After ethanol and carbon dioxide, glycerol is the most abundant product of yeast fermentation (Ribéreau-Gayon et al., 1998). Several parameters have been shown to influence the final glycerol levels in wine. These include the ripeness of grapes, the microbial flora on grape berries and cellar equipment, as well as the pH, fermentation temperature, the nitrogen source and the yeast strain (Scanes et al., 1998). Glycerol is typically found at concentrations of 4 -10 g/L in dry wine and in the case of the botrysized late harvest wines, levels in excess of 20 g/L are not uncommon (Ribéreau-Gayon et al., 1998). In the latter case, grape berries infected by Botrytis cinerea already contain significant amounts of glycerol as a result of the metabolism of the fungus, which explains the high glycerol levels commonly found in this wine style.
It is frequently suggested by winemakers, enologists and wine writers, that glycerol contributes positively to wine quality. The perceived contribution has been defined in terms of mouth-feel and texture properties, and is thought to be strongly dependent on the glycerol concentration in the wine. In general, higher glycerol levels are considered to improve wine quality. To date, the opinions regarding the relationship between glycerol and wine quality appear to be based on anecdotal and empirical evidence. In some instances, clear anomalies exist between the perceptions and actual data that have been obtained through experimental work. No positive relationship between glycerol per se and the mouth-feel attributes of wine has yet been established and several factors other than glycerol have been implicated in mouth-feel. These include the ethanol concentration, the yeast cell wall mannoproteins, barrel maturation, yeast autolysis, the yeast strain used, as well as phenolic compounds in red wines (Ribéreau-Gayon et al., 1998; Deltail & Jarry, 1992). Furthermore, at the concentrations at which glycerol is normally found in wine, the impact that it could have on the viscosity of wine would probably not be perceived by even the most experienced tasters (Noble & Bursick, 1984). Against this background it is quite possible that the perceived contribution of glycerol to mouth-feel can easily be over-emphasized.
Christin Carlson, 2004