Chemical Components of Grapes and Wine
Wine Aroma Compounds
The aroma of wines is represented by volatile compounds coming from several sources. Wine aroma is made up of several hundreds of volatile compounds with olfactory thresholds of these compounds with varying concentrations. These volatile compounds are derived from a number of sources including the grape, microbial flora, and biochemical changes due to aging process. They can also be impacted by grape variety, vineyard management practices, and winemaking procedures. The diversity of aromatic compounds in wine is immense and ranges in concentration from several milligrams per liter (mg/L) to a few nanograms per liter (ng/L). Consequently, the olfactory impact of the volatile compounds in wine depends both on concentration and type.
Primary Aroma Compounds
Although the overall composition of most grape varieties is very similar, there are clear and distinct aroma and flavor differences between most varieties. Table 1.1 lists common varietals with their familiar descriptors. These differences can mostly be attributed to relatively minor variations in the ratios of the compounds that constitute the aroma profile of a grape. Only a few aroma compounds have been directly linked to specific varietal flavors and aromas. Although most of these compounds are present at low concentrations in both grapes and in the fermented wine, they normally have a huge impact on the overall aroma profile.
Methoxypyrazines are grape-derived and important contributors to green pepper, asparagus, grassy, herbaceous and vegetative characteristics to Sauvignon Blanc, Semillon, and Cabernet Sauvignon wines. Isobutylmethoxypyrazine (IBMP) is considered to be the most important compound and is typically present in wines at concentrations of 5 to 30 ng/L.
Norisoprenoids are a diverse class of aromatic compounds that contribute to the varietal character of many wines, especially in aromatic varieties such as Riesling. The norisoprenoids identified in wine with important sensory properties are: g-damascenone, 1,1,6-trimethyl-1,2- dihydronaphthalene (TDN), and vitispirane. Betadamascenone has a pleasant, fruity aroma and seems to act as an aroma enhancer, boosting the intensity of other fruity-smelling compounds.
Terpenes are an important group of aromatic compounds characterizing the odor of many flowers, fruits, seeds, leaves, woods, and roots. Although the terpene concentration of healthy grapes is generally stable throughout fermentation, infection by B. cinerea can both reduce and modify grape terpene content.
Varietal thiols, often referred to as volatile thiols, which impart desirable citrus and tropical fruit notes, are of particular importance to certain white grape varieties, notably Sauvignon Blanc. These sulfur-containing compounds are one of the main reasons for the success observed in Sauvignon Blanc produced in Marlborough, New Zealand, with concentrations soaring to levels only imagined in most wine-producing countries. Although Sauvignon Blanc is the poster child for thiols, we know that the bound precursors have been found in the skin of many other white cultivars like Chardonnay, Colombard, Gewürztraminer, Grenache blanc, Pinot gris, Pinot blanc, and Sémillon.
Secondary and Tertiary Aroma Compounds
Acetaldehyde is the major wine aldehyde. Acetaldehyde is the most important and familiar sensory aldehyde in wine and constitutes about 90 percent of the total aldehyde content. The sensory descriptors of acetaldehyde vary with concentration; at low levels, it can impart a pleasant, fruity aroma, but with higher concentrations, it is reminiscent of nuts, and at still higher levels smells like rotten apple. In sherry/port wines the high acetaldehyde concentrations are considered to be a unique feature of that style. The very high acetaldehyde production levels in sherries are due to the fact that this wine style is produced under oxidative conditions.
Esters are the class of volatile compounds that are responsible for a general contribute sweet-fruity aromas and flavors in white, roses and red wines. They are some of the most abundant aromatic compounds within wine. Esters are found in grapes in small amounts, but most of the esters in wine are secondary or tertiary flavor compounds that are largely absent from grapes but are instead formed during fermentation and maturation, respectively. Esters can contribute positively to the aroma of a wine. In low concentrations these compounds are perceived as generically fruity or floral (e.g., banana, raspberry and pineapple) and can boost the awareness of the innate fruit and floral characteristics of the grape varietal.
Higher alcohols or fusel alcohols are quantitatively major volatile by-products of fermentation and are thought to contribute to the aromatic complexity of wine. At moderate concentrations (less than 300 mg/L) they are part of the wine flavor backbone, but at higher levels they can mask fruit and finesse. Higher concentrations can negatively impact on wine aroma by contributing harsh aroma and taste. Higher alcohol production during fermentation is influenced by yeast strain, temperature, oxygen levels, nutrition levels, and acidity.
Many ketones are produced during fermentation, but few appear to have sensory significance. The major exception is diacetyl. At low concentrations (less than 5mg/L), diacetyl may donate a buttery, nutty, or toasty flavor (Jackson,2008).However, at slightly above its sensory threshold, diacetyl may begin to donate a caramel-like attribute. Its ultimate sensory importance depends on its stability during aging, other volatile wine constituents, and the presence of sulfur dioxide. Diacetyl may be produced by yeasts, especially at high fermentation temperatures, but is most commonly associated with malolactic fermentation (See Chapter 7).
Oak Volatile Compounds
The use of oak for the maturation of premium wine products is commonplace in the wine industry. It is primarily used to increase complexity and to add oak flavors to wines. Oak species and origin, cooperage practices, and winemaking techniques all influence the oak aroma compounds present in a given wine. Volatile compounds derived from oak are important contributors to wine aroma and flavor.
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