Wine descriptions often come under fire for being fanciful. Can you really smell grass and grapefruit, rose petals and pepper in a wine?\r\n\r\nThe answer is yes, you can. Here\u2019s the science behind it.\r\n\r\nIt\u2019s all down to organic chemistry. Wine is made from grapes, and grapes draw on the same set of elements as all other fruits and plants. In unfermented grapes, most aroma molecules are bound up with sugar, so you cannot smell them. However, once fermentation turns sugar into alcohol, those volatile flavor compounds are set free and can be detected by our sense of smell.\r\n\r\nSome aromas in wine come from the grape itself and are the same compounds that occur elsewhere in nature. A type of chemical compounds found in Riesling, called terpenes, are also in citrus peel.\r\n\r\n\r\n\r\nOther aromas, like esters, are created by fermentation, while even more develop over time as wine\u2019s constituents react with each other. Then there\u2019s the influence of oak and winemaking practices, which can impart compounds that convey vanilla, caramel or coconut.\r\n\r\nAll these factors contribute to the complex array of aromas we get every time we smell wine. Our brains can decipher many of them, depending on our sensitivity and the concentration of the compounds.\r\n\r\nHere\u2019s a list of noteworthy compounds found in wine, and what common aromas and flavors you may not realize come from them.\r\n\r\n\r\nTerpenes\r\nIf you smell rose petals or citrus in your wine, it\u2019s due to terpenes, which reside primarily in grape skins and are abundant elsewhere in nature: in blossoms, fruits and leaves of myriad plants.\r\n\r\nTerpenes are a class of compound that make wines like Muscat and Gew\u00fcrztraminer fragrant and give characteristic citrus flavours to Riesling. Here are some you will recognize.\r\n\r\nLinalool: This terpene, in connection with others, causes the perception of lavender, orange blossom, lily and even bay leaves.\r\n\r\nGeraniol: The compound behind the tell-tale smell of rose petals.\r\n\r\nNerol and citronellol: Both these compounds create the floral, citrusy scents that occur in numerous flowers and fruits, and wine.\r\n\r\nLimonene and citral: These zesty flavors in wine are derived from the same compound found in citrus peel.\r\n\r\nHotrienol: The smell of linden blossom is also a constituent of that heady elderflower smell in Sauvignon Blanc.\r\n\r\n1,8-cineole and alpha-pinene: If you regularly smell eucalyptus in Australian red wines, or the scent of garrigue scrubland in the red wines of Southern France, you\u2019re not fantasizing. The compound 1,8-cineole gives eucalyptus trees their distinctive smell, while alpha-pinene is found in aromatic shrubs like juniper and rosemary. Both are airborne, highly volatile aroma compounds that can cling to the bloom on the skin of grapes.\r\n\r\nRotundone: Since red wines are fermented on their skins, highly aromatic and stable compounds can make it into the wine. As a sesquiterpene, rotundone is better known for the pungent aroma it gives peppercorns, particularly white pepper. If your Shiraz, Syrah or Gr\u00fcner Veltliner gives you a whiff of a recently used pepper mill, you are smelling this compound.\r\n\r\n\r\nAldehydes\r\nHexanal and hexenal: These two aldehydes are responsible for scents of freshly cut grass and tomato leaf in Sauvignon Blanc.\r\n\r\nVanillin: Another famous aldehyde, this is the chief flavor of vanilla beans. In wine, it derives from fermenting or aging in oak barrels. American oak (Quercus alba) tends to have more vanillin than French oak (Quercus robur), so you\u2019re not mistaken if that oaked California Zinfandel seems to particularly smell of vanilla.\r\n\r\n\r\n\r\nBenzaldehyde: If you come across the scent of bitter almond or marzipan in aged Pinot Gris or some Italian white wines, you are smelling this compound.\r\n\r\nFurfural: Reminiscent of dried wood, caramel and wheat bran, this chemical often occurs in oak-aged wines.\r\n\r\n\r\nPyrazines\r\nMethoxypyrazines: Ever smelled green bell pepper in Cabernet Sauvignon or leafiness in Carmen\u00e8re? What you\u2019re sniffing are methoxypyrazines. These incredibly stable compounds form part of the varietal character of some grapes, especially those in the Sauvignon family.\r\n\r\nMethoxypyrazines can also be a sign of underripeness in other grape varieties, where they present as an almost acrid herbaceousness. After all, green bell pepper is just the unripe version of red and yellow bell peppers.\r\n\r\n\r\nEsters\r\nEsters are volatile flavor compounds created by reactions between alcohols and acids. Esters are responsible for primary fruit flavors we get in very young wine.\r\n\r\nIsoamyl acetate and ethyl acetate: Very young, white wines often smell of pear-drop candy, or a mix of banana and pear flavor. This is the result of these two very common esters.\r\n\r\nOctyl acetate: Expect aromas of oranges and citrus fruits.\r\n\r\nButyl acetate: This ester smells of Red Delicious apples.\r\n\r\n\r\nKetones and diketones\r\nBeta-ionone: This compound creates the haunting scent of violets in Pinot Noir or Syrah.\r\n\r\nBeta-damascenone: Floral aromas with an undertow of red fruit come from this ketone.\r\n\r\n\r\n\r\nDiacetyl: The most obvious diketone, diacetyl smells like melted butter and can also come across as creaminess in wine. Diacetyl is a byproduct of malolactic fermentation where bacteria transforms sharp malic acid in wine into much softer lactic acid. That is why many Chardonnays smell buttery and creamy.\r\n\r\nWhen Chardonnay that went through malolactic fermentation is aged in new American oak, which imparts vanilla and nutty aromas, it can easily be reminiscent of buttered popcorn.\r\n\r\n\r\nMercaptans\r\nThese volatile sulfur compounds occur in grapes and are released by fermentation.\r\n\r\n3MH (3-mercaptohexan-1-ol): The 3MH mercaptan gives intense passion fruit notes to Sauvignon Blanc.\r\n\r\n3MHA (3-mercaptohexyl acetate): Guava and gooseberry aromas as the result of fermentation are signatures of this compound.\r\n\r\n4MMP (4-mercapto-4-methylpentan-2-one): The leafy, fruity notes of blackcurrant, so often encountered in Cabernet Sauvignon, are the result of 4MMP.\r\n\r\n\r\nLactones\r\nSotolon: This lactone is formed in wines that have aged. Sotolon is associated with botrytised wines like Sauternes and long-aged wines like Madeira. Depending on its concentration, it can smell of curry spice, nuts, toast or maple syrup.\r\n\r\nOctalactone: This compound comes from aging on oak, and smells of coconut.\r\n\r\n\r\nOther common wine aroma compounds\r\nThe famous petrol or kerosene smell in mature Riesling is known as TDN (1,1,6-trimethyl-1,2-dihydronaphthalene\u2014now you know why it\u2019s abbreviated) and relates to sun-exposure in the grape skins. Phenols are derived from oak aging: guaiacol gives smoky, toasty and roasted aromas to wines while eugenol is responsible for spicy notes of clove.