Acetaldehyde (a major flavor compound in Sherry) diffuses from the yeast during this primary fermentation, usually as an intermediate product during the first 48 hours of fermentation. Levels are reduced (via conversion into ethanol and acetic acid) as fermentation and maturation proceeds beyond this point. High levels of acetaldehyde may be formed by the intentional incorporation of large volumes of air during fermentation, to ensure adequate yeast growth. Other significant components formed during fermentation are; esters (fatty acids esterified by ethanol); organic acids, produced by yeast upon deamination of amino acids ; and higher alcohols, formed by yeast deamination, decarboxylation and reduction of amino acids . Glycerol is also one of the more abundant alcohols at this point, at ~ 8g l-1.
Fermentation goes to completion over several weeks, leaving a dry wine with less than 2g/l of fermentable sugars and 11 – 12% alcohol by volume. Higher alcohol concentrations range from 207 – 405 mg l-1. Some strains of S. cerevisiae may produce large amounts of malic acid during fermentation (Figure 2); however significant concentrations of malate, between 1 – 8g/L are common in grape juice . The malic acid is converted by lactic acid bacteria found in the must, primarily altering the taste of the wine. Leuconostoc oenos is the only species capable this reaction in the Sherry base wine, as Henick-Kling (1993) and Jackson (1994b) both state it is the only species performing this malolactic conversion in wines with a pH of 3.5 or less. The sharp tasting L-malic acid is decarboxylated by a malolactic enzyme (malate carboxylase) possessed by the bacteria, to produce L-lactic acid, which has a softer taste. Malolactic fermentation also raises the pH of the wine by reducing the total acidity (removing one of the carboxylic acid groups in malic acid; Figure 3), which is also a characteristic of Sherry.
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