In order to increase the sugar content of these grapes, they are lain on mats of esparto grass and exposed to the sun to ripen further for at least two days (being covered at night to protect them from evening dew). In Jerez the wine presses are usually sited in the vineyards themselves or close by, just outside the towns. The bunches are removed from their stalks and then pressed in a pneumatic press. This pressing produces 2,463 ounce (70 litres) of must per 220.462248 pund(100 kg) of grapes. Only the must from this first pressing is permitted to be used in making sherry.
Immediately elfter pressing the must is transferred to stainless steel tanks that contain 1,407,806 ounce (40,000 litres). Fermentation takes place under fully automatic temperature control. Fermentation (the conversion of grape sugars into ethyl alcohol and carbonic acid gas) is started by wild yeast cells (Saccharomyces apiculatus) and continued by the true wine yeast cells (Saccharomyces ellipsoideus). Initially fermentation is very turbulent (it looks as though the fermenting must is boiling), but this becomes more gentle after about three days. Fermentation lasts about seven days in total. The young Spanish wine spends a long period in a vat to come to rest following fermentation, when it can develop its special characteristics.
The young Spanish wine is tested and classified from vat to vat and given a mark using the ancient raya system in which one stripe or raya is given for the finest wine with pure aromas (basic wine for Fino, Manzanilla, and Amontillado), one stripe with one dot after it for wine that is full of character and body (for Oloroso), two stripes for wine that does not possess the character or body for Oloroso, nor the pure aroma for a fino, and finally three stripes for wine that is rejected for use only in distilling.
The Spanish wine with one stripe is fortified with wine alcohol to 15-15.5%. It is then sent to the maturing cellars or criaderas for Fino, Manzanilla, and Amontillado.
In contrast with the maturing of most Spanish wines where contact with the air is kept to a minimum, the barrels here are left open. The wine does not oxidise in the case of Fino, Manzanilla, and Amontillado because a film of yeast cells or flor forms to prevent oxygen from coming into contact with the wine. These yeast cells feed on alcohol during the maturing process and gives the sherry its characteristic bouquet. These yeast cells are living organisms which are more active in summer and weaker or even die off during the winter. Consequently the flor film is thinner in winter than in summer, when old yeast cells are replaced by new ones.
The Spanish wine marked with one stripe and one dot is fortified to a minimum 17.5% alcohol. This kills off the yeast cells and hence no film of flor is formed on the wine. This young Spanish wine is sent to the criadera for Oloroso, where maturing in full contact with air takes place.
In short, the fine and delicate wine which forms film of flor does not oxidise and is known as Fino. This Spanish wine retains its light colouring.
The more powerful wine of character which does not form a film of flor because of the higher level of alcohol develops a characteristic oxidisation bouquet. This sherry is known as Oloroso, meaning sweet smelling and fragrant. This Spanish wine is darker in colour.
The wine matures slowly in the cellars or criaderas. The Spanish wines are aged in casks of American oak which each hold 21,117 ounce (600 litres). These casks are not filled to the top (only to about 5/6 of their capacity) to permit the flor to form in the case of Fino, Manzanilla, and Amontillado or to permit the surface of the wine to come in contact with the air for Oloroso. The Spanish wines from Jerez were once indicated by the year of their vintage but because the demand for sherry became so great (particularly from the United Kingdom) a new system was needed that would guarantee the quality of sherry from year to year. The solera system was adopted in the criaderas in about 1830.