|Tom Cannavan's wine-pages.com|
This is purely for reference, so there's no need to get bogged down here unless you want a user-friendly explanation of one of the following terms:
YEAST AUTOLYSIS, MAILLARD REACTION, MERCAPTANS, OTHER VOLATILE SULPHUR COMPOUNDS, PYRAZINES, TERPENES and VOLATILE PHENOLS.
Full yeast autolysis is unwelcome in most wines, but it is essential for Champagne. After remuage, Champagne may undergo a further period of ageing before the sediment is removed. The benefits of yeast-contact are derived from autolysis, which is the enzymatic breakdown of dead yeast cells. This occurs several months after the second fermentation, lasts for between four and five years, although up to 10 years is possible, and it:
After autolysis has finished, if a sparkling wine is kept on its lees, it merely remains fresher than the same wine disgorged at an earlier date, but the longer it is kept in this state, the more rapid the evolution after disgorgement. This is because the older a sparkling wine gets, the more sensitive it becomes to the sudden shock of exposure to the air during the disgorgement process.
Commonly associated with part of the raisining effect that darkens dried fruit, the importance of Maillard Reaction (MR) in the development of wine aromas is only just in the process of being discovered. It is, for example, an integral part of the toasting process in barrel-making and thereby contributes to many of the aromas closely associated with oak and lees-ageing. However, the most interesting aspect of MR under study at the moment is its importance in the marrying of the dosage of a sparkling wine. MR is now recognised as being responsible for many of the most complex aromas that develop after disgorgement. It involves a reaction between the sugar from the dosage and the amino acids created (and modified) during autolysis. We already know that some of the by-products of MR are responsible for toasted-roasted-vanilla post-disgorgement aromas, but precisely which amino acids are the precursors to specific aromas has yet to be established. Most of the results so far point to cysteine as a major precursor, but much work has still to be done.
Generally perceived as foul-smelling compounds, but like most groups of odoriferous compounds, mercaptans (thiols) can be good as well as bad. Some are even essential to what we believe to be the varietal character of certain grapes (e.g., 4MMP). Mercaptans' positive qualities may be due to the type of mercaptan, the level at which it is found, mitigating effects of other odoriferous compounds or a combination of any of these factors.
|Type of mercaptan|
The most common mercaptans in wine are methyl mercaptan (methanethiol) and ethyl mercaptan (ethanethiol). These are typically foul-smelling compounds. Methanethiol, for example, has the smell of stagnant water and is an active compound in the odour of halitosis, while ethanethiol has a raw onion stench. However, some mercaptans are responsible for extremely pleasant wine aromas. For example 4MMP or 4-mercapto-4- methyl-pentan-2-one is generally acknowledged as the compound responsible for the varietal character of Sauvignon Blanc (although not without a good dollop of pyrazine and ripe acidity), while mercaptohexanol can be redolent of blackcurrant, grapefruit or passion fruit, and thiophene-2-thiol (first detected in wine in 2000), 2-furanmethanethiol and 2-furfurylthiol can all have roasted coffee aromas.
OTHER VOLATILE SULPHUR COMPOUNDS
All sulphur compounds, including mercaptans, are divided into two basic categories: "light" (boiling point below 90ºC) and "heavy" (boiling point above 90ºC). Both can be foul smelling. There are exceptions, but as a rule of thumb, "light" volatile sulphur compounds have a much lower perception threshold (usually less than one part per million or ppm) than "heavy" ones (mostly between 50 and 1200 ppm). The only important exceptions to this rule as far as wine is concerned are the "light" dimethyl sulphide etc (5ppm) and "heavy" dimethyl disulphide (2.5ppm). However, since dimethyl sulphide has the aroma of quince and truffle and dimethyl disulphide quince and asparagus, it could be argued that their presence contributes positive notes to the complexity of a wine, thus the overlapping of their thresholds is somewhat academic.
As far as the most unpleasant smelling sulphur compounds are concerned, it requires only the tiniest quantities of a "light" compound to taint a wine. They are usually produced by yeast metabolism after fermentation in wines aged on their lees, but can be removed during the winemaking process (most recent method by introducing fresh lees - Lavigne, 1996). "Heavy" reduction faults are far less common, largely due to the much higher concentrations required, thus these compounds have rarely been studied. They are also produced by yeast metabolism, but they do not increase after fermentation. When they do occur, however, the wine will be ruined because of their low volatility.
|"Light" sulphur compounds
Carbon disulphide Rubber
Diethyl disulphide Raw onion, garlic, burnt rubber
Dimethyl sulphide Quince, truffle
Ethanethiol Onion, rubber
Hydrogen sulphide Rotten eggs
Methanethiol Stagnant water, halitosis
Carbon disulphide Rubber
|"Heavy" sulphur compounds
Dimethyl disulphide Quince, asparagus
Ethyl methionate Metallic
2-Mercaptoethanol Burnt rubber
Methional Pungent cooked cauliflower
Methionol Boiled cabbage
Methionyl acetate Mushroom
Methyl-2-tetrahydrothiophenone Natural gas
One of the most important groups of aromatic compounds, especially methoxypyrazines, which have typically green, leafy, grassy characteristics through to bell-pepper, green pea and asparagus. They become less abundant as grapes ripen and are considered a vital element of the varietal character of Sauvignon Blanc (see 4MMP in MERCAPTANS above). Dimethylpyrazines are more chocolaty, roasted nuts and can even be raw potato, although ethyl-n-methylpyrazines are even more earthy.
Also called terpenoids. isopentenyl pyrophosphate (IPP) is the five-carbon isoprenic unit from which all terpenes are built Thus they are all multiples of five units (5, 10, 15, 20 etc) and each group is named after its number of carbons: hemiterpenes (5), monoterpenes (10), sesquiterpenes (15) and so on up to caratenoids (40), after which they are lumped together as polyisoprenoids.
Although terpenes have been found in most grape varieties, it is the higher concentration of these compounds found in varieties such as Gewürztraminer, Muscat, Riesling and
various German crosses that makes them so aromatic. There are more than 400 naturally occurring terpene compounds in the plant world, but only about 40 have been found in
grapes or wine and relatively few of these are important components of aroma. For example, the characteristic aroma of Muscat, the most terpene-laden grape variety, is due to a
combination of just three terpenoid alcohols: geraniol, linaloöl and nerol, and of these geraniol is considered most important. All but one of the most
important terpenoid compounds for wine aromas are montoterpenes:
Limonene Used by the fragrance industry, limonene is one of the basic elements of aroma in bergamot and both orange oil and lemon oil, but is closer to orange than lemon.
Can also be very resinous.|
Myrcene The most herbal-resinous of simple hydrocarbon wine terpenes, myrcene is found in star-anise, coriander, hop, ginger, cinnamon,nutmeg, cardamom, bay leaves, basil, rosemary, sage, peppermint, spearmint, bell pepper, black pepper and grapefruit.
Citronellol Found in garden rose, geranium, ginger, black pepper, basil, peppermint and cardamom. Also plays a supporting role to citronellal in the aroma of Lemon
Eugenol The most herbal aroma of all terpenoid alcohols, eugenol is found in bay leaves, cloves and allspice.
Farnesol Whilst all the other terpene compounds important to grape and winearoma are monoterpene compounds, farnesol is a sesquiterpene alcohol (i.e., 15 carbon atoms). Farnesol is found in Linden Oil andis a constituent of garden rose aroma.
Geraniol Found in nutmeg, ginger, basil, rosemary, sage, cardamom and grapefruit, geraniol is one of the three terpene compounds principally responsible for Muscat
Hotrienol Has the aroma of Linden or Lime Tree, but levels higher than 30 ug/lindicate premature ageing, probably due to poor storage conditions.
Linaloöl Also spelt linalol, it is found in lavender, bergamot, jasmine, basil, rosemary, sage, star-anise, cinnamon, clove, nutmeg, coriander, cardamom, ginger, black
pepper and mandarin. This is one of the three terpene compounds principally responsible for Muscat aroma.
Nerol Found in orange blossom, ginger, basil, cardamom, mint and mandarin, nerol is one of the three terpene compounds principally responsible for the Muscat aroma.
Citronellal Found in ginger, black pepper, geranium and peppermint, citronellal is however overwhelmingly lemony-resinous in character, representing a minimum
of 82% of Lemon Eucalyptus Oil.
Geranial Found in cinammon, clove, ginger, basil and peppermint.
Generally considered to be faults, although ethyl-4-guaiacol and to a lesser degree vinyl-4-guaiacol can contribute attractive elements of aroma to a wine's bouquet, and this positive effect may vary from grape variety to grape variety (e.g.,vinyl-4-guaiacol is perceived as a defect in Kerner, yet it is thought to contribute in a positive sense to the varietal character of Gewürztraminer). Almost one-third of all French wines tested have had volatile phenols above the level of perception, so they cannot always be bad. The amount of ethyl and vinyl phenols present in a wine is increased by harsh methods of pressing (particularly continuous presses), insufficient settling, particular strains of yeast and, to a lesser extent, increased skin-contact. Some yeasts, such as Zymaflore VL1, are specifically designed to produce a wine without any phenol off-flavours, although they are, rather confusingly, known as Poff (for phenol off-flavour) strains.
Ethyl-4-phenol Stables, horsey, sweaty-saddles
Vinyl-4-phenol Band-Aid (sticking plaster)