Odoriferous Compounds in Grape Berry and Wine - Varietal Aromas

Wine aromas are made up of several hundreds of volatile compounds, at concentrations ranging from several mg/l to a few ng/l, or even less.
The complexity of wine aromas is due to the diversity of the mechanisms involved in their development:

• (i) grape metabolism, depending on the variety, as well as climate, soil and vineyard management techniques
• (ii) biochemical phenomena (oxidation and hydrolysis) occurring prior to fermentation, triggered during extraction of the juice and maceration
• (iii) the fermentation metabolisms of the microorganisms responsible for alcoholic and malolactic fermentations
• (iv) chemical or enzymatic reactions occurring after fermentation, during aging of the wine in vat, barrel and bottle

Yeast Fermentation produces volatile metabolites that are derived from sugar and amino acid metabolism and comprise esters, higher alcohols, carbonyls, volatile fatty acids and sulfur compounds, giving wine its vinous character Grapes produce odorifeous compounds that reflect the particular grape variety and play a more decisive role in the quality and regional character of wines than any other aroma component

• these include mainly monoterpenes, C13-norisoprenoids, methoxypyrazines, and sulfur compounds, and are responsible for the varietal aroma of wines
• numerous studies have shown that the terpenoid compounds form the basis of the sensory expression of the wine bouquet, and they can be used to differentiate grape varieties
• terpene compounds belong to secondary plant constituents, whose biosynthesis begins with acetyl-coenzyme A (CoA), and are located in the berry skin
• at present, about 50 monoterpene compounds are known, dominating monoterpene alcohols, particularly from Muscat varieties, are linalool, geraniol, nerol, citronellol, α-terpineol and hotrienol
  

A general classification of grape varieties based on monoterpene concentration allows division into:

• (1) intensely-flavoured muscats, in which total free monoterpene concentrations can be as high as 6 mg/L
• (2) non-muscat but aromatic varieties with total monoterpene concentration of 1-4 mg/L, including Traminer, Huxel and Riesling varieties
• (3) more neutral varieties not dependent upon monoterpenes for their flavor, including numerous cultivars such as Cabernet Sauvignon, Sauvignon Blanc, Merlot, Shiraz and Chardonnay

There are three categories of monoterpenes with some interrelationships between them:
(1) free aroma compounds

• commonly dominated by linalool, geraniol, and nerol, together with the pyran and furan forms of the linalool oxides
• depending on how juice has been treated and on other factors, which may include climate, many additional monoterpenes can be found in this group, i.e. citronellol, α-terpineol, hotrienol, nerol oxide, myrcenol, the ocimenols plus several other oxides, aldehydes and hydrocarbons
• in wines, several monoterpene ethyl ethers and acetate esters have also been found among free aroma compounds
• the most odoriferous of the monoterpene alcohols are citronellol and linalool
• olfactory impact of terpene compounds is synergistic

(2) free odorless polyols (diols and tiols)

• most significant feature of the polyols is that some of them are reactive and can break down easily to give pleasant and potent volatiles, i.e. diendiol (3,7-dimethylocta-1,5-diene-3,7-diol) which can give hotrienol and nerol oxide
• these monoterpene polyols are present in grapes at concentrations up to 1 mg/L

(3) glycosidically conjugated monoterpenes

• make no direct contribution to the aroma of the grape
• glycosides are, in most cases, more abundant than the unglycosilated forms of individual mono- terpenes and polyols
• glycon moiety is composed by glucose or by a disaccharide constituted by glucose and rhamnose or arabinose or apiose
• enzyme hydrolysis has been used to enrich flavour by release of free aromatic compounds from natural glycoside precursors producing a more “natural” flavour in the wine

Other information:

• free and bonded forms of terpenols accumulate in ripening grapes from the colour change onwards
• free monoterpenes start to decrease before maximum sugar level is reached
• vineyard conditions during ripening, such as temperature and water supply, may influence aroma development during ripening
• methoxypyrazines contributes herbaceous aroma in certain grape varieties, such as Cabernet Sauvignon, Cabernet Franc and Sauvignon Blanc
• the compounds 2-isopropyl-3-methoxypyrazine, 2-sec-butyl-3-methoxypyrazine and 2-isobutyl-3-methoxypyrazine, have odors reminiscent of green pepper and asparagus, or even earthy overtones
• a characteristic component of the aroma of Sauvignon Blanc wines is 4-mercapto-4-methyl-pentan-2-one, with a marked smell of boxwood and broom and concentrations may reach 40 ng/l
• the excessive hydrocarbon smells (produced by the non-megastigmane C₁₃-norisoprenoid derivative TDN, 1,1,6-trimethyl-1,2-dihydronaphtalene) that sometimes develop as Riesling wines age are related to extremely high temperatures, especially during the grape ripening periods

References:
1. Carlos Conde, Paulo Silva, Natacha Fontes, Alberto C. P. Dias, Rui M. Tavares, Maria J. Sousa, Alice Agasse, Serge Delrot, Hernâni Gerós, Biochemical changes throughout Grape Berry development and fruit and wine quality, Food, 1(1), 1-22 ©2007 Global Science Books.