Wine exhibits remarkable diversity in both flavour and aroma. Even wines produced in the same region during the same vintage never show completely identical sensory profiles. Moreover, when a single producer vinifies grapes from the same vineyard under identical conditions in two separate vessels, the finished wines still display perceptible differences in taste and aroma.
These variations arise from differences in the types and concentrations of numerous chemical constituents present in wine. In some cases, trace components amounting to less than one percent of the total composition can decisively influence a wine’s character. If such minor constituents exert substantial effects, then more abundant components cannot be ignored in evaluating wine quality.
This article therefore focuses on glycerol (IUPAC: propane-1,2,3-triol), the third most abundant compound in wine after water and ethanol. In most wines, glycerol is present in greater quantities than any component other than water and ethanol, making it one of the principal non-volatile constituents of the matrix.
Composition of Wine and Relative Proportions of Its Components
Reproducing a wine exactly by artificial means is far from simple. By contrast, creating a beverage that is merely similar to wine is not especially difficult. Historical records even contain “recipes” for wine-like drinks, and anecdotes recount winemakers revealing on their deathbeds that “wine can also be made from grapes”, underscoring how loosely some beverages once corresponded to wine.
Wine contains several hundred identifiable aroma compounds, and even with modern analytical techniques it remains difficult to determine all of them and quantify their concentrations precisely. However, when these components are viewed as groups rather than as individual molecules, the overall structure of wine appears surprisingly simple.
Water constitutes approximately 70–85% of a wine’s total volume. The next largest component is ethanol, typically accounting for around 10–15%. The remainder—very broadly about 15–20%—is often discussed in terms of total solids. When wine is broken down at this highest level of abstraction, it can be described as consisting of water, alcohol and solids.
Solids and Extract
The term “total solids” is rarely used on consumer-facing labels, but in enological analysis it encompasses acids, sugars and extract (also called dry extract). Acid content and residual sugar vary substantially depending on wine style; extremely sweet wines with very high residual sugar also show proportionally elevated levels of total solids.
Extract comprises all components other than water, alcohol, acids and sugars. This includes phenolic compounds such as anthocyanins—the pigments responsible for the colour of red wine—as well as volatile aroma compounds and a wide range of both organic and inorganic constituents.
In a dry wine with negligible residual sugar, total acidity typically falls between 6 and 15 g/L. In contrast, inorganic constituents in extract are reported at approximately 1.8–4 g/L, polyphenols at about 0.1–3.5 g/L and volatile aroma compounds at roughly 0.8–1.2 g/L.
Among all extract components, glycerol is generally the most abundant. Its concentration in typical table wines ranges from approximately 5 to 12 g/L, representing more than 30% of the total extract in many cases.
Elevated Glycerol in Botrytized Wines
Glycerol is not a familiar substance in everyday contexts, although the name “glycerin” is widely recognised from cosmetic and pharmaceutical products. Chemically, glycerol is a non-volatile sugar alcohol (polyol), colourless, odourless and characterised by high viscosity. It produces a distinct sweetness when tasted.
In most wines, glycerol levels correspond to about 7–10% of the ethanol concentration, though the actual range is wide. One factor known to cause a substantial increase in glycerol concentration is infection of grapes by Botrytis cinerea.
Botrytized sweet wines, most notably noble rot wines, require grapes colonised by Botrytis cinerea, a fungal species also known as grey mould. As berries dehydrate due to fungal activity, water evaporates and sugars concentrate. During this process, glycerol concentrations in the must also increase.
Whereas wines produced from healthy grapes generally contain approximately 1.36–14.7 g/L of glycerol, wines made from botrytized fruit show markedly elevated concentrations of 14.6–24.7 g/L.
Under normal circumstances, glycerol in wine originates almost exclusively from yeast metabolism during alcoholic fermentation. Pre-fermentation must contains negligible glycerol. In botrytized must, however, high glycerol concentrations are already present before fermentation begins. Fermentation then adds additional glycerol produced by yeast, resulting in exceptionally high total concentrations in noble rot wines.
Glycerol as a Fermentation By-Product
Except in the case of botrytized grapes, glycerol enters wine as a by-product of alcoholic fermentation.
Yeast generates ethanol through glycolysis, and glycerol is formed within this metabolic network as a redox-balancing metabolite. The amount produced depends not only on the yeast strain but also strongly on fermentation conditions. Must pH, fermentation temperature, grape maturity and the composition of the microbiota present on berry skins have all been shown to influence glycerol formation. By comparison, differences in grape variety or region exert relatively minor effects on glycerol productivity.
Some commercial active dry yeasts are marketed as “high-glycerol-producing” strains. However, even when identical strains are used under nominally identical conditions, considerable variation in glycerol production has been reported. These observations suggest that fermentation conditions may exert stronger control over glycerol formation than strain-specific genetic factors alone.
Sensory Implications of Glycerol in Wine
Because glycerol is both sweet-tasting and highly viscous, increased concentrations are commonly believed to enhance a wine’s sense of volume or body. The commercial development of high-glycerol-producing yeast strains is closely linked to this perception.
In red wines, positive correlations between glycerol concentration and sensory quality have been documented. These relationships are often interpreted to reflect the contribution of glycerol to roundness and smoothness, as well as its ability to modulate the balance between acidity and tannin-derived astringency through its intrinsic sweetness.
Conversely, both red and white wines suffer sensory deterioration when glycerol concentrations become excessively high. Thus, glycerol is not a linear quality driver.
A central difficulty in assessing glycerol’s sensory role is the relationship between its concentration and its sensory threshold. The gustatory threshold of glycerol is relatively high, meaning that its concentration in most wines does not reach levels detectable as sweetness. Below this threshold, its influence on perceived viscosity is similarly limited, except in special cases such as botrytized wines.
Red Wines and Higher Glycerol Concentrations
Among non-botrytized wines, red wines generally contain the highest glycerol concentrations. White wines—whether dry or off-dry—tend to have lower levels.
This difference is readily explained by fermentation conditions. Red wine fermentations typically occur at higher temperatures and at higher must pH relative to white wine fermentations, creating conditions in which yeast more readily produces glycerol.
As noted earlier, red wines often show a positive association between glycerol concentration and perceived quality. This provides partial support for the practice of maintaining relatively warm fermentation temperatures in red winemaking, at least from the perspective of glycerol production.
Moreover, some red wines taste sweet despite containing virtually no residual sugar. In such cases, elevated alcohol concentrations—which correlate with increased glycerol production—may account for the subtle sweetness perceived by tasters, arising from glycerol rather than from fermentable sugars.
Summary: Is More Glycerol Better?
Historically, glycerol was widely considered to improve wine quality by enhancing mouthfeel, leading to attempts to maximise glycerol concentrations. Subsequent research, however, has demonstrated that excessive glycerol can negatively affect quality, and within realistic concentration ranges, increases do not consistently yield meaningful sensory benefits. As a result, modern winemaking places far less emphasis on strategies aimed solely at increasing glycerol content.
In practical winemaking, fine control of glycerol production is difficult. Because glycerol formation depends on yeast metabolism, its levels exhibit substantial variability. Even with identical yeast strains and controlled conditions, consistent glycerol concentrations cannot be guaranteed.
If the objective were simply to “increase glycerol on average”, certain management strategies could push outcomes in that direction, albeit with significant inter-tank and inter-vintage variability. However, such an approach relinquishes precise control to chance, and overly high glycerol concentrations can compromise wine quality. Therefore, the enological value of strategies based on the principle that “more is better” is questionable.
Although glycerol is the third most abundant component of wine after water and ethanol, its sensory impact is far less forceful than that of volatile aroma compounds, many of which exert perceptible effects at much lower concentrations. Unlike volatile aromatics, whose intensities generally scale with concentration, glycerol does not behave according to a simple “more equals better” relationship, highlighting its distinct functional role within the wine matrix.
