These are terms used to describe the general weight, 'fullness' or overall feel of a wine in your mouth. Full-bodied wines are big and powerful. In contrast, light-bodied wines are more delicate and lean. Medium-bodied wines fall somewhere in between. There is no legal definition of where the cut-offs occur and many wines fall into the medium-to-high or light-to-medium body categories.
Alcohol and Extract - Key Influencing Factors
A number of factors determine the overall body or weight of a wine. Alcohol is typically the primary determinant of body. Alcohol contributes to the viscosity of a wine. The higher the alcohol in a wine, the weightier the mouthfeel, and the fuller the body. Wines with alcohol levels above 13.5% are typically considered full-bodied.
Extract is another important factor that contributes to body. Extract includes all the non-volatile solids in a wine such as the phenolics (e.g. tannins), glycerol, sugars, and acids.
In general red wines are more full bodied than white wines. If the wine is fermented or matured in oak, it adds further weight and body to a wine.
In white wines, certain winemaking techniques, such as leaving the wine on its lees (dead yeast cells) after fermentation, as well as bâtonnage (the periodic stirring of these lees) also add weight to a wine.
The Grape Variety
Certain grape varieties produce wines that are more full-bodied than others. Typically, there are varieties that when ripe have a high sugar content. Grenache and Gewürztraminer are two that immediately come to mind. Chardonnay wines in general are considered more full-bodied than Sauvignon Blanc or Riesling wines. However, not all Chardonnay wines are full-bodied. The body of a Chardonnay wine is quite influenced by the climate where the grapes are grown. Consider the difference between a crisp, lean Chablis (cool climate) and a barrel-fermented, oak aged Napa Chardonnay (warm climate). Regardless of grape variety, warmer regions, produce riper grapes with more sugar, hence higher potential alcohol - the primary determinant of body.
Thick-skinned varieties usually contain more extract than thin-skinned varieties. Red thin-skinned varieties include Gamay (think Beaujolais) and Barbera, while thick-skinned varieties include Cabernet Sauvignon, Merlot and Syrah/Shiraz.
Body and Quality
More or fuller body does not mean higher quality wine. Quality, which I will discuss another day is more to do with the balance of the wine's different components. For example, consider the very high quality of many light bodied Mosel Rieslings.
Examples of very light-bodied wines include German, Mosel Riesling, Asti and Moscato d'Asti with alcohol levels between 5.5% and 9%. Young Hunter Valley Semillon (Australia) and Vinho Verde (Portugal) wines at around 11% are also wines I consider light-bodied. Many more wines sit in the medium-bodied range with alcohol levels between 12% and 13.5%. Finally full-bodied wines typically come from warmer regions and would include most New World Reds, but also many Italian reds (especially Barolo and Southern Italian reds), Southern Rhône wines such as Gigondas or Châteauneuf-du-Pape, as well as Spanish reds from Priorat and Toro to name but a few.