The Beer Brewer

Sherry type wines produced outside Spain often utilize different production techniques than those described in previous sections.

A baking process is employed in to produce Sherry wines in California and Australia. Base wine is produced from red or white varieties with a pH of 3.6 and fermented to dryness. Fortification of the wine occurs (with neutral spirit), to a level dependant upon style, before being filled into tanks and exposed to a heat source (49°C) for a month, which may be a heated room or steam filled coil inserted within the barrel. Oxidation of the wine rapidly occurs, (particularly if oxygen is introduced in the system) which lead caramelisation of the wine and the production of Maillard type compounds that color the wine. Similarly oxidation of ethanol occurs, leading to the formation of required acetaldehyde. There is little or no loss of volatile and total acids during this process. The coarse/strong flavors produced by baking require maturation of the wine, often in oak barrels, for a period of six months. Quality Sherry styles may receive up to three years maturation. Blending and sweetening with fortified grape juice occurs prior to bottling.

Submerged culture type processes are used to produce Sherry style wine in Australia, although some non blended solera type wine is produced. Base wine of pH 3.2 is fortified to 15% alcohol, inoculated with flor yeast and added to a vessel equipped with aeration and agitation devices, to continually disperse the suspended yeast throughout the whole volume of wine rather than allowing a film to form. Temperature is maintained around 15°C. This system rapidly creates high acetaldehyde levels.

Fortification again occurs, post flor treatment, raising alcohol to 17 – 19%, before the wine is blended with baked sherry types, on account of its lack of complexity.

Other Commercial Styles
Cream and pale cream (11.5 – 14% reducing sugars), dry (< 4.5% reducing sugars)and medium type sherries (4.5 – 11.5%) are primarily sweetened oloroso wines containing small percentages of amontillado, with fino added to lighten the style. Sweetness is achieved in these oloroso wines via the addition of specific sweetening wines, which are fortified to 17 - 18% alcohol by volume at the beginning of fermentation, thus preserving the majority of the must sugars.

Spoilage
Spoilage is primarily via the action of heterofermentative lactic acid bacteria, particularly during flor maturation, although maintenance of hygienic practices minimizes this risk.

Stabilization and Bottling
Sherry wines are fined and cold stabilized for a couple of weeks at temperatures of – 9°C, to remove any precipitate and are filtered through a 1.2µm membrane prior to bottling, thus ensuring microbial stability.

This is the concluding part of the Maturation and Wine Characteristics. The work continues though!

A loss of the flor may occur in fino types not regularly replenished or of considerable age, which results in the production of richer amontillado type wines. Wines designated for this style are re-fortified to 17.5% alcohol by volume and aged in subsequent solera systems in the bodega for at least eight years, in butts filed to 95% capacity . Oxidation of the wine in the soleras occurs, transforming the pale wine into a dark gold color. Characteristic nutty flavors are also evident in the product, which are created by a combination of compound oxidation, esterification and concentration of fusel oils and other non volatile compounds due to the process of evaporation in the wooden butts. This wine displays a fino aroma, but a fuller body.

As previously mentioned, oloroso wines do not develop flor yeast, thus such types undergo a process of aging similar to amontillado types. Butts are filled to 95% of capacity and stored in warmer parts of the bodega, as temperature control is not needed to ensure flor formation. This warmth may aid extraction of phenolic compounds from the oak, which contribute to the oxidation of the wine. Oloroso solera systems contain less ciradera stages and blending is more limited, thus slightly more barrel to barrel variation may occur in these wines. Maturation is similar to the oxidative process described for the amontillado types above, with color and flavor forming via the oxidation of phenolic compounds in the wine and a concentration effect. Esters and volatile acid levels in the wine increase, due to the conversion of acetaldehyde to acetic acid, which may combine with ethanol to produce ethyl acetate. Wines emerge fairly dry, although sweetness and body is more evident in this style than in fino Sherry, as glycerol formed in initial fermentation is not utilized by the flor yeasts and is therefore present in concentrations between 7 – 9 g l. Oloroso wines contain 17- 18% alcohol by volume.

Raya style Sherry is a form of oloroso that is matured in direct sunlight, due to high phenol content of the wine. A very dark wine is formed from the oxidative reactions that subsequently occur. Raya are typically blended with oloroso wines before sale.

We’ll be moving on now ..

This continues the previous articcle on “Maturation and Individual Wine Characteristics”

Other products, such as terpenes and lactones are also formed at this stage by the yeast, which also contribute to the fino character. Terpenes are synthesized by the flor yeasts, and produce rose like, or floral aromas in the wine. Lactones are esters synthesized by yeast and appear to originate from an esterification reaction between α –ketobutyric acid and acetaldehyde. They are responsible for strong nutty aromas. Some of the lactone precursors may be extracted from the wooden cask material by the ethanol within the wine.
Although great care is taken not to disturb flor films, regular removal every three months and replenishment of wine (from the same anada or vintage) is required to replenish diminishing oxygen levels and nutrients, which are crucial to maintaining a flor coverage.

Wines with flor remain unblended for a year, after which time they become part of the unique solera system (Figure 3) common to all Sherries. This system is a form of blending, facilitated by topping up of older butts with young wine from more recent vintages, to achieve age and consistency in the final product. This system is composed of a number of stages or criadera, (which vary from producer to producer), each containing wine at a particular age, with the oldest stage being that of the solera stage or solera. As wine is removed from the solera for bottling, the partially empty butts are refilled with wine from the first (or next oldest) criadera or stage. The first criadera is thus replenished from the second (or next oldest) criadera and so on throughout the numerous criaderas, with the final, youngest criadera refilled with a suitable wine from the same vintage (anada). Wines drawn from individual butts in a criadera are blended together before being added to the next criadera, thus ensuring product consistency during production. This indicates solera systems for fino wines may involve up to 600 butts, arranged in one solera and five criaderas. Aging of fino types takes at least three years and may extend to eight.

Finos emerge with 15.5 – 17% alcohol by volume, a distinct pungent (green) aroma and a dryness that lacks acidity.

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.

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.

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.

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 …

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.

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.