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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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Processing of the grapes occurs soon after harvest, with whole bunches, (still containing stalks to aid drainage) being crushed in a roller crusher, before immediate pressing occurs, to reduce the extraction of phenolic compounds (expressed as gallic acid equivalents), which produce ‘rough’ or uncharacteristic tastes in the finished wine at concentrations greater than 850mg l-1.  Batch pressing is often utilized in modern establishments, as opposed to the more traditional screw pressing in vats (lagares) after treading occurred. Batch pressing allows juice to initially drain freely from the solids. This ‘free drain’ juice contains up to 200mg l-1 of total phenolics, and is kept separate from other pressed juice, and used primarily in the production of fino style Sherry. Further screw pressings in the batch process yield more juice (48% of total), although increased levels of tannic material are extracted. Extracts containing total phenolics to levels of 850mg l-1 are used to produce oloroso or associated Raya type wines, as the higher levels (of phenolics) assist in the subsequent oxidation of the wine. Extracts containing polyphenol levels of greater than 850mg l-1 are not used to make Sherry.
Prior to fermentation, juices are cooled to 12 - 15°C, to facilitate the settling of solids, thus maintaining low total solid levels (1%). Tartaric acid may be added to correct pH levels to approximately 3.45. Sulphur dioxide may be added to 100mg l-1 to inhibit undesirable spoilage microorganisms (predominantly the Lactic Acid Bacteria), and to prevent browning.

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This part 3 of the article about sherry and fortified wines.

Climate and soil characteristics of Jerez

The Jerez area has a characteristically warm climate with minimal cloud cover; mean monthly temperatures vary from 25°C in summer to 10.5°C in winter. Rainfall is varied, mostly occurring in the winter, and approximately 600mm per year . Wind factors significantly influence the local climate, the prevailing south westerlies increase relative humidity’s and cool temperatures , which may delay harvest, while the south easterly wind increases temperatures and decreases humidity’s, producing early harvests . The time of harvest has a significant influence upon the resulting wine, as is described in a subsequent section.
Soil is generally of poor quality, however it is capable of absorbing and maintaining moisture well, the latter an essential characteristic as irrigation of vines is not permitted. The most appropriate soils for viticulture are those found in the low hills of the region, which have high calcium carbonate (chalk) contents, a clay consistency and alkaline pH levels, between 7.5 – 8.5.

Grape Varieties

Four types of grape are permitted in the production of sherry:
Palomino fino is the primary type utilized, as varietal character of this cultivar is low, which is a prerequisite for the production of subtle flavours in the wine during maturation and blending. This variety also produces wines low in acid and sugar, which are essential characteristics for the maturation process.
Palomino de Jerez is an old cultivar no longer in use, due to low yields in comparison to Palomino fino vines.
Pedro Ximinez varieties, which may be sourced from outside the Jerez region, are sometimes incorporated in the musts for sweetening purposes.
Muscatel types are only used in small amounts, primarily for flavoring of the wine.

Viticulture and Harvesting

The majority of vines are trained upon wires and pruned low, leaving two fruiting branches, which have maximum exposure to the sun, thus ensuring grapes ripen to optimal conditions (i.e. contain maximum sugar and juice concentrations to assist in achieving wine with high alcohol levels and low acidity) prior to picking.
Due to the uniformity of grape variety (Palomino fino) and soil conditions, vines present optimally ripened fruit at the same time, usually the first week in September. Thus harvest is completed quickly, within 3 weeks, to preserve optimal juice/sugar levels.
European regulation states that sherry must is required to contain a natural alcoholic strength of no less than 10.5% by volume, although optimal must conditions are often defined as 11.5% by volume of potential alcohol, with 2.75g l-1 of total titratable acidity (as tartaric acid). However, titratable acidity is not accurate in defining must acidity, as high levels of potassium and polyphenol compounds may falsely elevate pH levels. Thus, further define must qualities, as represented by °Brix × (pH)2. Values between 270 – 295 indicate optimum quality grapes, in terms of alcohol and acidity.
Harvesting of the fruit is primarily by hand, as mechanical pickers may damage grapes. Fruit damage is undesirable, as it leads to enzymic browning (via polyphenol oxidase action) and may inhibit the metabolism and growth of flor yeasts during maturation. Small containers of 15 – 20kg capacity are used for harvested grapes, to minimize damage. Traditionally, post harvest, grapes were left to dry for 24 – 48hrs on mats in the sun to further concentrate sugars within the grape. This procedure is now only utilized in the production of sweet wines from Pedro Ximinez grape varieties. Modern practices ensure crushing occurs within four hours of picking.
A point of note is the fact that the prevailing environmental conditions during growing season dictate the sherry style that is produced that year: states that humid seasons delay ripening and lead to the production of predominantly fino types; early vintages produced by hot, dry conditions, favor the production of oloroso type wines. The characteristics of both of these styles are defined elsewhere in this paper.

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This is part 2 of an ongoing publication on fortified wines and sherry.
author: The Master Brewer English-Australian copyright 2007

History of Sherry and Fortified Wines

Wine has traditionally been produced in the Jerez area of Spain since the time of the Romans, however sherry in its present form seems to have evolved at some point during the mid 1700s to early 1800s , after the commercialization of distillation , although Voss (1993) and Reader and Dominguez (2003) indicate its origins may extend beyond this time period. The primary reason for fortification of this style (with a neutral grape spirit) seems to lie in the fact that the hot climate of this region, coupled with comparatively poor soils, created wines of poor quality . Fortification with a spirit served to significantly improve the quality of this wine . However some authors, Halliday and Johnson, (1992) and Jackson, (1994) indicate fortification was utilized to offer microbial stability to such wines during often lengthy ocean voyages to England (a primary consumer of such wines) in the 1700s.

What is certain is that the present style of Sherry, i.e. exclusively sourced from white wine and produced using a unique maturation system (solera) is a result of collusion between local producers and British traders.

Come back shortly for lots more info!

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This is part 1 of an ongoing publication on fortified wines and sherry.

author: The Master Brewer English-Australian copyright 2007

Definition of a Fortified Wine

Before proceeding to describe sherry and port wine production, if may be of benefit to define the category that these wines fall into: Both are classified as fortified wines (containing between 15 – 22% alcohol by volume), on account of both styles receiving fortification with a wine derived spirit at some point in the production process, rather than achieving final alcohol volumes by fermentation only.

Definition of Sherry and Sherry Style Wines

The term Sherry describes a range of fortified, white dessert wines, produced in the southern region of Spain (Andalusia), around the area of Jerez de la Frontera, in the province of Cadiz. This style of wine is a geographical appellation (European regulation), therefore only the wines produced in this designated area may bear the name Sherry . There are three main types of Spanish Sherry, fino, amontillados and oloroso, which are described hereafter.
Further European regulation stipulates dry and medium style sherries are required to contain a minimum of 15% alcohol, and cream sherries 15.5% . Outside Europe, wine styles similar to Sherry, that may or may not utilize similar production techniques to that used in Jerez, are often termed Sherry style, or carry the producing countries name appended to the term sherry.
The traditional method of sherry production utilized in Jerez (the solera method) is initially considered hereafter. Other methods will be briefly compared subsequently.

Check back for part 2 shortly ….

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Just a quick message to say that we’ll be publishing some great articles about beer and beverages shortly.

Thanks and keep an eye on us - we’ve got great plans afoot!

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