Caveat: I hated being given a page limit on this paper. But then, I suppose there’s no point in writing a book, because there are perfectly good books out there that cover it all, anyway. Still. To anyone who loves this subject as much as, or more, than I do, I offer my sincere apologies. This paper does none of it true justice.
The Science of Wine Flavor
The flavor of wine has been recognized by many as particularly complex. It is made up of both nonvolatile and volatile aroma compounds. The ratios in which these compounds interact creates that unique flavor of wine that can vary so greatly within a seemingly small category, such as of a single varietal.4 The nonvolatile compounds are directly related to the sensation of taste, whereas the volatile compound are responsible for both taste and aroma.5 A grape’s exposure to the environment in which it is grown results in characteristics in the finished wine, specific not only to its varietal inherently, but also to its terroir – that is, its environment holistically – its level of ripeness, and most importantly, the techniques used in the fermentation of its juice. Also, because of the complex interaction of both volatile and non-volatile compounds, when speaking of wine, ‘flavor,’ ‘aroma,’ and ‘bouquet’ are terms which are used interchangeably to reference the overall effect of the wine on our senses.4
The science behind wine’s complex flavor begins in the ground. Two fundamental aspects of terroir include the capacity of a specific area of ground to retain groundwater, and how much sunlight that specific area is exposed to.8 Land with higher groundwater retention ensures the regularity with which a vine is supplied with water, and encourages grapes to ripen earlier.8 More sunlight earlier in the year, specifically in the springtime, causes the soil to heat up more rapidly and causes the vine to develop more quickly.8 Some rocky vineyards in colder areas benefit from rocks which can retain heat and help grapes to ripen in spite of the lack of actual sun exposure.3 Lack of water and lack of sunlight result in stress on the vine, which plays a key role in flavor.3 Put very simplistically, vines have the same amount of flavor which they can potentially contribute to their grapes. If less grapes are produced due to environmental stress, the flavor becomes more concentrated in the grapes. Conversely, the flavor in grapes from a vine which produces grapes in relative abundance is considerably less concentrated, and the flavor spread out among the fruit.3
The fermentable sugars in wine grapes are fructose and glucose, and the three main acids found in wine grapes are tartaric, citric, and malic acid.3 Sugar and acid in grapes have an inverse relationship; as the sugar increases with ripeness, the acidity decreases.3 Acid in the mouth prompts the production of saliva.1 Relative acidity between two wines can easily be distinguished by the question, “Which makes your mouth water more?” Warmer weather or a longer hanging time on the vine results in a higher sugar content and a lower acidity content in the fruit.3 Either of these would also result in a higher concentration of tannins in red wines.3 Higher levels of sugars mean a higher level of alcohol in the finished wine, and potential for a higher level of residual sugars in the wine. The sugars not only contribute sweetness to the wine, but also contribute to the body of the wine.3 Glycerol, the concentration of which is partly dependent on the ripeness of the grape (and otherwise dependent on the yeast used in the fermentation process) directly affects the body and mouthfeel of a finished wine.1
Aromatic compounds are found in very small amounts in the skin cells of grapes, and each varietal has its own unique physiological makeup of these compounds.1 Blending grapes to try and both capture and balance these inherent varietal characteristics can be a fun and very rewarding process for the winemaker.
Tannins, another inherent varietal characteristic, are found in the skins, stems, and seeds of grapes.3 The thicker the skins of grapes, the higher the concentration of tannins.3 Tannins dry out one’s mouth by bonding with the proteins in saliva responsible for lubrication and preventing those proteins from playing their natural role.8 Tannins are only found in wines that are exposed to their skins during fermentation, or that are wooded during aging.3
Another factor affecting flavors is the method with which grapes are harvested. When grapes are harvested mechanically, underripe and rotten grapes are picked along with the good ones. Though it is considerably more time consuming and costly to hand-pick grapes due to the labor involved, the difference in the flavor and quality of the finished wine is significant.3
When the grapes are crushed, the microbial content of the resulting must – a term for grape juice prior to fermentation – depends on several factors. Some of these factors include the method with which the grapes were harvested, the time and temperature [and sulfite addition, if any] during transport, and the general condition of the grape prior to the crush with regards to hygiene and aeration.4 The potential for influence of yeast during fermentation depends on the microbial content of the must.4
For grape juice to become wine, it must undergo some type of alcoholic fermentation. It is claimed unanimously by winemakers that the yeast used and the conditions under which the juice is fermented are the two most important factors in influencing the flavor of wine.4 Different yeast strains have different properties, and they vary in characteristics such as alcohol tolerance, SO2 tolerance, temperature tolerance and preference, nutrient requirements, and H2S production, to name but a few.7 Some yeasts have very unique abilities, such as the ability to reduce acidity, or to produce higher levels of aromatic ester compounds, or to improve the mouthfeel of the finished wine.7 The three most common species of yeast that are used in winemaking are Saccharomyces cerevisiae, Saccharomyces bayannus, and Saccharomyces uvarum. From these three yeast species are derived hundreds of cultured strains.7 Local and/or native yeast strains may produce flavors and aromas in wine which are expressive of the terroir, but these strains tend to be conducive to spoilage due to their low tolerance levels for the conditions needed to successfully complete the fermentation process; they have a tendency to result in ‘stuck’ fermentations and leave a great deal of residual sugar and incompletely fermented must.7 Furthermore, some yeasts are specifically identified as spoilage yeasts which can result in off flavors and aromas.7 Instead of settling for the characteristics of a single yeast strain, winemakers can blend yeasts based on the characteristics which any number of yeasts are known to produce.2
The aforementioned aromatic compounds found in the skin cells of grapes are responsible for the dominant aroma characteristics which appear in the finished wine, yet it is the fermentation process which allows these compounds to surface, and otherwise, they appear mostly undetectable.1 In the fermentation process, yeast eats those natural sugars of the grape and multiplies.1 During yeast metabolism, nutrients are both assimilated and dissimilated; that is, in assimilation, the yeast consumes energy, and in dissimilation, the yeast generates energy.4 The byproducts of this process include the magnification of already existing flavor compounds, as well as the creation of others.1 The main compounds which we detect are organic acids, higher alcohols or fusel alcohols, esters, and sometimes aldehydes.4
Higher alcohols, or fusel alcohols, have a higher molecular weight and boiling point than ethanol.4 Though there are a few exceptions, like Semillon, white wine varieties in general contain lower concentrations of fusel alcohols than red varieties.4 Fusel alcohols have very specific concentration levels at which they can be perceived as desirable or otherwise.4 At below 300mg/L, they are usually able to contribute pleasantly to the complexity of wine, but at above 400mg/L, their overall strong pungent smell and taste is generally regarded as negative.4 Fusel alcohols are produced by yeasts through the conversion of specific amino acids present in the must. Aside from what they bring to the table flavor-wise, they are also important precursors of esters which are associated with very pleasant aromas.4
|Examples of Fusel Alcohols4|
|Active amyl alcohol||Isoleucine||marzipan|
|Tyrosol||Tyrosine||bees wax, honey-like|
|Phenethyl alcohol||Phenylalanine||floral, rose|
Esters are secondary products of fermentation, and the amount and concentration of their formation is directly related to the yeast strain which is used.4 Esters in wine can be divided into three groups based on boiling ranges.4 Some esters contribute mostly to aroma while others contribute to body.4
|Examples of Esters4|
|Ethyl acetate||varnish, nail polish, fruity|
|Isoamyl acetate||banana, pear|
|2-Phenethyl acetate||rose, honey, fruity, flowery|
|Ethyl isovalerate||apple, fruity|
|Ethyl butanoate||floral, fruity|
|Ethyl 2-methyl-butanoate||strawberry, pineapple|
|Ethyl hexanoate||apple, banana, violets|
|Ethyl octanoate||pineapple, pear|
Those conditions which favor the production of fusel alcohols also favor the production of aldehydes. Aldehydes can contribute aromas anywhere between apple- and citrus-like to nutty depending on the chemical structure.4 Oxidation of alcohols and fatty acids, and also the degradation of amino acids, can all produce aldehydes as well.4 The formation of aldehydes is especially important to consider during the aging phase of winemaking.
|Examples of Aldehydes4|
|Acetaldehyde||sour, green apple|
|Isobutanal||slightly apple like|
|Pentanal||cocoa, coffee-like, slightly fruity, choking at high levels|
|Isovaleraldehyde||warm, herbaceous, slightly fruity, nut-like, acrid at high levels|
As in cooking, time and temperature are key during fermentation. If the fermentation is too fast, aromas and flavors can be lost, but if the fermentation is too slow, it can become ‘stuck.’7 Finding the balance between flavor fermentation and a stuck fermentation is one of the challenges of winemaking.7 Commercial wine yeast is very delicate, and can produce unappealing medicinal aromas and flavors if stressed.7
Fermentation does not have to be a one-time process. Malolactic fermentation is an example of a secondary fermentation which is sometimes desired based on the style of wine being produced. Malolactic fermentation is induced by malolactic bacteria, which, unlike most spoilage organisms which are unable to grow in wine due to the high ethanol content, low pH, high acidity, and low nutrient content, it is actually able to feed on the wine’s acid and convert malic acid to lactic acid.6
The two main effects which malolactic fermentation is used to achieve are the reduced perception of acidity and the production of diacetyl. Grapes which are harvested with a very high acidity, either because of the varietal characteristics or the limitations of terroir, can be softened with malolactic fermentation.6 Diacetyl is a compound which we associate best with movie popcorn, and is the compound which makes those California Chardonnays buttery. Diacetyl can also contribute a creamy mouthfeel to wine.6 It should be noted that some yeasts have the ability to produce diacetyl, and the presence of diacetyl in a wine does not necessarily mean that that wine has undergone malolactic fermentation.6
Wine’s flavor changes and becomes even more complex with age. As tannin-sugar bonds increase over time, the perceived astringency of the tannins is muted.8 Aldehydes, which reach a maximum point during the fermentation process and fall near the end of fermentation, slowly increase with age depending on a wine’s exposure to oxygen.4 The larger the bottle, the less effect oxidation has on wine.8 When a bottle is corked, CO2 can escape the bottle, but oxygen can still get in, because the partial pressure outside of the bottle is greater than the partial pressure inside of it.8 Oxidation is often connected to a cooked fruit taste in wine. 8 If the wine is aged in oak, oak also has its own set of flavor characteristics. Oak has the potential to impart flavor to wine that can easily dominate wine if appropriate care is not taken.1 Oak is porous and allows wine to evaporate, albeit slowly.1 The polyphenols in oak contribute tannins to the wine, and this is the only source of tannins that white wines potentially have, since they are not exposed to their skins, seeds, or stems during fermentation.1 French oak versus American oak will impart different characteristics. 3 French oak gives vanilla, cinnamon, nutmeg, and clove notes. American oak is much more aggressive with its vanilla notes, and also gives notes of toasted coconut and dill.3 Whether or not the oak barrel itself was toasted can give it the potential to impart toasted bread, nut, roasted coffee, or dark chocolate notes to wine.3
Finally, how we serve and taste wine affects how we perceive its flavor. The release of odorant molecules is dependent on temperature, which is why we serve some wines at warmer temperatures, and some at colder ones.8 Some aromatic compounds remain bound to a nonaromatic part, It is necessary for these compounds to react with enzymes in our saliva in order for the aromatic and nonaromatic bonds to be broken. It is when these bonds are broken that we are then able to perceive their taste.
The science behind the flavor of wine is a truly fascinating subject, one which, due to the endless possibilities with regards to its process control points, can never be truly exhausted by study.
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3 Gibson, M. The Sommelier Prep Course: An Introduction to the Wines, Beers, and
Spirits of the World; John Wiley & Sons: Hoboken, 2010; p 3 – 234
4 Lambrecths, M.G.; Pretorius, I.S. South African Society for Enology & Viticulture.
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5 Longo, M.A.; Sanroman, M.A. Production of Food Aroma Compounds: Microbial and Enzymatic Methodologies. Food. Technol. Biotechnol. [Online] 2006, 44, 3.
http://www.ftb.com.hr/44/44-335.pdf (accessed Sep 10, 2012).
6 Mansell, T. Palate Press: The Online Wine Magazine.
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7 Nelson, C. Chrisman Mill Vineyards. http://www.chrismanmill.com/dr-nelsons-articles-
fermentation.html(accessed Sep 10, 2012)
8 This, H. Molecular Gastronomy; Columbia University Press: New York, 2002; p 57 – 265