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p r a c t i c a l w i n e r y & v i n e ya r d J a n U a r y 2 0 1 5 29 W I N E M A K I N G Russell Moss BY 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 bot- tle development, volatile esters decrease as they react hydrolytically and finally achieve equilibrium. 12 Non-volatile esters contribute rela- tively 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 Federweißer (a.k.a. neu wein) and Beaujoais Nouveau, rely heav- ily 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 sig- nificant 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 fermen- tation 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, etha- nol 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 produc- tion as the acetate esters. However, they are more significant in products of distil- lation. 19 Ellusive answers 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 acetyltrans- ferase (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 dur- ing 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 pres- ent 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 acetate:acetic 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 pro- duction of many wines, especially those made from Pinotage. If uncontrolled, this 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, vola le fa y acids, vola le phenols, sulfurous compounds and higher alcohols. The old adage "one man's trash is another man's treasure," holds true with these compounds. A winemaker may make a Sauvignon Blanc table wine within which they would like to have dominant aromas of grapefruit, gooseberry and passionfruit and perceptible acetaldehyde would be viewed as a fault. However, another winemaker may wish to make a wine that emulates sherry, at which point maximizing acet- aldehyde 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 fermenta- tion 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 dur- ing bottle development. Esters can be classified as either vola- tile 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 char- acters of wine. Acid-catalyzed esteri- fication may occur faster in wines of a lower pH. 3 Therefore, esters not only contribute significantly to the sensory impact of newly fermented wine, but the aged product as well. Microbial origins of key wine aromas E STERS and ALDEHYDES