The slide below – photographed at last year’s sherryfest during a masterclass by the great Cesar Saldana – is probably the best summary I have seen of the effects of biological vs oxidative or “traditional” ageing.
In biological ageing, the living flor protects the wine from the oxygen in the air and steadily eats away at the alcohol, sugar and glycerine, reducing the volatile acids and leaving behind the hangover inducing Acetaldehydes. The result is “fino” – fine, potent, dry wine with bready, nutty flavours.
In “traditional” ageing, these gears go into reverse. Residual alcohol, glycerine and sugar all increase as evaporation (the angels taking their share) does its work, and colour and volatile acids increase due to the interaction of the wine with air and oak barrel. Now the result is oloroso: a fragrant (oloroso literally means “smelly”), acidic, caramel flavoured wine.
And of course the great thing about jerez is that in some wines both processes are used: in a palo cortado there are a few months of biological ageing followed by a longer spell in the open air, and in an amontillado a longer spell of biological ageing (up to 7 or 8 years in some cases) followed by a good stretch of oxidation. The results can be spectacular.