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30 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 W I N E M A K I N G varietal wine is capable of developing a pungent, banana aroma from isoamyl acetate, which is produced during fer- mentation and at excessive concentra- tions may be viewed as a fault. 18 Esters can also be a major contributor to varietal aroma. This is especially true in Pinot Noir from Burgundy, which contains four particular esters that con- tribute 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 pro- duce 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 com- ponent of the fermentation bouquet, and they rapidly dissipate after fermentation. 1 Must conditions such as high solids and high fermentation temperatures (above 15º C), can decrease the amount of poten- tial 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 reten- tion of esters in the bottle. 20 Aldehydes 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 com- pound produced during the conversion of sugar to ethanol. Sugar is metabo- lized through glycolysis, which allows for the formation of ATP and NADH, providing cellular energy. The end prod- uct of glycolysis is two pyruvate mol- ecules. Pyruvate is then enzymatically decarboxylated to form acetaldehyde. Acetaldehyde is then enzymatically con- verted to ethanol. However, not all the acetaldehyde pro- duced 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 uti- lize ethanol as their primary carbon source for growth. Film yeast are reg- ularly 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 8º C to 12º C are ideal to restrain oxidative yeast film formation. 19 Aldehydes commonly convey a nutty or bruised apple aroma. 16 This com- pound is intrinsic to oxidative wine styles, such as sherry and Vin jaune (yellow wine). However, where these characteristics are desired in the afore- mentioned styles, they are viewed as a fault in typical table wines. Where aldehydes are intrinsic to the Savagnin- dominant Vin jaune wines of Jura, aldehydes in the Vin de paille (also Savagnin-dominant) wines from this region, would be viewed as a fault. Color link Besides affecting wine aroma, aldehydes may be intricately linked to color devel- opment of red wines. Aldehydes interact with phenolic compounds during bottle development, which promotes the forma- tion of tannin-anthocyanin polymeriza- tion. 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 acetal- dehyde concentrations then SO 2 is used to neutralize the aromatic impact of acetal- dehyde and form the less aromatic prod- uct, acetaldehyde--hydroxysulfonate. 19 It requires 1.45 mg of SO 2 per milligram of acetaldehyde for the latter to be com- pletely "bound." 5 Unfortunately, SO 2 is not always a posi- tive tool in decreasing the sensory impact of acetaldehyde. Increasing amounts of pre-fermentative SO 2 correlates with higher acetaldehyde production, since SO 2 inhibits aldehyde dehydrogenase, which converts acetaldehyde to etha- nol. 4,13 Further, incorrect timing of the SO 2 addition leads to degradation of acetaldehyde--hydroxysulfonate by lac- tic acid bacteria, thereby releasing SO 2 and halting, or prolonging malolactic fermentation. 11 Conclusion Wine is commonly referred to as a "com- plex matrix." By breaking wine down into its fundamental components, we can begin to understand how to better man- age our vineyards and wineries to attain the wine styles that our markets desire. Esters and aldehydes could be con- sidered 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. PWV This text was edited from first publica- tion in the Australian & New Zealand Grapegrower & Winemaker, September 2014 with permission of the publisher, Winetitles. Bibliography 1. Boulton, R., V. Singleton, L. Bisson, and R. Kunkee. 1996 Principles and Practices of Winemaking. New York City: Springer Science and Business Media Inc. 2. Davis, C., D. Wibowo, R. Eschenbruch, T. Lee, and G. Fleets. 1985 "Practical implications of malolactic fermentation: a review." Am. J. of Enol. & Vit., 36 (4), 290–301. 3. Edwards, T., V. Singleton, and R. Boulton. 1985 "Formation of ethyl acids of tartaric acid during wine aging chemical and sensory effects." Am. J. of Enol. & Vit. (36), 118 –124. 4. Frivik, S., and S. Ebeler. 2003 "Influence of sulfur dioxide on formation of aldehydes in white wine." Am. J. of Enol. & Vit., 54 (1), 21– 38. 5. Hornsey, I. 2007 The Chemistry and Biology of Winemaking. Cambridge: The Royal Society of Chemistry. 6. Lamikanra, O., C. Grimm, and I. Inyang. 1996 "Formation and occurrence of flavor components in noble muscadine wine." Food Chemistry, 56, 373–376. 7. Lema, C., C. Jarres, I. Orriols, and L. Angulo. 1996 "Contribution of saccharomyces and non-saccharomyces to the production of some components of Albarino wine aroma." Am. J. of Enol. & Vit., 47 (2), 206–216. 8. Margalit, Y. 2004 Concepts in Wine Chemistry (2nd ed.). (J. Crum, Ed.) San Francisco: The Wine Appreciation Guild. 9. Mason, A., and J. Dufour. 2000 "Alcohol acetyltransferases and the significance of ester synthesis in yeast." Yeast, 1287–1298. 10. Moio, L., and P. Etievant. 1995 "Ethyl Anthranilate, Ethyl Cinnamate, 2,3-Dihydrocinnamate, and