Glycerine in Sherry

Glycerine, from the Greek “glukus” = sweet and often known as Glycerol, is a natural by-product of fermentation of sugar in the grape juice and is abundant in wine. This colourless, odourless, viscous liquid which has a slightly sweet taste is responsible for roundness and smoothness on the palate as it also tends to mask acidity. In conjunction with alcohol, it forms the legs or tears on the glass through the Marangoni effect whereby as the alcohol evaporates up the side of the glass it takes water and glycerine with it until the weight of the latter makes it fall back down into the glass. The more sugar in the must, the more glycerine the wine will contain, and it is more evident in warmer countries where grapes ripen more fully, like Spain for example.

A wine’s glycerine content can vary according to the yeast strain, however. Wine yeasts are from the Saccahromyces or (sugar-eating) group and some are hungrier sugar eaters than others. While Saccaromyces Cerevisiae is responsible for most wine fermentations, it is not the only yeast involved and different places have different native yeast populations which help to create so many different styles of wine. In the Marco de Jerez the flor yeasts are rapacious.

Levels of glycerine in Sherry vary enormously due to the actions (or not) of flor yeasts. A newly fermented wine such as a mosto destined for Fino will contain somewhere around 7.3 g/l, but after say five years under flor this level will have diminished to close to zero, while volatile acidity and alcohol, which flor is also fond of, will have dropped to a quarter and by half a degree respectively from their original levels. The resulting Fino is therefore bone dry, low in acidity and viscosity.

When that Fino is fortified again to become an Amontillado, things go into reverse once flor is no longer present. Through the concentration caused by transpiration of water from the butts, everything slowly becomes less dilute, so the tiny glycerine content becomes more apparent again, though rarely beyond 2 g/l, while the alcohol level rises and oxidation increases volatile acidity again. The finished wine remains very dry and with more bite yet that hint of glycerine rounds it off.

Oloroso starts off with much the same glycerine content but, never having been subjected to flor, that content rises by perhaps a third during ageing to reach some 9 g/l. Meanwhile the alcohol level rises and continuous oxidation causes volatile acidity levels to rise also. This gives the fuller bodied, generous, darker, and apparently sweeter style we know and love.

To get the feel of the effect of glycerine, you could try a sip of Fino or Manzanilla followed by a sip of a normal table wine, and see how sweet the latter seems despite containing nearly double the level of acidity.