Pascal Ribéreau-Gayon - Handbook of Enology, Volume 2

FIGURE 2.3 Oxidation–reduction equilibrium of the thiol/disulfide system.

Methanol is always present in wine in very small quantities, between 60 and 150 mg/l. It has no organoleptic impact. Methanol is not formed by alcoholic fermentation, but results exclusively from enzymatic hydrolysis of the methoxyl groups of pectins during fermentation to form pectic acids:

As grapes have a relatively low pectin content, wine is the fermented beverage with the lowest methanol concentration.

Methanol content depends on the extent to which the grape solids, especially the pectin‐rich skins, are macerated. Red wines have a higher concentration (152 mg/l) than rosés (91 mg/l), while white wines have even less (63 mg/l) (Ribéreau‐Gayon et al ., 1982). Wines made from hybrid grape varieties have a higher methanol content than those made from Vitis vinifera . This is due to the higher pectin content of hybrid grape skins. The use of pectolytic enzymes to facilitate extraction or clarification of the must may cause an increase in methanol as a result of the pectin esterase activity.

Methanol's toxicity is well known. Following ingestion, it oxidizes to produce formaldehyde and formic acid, both toxic to the central nervous system. Formaldehyde damages the optical nerve, causing blindness. Wines made in the normal way never have methanol concentrations anywhere near hazardous levels (Skrzydlewska, 2003).

Alcohols with more than two carbon atoms are known as higher alcohols or fusel oil (Table 2.1). Several of these are produced during fermentation. For reasons of simplicity, they are generally referred to collectively, reaching total concentrations on the order of 150–550 mg/l in wine (Ribéreau‐Gayon et al ., 1982; Jackson, 1994). These alcohols and their esters have intense odors that play a role in wine aromas. The main higher alcohols of fermentation origin are isobutyl alcohol (2‐methyl‐1‐propanol) and amyl alcohols (a mixture of 2‐methyl‐1‐butanol and 3‐methyl‐1‐butanol). At low concentrations (less than 300 mg/l), they contribute to a wine's aroma complexity. At higher levels, their odors mask the wine's aroma finesse. Acetates of these alcohols, especially isoamyl acetate, have a banana odor that may play a positive role in the aroma of some young red wines ( nouveau ‐type).

Higher alcohols are formed by yeast, either directly from sugars or from grape amino acids by the Ehrlich reaction ( Figure 2.4). This reaction is caused by the activity of a FAD +dehydrogenase, which oxidizes amino acids into imino acids. These are hydrolyzed into α ‐ketoacids and then subjected to the action of a decarboxylase with thiamine pyrophosphate (TPP) coenzymes. Via this pathway, leucine produces isoamyl alcohol, and isoleucine results in optically active amyl alcohol with an asymmetrical carbon. The higher alcohol content of fermentation origin in wine varies according to fermentation conditions, especially yeast strain. In general, factors that increase the fermentation rate (yeast biomass, oxygenation, high temperature, and the presence of matter in suspension) also increase the formation of higher alcohols.

TABLE 2.1Simple Alcohols Originating from Grapes and Yeast (Ribéreau‐Gayon et al., 1982)

Formula	Name	Boilingpoint (°C)	Concentration(g/l)	Comments oralternative name
H−CH 2OH	Methanol	65	0.1	Produced by hydrolysisof pectins, not fermentation
CH 3−CH 2−OH	Ethanol	78	100
CH 3−CH 2−CH 2OH	1‐Propanol	97	0.03
CH 3−CHOH−CH 3	2‐Propanol	82	Traces	Isopropyl alcohol
CH 3−CH 2−CH 2−CH 2OH	1‐Butanol	117	Traces
	2‐Methyl‐1‐propanol	107	0.1	Isobutyl alcohol
	2‐Methyl‐2‐propanol	82	?
CH 3−CH 2−CHOH−CH 3	2‐Butanol	99	Traces
CH 3−CHOH−CHOH−CH 3	2,3‐Butanediol	183	1
CH 3−CH 2−CH 2−CH 2−CH 2OH	1‐Pentanol	137	Traces
CH 3−CH 2−CH 2−CHOH−CH 3	2‐Pentanol	119	Traces
CH 3−CH 2−CHOH−CH 2−CH 3	3‐Pentanol	115	?
	3‐Methyl‐1‐butanol	131	0.2	Isoamyl alcohol
	2‐Methyl‐1‐butanol	129	0.05	Active amyl alcohol
	3‐Methyl‐2‐butanol	112	?
	cis‐3‐Hexen‐1‐ol	156		Present only in grapes withherbaceous odors
CH 3−(CH 2) 4−CH 2OH	1‐Hexanol	158	0.01
CH 3−(CH 2) 3−CHOH−CH 3	2‐Hexanol	138	?
CH 3−(CH 2) 5−CH 2OH	1‐Heptanol	177	Traces
CH 3−(CH 2) 4−CHOH−CH 3	2‐Heptanol	160	?
CH 3−(CH 2) 6−CH 2OH	1‐Octanol	194	?
CH 3−(CH 2) 5−CHOH−CH 3	2‐Octanol	180	?
CH 3−(CH 2) 7−CH 2OH	1‐Nonanol	212	?
CH 3−(CH 2) 6−CHOH−CH 3	2‐Nonanol		?
CH 3−(CH 2) 8−CH 2OH	1‐Decanol	229	?
Φ−CH 2−CH 2OH	2‐Phenylethanol	219	0.05	Fermentation alcohol(rose odor)
OH−Φ−CH 2−CH 2OH	Tyrosol
CH 3−(CH 2) 4−CHOH−CH=CH 2	1‐Octen‐3‐ol			Mushroom odor

Φ, benzene ring.