The Beer Brewer

As the Saccaromyces species decline once the primary fermentation is completed (2 months), the secondary or lambification (acidifcation) phase, initially involving the hop tolerant Pediococcus damnosus, takes place. P. damnosus is the primary producer of lactic acid in lambic beer, as it is a homofermentative organism producing only lactic acid from the metabolism of glucose, via the EMP pathway. As lactic acid is a key component in lambic flavour, its production is considered here: Glucose undergoes enzymatic cleavage and substrate level phosphorlyation, producing glcyeraldehyde 3 - phosphate, a proportion of which is converted to pyruvate. Lactic acid is produced from pyruvate by the the enzyme lactate dehydrogenase. No gas is evolved. The taste threshold for lactic acid is 400ppm. Final concentrations are indicated as between 492 - 3677ppm.

Fermentation at temperatures above 20oC are required for P. damnosus growth, therefore increasing temperatures beyond this level will stimulate lactic acid production as cell numbers increase. P. damnosus is a also a facultative anaerobe, thus the exclusion of air during P. damnosus activity will presumably facilitate the further production of lactic acid.

P. damnosus is also a producer of diacetyl, prodcuing up to 200ppb during this period. Acetoin is also produced as a consequence . The production of large amounts of lactic acid result in a drop in beer pH from 4 to 3, with a gradual lowering of residual extract.

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Upon completion of fermentation, the wines are racked and fortified. Clarification upon racking is not required, as flocculation of the yeast leaves a bright product. Wine destined for fino types are a pale yellow, dry, with a volatile acidity (as acetic acid) of 0.2 – 0.5 g l-1 a pungent aroma, and low phenolics. Wines marked as oloroso and associated types are darker in colour (higher phenolics), fuller bodied, with higher volatile acidity and a more vinous aroma. The fortifying spirit is a neutral type, (and thus has little impact upon the flavour of the product), of 96% alcohol by volume, which is distilled from wine or associated pomace. It is sourced from outside the Jerez region. The spirit is blended with a quantity of wine three days prior to fortification, to prevent clouding (Bakker, 2003). Fino types are fortified to 15.5% alcohol by volume and oloroso types to 18.5%, with the latter higher alcohol content required to prevent the formation of flor yeasts, which will develop upon the fino types during maturation.

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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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The must (Juice) is transferred into open, stainless steel cylindrical fermentation vessels to begin the initial alcoholic fermentation, at temperatures around 25°C, to produce the white wine used as the Sherry base. Fermentation temperatures are slightly higher than those used for white table wines, consequently higher alcohols, such as isoamyl and phenethyl alcohol, are formed. Fermentation is initiated spontaneously by the yeasts that are part of the grape micro flora, or by the addition of specific inoculum of dry wine yeast. In spontaneously initiated fermentations, Kloeckera and Hanseniaspora yeasts have been identified as fermentation initiators, although the ubiquitous Saccharomyces cerevisiae ultimately dominates the fermentation, due to its comparative tolerance of alcohol. Kloeckera activity in the must leads to the production of glycerol, acetic acid and a variety of esters, before the species declines as S. cerevisiae predominates for the remainder of the fermentation.

S. cerevisiae metabolizes sugars contained in the must, primarily glucose and fructose, into ethanol and CO2 via the glycolytic pathway (shown in Figure 1), generating cellular ATP in the process.

Glycolysis provides the substrates that are utilized in respiration or fermentation. Preceding glycolysis, the hexose sugar is transported into the yeast cell via membrane bound permease transporters. During glycolysis, the hexose sugar undergoes a series of phosphorylation steps and cleavage to form triose phosphates, which subsequently form pyruvate. Respiration occurs only briefly in the must, as the yeast utilizes molecular oxygen to synthesize membrane material and to increase biomass. Respiration involves the generation of ATP from pyruvate via the TCA cycle (Figure 2). Under anaerobic conditions, which prevail in the must, ATP is generated predominantly via glycolysis (Figure 1). Fermentation (during glycolysis) commences with pyruvate, which is decarboxylated into acetaldehyde (and CO2), which in turn is reduced by the enzyme alcohol dehydrogenase to ethanol and rapidly exported from the cell and into the medium. The fermentation of pyruvate occurs as a redox balancing system to continue the fermentation process, with NADH re-oxidized to NAD+.

Continuing on the fermentation of sherry shortly …

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