The Science Behind Sherry Aroma In Wines

Sherry is a fortified wine made from white grapes grown near the city of Jerez de la Frontera in Andalusia, Spain. It is produced in a variety of styles, from dry to sweet and light to intense, with nutty, dried fruit, and saline flavours. Sherry is made primarily from the Palomino grape, with Moscatel and Pedro Ximénez also used for sweet wines. The wine is fortified with grape spirit to increase its final alcohol content, with wines classified as suitable for ageing as fino and Manzanilla fortified to 15.5% alcohol, and those classified to undergo ageing as oloroso fortified to at least 17%. The former is aged in barrels under a cap of flor yeast to prevent contact with the air, while the latter does not develop flor and so oxidises slightly as it ages, giving it a darker colour. Sherry is then aged in the solera system, a multi-barrel method of fractional blending and ageing, with new wine put into wine barrels at the beginning of a series of three to nine barrels. Sherry is regarded by some wine writers as underappreciated and a neglected wine treasure.

The type of grape used

In addition to the type of grape, other factors that contribute to the aroma of sherry include the climate of the region, vinification techniques, and ageing processes. The dynamic system of biological or oxidative ageing, in particular, plays a significant role in developing the wine's organoleptic characteristics.

The type of ageing

The solera system is a multi-barrel method of fractional blending and ageing. Each type of sherry has its own solera. The bottom barrels (called the solera) contain the oldest wine, and a portion of this is drawn off when the producer needs to bottle. The empty space is then filled with wine from the next level of barrels, called the 1st criadera. These barrels are topped up with wine from the 2nd criadera, and so on. Barrels are never emptied, and the top layer of barrels is replenished from newly produced wine called sobretable. As one might imagine, a sizable solera can result in wines with impressive complexity.

The alcohol content

Fino

Fino is a dry sherry with a characteristic almond aroma and a very dry flavour. It typically has an alcohol content of around 15-17%.

Manzanilla

Manzanilla is a type of fino sherry with a similar alcohol content of 15-17%. It is produced only in the seaside town of Sanlúcar de Barrameda and has a distinct ocean aroma with a salty, sea-spray quality.

Amontillado

Amontillado is an off-dry sherry with a nutty flavour and deeper brown colour. It typically has an alcohol content of around 18% but can range from 16-22%.

Oloroso

Oloroso is a dry, oxidative-style sherry with a dark appearance and a full body. It typically has an alcohol content of 18-20% but can range from 18-22%.

Palo Cortado

Palo Cortado is a rare and complex sherry with a reddish-brown colour and a dry palate. It typically has an alcohol content of 17-22%.

Cream Sherry

Cream sherry is a sweet, full-bodied sherry made from Amontillado or Oloroso and sweetened with PX. It typically has an alcohol content of 15.5-22%.

Pedro Ximénez (PX)

Pedro Ximénez is an ultra-sweet dessert sherry made from sun-dried grapes. It typically has an alcohol content of 15-22%.

The role of the flor velum

The flor velum is a natural biofilm of a strain of Saccharomyces cerevisiae yeast that grows on the surface of the wine during the biological ageing process. The flor velum is a film-forming culture composed of 95% different strains of Saccharomyces cerevisiae, which grow under harsh conditions such as low oxygen concentrations, high ethanol concentrations, low pH levels, and low fermentable sugar concentrations.

The flor velum develops an oxidative metabolism, where ethanol consumption as a primary carbon source has been widely studied, as well as glycerol, acetic, and lactic acid. In this way, acetaldehyde, 2,3-butanediol, fusel alcohols, diacetyl, and acetoin, among others, are formed as reaction products.

The flor velum's hydrophobicity increases with the bee pollen dose, which is significant for doses of 1 g/L and above. This increase in hydrophobicity may be due to the presence of fatty acids in bee pollen, which are responsible for the yeast cell wall hydrophobicity.

The addition of lysozyme does not affect the flor yeast during the fermentative stage or biofilm stage. However, if yeast inoculation is carried out under submerged culture conditions during biological aging, low doses of lysozyme (≥12.5 g/hL) affect cell multiplication and the membrane hydrophobicity of the yeast, inhibiting their aggregation, flotation, and subsequent development of the flor velum.

The rate of flor formation varies with the strain, appearing sooner in more ethanol-sensitive strains.

The type of wood used for ageing

Oak Wood

Oak is widely used in winemaking to alter the wine's colour, flavour, tannin profile, and texture. It is introduced during the fermentation or ageing periods in the form of barrels, free-floating chips, or staves added to the wine. Oak barrels impart distinct qualities to the wine through evaporation and low-level oxygen exposure, allowing for concentration of flavour and aroma compounds. The porous nature of oak enables controlled oxidation, softening the wine's tannins.

The chemical composition of oak lends vanilla-like flavours and tea or sweet notes to the wine. The degree of "toast" on the barrel further influences tannin levels and wood flavours. The presence of hydrolyzable tannins, derived from lignin structures, also helps protect the wine from oxidation and reduction.

American oak, or Quercus alba, is commonly used due to its faster growth, wider grains, and lower wood tannins. It imparts more intense flavours, with sweet and vanilla notes, compared to French oak. French oak, including Quercus robur and Quercus petraea, is considered superior for its finer grain and richer aromatic contribution, such as vanillin, methyl-octalactone, tannins, phenols, and volatile aldehydes.

The choice of oak type and source depends on the desired wine characteristics. American oak is often selected for bold, powerful reds or warm-climate Chardonnays, while French oak is favoured for its ability to impart silkier tannins and fruitier flavours.

Other Wood Types

While oak is predominant, other wood types such as chestnut, ash, acacia, and cherry have been used for ageing wine. Chestnut wood, for example, is known for its high tannin release, making it suitable for wines produced in southern Italy. Acacia wood, on the other hand, has low tannin release and is used for fragrant whites like Sauvignon Blanc.

The choice of wood type depends on the desired characteristics for the wine. Each wood variety imparts distinct flavours, aromas, and textures to the wine during the ageing process.

Frequently asked questions