Microbial origins of key wine aromas papt I: esters and aldehydes.
However, another winemaker may wish to make a wine that emulates sherry, at which point maximizing acetaldehyde production would be intrinsic to the desired wine style.
The same is also true with esters. A winemaker may wish to create a young wine that is similar to Beaujolais Nouveau or a fresh white wine that is meant for immediate sale. The formation and retention of esters during fermentation will be critical to achieve the desired wine style.
However, another vigneron may wish to make Pinotage table wine that has deep flavors of smoke, earth and a varied assortment of berries. The winemaker might view isoamyl acetate (an ester) as a fault and attempt to minimize its impact on the wine by controlling viticultural and winemaking practices which will influence this compound.
What are esters?
Esters are the class of volatile compounds that are responsible, in part, for a general "fruity" smell in wines.
They are some of the most abundant aromatic compounds within wine. (12) Esters are found in grapes in small amounts, but most of the esters in wine are formed during fermentation or during bottle development.
Esters can be classified as either volatile esters (or neutral esters) and acid esters (or non-volatile esters). Neutral esters are produced through enzymatic reactions.
Acid esters are formed in simple hydrogen-ion-catalysed esterification. (8) This simple acid-catalyzed reaction is slower than enzymatic esterification, but may be responsible for aged characters of wine. Acid-catalyzed esterification may occur faster in wines of a lower pH.' Therefore, esters not only contribute significantly to the sensory impact of newly fermented wine, but the aged product as well.
Volatile esters are produced in such high quantities during fermentation that the concentration surpasses the synthe-sis/hydrolysis equilibrium point, and they cannot be maintained. During bottle development, volatile esters decrease as they react hydrolytically and finally achieve equilibrium. (12)
Non-volatile esters contribute relatively negligible aromas and flavors in wine; however they may somewhat soften the tartness of highly acidic white wines such as from Chablis in France. (8)
Volatile esters are a major component of fermentation bouquet and rapidly dissipate after fermentation. (1) Therefore, wines destined for early release such as those sold in the northern hemisphere's autumn such as Federweifier (a.k.a. nett zvein) and Beaujoais Nouveau, rely heavily on fermentation esters as part of their intrinsic character.
Ester production and the proportion of each ester is dependent upon yeast strain. (22) Therefore, the winemaker might select a specific yeast strain in order to enhance or downplay certain esters. For instance, one might use Enoferm M1 to produce a wine with a "fruit punch" type of aroma or Lalvin V1116 (K1) to impart a "floral aroma" from the esters which these yeasts are known to produce.
Types of esters
There are two groups of esters, aliphatic and phenolic. Aliphatic esters are those formed with straight chain/non-cyclic molecules (such as alcohols and fatty acids). Phenolic esters are formed from phenolic compounds, which are cyclic in nature.
However, only the aliphatic monocar-boxylic esters make a significant impact in wine. The monocarboxylic acid esters can be further broken down into those formed from ethanol and saturated fatty acids. The second group are those formed from acetic acid and higher alcohols.
Monocarboxylic acids are the most significant esters for most wines. However meth- and ethanolic esters have been found to be associated in the aroma of Muscadine wines. (6) The physiological function of esters formed during fermentation is unclear. (12)
Esters can arise in two ways: from acetates, ethanol and higher alcohols or from ethanol and straight-chained fatty acids.
Esters which form from acetates, ethanol and higher alcohols include:
* Ethyl acetate, isobutyl acetate, isoamyl acetate and 2 phenethyl acetate.
Esters which form from ethanol and straight chain fatty acids include:
* Ethyl hexanoic acid, ethyl octanoic acid and ethyl decanoid acid.
The esters formed from fatty acids are not nearly as important in wine production as the acetate esters. However, they are more significant in products of distillation. (19)
The mechanism by which yeasts form esters has been theorized by many, but a consensus has not been reached. Some believe that the reaction is catalyzed by an enzyme called alcohol acetyltransferase (AAT). This reaction uses alcohol (as a substrate), co-enzyme A and ATP to form an ester. (1), (9), (19) Esters may also be formed through simple hydrogen ion-catalyzed reactions.
Oenoccoccus oeni and other lactic acid bacteria have esterases and can affect the ester concentration of a wine during malolactic fermentation. This occurs through ester synthesis or hydrolysis, which will complement or detract from wine aroma, depending on the esters produced or metabolized by the strain. (2)
Esters are usually associated with "general fruit" rather than attributing a specific aroma; however, they are not always pleasant (such as ethyl acetate). (12), (19) Ethyl acetate, which has a detection threshold of 12 to 14 mg/L, is also present in acetic acid and contributes to the vinegar (or nail polish) aroma at 120 to 160 mg/L. The perception of volatile acidity as a fault is a function of the ethyl acetateacetic acid ratio. (8), (12)
Significance less understood
Esters are generally thought to be more important to the aroma of white wines; their significance in red wine aroma is less understood.
However, esters are critical in the production of many wines, especially those made from Pinotage. If uncontrolled, this varietal wine is capable of developing a pungent, banana aroma from isoamyl acetate, which is produced during fermentation and at excessive concentrations may be viewed as a fault. (18)
Esters canalso be a major contributor to varietal aroma. This is especially true in Pinot Noir from Burgundy, which contains four particular esters that contribute to its characteristically fruity aroma. (10)
Native yeasts such as Hansenula ano-mala and Kloeckera apiculate produce an abundance of ethyl acetate. Therefore, yeast strain can affect the formation of certain esters. C. Lema found that the concentration of total esters was more dependent on the size of the initial yeast culture, rather than the yeast strain itself. (7) However, the concentration of the esters produced was different from strain to strain.
Saccharomyces yeast generally produce roughly the same concentrations of esters, but their distribution differs. Non-Saccharomyces yeast can produce many more esters than Saccharomyces, but may not always be pleasant. Nonetheless, this may be a reason why natural fermentations produce wines of greater complexity. (1)
Volatile esters are an important component of the fermentation bouquet, and they rapidly dissipate after fermentation. (1) Must conditions such as high solids and high fermentation temperatures (above 15[degrees] C), can decrease the amount of potential esters formed during fermentation. (1), (8)
Further, if oxygen is dissolved within the must, this may minimize ester produc-tion. (21) The use of sulfur dioxide and other antioxidants such as glutathione, caffeic acid and gallic acid can aid in the retention of esters in the bottle. (20)
Acetaldehyde constitutes around 90% of all the aldehydes found in wine. It is a normal yeast fermentation by-product and is an intermediary in the process of diacetyl forming from pyruvic acid. (12)
Acetaldehyde is the penultimate compound produced during the conversion of sugar to ethanol. Sugar is metabolized through glycolysis, which allows for the formation of ATP and NADH, providing cellular energy. The end product of glycolysis is two pyruvate molecules. Pyruvate is then enzymatically decarboxylated to form acetaldehyde. Acetaldehyde is then enzymatically converted to ethanol.
However, not all the acetaldehyde produced by the yeast cell is converted to ethanol, as it is used to maintain a redox balance within the cell. Some acetaldehyde remains in the cell, some is excreted, and the remainder is converted into alcohol. Notably, ethanol is able to oxidize back into an aldehyde. (16)
Acetaldehyde can also increase in wine through enzymatic oxidation of ethanol by film yeast. These yeasts utilize ethanol as their primary carbon source for growth. Film yeast are regularly exploited in the production of sherry, but must be controlled when creating table wine.
Yeast differ widely in their ability to produce acetaldehyde. In general, low acetaldehyde-producing yeast generate less acetic acid and acetoin than their higher-producing cousins. (14) Therefore, these yeast can be selected to create a more "fresh" wine style.
Cellar temperature during bulk wine storage is critical for control of film yeast. Temperatures of 8[degrees] C to 12[degrees] C are ideal to restrain oxidative yeast film forma tion. (19)
Aldehydes commonly convey a nutty or bruised apple aroma. (16) This compound is intrinsic to oxidative wine styles, such as sherry and Viii jaune (yellow wine). However, where these characteristics are desired in the aforementioned styles, they are viewed as a fault in typical table wines. Where aldehydes are intrinsic to the Savagnin-dominant Viii jaune wines of Jura, aldehydes in the Vin de paille (also Savagnin-dominant) wines from this region, would be viewed as a fault.
Besides affecting wine aroma, aldehydes may be intricately linked to color development of red wines. Aldehydes interact with phenolic compounds during bottle development, which promotes the formation of tannin-anthocyanin polymerization. However, the role of acetaldehyde in wine color stability may be of little to no significance. (15), (16), (17)
Free acetaldehyde in young wine is usually less than 75 mg/L. Although, if oxidative reactions induce higher acetaldehyde concentrations then S[O.sub.2] is used to neutralize the aromatic impact of acetaldehyde and form the less aromatic product, acetaldehyde-a-hydroxysulfonate. (19) It requires 1.45 mg of S[O.sub.2] per milligram of acetaldehyde for the latter to be completely "bound." (5)
Unfortunately, S[O.sub.2] is not always a positive tool in decreasing the sensory impact of acetaldehyde. Increasing amounts of pre-fermentative S[O.sub.2] correlates with higher acetaldehyde production, since S[O.sub.2] inhibits aldehyde dehydrogenase, which converts acetaldehyde to etha-no1. (4), (13) Further, incorrect timing of the S[O.sub.2] addition leads to degradation of acetaldehyde-a-hydroxysulfonate by lactic acid bacteria, thereby releasing S[O.sub.2] and halting, or prolonging malolactic fermentation. (11)
Wine is commonly referred to as a "complex matrix." By breaking wine down into its fundamental components, we can begin to understand how to better manage our vineyards and wineries to attain the wine styles that our markets desire.
Esters and aldehydes could be considered a fault or aromas that are intrinsically valuable to a wine style, depending upon what we are trying to achieve.
It is crucial to understand how these compounds arise and how vintners can manage them effectively and efficiently.
This text was edited from first publication in the Australian & New Zealand Grapegrower & Winemaker, September 2014 with permission of the publisher, Winetitles.
This is Part I of a three-part series that covers the fungal and bacterial origins of wine aromas. The series will detail esters, aldehydes, volatile fatty acids, volatile 1 phenols, sulfurous compounds and higher 1 alcohols. The old adage "one man's trash is another man's treasure," holds true with these compounds.
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|Comment:||Microbial origins of key wine aromas papt I: esters and aldehydes.|
|Publication:||Wines & Vines|
|Date:||Jan 1, 2015|
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