermentation/conversion responsible for the butter/cream/dairy notes in Chardonnay. Recognizing diacetyl is important not only for Chardonnay-based wines, but other white wines that undergo Chardonnay “treatment” such as Viognier.
New oak: imparts a wide range of aromas and flavors including vanilla, baking spices, chocolate, coconut, dill, smoke, toast, coffee, tea, and more. It’s important for the student to be able to identify the markers of new oak vs. a wine without oak-aging--and to be able to describe them.
Carbonic maceration: a winemaking technique often associated with wines from Beaujolais. The intent of carbonic maceration is to extract maximum color and fruit from the grapes without excessive tannin. Wines made with carbonic maceration display overtly fruity aromas and flavors that are often described as candied or artificial. Jolly Rancher candy, bananas, and Hawaiian Punch are common descriptors. Carbonic wines also tend to display green woody notes as stems are naturally included during fermentation. Aside from Beaujolais, carbonic maceration is often used for some Côte du Rhône red wines, mass-market commercial Zinfandel, Shiraz blends, and others.
Stem inclusion: the use of stems during fermentation is commonly practiced in red winemaking in Burgundy as well as Beaujolais. In the latter, the practice of stem inclusion is the result of fermenting whole clusters of grapes via carbonic and semi-carbonic maceration. Aromas and flavors from stem inclusion can best be described as green and woody.
Brettanomyces: a yeast whose origins have been traced to the Senne Valley in Belgium. Brett, as it is often called, originates in vineyard soils and can also be found in winery environments. Brett smells like barnyard, animal, fecal, and medicinal/Band Aid. The threshold for detecting Brett as well as tolerance--even preference—of it varies dramatically with the individual, ranging from zero-tolerance to strong preference. However, it’s also important to note that Brett is highly contextual in wine. The presence of higher levels of Brett in certain classic Old-World wines is far from unusual. Bordeaux, Burgundy, Rhône wines (both the North and South), and other traditional Old-World reds fall into this category. For the student, it’s important to be able to detect Brett-related aromas and connect them to these possible wines/origins.
High VA: VA, or volatile acidity, is a by-product of fermentation and therefore present in all wines in trace amounts. Detectable acetic acid is usually considered a fault. However, like Brettanomyces, VA is highly contextual. In wines such as Barolo and Gran Reserva Rioja, higher levels of VA are acceptable and viewed as facet of wine style.
Raisination: wines displaying raisinated fruit characteristics can be caused by several different factors: grape varieties that ripen unevenly, making wine from over-ripe grapes, or making wine from dried grapes. In New World red wines, raisination is often associated with Zinfandel and Syrah/Shiraz, as both tend to ripen unevenly. Both wines will also tend to display elevated alcohol levels as they are often harvested late in the season. In the Old World, raisination can be found in certain Italian red wines, most notably Recioto della Valpolicella Amarone, which offers elevated alcohol, residual sugar, and possible elevated levels of volatile acidity. Raisinated qualities can further be found in other Italian passito wines (Passito di Pantelleria) as well as sweet fortified dessert wines such as Sherry, Port, and Vin Doux Naturels.
Tasting Practice with Impact Compounds
As mentioned, it’s absolutely vital that students learn to recognize impact compounds and further be able to connect the dots to the appropriate grape varieties and wines. With that, here are some recommended strategies for tasting practice with the compounds.
Use a Coravin
As I’ve stated before, practically every wine accessory is somewhere between almost useful and utter crap. An exception to the rule is the Coravin—simply a brilliant invention. Using a surgical-quality needle and a small cannister of argon gas, the Coravin allows you to tap into any bottle, take out as much wine as desired, and leave the rest preserved for months. In short, it allows anyone to taste a wine repeatedly over time without compromising the rest of the contents in the bottle. With a Coravin, you can purchase top quality examples of classic wines and taste them multiple times--over a long duration of time. And that will save a lot of money.
Taste Wine in Pairs
I find that our brains easily learn differences in side-by-side comparisons. With that, taste wines in pairs that show different compounds. The Coravin will allow you to taste pairs of wines multiple times until the desired “Aha!” moment occurs. Here are some suggested pairs of wines to taste that display different compounds or winemaking techniques:
- Oak vs. no oak: Chablis 1er Cru vs. California Chardonnay
- Non-ML vs. ML: Chablis 1er Cru vs. California Chardonnay
- Mineral vs. no mineral: Chablis 1er Cru vs. California Chardonnay
- Pyrazines vs. terpenes: New Zealand Sauvignon Blanc vs. Alsace Gewurztraminer
- Used oak vs. phenolic bitterness: Vouvray Sec vs. Alsace Gewurztraminer
- Carbonic vs. non-carbonic: Beaujolais Villages vs. AC Red Burgundy
Calibrating Structural Extremes
Using the Coravin again, here are suggested pairs of wines to taste that show extremes in structural elements:
- Acidity - lower vs high: Alsace Gewurztraminer vs. Australian Riesling
- Alcohol - low vs. high: Mosel Riesling vs. Central Coast Viognier
- Tannin - low vs. high: Beaujolais Villages vs. Barolo
- Phenolics - low vs. high: Chablis vs. Alsace Gewurztraminer
Non-Tasting Sensory Work
Finally, I recommend daily practice of your memories of both common wine aromas and impact compounds. Using the list above, go through your memories of all the compounds at least once, if not multiple times a day. It takes very little time and the rewards are considerable. Be mindful of the fact that you use images for all your olfactory memories. When you go through the list, make the image of the compound you’re working with at the moment internally as large, close in proximity, and as bright as you can. It will make the exercise a lot easier and far more effective. I strongly believe any student needs to do sensory memory work five times more than actually tasting. After all, the entire tasting process is all about perception, recognition, and memory—and all can be easily practiced without physically tasting.