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August 2015 P R A C T I C A L W I N E R Y & V I N E YA R D 71 W I N E M A K I N G formation of 1-octen-3-ol, which creates a pleasant mushroom odor due to the enzymatic degradation of linoleic acid. Higher alcohols generally have little influence on the sensory impact of table wines. However, their impact is much greater in products of distillation. 3 The high molecular weight of fusel alco- hols gives them higher boiling points. Therefore, it is crucial to maintain a tem- perature of around 80° C ± 5° C during distillation to attain ethanol. If the tem- perature surpasses this range, a distil- late of concentrated fusel oil may result. Higher alcohols may be most fully appre- ciated in fortified wines such as Port. 18 Although it varies based on aromatic intensity, the impact of higher alcohol on the sensory characteristics of table wine may be limited. If harvest rains dilute the must, then the aromatic intensity will decrease, and the impact of fusel alcohols will increase. 20 Straight-chain high alcohols have a low threshold of detection. At low concentra- tions (less than 0.3 g/L) they can contrib- ute to the complexity of wine aroma. 18 At higher concentrations, however they can be overpowering. Likely, the most significant aspect of higher alcohols is their function in the formation of esters. Fusel alcohols affect wine aging by undergoing esterification with organic acids. However, the pace of these reac- tions is slower than the esterification that occurs during fermentation. Therefore, it takes a prolonged period of bottle aging before these tertiary bottle esters can become perceivable. 13 Higher alcohols may account for half of aromatic constituents in a wine and can even contribute a unique herbaceous aroma. 11 R. Boulton et al. claim a low fusel component in dessert wines results in a thin and simple product. 3 Some higher alcohols are derived from phenolics, such as 2-phenylethanol or phenethyl alcohol. Although this alcohol is not generally classified as a higher alcohol, many believe it should be con- sidered as such, as it is highly aromatic and causes a rose-like scent. 3 Aside from phenethylethanol, most higher alcohols will have an unpleasant odor at high concentrations. Winery practices can influence the rate of higher alcohol formation dur- ing fermentation. Elevated fermentation W hen a winemaker typically thinks of volatile phenols, usually the first com- pounds to come to mind are 4-EP and 4-EG, which produce an unpleasant "Band-Aid" or "barnyard" like aroma. However, there are volatile phenols that are intrinsic to the aroma of some wines; such is the case with 4-vinylphenol and 4-vinylguiacol, which provide Gewürztraminer with a carna- tion and clove-like aroma. In terms of sensorial impact, higher alcohols (fusel alcohols) in distillates are a major concern. These compounds can provide the consumer of the distilled product with an unpleasant "solvent- like" aroma. In wine, these compounds can produce a pleasant earthy/mush- room aroma that is commonly associated with wines made from botrytis-affected fruit. Also, fusel alcohols can contribute positively to the aroma of an aged wine. Higher alcohols Alcohols (a chain of carbons and hydro- gen with at least one –OH group) are classified as higher alcohols when they have more than two carbon atoms. 11,25 "Fusel oil" is the term given to all the higher alcohols present within a wine. 3 The German word "fusel" literally trans- lates to "rot gut" or "bad liquor." Although most higher alcohols are a byproduct of fermentation by yeast, some are found in the grape and are sustained through the fermentation pro- cess (such as 2-ethlyl-1-hexanol, benzyl alcohol, 2-phenylethanol, 3-octanol and 1-octen-3-ol). The straight-chain higher alcohols are considered to have the most significant sensorial impact (1-propanol, 2-methyl-1-propanol (isobutyl alcohol), 2-methyl-1-butanol and 3-methyl-1-buta- nol (isoamyl alcohol). 18 Yeast commonly synthesize the follow- ing higher alcohols during normal fer- mentation: isoamyl alcohol (solvent-like odor), active amyl alcohol, isobutyl alco- hol and 1-propanol, listed in descending concentration. The function of these alco- hols is currently unknown. 3,20 These higher alcohols are mostly pro- duced from the deamination of nitrogen from amino acids, for example, arginine, which has a free α-amino group. In con- trast, yeast cannot cleave the aromatic ring around the nitrogen atom in other amino acids, such as proline. 8,12,21 The biochemical pathway for creation of fusel alcohols is similar to the forma- tion of amino acids, such as leucine and valine, aside from the last few steps. The transformation from amino acid to fusel alcohol occurs by way of the amino acid becoming a keto-acid (a molecule that has a carboxyl and ketone group). Then, the compound is debcarboxylated to form an aldehyde and reduced by NADH to form its corresponding alcohol. The subsequent reduction of aldehyde into alcohol maintains the redox balance within the cell. 16,18 If nitrogenous com- pounds are lacking in the must (ammo- nia or free amino nitrogen), the yeast will be forced to scavenge nitrogen from available amino acids, and the transami- nation process will leave behind fusel alcohols as a byproduct. 3,16,20,24 The con- verse is also true: An excess of nitrogen leads to less formation of fusel alcohols. 2 The formation of higher alcohols does not always require an amino acid precur- sor. For example, α-Ketoisocaproate forms as a reaction between α-Ketobutyrate (a product of pyruvate) and acetyl CoA. This product is a precursor to leucine and isoamyl alcohol. 20 Anabolically formed higher alcohol from sugars can occur during fermenta- tion using the aforementioned pathways. 3 Hence, chaptilization can increase fusel alcohol concentration. 12 Approximately 35% of the fusel alcohols found in wine come from glucose. 25 As the amino acid content of a juice rises, the concentra- tion of catabolically formed higher alco- hols increases (those formed through the Ehrlich mechanism), which spurs a corresponding decrease in anabolically formed higher alcohols. 9 Higher alcohols are also produced by spoilage yeast and bacteria, which cause off-aromas. Noble rot can lead to the Russell Moss BY Microbial origins of key wine aromas: HIGHER ALCOHOLS and VOLATILE PHENOLS This ar cle is part of a series that covers the fungal and bacterial origins of wine aromas. Part I covered esters and alde- hydes (January 2015). Part II included volatile fatty acids and sulfurous com- pounds (March 2015). The old adage "one man's trash is another man's treasure" holds true with these compounds.