The Beer Brewer

Wine maturation occurs in American oak butts of 500 – 600 l capacity, which are stacked in tall, well ventilated buildings (Bodegas) which maintain a cool environment. Internal humidity is maintained above 60% by frequent watering, preventing excessive evaporation of water (through the wooden maturation vessels) from the wine.
Butts containing fino type wines are filled to 80% capacity, to maintain sufficient air/liquid interface to allow the spontaneous formation of the flor yeasts, so crucial in producing quality fino Sherry with the required characteristics. Temperatures must be maintained between 15 – 20°C to ensure flor initiation will occur. The flor is a film of yeast, created by the activity of S. cerevisiae type yeasts (S. beticus, S. cheresiensis), that float on the surface of the wine, preventing the exposure of wine to oxygen, (which may cause browning due to the oxidation of phenols) and film forming, acetic acid producing spoilage organisms. Flor yeasts possess an aerobic metabolism which leads to the metabolism of compounds within the wine: ethanol is consumed as a carbon source (1 – 1.5% v/v) and is replenished accordingly (via refortification), acetic and lactic acids are reduced (to acetaldehyde and esters), from 0.4 g l-1 to 0.04 g l-1, and glycerol is reduced from 8g l-1 to 0.5 g l-1. The large amounts of acetaldehyde (responsible for the pungent “green” aroma of the wine) that are common in such wines, 260 – 360 mg l-1, are the result of oxidative alcohol dehydrogenation by the flor yeasts. Acetaldehyde derivatives (which contribute to fino character), such as 1,1 diethoxyethane and acetoin also increase during flor maturation, to 40 – 75mg and 8 – 19 mg l-1 respectively. 1,1 diethoxyethane is an acetal compound formed via a chemical pathway from acetaldehyde and is partly responsible for the fruity aroma of the wine. A proposed pathway for the formation of acetoin by yeast involves a reaction between active acetaldehyde (acetaldehyde-TPP complex, formed from the decarboxylation of pyruvate) and acetyl coA, via the action of diacetyl synthetase enzyme, to produce diacetyl. The enzyme diacetyl reductase then reduces diacetyl to acetoin.