The Ageing of Sherry: Oxidative v. Biological

One of Sherry´s unique characteristics is that it is aged in two distinct ways, biologically and oxidatively, and the resulting styles, while quite different, are equally delicious. The fundamental difference is oxygen, the abundanceof it or lack ot it.

The driver of biological ageing is flor yeast. It begins to cover the surface of the wine soon after the end of fermentation, trying to adapt to a new situation which now contains alcohol and with little or no sugar remaining. The amazing thing is that local strains have evolved a means of survival in this new hostile environment by modifying their diet. In place of consuming sugars from the must they turn their attention to metabolising volatile acidity, glycerine and alcohol, breathing vast quantities of oxygen as they do so. This creates a strongly reductive atmosphere in the butts making oxidation impossible so long as the wine contains no more than 16° of alcohol which would kill the yeast.

Biological ageing produces many changes in the wine over time. The volatile acidity level falls by about a quarter over a maturation peroid of say five years while total acidity also falls, if less dramatically. The glycerine content plummets to almost zero while dry extract is halved. The alcohol leve is gradually reduced since it is now the yeast´s main source of carbon, and Finos and Manzanillas need to be checked for strength. The most inpressive change however is acetaldehyde which rockets from around 50 mg/l to over 400, far more than in almost any other wine. This aldehyde is responsible for the bitter almondy carácter in the wine. Reading this back you can see the formation of a biologically aged wine: it is losing sugar, glycerine, extract and acidity and gaining in bitter yeastiness.

In contrast to the reductive conditions of the biologically aged wines, Olorosos develop in aerobic conditions. With no yeast to metabolise it, glycerine produced during fermentation remains in the wine, and indeed grows by concentrarionn, giving body and a slight impresision of sweetness or roundness. In the ocasional extreme example the wine is known as “Pata de Gallina” (hen´s foot) from the symbol used to mark the butt. Total acidity is much the same in Finos and Olorosos, but the glycerine obscures it somewhat in the latter. Exposure to oxygen causes the wine´s colour to darken and raises the level of volatile acidity while that of acetaldehyde falls. While biological wines are best stored in cooler more humid conditions closer to the ground, Olorosos can be - and are - stored higher up, and sometimes above the biological ones. 

Another key difference is the alcohol level. In order to promote flor growth, biological wines are fortified to 15-16°, while in order to prevent it Oloroso are fortified to 18-19°, and as they age evaporation concentrates their alcohol content. In Jerez the humidity is much less than 75% which means it is water which evaporates. (In Scotland the humidity is much higher so it is Whisky which evaporates but either way you´re losing product). It puts the yeast activity into sharp focus when, despite concentration caused by evaporation, the alcohol level drops. Another thing which increases is dry extract, the notional powder remaining if the wine were centrifuged dry, a sign of concentration.