Glycerol, which is also known as glycerine, is a naturally occurring chemical with the formula C3H8O3. In wine production, it is a fermentation by-product and can be found as a trace compound in most wines as a colourless, odourless liquid.
If it’s colourless and odourless, how do we know it’s there?
It may not taste of anything or change the colour of wine, but glycerol is important as it is a viscous liquid and can contribute to a wine’s texture, body and mouthfeel. The weight that it brings can help to smooth off a wine’s rough edges and while it has no taste it can bring a slight sweet sensation to wines, which can be favourable especially in wines like Amarone.
After alcohol and CO2 it is the most abundant product of yeast fermentation and various factors can influence its prevalence in wine, including ripeness of fruit, grape variety, juice pH and grape nitrogen levels and source.
It also plays an important role during fermentation. ‘The production of glycerol has two important functions for yeast: to combat osmotic stress and to maintain the oxidation-reduction balance,’ writes Dr Sylvie Dequin, director of research at the Institut National de la Recherche Agronomique (INRA) in Paris, in a study for Lallemand Wine.
It is typically found at concentrations of 4-10 g/L in dry wine but can reach levels of 20 g/L+ in late harvest wines which have been affected by grey rot (Botrytis cinerea). Writing for Decanter in 2009 Jeff Cox explains why botrytised wines have increased levels of glycerol: ‘As the grape rots botrytis digests its sugar and acid, and excretes glycerol, which contributes to the silky mouthfeel of the wines.’
Amarone is another style where higher than normal glycerol levels can be found and, by some producers, actively encouraged. The red grapes dried to make Amarone are often more prone to botrytis which in turn leads to heightened glycerol levels. Amarone wines with more glycerol content can offer a favourable textural viscosity and perception of sweetness despite being very dry.
What does it offer to dry white wines?
Some grapes benefit from the oily mouthfeel and texture that glycerol brings. Take a look at this tasting note from Malu Lambert writing for Decanter in June 2020 of a South African Semillon Gris. ‘There’s a tension; a pull between chalkiness and an oily glycerol roundness, with a saline edge and bright, pithy acidity.’
Ian D’Agata, also writing for Decanter, found joy in how glycerol contributed texture to this Soave: ‘Rich creamy dense maybe not the most refined Soave but lovely glycerol, almost tannic texture.’
Is glycerol responsible for the formation of legs, or tears, on my wine glass?
It was once believed to be the case that glycerol contributed to these ribbons of wine running down the inside of the glass, but according to Ronald Jackson’s book Wine Tasting: A Professional Handbook it is ethanol, not glycerol that generates wine legs/tears.